IJSPT
ORIGINAL RESEARCH
TRUNK AND HIP ELECTROMYOGRAPHIC ACTIVITY DURING SINGLE LEG SQUAT EXERCISES DO SEX DIFFERENCES EXIST? Lori Bolgla, PT, PhD, MAcc, ATC1 Naomi Cook, PT, DPT1 Kyle Hogarth, PT, DPT1 Jennifer Scott, PT, DPT1 Cary West, PT, DPT1
ABSTRACT Purpose/Background: Researchers have identified sex-differences in lower extremity muscle activation during functional activities that involve landing and cutting maneuvers. However, less research has been conducted to determine if muscle activation differences occur during rehabilitation exercises. The purpose of this investigation was to determine if sex-differences exist for activation amplitudes of the trunk and hip muscles during four single leg squat (SLS) exercises. Methods: Eighteen males and 16 females participated. Surface electromyography (EMG) was used to determine muscle activity of the abdominal obliques (AO), lumbar extensors (LE), gluteus maximus (GMX), and gluteus medius (GM) during four SLS exercises. Data were expressed as a percentage of a maximum voluntary isometric contraction (% MVIC). A 2 X 4 mixed-model analysis of variance with repeated measures was used to determine the interaction between sex and exercise on each muscle’s activity. Results: No interaction effect existed between sex and exercise. A main effect for sex existed for the GM and LE. On average, females generated 39% greater GM (27.6 ± 10.4 % MVIC versus 19.8 ± 10.5 % MVIC) and 40% greater LE (8.0 ± 2.8 % MVIC versus 5.7 ± 2.8 % MVIC) activity than males. All subjects, regardless of sex, demonstrated similar GMX and AO activity. Overall EMG values ranged from 11.0 % MVIC to 14.7 % MVIC for the GMX and 5.7 % MVIC to 8.8 % MVIC for the AO. Conclusions: None of the subjects generated sufficient EMG activity for strength gains. Females generated a moderate level of GM activity appropriate for neuromuscular re-education/endurance. Males generated a low level of GM activity that may not necessarily be sufficient to improve GM function. Subjects exhibited low levels of EMG activity for the other muscles. These findings suggest that clinicians modify and/or prescribe different exercises than those studied herein for the purpose of improving GM, GMX, AO, and LE function. Key words: electromyography, exercise, hip, sex Level of Evidence: 3b
1
Georgia Regents University, Augusta, GA, USA
This investigation was conducted at the Medical College of Georgia (now Georgia Regents University) and completed for partial fulfillment of a degree. No grant monies were received to support this investigation. All subjects signed an informed consent document approved by the Medical College of Georgia Human Assurance Committee prior to participating in this investigation. The authors would like to thank the following individuals for assistance with data collection: Mario Cruz, PT, DPT, SCS, ATC; Lauren Hayes Roberts, PT, DPT; Angela Minning Buice, PT, DPT; and Tori Smith Pou, PT, DPT
CORRESPONDING AUTHOR Lori Bolgla, PT, PhD, MAcc, ATC Associate Professor EC-1334 Department of Physical Therapy Georgia Regents University Augusta, GA 30912 E-mail: [email protected]
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 756
INTRODUCTION Hip strengthening exercise represents an important intervention strategy for the rehabilitation of a variety of low back and lower extremity pathologies.1-4 Many exercises address gluteus medius (GM) and gluteus maximus (GMX) weakness that commonly accompanies these pathologies. Clinicians routinely prescribe weight bearing exercises because they better simulate muscle activation during activities of daily living.5,6 A single-leg squat (SLS) exercise represents one of the most commonly used exercises.6-8 Researchers have used surface electromyography (EMG) to quantify GM and GMX muscle activation during a SLS.5-7,9-12 The common belief is that that exercises that require more EMG activity result in greater potential for strength gains.13 Although the SLS requires hip muscle activity, it also requires trunk muscle activation. For example, when performing a SLS, an individual activates the GM to prevent excessive contralateral pelvic drop.14 The individual also must simultaneously activate the ipsilateral abdominal oblique (AO) and lumbar extensor (LE) muscles. Ipsilateral contraction of these muscles not only stabilizes the spine15 but also produces trunk lateral flexion, an action that can prevent excessive contralateral pelvic drop when performing a SLS.16 To date, minimal data exist regarding AO and LE activation during a SLS.17,18 Researchers19-21 have identified sex-differences with respect to muscle activation during functional tasks that involve landing and cutting maneuvers. However, limited information exists regarding EMG activity comparisons between males and females during rehabilitation exercises. Zeller et al10 reported general overall higher GM and GMX activity in females than males who performed a SLS. Dwyer et al11 also reported overall greater eccentric GMX activation in females during three weight bearing rehabilitation tasks (e.g., SLS, a lunge, and step-over maneuver). Findings from these works have provided preliminary evidence for different levels of muscle activity occurring between males and females during a SLS. Further exploration and identification of these differences may provide important information for the need for sex-specific exercise prescription. The purpose of this investigation was to determine if sex-differences existed for activation amplitudes of the trunk and hip muscles during four SLS exer-
cises. The authors hypothesized that females would exhibit higher activation amplitudes than males during these exercises. METHODS Subjects Thirty-four healthy, recreationally active individuals, 18 males (mean age 24.3 ± 3.4 yr, mass 81.2 ± 9.7 kg, and height 1.8 ± .1 m) and 16 females (mean age 24.0 ± 1.5 yr, mass 59.9 ±8.8 kg, and height 1.65 ± .1 m), volunteered for this study. A sample of convenience was recruited from a local university setting. Subjects participated if they met the following inclusion criteria: a) no history of surgery for the spine or lower extremities; b) the ability to stand on each lower extremity (e.g. perform a single leg stance on each lower extremity) at least 30 seconds while keeping their eyes open; and c) demonstrated normal lower extremity range of motion and strength with manual muscle testing. Exclusion criteria included the following: a) inability to stand on a single lower extremity less than 30 seconds while keeping their eyes open; b) history of disease affecting the spine and lower extremities such as diabetes, peripheral neuropathy, arthritis, or fibromyalgia; c) history of significant spine or lower extremity injury in the previous year, or d) history of allergic reaction to adhesive tape. The investigators explained the benefits and risks of this study to all participants. The Medical College of Georgia Human Assurance Committee approved the study protocol and all subjects signed the informed consent document. Procedures After obtaining informed consent, subjects participated in a warm-up session. They rode a stationary bike for three minutes at a sub-maximal speed and performed gentle stretching to the trunk extensor, trunk rotator, hamstrings, quadriceps, and calf muscles. Stretching consisted of three repetitions of each stretch with a 15-second hold. Then, an investigator instructed subjects in the four SLS exercises: wall squat, mini-squat, lateral step-down, and front stepdown (Figures 1-4). Subjects performed each exercise using the dominant limb, defined as the leg with which the subject naturally kicked a ball.22 The subject’s skin was prepared for the surface EMG electrodes by shaving (if needed) and cleaning
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 757
Figure 1. Unilateral Wall-Squat. Subjects stood with their backs against a wall, the knee of the stance limb extended, and the heel of the dominant limb a 30-cm distance away from the wall. They positioned the other leg (non-dominant leg) in front of them with the knee and the hip flexed so that the heel of this leg did not touch the floor. During this exercise, subjects lowered themselves 15 cm (by sliding their back down the wall while bending the knee of the stance leg) and returned to the start position. A full-length mirror was placed in front of the subjects as they performed each exercise. The mirror provided subjects visual feedback to help them maintain a vertical trunk position.
the skin with isopropyl alcohol over the following muscles: 1) AO; 2) LE; 3) GMX; and 4) GM. Bi-polar Ag-AgCl surface electrodes (Medicotest, Rolling Meadows, IL), measuring 5 mm in diameter with an interelectrode distance of approximately 20 mm, were placed in parallel alignment over the belly of each muscle.23,24 A ground electrode was placed on the ulnar styloid process on the same side of the instrumented leg. Electrodes were then secured with tape to minimize slippage during testing. Placement was confirmed by observing the electrical signal on an oscilloscope during common manual muscle testing techniques.25 A 3-second standing “rest” file was taken to exclude ambient noise.
Figure 2. Unilateral Mini-Squat. Subjects stood solely on the stance limb while bending the knee of the non-stance limb enough to keep the non-stance foot off the floor. While keeping the trunk vertical, they bent the knee of the stance limb to lower the trunk 15 cm and returned to the start position. A full length mirror was placed in front of the subjects as they performed each exercise. The mirror provided subjects visual feedback to help them maintain a vertical trunk position.
The subjects performed two maximum voluntary isometric contractions (MVIC) for each muscle to enable normalization of the raw EMG data using common manual muscle testing techniques (Table 1). Subjects generated the MVIC using the “make” test26 to the beat of a metronome. They generated force over a 2-second period and held the maximum force for an additional 5-second period. Subjects performed 1 practice27 and 2 test trials. They received strong verbal encouragement28 during each test trial and rested 30 seconds between each MVIC. A computer algorithm determined the maximum root-mean-square (RMS) amplitude recorded across a moving 500-millisecond average window across the MVICs.29 The window having the greatest amplitude was assumed to represent 100% of the maximum voluntary isometric contract (% MVIC) and was used to express all data as a % MVIC for statistical analysis. Pilot testing
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 758
Figure 3. Lateral Step-Down. Subjects stood solely on the stance limb on the edge of a 6-inch step. They positioned the non-stance limb with the knee extended, foot dorsiflexed, and the hip in a neutral position. The foot of the non-stance limb did not contact the step. While keeping the trunk vertical, subjects bent the knee of the stance limb until the heel of the nonstance limb touched the ground and returned to the start position (a 15-cm excursion). A full-length mirror was placed in front of the subjects as they performed each exercise. The mirror provided subjects visual feedback to help them maintain a vertical trunk position.
Figure 4. Front Step-Down. Subjects stood solely on the stance limb on the edge of a 6-inch step facing away from the step. They positioned the non-stance limb with the knee extended, foot dorsiflexed, and the hip in a slightly flexed position. The foot of the non-stance limb did not contact the step. While keeping the trunk vertical, subjects bent the knee of the stance limb until the heel of the non-stance limb touched the ground and returned to the start position (a 15-cm excursion). A full-length mirror was placed in front of the subjects as they performed each exercise. The mirror provided subjects visual feedback to help them maintain a vertical trunk position.
showed acceptable reliability for these procedures as evidenced by intraclass correlation coefficients (ICC [3,1]) ranging from 0.80 to 0.97.
gators used the visual feedback to minimize the risk of subjects using excessive ipsilateral trunk lean during exercise that could reduce demands placed on the GM.30 Upon completion of testing, subjects were instructed to refrain from any physical activity, other than normal walking, for a 24-hour period to minimize the potential for muscle and joint soreness.
Next, subjects performed 15 repetitions of each exercise to the beat of a metronome set at 40 beats per minute6 while the investigators collected EMG data. During each exercise, the investigators used an external trigger switch to delineate the beginning and end of each repetition performed. The order of testing was randomly determined to reduce ordering effects. Subjects rested three minutes between each exercise to minimize fatigue. Subjects were instructed to perform all the exercises with the trunk in a vertical position and received visual feedback via use of a full-length mirror. Based on prior work, the investi-
EMG Analysis An 8-channel EMG system (Run Technologies, Mission Viejo, CA) recorded all muscle activity. Subjects wore a Myopac-Jr transmitter belt unit (Run Technologies) that transmitted raw EMG data at 2000 Hz via a fiber optic cable to its receiver unit. Unit specifications included a common mode rejection
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 759
Table 1. Summary of test positions for determining the electromyographic activity during a maximum voluntary isometric contraction.25,30,42 Muscle Abdominal Obliques
Lumbar Extensors
Gluteus Maximus
Gluteus Medius
Test Position Subjects were positioned on a plinth in hook-lying with the arms crossed in front of the chest. An investigator stabilized the lower extremities by pressing the subjects’ feet firmly on the plinth. For testing, subjects performed an oblique trunk curl (i.e. sit-up) by lifting the test-side scapula off the plinth and toward the opposite knee while another investigator applied manual resistance to the anterior aspect of the test shoulder. Subjects did not lift the other scapula off the plinth during testing. Subjects were positioned on a plinth in prone with the arms crossed in front of the chest. The lower extremities were stabilized by placing an immovable strap across the back of the legs and pelvis and securing the strap to the plinth. For testing, subjects lifted the chest off the table while an investigator applied manual resistance at the scapulae. Subjects were positioned on a plinth in prone with the upper extremities “hugging” the plinth and the test-side knee flexed to 90°. Resistance was provided by an immovable strap placed across the distal aspect of the thigh and secured to the plinth. Subjects were positioned on a plinth in sidelying with 2 pillows placed between the thighs and the test limb on top. Resistance was provided by an immovable strap placed across the distal aspect of the thigh and secured to the plinth.
ratio exceeding 90 dB, amplifier gain of 2000, and input impedance exceeding 1 MOhm. Raw EMG data were band pass filtered between 20 and 500 Hz using Datapac software (Run Technologies), stored on a personal computer, and analyzed using Datapac software. For each exercise, the authors determined the RMS amplitude for each repetition. Data were then expressed as a % MVIC. The last 10 repetitions during each exercise were averaged and used for statistical analysis.30 Statistical Analysis Separate 2 (sex) X 4 (exercise) mixed-model analyses of variance (ANOVA) with repeated measures were used to determine between-sex differences in muscle amplitudes during each exercise. The level of significance was established at the 0.05 level. Since this investigation was exploratory in nature, the decision was made not to adjust the p-value for multiple pairwise comparisons at risk of incurring a Type II error.31 Effect sizes (Cohen’s d) also were calculated for pairwise comparisons deemed significant. Cohen32 has interpreted effect sizes as follows: 0.20 (small); 0.50 (medium); and 0.80 (large). We considered an effect size of 0.50 and greater as representing a clinically meaningful difference.6
RESULTS Table 2 summarizes normalized EMG activity descriptive statistics for males and females during each exercise. EMG activity ranged from 5.4 to 8.8 % MVIC and 5.1 to 9.4 % MVIC for the AO and LE, respectively. No interaction effect existed for the AO (p=0.40) or LE (p=0.08); however, a main effect for sex did exist for the LE (p=0.02). Cohen’s d of 0.84 suggested a large effect and clinically meaningful difference in LE activity. EMG activity for the GMX ranged from 8.4 to 20.4 % MVIC; yet no interaction (p=0.77) or main (p=0.12) effect for sex existed. EMG activity for the GM ranged from 18.5 to 32.0 % MVIC. While an interaction effect did not exist (p=0.54), a main effect for sex did (p=0.04). Cohen’s d of 0.75 suggested a medium-to-large effect and clinically meaningful difference in GM activity. DISCUSSION The purpose of this study was to determine if sexdifferences existed for EMG amplitudes of the trunk and hip muscles during four SLS exercises. The authors hypothesized that females would generate greater EMG muscle activity than males for all exercises. Findings from this study partially supported this hypothesis. No sex-differences existed with
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 760
Table 2. Comparison of mean (+ standard deviation) electromyographic activity during exercise between males and females expressed as 100% of a maximum voluntary isometric contraction. Effect sizes (d) reported to provide information regarding clinical significance. Wall Slide
Abdominal Obliques Lumbar Extensors Gluteus Maximus Gluteus Medius
Mini-Squat
Lateral Step-Down
Front Step-Down
Male 5.4 (4.0)
Female 5.9 (2.2)
Male 6.2 (4.1)
Female 8.8 (5.6)
Male 5.6 (3.8)
Female 5.8 (1.9)
Male 6.0 (4.2)
Female 6.9 (2.5)
Grand Mean for all Exercises Male Female 5.8 (3.3) 6.8 (3.4)
5.8 (2.8)
9.4 (5.1)
6.8 (2.8)
7.9 (4.1)
5.1 (2.2)
7.4 (4.1)
5.2 (2.1)
7.3 (4.3)
5.7 (2.8)
8.0 (2.7)*
17.6 (13.7)
20.4 (9.3)
9.0 (5.5)
12.8 (9.7)
8.4 (4.7)
12.3 (7.5)
8.8 (5.1)
13.3 (8.1)
11.0 (6.7)
14.7 (6.8)
21.6 (8.6)
32.0 (13.1)
20.3 (11.2)
26.6 (12.8)
18.5 (10.2)
24.6 (10.6)
19.0 (9.2)
27.2 (13.9)
19.8 (10.5)
27.6(10.4)†
*Females generated significantly greater overall lumbar extensor activity than males (p=.02, d=0.84) † Females generated significantly greater overall gluteus medius activity than males (p = 0.04, d = 0.75)
respect to EMG activity during any individual exercise, suggesting no interaction effect between sex and exercise. When analyzing overall muscle activation across all exercises, females generated 1.4 times greater LE and GM activity than males. Few researchers10,11,18 have compared EMG activity between males and females during rehabilitation exercises. Furthermore, most used different variations of the SLS exercises than used in the current study, limiting the ability to make conclusive comparisons to prior works. However, patterns of EMG activity reported may provide important clinical information. While not statistically significant for all comparisons, females in the current study generated relatively greater EMG activity during each individual exercise. These findings partially agreed with Zeller et al10 and Dwyer et al.11 Both investigators collected EMG data as subjects squatted as low as possible on a single limb, a task that was more demanding than the exercises used in the current study. EMG amplitudes in the Zeller and Dwyer studies were much higher and most likely reflected greater demands placed on the lower extremity muscles during a deep squat. Females also exhibited greater GMX activity than males. This pattern suggested that females in both studies generated more EMG activity to control hip flexion during the deep squat. Conflicting data existed with respect to GM activity. Zeller et al10 found higher GM activity in males than females whereas Dwyer et al11 found no differences. One possible explanation for conflicting results
could be due to trunk position during the SLS. Subjects who performed the SLS with increased lateral trunk flexion over the stance limb would generate less GM activity.30,33 Neither Zeller et al nor Dwyer et al reported trunk kinematics or methods to limit trunk movements, which precluded the ability to make this determination. To control for this compensation, subjects in the current study received visual feedback for maintaining a vertical trunk position.30 Using this procedure, the authors found that females generated greater GM activity across all exercises than males. Leetun et al34 compared hip abductor strength in males and females and found that males exhibited significantly greater strength. Females in the current study could have had less hip abductor strength than males. This relative weakness could have required them to use greater GM activity to keep the pelvis level. This determination cannot be made conclusively, because hip abductor strength was not measured. Structural differences between males and females also may provide another explanation for greater overall GM during the SLS exercises. Although a common belief, evidence does not support the presumption that females have wider pelvises than males.35,36 However, Livingston and Gahagan35 found that females have a significantly less femur length compared to males. An increased pelvic width-to-femoral length ratio may reflect a wider pelvis, relative to femur length, in females than males. This relationship may explain the increased demands needed for
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 761
females to maintain a level pelvis over the femur during the SLS exercises.. Interestingly, Bouillon et al18 collected EMG data as subjects performed a front step-down in a similar manner as the current study. They reported EMG activity ranging from 12 % MVIC to 14 % MVIC for the AO, LE, GMX, and GM. Compared to other works,7,9,10,12 these GMX and GM values appeared very low. However, lower AO and LE values agreed with values obtained during the current study. Subjects in Bouillon et al and the current study performed the SLS task with the trunk in a vertical position over the pelvis. Maintaining good trunk alignment would minimize the need for AO and LE activation.14 Another reason for relatively low activation may have resulted from the stabilizing effects from other muscles, like the multifidi and transversus abdominis, not assessed in the Bouillon et al18 or the current investigation. Cholewicki and Van Vliet37 reported that no single muscle contributed to more than 30% of the overall stability of the lumbar spine during trunk stabilization exercises. The multifidi and transversus abdominis most likely helped stabilize the trunk during the SLS exercises, resulting in the need for less AO and LE activity. Future investigations are needed to understand the role of other trunk muscles during the SLS exercises. Finally, it was noteworthy that females in the current study generated greater overall LE activity than males (8.0 % MVIC versus 5.7 % MVIC) during the exercises. Although significantly greater with a large effect (Cohen’s d = 0.84), this very low magnitude suggested minimal, if any, clinically important difference.8,38 Clinical Applications Findings from this investigation may have important clinical implications when designing and implementing an exercise program designed to improve strength of the trunk and hip muscles in males and females. Reiman et al8 systematically reviewed GMX and GM activity during various rehabilitation exercises. For this purpose, they classified EMG activity as follows: low (0-20% MVIC), moderate (21-40% MVIC), high (41-60% MVIC), and very high (> 60% MVIC). Moderate activation would be required for neuromuscular re-education/endurance8,17 whereas high and very high activity would be necessary for meaningful strength gains.39,40
Using these threshold values, males generated low (19.8% MVIC) and females moderate (27.6% MVIC) GM activity. These results suggested that females would have greater benefit for improving GM function during our exercises than males. Depending on the therapeutic goals, clinicians should consider applying resistance above and beyond body weight to better activate the GM, especially for males, when prescribing the exercises used in our study. Future investigations are needed to determine the amount of additional load needed for strengthening effects. Findings from the current study showed no differences in average GMX activation between males (11.0 % MVIC) and females (14.7 % MVIC). These values were much lower than those reported by Ayotte et al6 (values ranging from 56 % MVIC to 86 % MVIC). Even though the exercises used in their study were replicated in the current study, higher GMX values most likely reflected methodological differences. Subjects in the current study performed a total of 15 repetitions instead of three repetitions, in order to better represent exercise prescription. Average EMG activity for the last 10 repetitions instead of average concentric phase activation during the last two repetitions was analyzed. Researchers39,41 have compared isometric, concentric, and eccentric muscle activity and noted relatively greater EMG activity during a concentric action. The current findings have provided evidence that clinicians apply additional resistance if using these exercises for GMX neuromuscular re-education/endurance or strengthening effects. Regardless of sex, AO and LE values were low (ranging from 5.7 % MVIC to 8.0 % MVIC) and similar to the findings of prior works.17,18 Low values most likely represented a stabilizing, rather than a torque generating, effect. The SLS exercises used in this study would provide minimal, if any, therapeutic benefit for neuromuscular re-education/endurance or strengthening purposes. Limitations The present study has limitations. One limitation was the possibility of signal crosstalk from the use of surface EMG. Signal crosstalk was minimized by applying electrodes in a standardized and recommended manner.23 The principal investigator has extensive experience with electrode placement and
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 762
supervised all placements. Furthermore, proper placement was confirmed by observing EMG activity on an oscilloscope during specific manual muscle testing procedures. Another limitation was generalizability. Subjects in this investigation were recreationally active and healthy. Therefore, the results cannot be used to analyze EMG activity levels that individuals with pathology or weakness would generate. CONCLUSION Results from this study demonstrated an overall between-sex difference in GM and LE activation during four SLS exercises. Information gained from this study may enhance exercise prescription, highlighting that these exercises may benefit females more than males for addressing GM function or as a stimulus for strengthening. These findings also showed that these exercises would provide minimal, if any, stimulus for strengthening of the GMX, AO, and LE or enhancing their function. Clinicians should develop and implement other exercises in order to effectively provide strengthening stimuli for these muscles. In summary, none of the SLS exercises challenged any of the muscles enough to provide sufficient stimulus to induce strength gains (e.g., > 40 % MVIC).8 REFERENCES 1. Spencer-Gardner L, Eischen JJ, Levy BA, Sierra RJ, Engasser WM, Krych AJ. A comprehensive five-phase rehabilitation programme after hip arthroscopy for femoroacetabular impingement. Knee Surg Sports Traumatol Arthrosc. 2014;22(4):848-859. 2. Bolgla LA, Boling MC. An update for the conservative management of patellofemoral pain syndrome: a systematic review of the literature from 2000 to 2010. Int J Sports Phys Ther. 2011;6(2):112-125.
extermity muscles during 5 unilateral weight-bearing exercises. J Orthop Sports Phys Ther. 2007;37(2):48-55. 7. Distefano LJ, Blackburn JT, Marshall SW, Padua DA. Gluteal muscle activation during common therapeutic exercises. J Orthop Sports Phys Ther. 2009;39(7):532-540. 8. Reiman MP, Bolgla LA, Loudon JK. A literature review of studies evaluating gluteus maximus and gluteus medius activation during rehabilitation exercises. Physiother Theory Pract. 2012;28(4):257-268. 9. Boren K, Conrey C, Le Coguic J, Paprocki L, Voight M, Robinson TK. Electromyographic analysis of gluteus medius and gluteus maximus during rehabilitation exercises. Int J Sports Phys Ther. 2011;6(3):206-223. 10. Zeller BL, McCrory JL, Kibler WB, Uhl TL. Differences in kinematics and electromyographic activity between men and women during the singlelegged squat. Am J Sports Med. 2003;31(3):449-455. 11. Dwyer MK, Boudreau SN, Mattacola CG, Uhl TL, Lattermann C. Comparison of lower extremity kinematics and hip muscle activation during rehabilitation tasks between sexes. J Athl Train. 2010;45(2):181-190. 12. Boudreau SN, Dwyer MK, Mattacola CG, Lattermann C, Uhl TL, McKeon JM. Hip-muscle activation during the lunge, single-leg squat, and step-up-andover exercises. J Sport Rehabil. 2009;18(1):91-103. 13. Anderson L, Magnusson S, Neilsen M, Haleem J, Poulsen K, Aagaard P. Neuromuscular activation in conventional therapeutic exercises and heavy resistance exercise: implications for rehabilitation. Physical Therapy. 2006;86(5):683-697. 14. Neumann DA. Hip. In: Neumann DA, ed. Kinesiology of the Musculoskeletal System. St. Louis: Mosby; 2010:465-519. 15. Bergmark A. Stability of the lumbar spine. A study in mechanical engineering. Acta Orthop Scand Suppl. 1989;230:1-54.
3. Webster KA, Gribble PA. A comparison of electromyography of gluteus medius and maximus in subjects with and without chronic ankle instability during two functional exercises. Phys Ther Sport. 2013;14(1):17-22.
16. Neumann DA. Axial skeleton: muscle and joint interactions. In: Neumann DA, ed. Kinesiology of the Musculoskeletal System. 2nd ed. St Louis: Mosby; 2010:379-422.
4. Reiman MP, Weisbach PC, Glynn PE. The hips influence on low back pain: a distal link to a proximal problem. J Sport Rehabil. 2009;18(1):24-32.
17. Ekstrom R, Donatelli RA, Carp K. Electromyographic analysis of core trunk, hip, and thigh muscles during 9 rehabilitation exercises. J Orthop Sports Phys Ther. 2007;37(12):754-762.
5. Krause DA, Jacobs RS, Pilger KE, Sather BR, Sibunka SP, Hollman JH. Electromyographic analysis of the gluteus medius in five weight-bearing exercises. J Strength Cond Res. 2009;23(9):2689-2694. 6. Ayotte NW, Stetts DM, Keenan G, Greenway EH. Electromyographical analysis of selected lower
18. Bouillon LE, Wilhelm J, Eisel P, Wiesner J, Rachow M, Hatteberg L. Electromyographic assessment of muscle activity between genders during unilateral weight-bearing tasks using adjusted distances. International Journal of Sports Physical Therapy. 2012;7(6):595-605.
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 763
19. Zazulak BT, Ponce PL, Straub SJ, Medecky MJ, Avedisian L, Hewett TE. Gender comparison of hip muscle activity during single-leg landing. J Orthop Sports Phys Ther. 2005;35(5):292-299. 20. Decker MJ, Torry MR, Wyland DJ, Sterett WI, Steadman JR. Gender differences in lower extremity kinematics, kinetics, and energy absorption during landing. Clin Biomech. 2003;18:662-669. 21. Landry SC, McKean KA, Hubley-Kozey CL, Stanish WD, Deluzio KJ. Neuromuscular control and lower limb biomechanical differences exist between male and female elite adolescent soccer players during an unanticipated run and crosscut maneuver. Am J Sports Med. 2007;35(11):1901-1911. 22. Bolgla LA, Keskula DR. Reliability of lower extremity functional performance tests. J Orthop Sports Phys Ther. 1997;26(3):138-142. 23. Rainoldi A, Melchiorri G, Caruso I. A method for positioning electrodes during surface EMG recordings in lower limb muscles. J Neurosci Methods. 2004;134:37-43. 24. Cram JR, Kasman GS. Introduction to Surface Electromyography. Gaithersburg, MD: Aspen Publishers, Inc.; 1998. 25. Kendall FP, McCreary EK, Provance P, Rodgers MM, Romani WA. Muscles. Testing and Function with Posture and Pain. 5th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2005. 26. Andrews AW, Thomas MW, Bohannon RW. Normative values for isometric muscle force measurements obtained with hand-held dynamometers. Phys Ther. 1996;76(3):248-259. 27. Mohr KJ, Kvitne RS, Pink MM, Fideler B, Perry J. Electromyography of the quadriceps in patellofemoral pain with patellar subluxation. Clin Orthop. 2003;415:261-271. 28. Campenella B, Mattacola CG, Kimura IF. Effect of visual feedback and verbal encouragement on concentric quadriceps and hamstrings peak torque of males and females. Isokin Exer Sci. 2000;8(1):1-6. 29. Bamman MM, Ingram SG, Caruso JF, Greenisen MC. Evaluation of surface electromyography during maximal voluntary contraction. J Strength Cond Res. 1997;11(2):68-72. 30. Bolgla LA, Uhl TL. Electromyographic analysis of hip rehabilitation exercises in a group of healthy subjects. J Orthop Sports Phys Ther. 2005;35(8):487-494.
31. Feise RJ. Do multiple outcome measures require p-value adjustment? BMC Med Res Methodol. 2002;2:8. 32. Cohen J. Statistical Power Analysis for Behavioral Sciences. 2nd ed. Hillsdale, NJ: Lawrence Erlbaum Associates; 1988. 33. Neumann DA, Hase AD. An electromyographic analysis of the hip abductors during load carriage: implications for hip joint protection. J Orthop Sports Phys Ther. 1994;19(5):296-304. 34. Leetun DT, Ireland ML, Willson JD, Ballantyne BT, Davis IM. Core stability measures as risk factors for lower extremity injury in athletes. Med Sci Sports Exer. 2004;36(6):926-934. 35. Livingston LA, Gahagan JC. The wider gynaecoid pelvis-- larger Q angle-- greater predisposition to ACL injury relationship: myth or reality? Clin Biomech. 2001;16:951-952. 36. Horton MG, Hall TL. Quadriceps femoris muscle angle: normal values and relationships with gender and selected skeletal measures. Phys Ther. 1989;69(11):897-901. 37. Cholewicki J, VanVliet JJ. Relative contribution of trunk muscles to the stability of the lumbar spine during isometric exertions. Clin Biomech. 2002;17(2):99-105. 38. Jaeschke R, Singer J, Guyatt GH. Measurement of health status. Ascertaining the minimal clinically important difference. Control Clin Trials. 1989;10(4):407-415. 39. Anderson KG, Behm DG. Maintenance of EMG activity and loss of force output with instability. J Strength Cond Res. 2004;18(3):637-640. 40. Escamilla RF, Lewis C, Bell D, et al. Core muscle activation during Swiss ball and traditional abdominal exercises. J Orthop Sports Phys Ther. 2010;40(5):265-276. 41. Selseth A, Dayton M, Cordova ML, Ingersoll CD, Merrick MA. Quadriceps concentric EMG activity is greater than eccentric EMG activity during the lateral step-up exercise. J Sport Rehabil. 2000;9(2):124-134. 42. Souza GM, Baker LL, Powers CM. Electromyographic activity of selected trunk muscles during dynamic spine stabilization exercises. Arch Phys Med Rehabil. 2001;82(11):1551-1557.
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 764
ORIGINAL RESEARCH
TRUNK AND HIP ELECTROMYOGRAPHIC ACTIVITY DURING SINGLE LEG SQUAT EXERCISES DO SEX DIFFERENCES EXIST? Lori Bolgla, PT, PhD, MAcc, ATC1 Naomi Cook, PT, DPT1 Kyle Hogarth, PT, DPT1 Jennifer Scott, PT, DPT1 Cary West, PT, DPT1
ABSTRACT Purpose/Background: Researchers have identified sex-differences in lower extremity muscle activation during functional activities that involve landing and cutting maneuvers. However, less research has been conducted to determine if muscle activation differences occur during rehabilitation exercises. The purpose of this investigation was to determine if sex-differences exist for activation amplitudes of the trunk and hip muscles during four single leg squat (SLS) exercises. Methods: Eighteen males and 16 females participated. Surface electromyography (EMG) was used to determine muscle activity of the abdominal obliques (AO), lumbar extensors (LE), gluteus maximus (GMX), and gluteus medius (GM) during four SLS exercises. Data were expressed as a percentage of a maximum voluntary isometric contraction (% MVIC). A 2 X 4 mixed-model analysis of variance with repeated measures was used to determine the interaction between sex and exercise on each muscle’s activity. Results: No interaction effect existed between sex and exercise. A main effect for sex existed for the GM and LE. On average, females generated 39% greater GM (27.6 ± 10.4 % MVIC versus 19.8 ± 10.5 % MVIC) and 40% greater LE (8.0 ± 2.8 % MVIC versus 5.7 ± 2.8 % MVIC) activity than males. All subjects, regardless of sex, demonstrated similar GMX and AO activity. Overall EMG values ranged from 11.0 % MVIC to 14.7 % MVIC for the GMX and 5.7 % MVIC to 8.8 % MVIC for the AO. Conclusions: None of the subjects generated sufficient EMG activity for strength gains. Females generated a moderate level of GM activity appropriate for neuromuscular re-education/endurance. Males generated a low level of GM activity that may not necessarily be sufficient to improve GM function. Subjects exhibited low levels of EMG activity for the other muscles. These findings suggest that clinicians modify and/or prescribe different exercises than those studied herein for the purpose of improving GM, GMX, AO, and LE function. Key words: electromyography, exercise, hip, sex Level of Evidence: 3b
1
Georgia Regents University, Augusta, GA, USA
This investigation was conducted at the Medical College of Georgia (now Georgia Regents University) and completed for partial fulfillment of a degree. No grant monies were received to support this investigation. All subjects signed an informed consent document approved by the Medical College of Georgia Human Assurance Committee prior to participating in this investigation. The authors would like to thank the following individuals for assistance with data collection: Mario Cruz, PT, DPT, SCS, ATC; Lauren Hayes Roberts, PT, DPT; Angela Minning Buice, PT, DPT; and Tori Smith Pou, PT, DPT
CORRESPONDING AUTHOR Lori Bolgla, PT, PhD, MAcc, ATC Associate Professor EC-1334 Department of Physical Therapy Georgia Regents University Augusta, GA 30912 E-mail: [email protected]
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 756
INTRODUCTION Hip strengthening exercise represents an important intervention strategy for the rehabilitation of a variety of low back and lower extremity pathologies.1-4 Many exercises address gluteus medius (GM) and gluteus maximus (GMX) weakness that commonly accompanies these pathologies. Clinicians routinely prescribe weight bearing exercises because they better simulate muscle activation during activities of daily living.5,6 A single-leg squat (SLS) exercise represents one of the most commonly used exercises.6-8 Researchers have used surface electromyography (EMG) to quantify GM and GMX muscle activation during a SLS.5-7,9-12 The common belief is that that exercises that require more EMG activity result in greater potential for strength gains.13 Although the SLS requires hip muscle activity, it also requires trunk muscle activation. For example, when performing a SLS, an individual activates the GM to prevent excessive contralateral pelvic drop.14 The individual also must simultaneously activate the ipsilateral abdominal oblique (AO) and lumbar extensor (LE) muscles. Ipsilateral contraction of these muscles not only stabilizes the spine15 but also produces trunk lateral flexion, an action that can prevent excessive contralateral pelvic drop when performing a SLS.16 To date, minimal data exist regarding AO and LE activation during a SLS.17,18 Researchers19-21 have identified sex-differences with respect to muscle activation during functional tasks that involve landing and cutting maneuvers. However, limited information exists regarding EMG activity comparisons between males and females during rehabilitation exercises. Zeller et al10 reported general overall higher GM and GMX activity in females than males who performed a SLS. Dwyer et al11 also reported overall greater eccentric GMX activation in females during three weight bearing rehabilitation tasks (e.g., SLS, a lunge, and step-over maneuver). Findings from these works have provided preliminary evidence for different levels of muscle activity occurring between males and females during a SLS. Further exploration and identification of these differences may provide important information for the need for sex-specific exercise prescription. The purpose of this investigation was to determine if sex-differences existed for activation amplitudes of the trunk and hip muscles during four SLS exer-
cises. The authors hypothesized that females would exhibit higher activation amplitudes than males during these exercises. METHODS Subjects Thirty-four healthy, recreationally active individuals, 18 males (mean age 24.3 ± 3.4 yr, mass 81.2 ± 9.7 kg, and height 1.8 ± .1 m) and 16 females (mean age 24.0 ± 1.5 yr, mass 59.9 ±8.8 kg, and height 1.65 ± .1 m), volunteered for this study. A sample of convenience was recruited from a local university setting. Subjects participated if they met the following inclusion criteria: a) no history of surgery for the spine or lower extremities; b) the ability to stand on each lower extremity (e.g. perform a single leg stance on each lower extremity) at least 30 seconds while keeping their eyes open; and c) demonstrated normal lower extremity range of motion and strength with manual muscle testing. Exclusion criteria included the following: a) inability to stand on a single lower extremity less than 30 seconds while keeping their eyes open; b) history of disease affecting the spine and lower extremities such as diabetes, peripheral neuropathy, arthritis, or fibromyalgia; c) history of significant spine or lower extremity injury in the previous year, or d) history of allergic reaction to adhesive tape. The investigators explained the benefits and risks of this study to all participants. The Medical College of Georgia Human Assurance Committee approved the study protocol and all subjects signed the informed consent document. Procedures After obtaining informed consent, subjects participated in a warm-up session. They rode a stationary bike for three minutes at a sub-maximal speed and performed gentle stretching to the trunk extensor, trunk rotator, hamstrings, quadriceps, and calf muscles. Stretching consisted of three repetitions of each stretch with a 15-second hold. Then, an investigator instructed subjects in the four SLS exercises: wall squat, mini-squat, lateral step-down, and front stepdown (Figures 1-4). Subjects performed each exercise using the dominant limb, defined as the leg with which the subject naturally kicked a ball.22 The subject’s skin was prepared for the surface EMG electrodes by shaving (if needed) and cleaning
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 757
Figure 1. Unilateral Wall-Squat. Subjects stood with their backs against a wall, the knee of the stance limb extended, and the heel of the dominant limb a 30-cm distance away from the wall. They positioned the other leg (non-dominant leg) in front of them with the knee and the hip flexed so that the heel of this leg did not touch the floor. During this exercise, subjects lowered themselves 15 cm (by sliding their back down the wall while bending the knee of the stance leg) and returned to the start position. A full-length mirror was placed in front of the subjects as they performed each exercise. The mirror provided subjects visual feedback to help them maintain a vertical trunk position.
the skin with isopropyl alcohol over the following muscles: 1) AO; 2) LE; 3) GMX; and 4) GM. Bi-polar Ag-AgCl surface electrodes (Medicotest, Rolling Meadows, IL), measuring 5 mm in diameter with an interelectrode distance of approximately 20 mm, were placed in parallel alignment over the belly of each muscle.23,24 A ground electrode was placed on the ulnar styloid process on the same side of the instrumented leg. Electrodes were then secured with tape to minimize slippage during testing. Placement was confirmed by observing the electrical signal on an oscilloscope during common manual muscle testing techniques.25 A 3-second standing “rest” file was taken to exclude ambient noise.
Figure 2. Unilateral Mini-Squat. Subjects stood solely on the stance limb while bending the knee of the non-stance limb enough to keep the non-stance foot off the floor. While keeping the trunk vertical, they bent the knee of the stance limb to lower the trunk 15 cm and returned to the start position. A full length mirror was placed in front of the subjects as they performed each exercise. The mirror provided subjects visual feedback to help them maintain a vertical trunk position.
The subjects performed two maximum voluntary isometric contractions (MVIC) for each muscle to enable normalization of the raw EMG data using common manual muscle testing techniques (Table 1). Subjects generated the MVIC using the “make” test26 to the beat of a metronome. They generated force over a 2-second period and held the maximum force for an additional 5-second period. Subjects performed 1 practice27 and 2 test trials. They received strong verbal encouragement28 during each test trial and rested 30 seconds between each MVIC. A computer algorithm determined the maximum root-mean-square (RMS) amplitude recorded across a moving 500-millisecond average window across the MVICs.29 The window having the greatest amplitude was assumed to represent 100% of the maximum voluntary isometric contract (% MVIC) and was used to express all data as a % MVIC for statistical analysis. Pilot testing
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 758
Figure 3. Lateral Step-Down. Subjects stood solely on the stance limb on the edge of a 6-inch step. They positioned the non-stance limb with the knee extended, foot dorsiflexed, and the hip in a neutral position. The foot of the non-stance limb did not contact the step. While keeping the trunk vertical, subjects bent the knee of the stance limb until the heel of the nonstance limb touched the ground and returned to the start position (a 15-cm excursion). A full-length mirror was placed in front of the subjects as they performed each exercise. The mirror provided subjects visual feedback to help them maintain a vertical trunk position.
Figure 4. Front Step-Down. Subjects stood solely on the stance limb on the edge of a 6-inch step facing away from the step. They positioned the non-stance limb with the knee extended, foot dorsiflexed, and the hip in a slightly flexed position. The foot of the non-stance limb did not contact the step. While keeping the trunk vertical, subjects bent the knee of the stance limb until the heel of the non-stance limb touched the ground and returned to the start position (a 15-cm excursion). A full-length mirror was placed in front of the subjects as they performed each exercise. The mirror provided subjects visual feedback to help them maintain a vertical trunk position.
showed acceptable reliability for these procedures as evidenced by intraclass correlation coefficients (ICC [3,1]) ranging from 0.80 to 0.97.
gators used the visual feedback to minimize the risk of subjects using excessive ipsilateral trunk lean during exercise that could reduce demands placed on the GM.30 Upon completion of testing, subjects were instructed to refrain from any physical activity, other than normal walking, for a 24-hour period to minimize the potential for muscle and joint soreness.
Next, subjects performed 15 repetitions of each exercise to the beat of a metronome set at 40 beats per minute6 while the investigators collected EMG data. During each exercise, the investigators used an external trigger switch to delineate the beginning and end of each repetition performed. The order of testing was randomly determined to reduce ordering effects. Subjects rested three minutes between each exercise to minimize fatigue. Subjects were instructed to perform all the exercises with the trunk in a vertical position and received visual feedback via use of a full-length mirror. Based on prior work, the investi-
EMG Analysis An 8-channel EMG system (Run Technologies, Mission Viejo, CA) recorded all muscle activity. Subjects wore a Myopac-Jr transmitter belt unit (Run Technologies) that transmitted raw EMG data at 2000 Hz via a fiber optic cable to its receiver unit. Unit specifications included a common mode rejection
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 759
Table 1. Summary of test positions for determining the electromyographic activity during a maximum voluntary isometric contraction.25,30,42 Muscle Abdominal Obliques
Lumbar Extensors
Gluteus Maximus
Gluteus Medius
Test Position Subjects were positioned on a plinth in hook-lying with the arms crossed in front of the chest. An investigator stabilized the lower extremities by pressing the subjects’ feet firmly on the plinth. For testing, subjects performed an oblique trunk curl (i.e. sit-up) by lifting the test-side scapula off the plinth and toward the opposite knee while another investigator applied manual resistance to the anterior aspect of the test shoulder. Subjects did not lift the other scapula off the plinth during testing. Subjects were positioned on a plinth in prone with the arms crossed in front of the chest. The lower extremities were stabilized by placing an immovable strap across the back of the legs and pelvis and securing the strap to the plinth. For testing, subjects lifted the chest off the table while an investigator applied manual resistance at the scapulae. Subjects were positioned on a plinth in prone with the upper extremities “hugging” the plinth and the test-side knee flexed to 90°. Resistance was provided by an immovable strap placed across the distal aspect of the thigh and secured to the plinth. Subjects were positioned on a plinth in sidelying with 2 pillows placed between the thighs and the test limb on top. Resistance was provided by an immovable strap placed across the distal aspect of the thigh and secured to the plinth.
ratio exceeding 90 dB, amplifier gain of 2000, and input impedance exceeding 1 MOhm. Raw EMG data were band pass filtered between 20 and 500 Hz using Datapac software (Run Technologies), stored on a personal computer, and analyzed using Datapac software. For each exercise, the authors determined the RMS amplitude for each repetition. Data were then expressed as a % MVIC. The last 10 repetitions during each exercise were averaged and used for statistical analysis.30 Statistical Analysis Separate 2 (sex) X 4 (exercise) mixed-model analyses of variance (ANOVA) with repeated measures were used to determine between-sex differences in muscle amplitudes during each exercise. The level of significance was established at the 0.05 level. Since this investigation was exploratory in nature, the decision was made not to adjust the p-value for multiple pairwise comparisons at risk of incurring a Type II error.31 Effect sizes (Cohen’s d) also were calculated for pairwise comparisons deemed significant. Cohen32 has interpreted effect sizes as follows: 0.20 (small); 0.50 (medium); and 0.80 (large). We considered an effect size of 0.50 and greater as representing a clinically meaningful difference.6
RESULTS Table 2 summarizes normalized EMG activity descriptive statistics for males and females during each exercise. EMG activity ranged from 5.4 to 8.8 % MVIC and 5.1 to 9.4 % MVIC for the AO and LE, respectively. No interaction effect existed for the AO (p=0.40) or LE (p=0.08); however, a main effect for sex did exist for the LE (p=0.02). Cohen’s d of 0.84 suggested a large effect and clinically meaningful difference in LE activity. EMG activity for the GMX ranged from 8.4 to 20.4 % MVIC; yet no interaction (p=0.77) or main (p=0.12) effect for sex existed. EMG activity for the GM ranged from 18.5 to 32.0 % MVIC. While an interaction effect did not exist (p=0.54), a main effect for sex did (p=0.04). Cohen’s d of 0.75 suggested a medium-to-large effect and clinically meaningful difference in GM activity. DISCUSSION The purpose of this study was to determine if sexdifferences existed for EMG amplitudes of the trunk and hip muscles during four SLS exercises. The authors hypothesized that females would generate greater EMG muscle activity than males for all exercises. Findings from this study partially supported this hypothesis. No sex-differences existed with
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 760
Table 2. Comparison of mean (+ standard deviation) electromyographic activity during exercise between males and females expressed as 100% of a maximum voluntary isometric contraction. Effect sizes (d) reported to provide information regarding clinical significance. Wall Slide
Abdominal Obliques Lumbar Extensors Gluteus Maximus Gluteus Medius
Mini-Squat
Lateral Step-Down
Front Step-Down
Male 5.4 (4.0)
Female 5.9 (2.2)
Male 6.2 (4.1)
Female 8.8 (5.6)
Male 5.6 (3.8)
Female 5.8 (1.9)
Male 6.0 (4.2)
Female 6.9 (2.5)
Grand Mean for all Exercises Male Female 5.8 (3.3) 6.8 (3.4)
5.8 (2.8)
9.4 (5.1)
6.8 (2.8)
7.9 (4.1)
5.1 (2.2)
7.4 (4.1)
5.2 (2.1)
7.3 (4.3)
5.7 (2.8)
8.0 (2.7)*
17.6 (13.7)
20.4 (9.3)
9.0 (5.5)
12.8 (9.7)
8.4 (4.7)
12.3 (7.5)
8.8 (5.1)
13.3 (8.1)
11.0 (6.7)
14.7 (6.8)
21.6 (8.6)
32.0 (13.1)
20.3 (11.2)
26.6 (12.8)
18.5 (10.2)
24.6 (10.6)
19.0 (9.2)
27.2 (13.9)
19.8 (10.5)
27.6(10.4)†
*Females generated significantly greater overall lumbar extensor activity than males (p=.02, d=0.84) † Females generated significantly greater overall gluteus medius activity than males (p = 0.04, d = 0.75)
respect to EMG activity during any individual exercise, suggesting no interaction effect between sex and exercise. When analyzing overall muscle activation across all exercises, females generated 1.4 times greater LE and GM activity than males. Few researchers10,11,18 have compared EMG activity between males and females during rehabilitation exercises. Furthermore, most used different variations of the SLS exercises than used in the current study, limiting the ability to make conclusive comparisons to prior works. However, patterns of EMG activity reported may provide important clinical information. While not statistically significant for all comparisons, females in the current study generated relatively greater EMG activity during each individual exercise. These findings partially agreed with Zeller et al10 and Dwyer et al.11 Both investigators collected EMG data as subjects squatted as low as possible on a single limb, a task that was more demanding than the exercises used in the current study. EMG amplitudes in the Zeller and Dwyer studies were much higher and most likely reflected greater demands placed on the lower extremity muscles during a deep squat. Females also exhibited greater GMX activity than males. This pattern suggested that females in both studies generated more EMG activity to control hip flexion during the deep squat. Conflicting data existed with respect to GM activity. Zeller et al10 found higher GM activity in males than females whereas Dwyer et al11 found no differences. One possible explanation for conflicting results
could be due to trunk position during the SLS. Subjects who performed the SLS with increased lateral trunk flexion over the stance limb would generate less GM activity.30,33 Neither Zeller et al nor Dwyer et al reported trunk kinematics or methods to limit trunk movements, which precluded the ability to make this determination. To control for this compensation, subjects in the current study received visual feedback for maintaining a vertical trunk position.30 Using this procedure, the authors found that females generated greater GM activity across all exercises than males. Leetun et al34 compared hip abductor strength in males and females and found that males exhibited significantly greater strength. Females in the current study could have had less hip abductor strength than males. This relative weakness could have required them to use greater GM activity to keep the pelvis level. This determination cannot be made conclusively, because hip abductor strength was not measured. Structural differences between males and females also may provide another explanation for greater overall GM during the SLS exercises. Although a common belief, evidence does not support the presumption that females have wider pelvises than males.35,36 However, Livingston and Gahagan35 found that females have a significantly less femur length compared to males. An increased pelvic width-to-femoral length ratio may reflect a wider pelvis, relative to femur length, in females than males. This relationship may explain the increased demands needed for
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 761
females to maintain a level pelvis over the femur during the SLS exercises.. Interestingly, Bouillon et al18 collected EMG data as subjects performed a front step-down in a similar manner as the current study. They reported EMG activity ranging from 12 % MVIC to 14 % MVIC for the AO, LE, GMX, and GM. Compared to other works,7,9,10,12 these GMX and GM values appeared very low. However, lower AO and LE values agreed with values obtained during the current study. Subjects in Bouillon et al and the current study performed the SLS task with the trunk in a vertical position over the pelvis. Maintaining good trunk alignment would minimize the need for AO and LE activation.14 Another reason for relatively low activation may have resulted from the stabilizing effects from other muscles, like the multifidi and transversus abdominis, not assessed in the Bouillon et al18 or the current investigation. Cholewicki and Van Vliet37 reported that no single muscle contributed to more than 30% of the overall stability of the lumbar spine during trunk stabilization exercises. The multifidi and transversus abdominis most likely helped stabilize the trunk during the SLS exercises, resulting in the need for less AO and LE activity. Future investigations are needed to understand the role of other trunk muscles during the SLS exercises. Finally, it was noteworthy that females in the current study generated greater overall LE activity than males (8.0 % MVIC versus 5.7 % MVIC) during the exercises. Although significantly greater with a large effect (Cohen’s d = 0.84), this very low magnitude suggested minimal, if any, clinically important difference.8,38 Clinical Applications Findings from this investigation may have important clinical implications when designing and implementing an exercise program designed to improve strength of the trunk and hip muscles in males and females. Reiman et al8 systematically reviewed GMX and GM activity during various rehabilitation exercises. For this purpose, they classified EMG activity as follows: low (0-20% MVIC), moderate (21-40% MVIC), high (41-60% MVIC), and very high (> 60% MVIC). Moderate activation would be required for neuromuscular re-education/endurance8,17 whereas high and very high activity would be necessary for meaningful strength gains.39,40
Using these threshold values, males generated low (19.8% MVIC) and females moderate (27.6% MVIC) GM activity. These results suggested that females would have greater benefit for improving GM function during our exercises than males. Depending on the therapeutic goals, clinicians should consider applying resistance above and beyond body weight to better activate the GM, especially for males, when prescribing the exercises used in our study. Future investigations are needed to determine the amount of additional load needed for strengthening effects. Findings from the current study showed no differences in average GMX activation between males (11.0 % MVIC) and females (14.7 % MVIC). These values were much lower than those reported by Ayotte et al6 (values ranging from 56 % MVIC to 86 % MVIC). Even though the exercises used in their study were replicated in the current study, higher GMX values most likely reflected methodological differences. Subjects in the current study performed a total of 15 repetitions instead of three repetitions, in order to better represent exercise prescription. Average EMG activity for the last 10 repetitions instead of average concentric phase activation during the last two repetitions was analyzed. Researchers39,41 have compared isometric, concentric, and eccentric muscle activity and noted relatively greater EMG activity during a concentric action. The current findings have provided evidence that clinicians apply additional resistance if using these exercises for GMX neuromuscular re-education/endurance or strengthening effects. Regardless of sex, AO and LE values were low (ranging from 5.7 % MVIC to 8.0 % MVIC) and similar to the findings of prior works.17,18 Low values most likely represented a stabilizing, rather than a torque generating, effect. The SLS exercises used in this study would provide minimal, if any, therapeutic benefit for neuromuscular re-education/endurance or strengthening purposes. Limitations The present study has limitations. One limitation was the possibility of signal crosstalk from the use of surface EMG. Signal crosstalk was minimized by applying electrodes in a standardized and recommended manner.23 The principal investigator has extensive experience with electrode placement and
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 762
supervised all placements. Furthermore, proper placement was confirmed by observing EMG activity on an oscilloscope during specific manual muscle testing procedures. Another limitation was generalizability. Subjects in this investigation were recreationally active and healthy. Therefore, the results cannot be used to analyze EMG activity levels that individuals with pathology or weakness would generate. CONCLUSION Results from this study demonstrated an overall between-sex difference in GM and LE activation during four SLS exercises. Information gained from this study may enhance exercise prescription, highlighting that these exercises may benefit females more than males for addressing GM function or as a stimulus for strengthening. These findings also showed that these exercises would provide minimal, if any, stimulus for strengthening of the GMX, AO, and LE or enhancing their function. Clinicians should develop and implement other exercises in order to effectively provide strengthening stimuli for these muscles. In summary, none of the SLS exercises challenged any of the muscles enough to provide sufficient stimulus to induce strength gains (e.g., > 40 % MVIC).8 REFERENCES 1. Spencer-Gardner L, Eischen JJ, Levy BA, Sierra RJ, Engasser WM, Krych AJ. A comprehensive five-phase rehabilitation programme after hip arthroscopy for femoroacetabular impingement. Knee Surg Sports Traumatol Arthrosc. 2014;22(4):848-859. 2. Bolgla LA, Boling MC. An update for the conservative management of patellofemoral pain syndrome: a systematic review of the literature from 2000 to 2010. Int J Sports Phys Ther. 2011;6(2):112-125.
extermity muscles during 5 unilateral weight-bearing exercises. J Orthop Sports Phys Ther. 2007;37(2):48-55. 7. Distefano LJ, Blackburn JT, Marshall SW, Padua DA. Gluteal muscle activation during common therapeutic exercises. J Orthop Sports Phys Ther. 2009;39(7):532-540. 8. Reiman MP, Bolgla LA, Loudon JK. A literature review of studies evaluating gluteus maximus and gluteus medius activation during rehabilitation exercises. Physiother Theory Pract. 2012;28(4):257-268. 9. Boren K, Conrey C, Le Coguic J, Paprocki L, Voight M, Robinson TK. Electromyographic analysis of gluteus medius and gluteus maximus during rehabilitation exercises. Int J Sports Phys Ther. 2011;6(3):206-223. 10. Zeller BL, McCrory JL, Kibler WB, Uhl TL. Differences in kinematics and electromyographic activity between men and women during the singlelegged squat. Am J Sports Med. 2003;31(3):449-455. 11. Dwyer MK, Boudreau SN, Mattacola CG, Uhl TL, Lattermann C. Comparison of lower extremity kinematics and hip muscle activation during rehabilitation tasks between sexes. J Athl Train. 2010;45(2):181-190. 12. Boudreau SN, Dwyer MK, Mattacola CG, Lattermann C, Uhl TL, McKeon JM. Hip-muscle activation during the lunge, single-leg squat, and step-up-andover exercises. J Sport Rehabil. 2009;18(1):91-103. 13. Anderson L, Magnusson S, Neilsen M, Haleem J, Poulsen K, Aagaard P. Neuromuscular activation in conventional therapeutic exercises and heavy resistance exercise: implications for rehabilitation. Physical Therapy. 2006;86(5):683-697. 14. Neumann DA. Hip. In: Neumann DA, ed. Kinesiology of the Musculoskeletal System. St. Louis: Mosby; 2010:465-519. 15. Bergmark A. Stability of the lumbar spine. A study in mechanical engineering. Acta Orthop Scand Suppl. 1989;230:1-54.
3. Webster KA, Gribble PA. A comparison of electromyography of gluteus medius and maximus in subjects with and without chronic ankle instability during two functional exercises. Phys Ther Sport. 2013;14(1):17-22.
16. Neumann DA. Axial skeleton: muscle and joint interactions. In: Neumann DA, ed. Kinesiology of the Musculoskeletal System. 2nd ed. St Louis: Mosby; 2010:379-422.
4. Reiman MP, Weisbach PC, Glynn PE. The hips influence on low back pain: a distal link to a proximal problem. J Sport Rehabil. 2009;18(1):24-32.
17. Ekstrom R, Donatelli RA, Carp K. Electromyographic analysis of core trunk, hip, and thigh muscles during 9 rehabilitation exercises. J Orthop Sports Phys Ther. 2007;37(12):754-762.
5. Krause DA, Jacobs RS, Pilger KE, Sather BR, Sibunka SP, Hollman JH. Electromyographic analysis of the gluteus medius in five weight-bearing exercises. J Strength Cond Res. 2009;23(9):2689-2694. 6. Ayotte NW, Stetts DM, Keenan G, Greenway EH. Electromyographical analysis of selected lower
18. Bouillon LE, Wilhelm J, Eisel P, Wiesner J, Rachow M, Hatteberg L. Electromyographic assessment of muscle activity between genders during unilateral weight-bearing tasks using adjusted distances. International Journal of Sports Physical Therapy. 2012;7(6):595-605.
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 763
19. Zazulak BT, Ponce PL, Straub SJ, Medecky MJ, Avedisian L, Hewett TE. Gender comparison of hip muscle activity during single-leg landing. J Orthop Sports Phys Ther. 2005;35(5):292-299. 20. Decker MJ, Torry MR, Wyland DJ, Sterett WI, Steadman JR. Gender differences in lower extremity kinematics, kinetics, and energy absorption during landing. Clin Biomech. 2003;18:662-669. 21. Landry SC, McKean KA, Hubley-Kozey CL, Stanish WD, Deluzio KJ. Neuromuscular control and lower limb biomechanical differences exist between male and female elite adolescent soccer players during an unanticipated run and crosscut maneuver. Am J Sports Med. 2007;35(11):1901-1911. 22. Bolgla LA, Keskula DR. Reliability of lower extremity functional performance tests. J Orthop Sports Phys Ther. 1997;26(3):138-142. 23. Rainoldi A, Melchiorri G, Caruso I. A method for positioning electrodes during surface EMG recordings in lower limb muscles. J Neurosci Methods. 2004;134:37-43. 24. Cram JR, Kasman GS. Introduction to Surface Electromyography. Gaithersburg, MD: Aspen Publishers, Inc.; 1998. 25. Kendall FP, McCreary EK, Provance P, Rodgers MM, Romani WA. Muscles. Testing and Function with Posture and Pain. 5th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2005. 26. Andrews AW, Thomas MW, Bohannon RW. Normative values for isometric muscle force measurements obtained with hand-held dynamometers. Phys Ther. 1996;76(3):248-259. 27. Mohr KJ, Kvitne RS, Pink MM, Fideler B, Perry J. Electromyography of the quadriceps in patellofemoral pain with patellar subluxation. Clin Orthop. 2003;415:261-271. 28. Campenella B, Mattacola CG, Kimura IF. Effect of visual feedback and verbal encouragement on concentric quadriceps and hamstrings peak torque of males and females. Isokin Exer Sci. 2000;8(1):1-6. 29. Bamman MM, Ingram SG, Caruso JF, Greenisen MC. Evaluation of surface electromyography during maximal voluntary contraction. J Strength Cond Res. 1997;11(2):68-72. 30. Bolgla LA, Uhl TL. Electromyographic analysis of hip rehabilitation exercises in a group of healthy subjects. J Orthop Sports Phys Ther. 2005;35(8):487-494.
31. Feise RJ. Do multiple outcome measures require p-value adjustment? BMC Med Res Methodol. 2002;2:8. 32. Cohen J. Statistical Power Analysis for Behavioral Sciences. 2nd ed. Hillsdale, NJ: Lawrence Erlbaum Associates; 1988. 33. Neumann DA, Hase AD. An electromyographic analysis of the hip abductors during load carriage: implications for hip joint protection. J Orthop Sports Phys Ther. 1994;19(5):296-304. 34. Leetun DT, Ireland ML, Willson JD, Ballantyne BT, Davis IM. Core stability measures as risk factors for lower extremity injury in athletes. Med Sci Sports Exer. 2004;36(6):926-934. 35. Livingston LA, Gahagan JC. The wider gynaecoid pelvis-- larger Q angle-- greater predisposition to ACL injury relationship: myth or reality? Clin Biomech. 2001;16:951-952. 36. Horton MG, Hall TL. Quadriceps femoris muscle angle: normal values and relationships with gender and selected skeletal measures. Phys Ther. 1989;69(11):897-901. 37. Cholewicki J, VanVliet JJ. Relative contribution of trunk muscles to the stability of the lumbar spine during isometric exertions. Clin Biomech. 2002;17(2):99-105. 38. Jaeschke R, Singer J, Guyatt GH. Measurement of health status. Ascertaining the minimal clinically important difference. Control Clin Trials. 1989;10(4):407-415. 39. Anderson KG, Behm DG. Maintenance of EMG activity and loss of force output with instability. J Strength Cond Res. 2004;18(3):637-640. 40. Escamilla RF, Lewis C, Bell D, et al. Core muscle activation during Swiss ball and traditional abdominal exercises. J Orthop Sports Phys Ther. 2010;40(5):265-276. 41. Selseth A, Dayton M, Cordova ML, Ingersoll CD, Merrick MA. Quadriceps concentric EMG activity is greater than eccentric EMG activity during the lateral step-up exercise. J Sport Rehabil. 2000;9(2):124-134. 42. Souza GM, Baker LL, Powers CM. Electromyographic activity of selected trunk muscles during dynamic spine stabilization exercises. Arch Phys Med Rehabil. 2001;82(11):1551-1557.
The International Journal of Sports Physical Therapy | Volume 9, Number 6 | November 2014 | Page 764
