Knee Surg Sports Traumatol Arthrosc DOI 10.1007/s00167-014-3121-3
KNEE
Symmetry of squatting and the effect of fatigue following anterior cruciate ligament reconstruction Kate E. Webster • Darren C. Austin • Julian A. Feller • Ross A. Clark • Jodie A. McClelland
Received: 3 December 2013 / Accepted: 3 June 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose To evaluate weight-bearing and joint symmetry during the double-leg squat exercise at baseline and after fatigue in patients who have undergone anterior cruciate ligament reconstruction (ACLR) compared to uninjured controls. Methods Ten males who had returned to sport after primary ACLR participated along with ten uninjured male controls. Lower limb kinematic, kinetic and ground reaction force (GRF) data were collected during double-leg squats at baseline and after a generalised fatigue protocol. Symmetry indices were calculated for hip and knee external flexion moments and the vertical GRF (weight-bearing symmetry) at maximum squat depth. These were compared between ACLR and control groups before and after fatigue using ANOVA models. Results The ACL group preferentially unloaded the reconstructed limb at baseline, but changed to a more symmetrical load distribution to perform the squat exercise in the fatigue condition. This same loading pattern was seen at both the knee and hip joints. The control group did
K. E. Webster (&) J. A. Feller School of Allied Health, La Trobe University, Melbourne, VIC 3086, Australia e-mail: [email protected] D. C. Austin J. A. McClelland Department of Physiotherapy, La Trobe University, Melbourne, Australia J. A. Feller Epworth HealthCare, Richmond, Australia R. A. Clark School of Exercise Science, Australian Catholic University, Melbourne, Australia
not show any effect of fatigue. For both groups, symmetry indices were closer to zero (which indicated perfect symmetry) in the fatigue condition. Conclusions When prescribing squat exercises, it should be recognised that initially, patients with ACLR tend to unload the affected knee. More symmetrical loading patterns may be achieved by inducing bilateral fatigue. When fatigued, loading symmetry was similar between this patient group and controls. This is relevant information for those who implement rehabilitation training programmes. Level of evidence III. Keywords ACL Knee joint Rehabilitation Weightbearing asymmetry Biomechanics
Introduction The squat exercise is commonly used in anterior cruciate ligament reconstruction (ACLR) rehabilitation programmes to restore quadriceps muscle mass and strength. Biomechanical studies support the use of the squat for knee rehabilitation as anterior shear forces are generally low during this movement [8]. Double-leg squat exercises may also help to maintain and even improve strength in the unaffected limb. Studies have, however, shown that patients who undergo ACLR tend to have asymmetrical loading patterns during doubleleg activities, where usually load is reduced on the surgical side [5, 18]. During multiple joint exercises, such as the squat, patients may redistribute the effort from the targeted muscle group (the knee extensors) to another muscle group (i.e. hip extensors). Salem et al. [24] reported exactly this in a group of eight patients who were approximately 7–8 months post-ACLR surgery. In this
123
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study, the patient group increased the demand for muscular effort at the hip and reduced the effort at the knee on the operated side [24]. A later study by Castanharo et al. [3] also showed that deficits in knee power were partially substituted by the hip during loaded squats in a group of males who were more than 2 years following ACLR surgery. These results suggest that muscular imbalances persist after ACLR, despite rehabilitation efforts. These findings also suggest that it may be beneficial to investigate symmetry about individual joints, in addition to overall weight-bearing symmetry. Strengthening exercises are expected to induce some degree of fatigue in order for physiological adaption to occur [19]. There is, however, no data to show whether fatigue affects how the squat exercise is performed in people who have had an ACLR, as previous studies assessed the squat technique only in a non-fatigued state. It is important to understand how the body responds to fatigue as it may, for example, alter the effectiveness of strengthening exercises, or cause compensations to occur which may negatively affect performance or increase risk of injury. This study aimed to analyse weight-bearing symmetry along with hip and knee joint symmetry during a doubleleg squat in a group who had undergone ACLR surgery, both at baseline and following fatigue, and compare symmetry to an uninjured control group. As stated above, examining squatting in the fatigued state has not been done previously in this patient group and will therefore provide new information which may be of relevance for rehabilitation programmes. It was hypothesised that the ACLR group would unload the reconstructed knee joint and therefore perform the squat movement with greater asymmetry than uninjured controls and become more asymmetrical when fatigued.
the study when they attended the clinic for routine postoperative review. Participants in the control group were recruited via advertisement. ACLR procedures were performed arthroscopically by the same experienced knee surgeon. All patients received hamstring tendon grafts (doubled semitendinosus and doubled gracilis) which were secured with an Endobutton (Smith & Nephew, Memphis, TN) on the femoral side and an interference screw on the tibial side. Three patients had meniscal tears resected (one medial, two laterals). The post-operative rehabilitation protocol emphasised restoration of knee range of motion and quadriceps function, in particular that of the vastus medialis. Quadricepsstrengthening exercises were restricted to closed kinetic chain for the first 3 months. Running was typically permitted at 10 weeks with sports-specific drills from 3 months. Return to full competitive sports participation was usually permitted at 9 months as long as a full postoperative rehabilitation programme had been completed, and the patient had achieved full knee range of motion, adequate knee stability, functional quadriceps control, and there was no effusion. The inclusion criteria for both groups required the participants to be engaged in sports activities on a weekly basis. The exclusion criteria applicable to the ACLR group were any other surgeries to the lower limbs, including previous ACLR surgery to the same or contralateral knee. Exclusion criteria applicable to the control group were any previous history of lower limb surgery or any injury within 6 months prior to participation. At the time of participation, all in the ACLR group had returned to competitive sport and were part of a previous study investigating the effect of fatigue on landing biomechanics [27]. Instrumentation
Materials and methods Subjects Twenty male volunteers took part in the study. Ten participants had undergone primary ACLR surgery (mean age 23 ± 3 years, height 1.80 ± 0.09 m, and body mass 74 ± 8 kg) and were matched by gender, age (±4 years) and sport participation to ten healthy participants (control group: mean age 23 ± 2 years, height 1.82 ± 0.04 m and body mass 77 ± 7 kg). None of these demographic variables were significantly different between the groups. For all participants in the ACLR group, the surgical procedure had been performed a minimum 52 weeks prior to participation and the mean post-operative time was 69 ± 9 weeks. Participants in this group were invited into
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Concurrent bilateral ground reaction force (GRF) and 3-dimensional kinematic data were collected using two floor-embedded force plates (Kistler Instruments AG, Winterthur, Switzerland; AMTI, Watertown, MA, USA) and 10 infrared-sensitive cameras (100 Hz) (Vicon-MX3, Oxford Metrics Ltd, Oxford, UK). Experimental procedures Although this was a kinetic study, inverse dynamics was used to calculate joint moment data from kinematic data; therefore, reflective markers were attached to each participant based on the Helen Hayes system [12]. A further two markers were placed bilaterally over the iliac crest, and an additional marker was placed over the tenth thoracic vertebra (T10). A reference point for the markers was obtained
Knee Surg Sports Traumatol Arthrosc
by taking a static trial with the subject in standing for approximately 5 s using knee alignment devices as previously described [28]. During testing, participants were asked to stand with one foot on each force plate with a comfortable stance width. Ten consecutive double limb squats were then performed at a steady pace (4 s/squat; 2 s descent, 2 s ascent). Participants were instructed to descend to a level where the thighs were parallel to the floor, whilst keeping the arms parallel to the ground and then ascend to a full upright position. Bilateral GRF and joint kinetic data were recorded during the squats, both before and after exposure to a generalised fatigue protocol. The fatigue protocol has been described previously and consisted of the subject performing a set of squats (910), two vertical jumps and 10 drop landings (5 left leg, 5 right leg) [27]. Jump height was measured from the two vertical jumps and used to determine maximal fatigue which was operationally defined as either the point where jump height reduced by 20 % of that of the highest previous attempt or when the subject could no longer complete the fatigue protocol [1, 2, 9, 13]. The series of 10 drop landings, 10 squats and 2 vertical jumps was repeated five times and then to expedite fatigue; the number of squats in each set was increased by multiples of 10 (i.e. 20, 30) until fatigue was reached. At no point during the fatigue protocol were participants permitted to rest and participants transitioned immediately between the drop landing, squat and vertical jump components. Data analysis Vicon Plug-in-Gait (Oxford Metrics) biomechanical modelling software was used to process and output kinetic profiles. This model uses inverse dynamic analysis as described by Kadaba et al. [12] and Davis et al. [6] to calculate joint kinetics, which were normalised to body mass. If the anterior superior iliac spine markers were occluded during the squat movement, markers from the iliac crest were used to reconstruct the pelvis based on a previously published procedure [16]. Previous reports have investigated measurement accuracy of three-dimensional motional analysis systems [22, 30]. Data from the first set of squats were not analysed and were considered a familiarisation trial. Data from the second and third set of ten squats were exported into LabView (LabView 8.5, National Instruments, Austin, TX, USA), and using a custom-made programme, a sinusoidal-type wave was constructed using the T10 marker’s Z plane (vertical) displacement. From this wave, the three consecutive squats that had the lowest coefficient of variation in vertical displacement were selected from each set. This totalled six squats (three from each set) which were averaged as the baseline (pre-fatigue) condition. Using the
same method, three consecutive squats were chosen from the final two sets of squats immediately prior to cessation of testing and these six squats were averaged as the fatigue condition. This analysis technique was implemented to identify and focus on the most consistent repetitions within each set. For each limb, sagittal plane external joint moments were recorded at the point of maximum squat depth for the hip and knee for both the baseline and fatigue conditions, along with the peak vertical GRF. From this data, symmetry indices were calculated according to the method of Robinson et al. [23] using the following equation: Vaffected Vunaffected 100 2 ðVaffected þ Vunaffected Þ
SI ¼ 1
where Vaffected is the biomechanical variable recorded for the ACLR limb and Vunaffected is the biomechanical variable recorded for the contralateral limb in the ACL group. As control subjects do not have an affected and unaffected side, for comparison purposes, they were matched to the ACL group in which there were seven right-sided reconstructions and three left. Therefore, for seven control subjects, the right side was designated as ‘affected’, and for three, the left was designated as ‘affected’. The symmetry index (SI) that was calculated from the vertical GRF was referred to as weight-bearing symmetry. The magnitude of the SI reflects the degree of symmetry (ranges from -200 to 200 %) and the sign (positive or negative) the pattern of symmetry. A value of zero represents perfect symmetry. A positive SI indicates that the magnitude of that variable is larger on the affected side, whilst a negative SI value indicates a larger magnitude on the unaffected side. Consent was obtained for all subjects, and the procedures were approved by the Faculty of Health Sciences Ethics Committee at La Trobe University (FHEC09/162). Statistical analysis The sample size was consistent with a previous study investigating squatting following ACLR [24] and with statistical significance set at a two-sided level of 0.05, a power of 0.8, and the correlation of repeated measures at 0.5, a minimum of ten subjects was required for each group in order to detect a medium–large (f = 0.3) effect size. Symmetry indices were compared between the ACLR and control groups using a two-factor ANOVA model with condition (baseline, fatigue) as a within-subjects factor and group (ACL, control) as a between-subjects factor. An alpha level p B 0.05 was used to represent statistical significance. In the case of significant main effects or interactions, planned comparisons were made using paired tests for within-group comparisons and independent t tests for
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between-group comparisons with an adjustment of 0.025 made to the alpha level due these multiple comparisons.
Baseline
A
10
Fatigue
ACLR control
5
Knee and hip joint moment symmetry There was a significant group by condition interaction for knee joint moment symmetry [F(1,18) = 4.3, p = 0.05, g2p = 0.19, power = 0.5]. In the ACLR group, the knee joint moment was higher about the unaffected knee in the baseline condition, but in the fatigue condition, the knee joint moment was higher about the affected knee (this change was significant at the 0.05 level, but not at the alpha adjusted level of 0.025), whereas there was no change in the control group. In both groups, knee joint moment symmetry values were closest to zero in the fatigue condition (Table 1; Fig. 1b). Table 1 Mean (SD) symmetry values for the ACL and control groups at baseline and after fatigue Baseline
Fatigue
0
-5
-10
-15
B
20 15
0 -5 -10 -15 -20 -25
C
15 10
Control
-9.1*
1.9
-0.6
1.7
(10.4)
(15.2)
(16.4)
(14.4)
Knee
-12.0 (14.7)
3.8 (25.5)
5.7 (23.4)
3.5 (21.9)
-15
Hip
-11.7*
-6.5
4.0
2.8
-20
(17.2)
(20.2)
(26.1)
(26.6)
123
control
0
ACL
* Significant difference between baseline and fatigue in ACL group p \ 0.025
ACLR
5
Control
Note that a value of 0 represents equal symmetry
control
5
ACL vGRF
ACLR
10
Symmetry Index
There was a significant group by condition interaction for weight-bearing symmetry [F(1,18) = 5.3, p = 0.03, g2p = 0.199, power = 0.6]. Examination of the interaction showed that the ACLR group changed significantly between baseline and fatigue conditions (p = 0.02). This patient group placed more weight over the unaffected limb at baseline, but became more symmetrical when fatigued, whereas there was no change in the control group which performed the squat task with an average SI of less than 2 in both conditions (Table 1; Fig. 1a). The preference to unload the affected limb at baseline was consistent within the ACL group with 9 out of the 10 participants placing greater weight on the unaffected limb.
Symmetry Index
Weight-bearing symmetry
Symmetry Index
Results
-5 -10
Fig. 1 Mean (±SEM) weight-bearing symmetry (a), knee joint moment symmetry (b) and hip joint moment symmetry (c) for ACL reconstruction (ACLR) and control groups at baseline and after fatigue
Knee Surg Sports Traumatol Arthrosc
There was a significant condition effect for hip joint moment symmetry [F(1,18) = 11.5, p = 0.003, g2p = 0.389, power = 0.9]. Post hoc testing showed that the ACLR group significantly changed from a higher moment on the unaffected side at baseline to a higher moment on the affected side when fatigued (p = 0.004). In the control group, the difference between conditions was not significant. Again, in both groups, average hip joint moment symmetry values were closest to zero in the fatigue condition (Table 1; Fig. 1c).
Discussion The most important finding of this study was that a group of male athletes who had ACLR surgery performed a double-leg squat movement with greater symmetry when fatigued compared to a baseline non-fatigued condition. That this patient group initially performed the squat movement with asymmetry is consistent with the limited amount of work that has previously been conducted [3, 24] and confirms that even after having undergone a rehabilitation programme and returning to sport, asymmetries still remain. The new finding from this study was that these baseline asymmetries are significantly reduced after inducing fatigue and the hypothesis that ACLR patients would become more asymmetrical when fatigued was therefore not supported. Asymmetrical lower extremity loading is a topic that has received significant attention and has been reported in a variety of activities following ACLR [7, 14, 15, 20, 25, 26]. Although loading asymmetry has been postulated to contribute to the development or progression of post-traumatic knee osteoarthritis [4], perhaps the most serious short-term consequence of loading asymmetry is the apparent increased risk of re-injury to either the reconstructed or contralateral knee [10, 21]. Therefore, the baseline asymmetry that was present in this group of patients that had made a successful return to sport is relevant as this further highlights the challenge of achieving symmetry. From a clinical and rehabilitation perspective, the finding that the ACLR group became more symmetrical when fatigued has implications as it suggests that the initial asymmetry and tendency to unload the affected limb can be overcome. The current data showed that if sufficient repetitions of the squat movement are performed as part of a generalised fatigue protocol, loading will be increased on the reconstructed side and also specifically at the knee. Whilst this is relevant information for those who implement rehabilitation training programmes, the current data are nonetheless preliminary and their clinical relevance is unclear. The current data raise a number of interesting
questions that future research should address, such as whether it is worthwhile fatiguing (be it to a partial level), the unaffected limb during rehabilitation before double limb activities are performed. Any such changes to rehabilitation protocols should always be closely monitored, and current trials are investigating return to sport-specific training following ACLR [29]. Whilst this study only assessed patients when they were at maximum fatigue, it is possible that the effects of fatigue are seen at a much earlier time point. Previous studies have, for example, shown changes in lower limb biomechanical parameters when participants were only partially fatigued (50 % fatigue) [2, 17, 27]. It should, however, be noted that whilst squatting is not a novel task, it is possible that it was the learning adaptation with the repetitions of the movement that caused changes in weight-bearing and joint symmetry rather than fatigue per se. However, if this were the case, some change might also have been expected in the control group, but this was not observed. This study cannot specifically determine the reason why the patient group became more symmetrical with fatigue. One possible explanation is that the unaffected limb can only sustain the greater load for a limited time, after which there is no choice, but to distribute some weight onto the affected side. In future studies, single-leg fatigue protocols could be employed to examine this possibility. These results cannot be readily compared to others as this is the only study we are aware of which has evaluated the effect of fatigue on double-leg squatting in this patient group. The current results are, however, consistent with a previous study which also showed that weight-bearing asymmetries were less during a fatigued condition when a group of uninjured athletes performed weighted barbell squats [11]. A strength of the current study is that a number of biomechanical outcome measures were used to assess squat symmetry. This allowed us to not only examine overall weight bearing, but also the degree of symmetry at both the knee and hip joints. As most settings are not equipped to conduct three-dimensional motion analysis of a squat exercise, it is noteworthy that data from the force plate alone was representative of the overall findings in this study. The current study has some limitations. Only males participated, and therefore, the findings are not generalisable to females. As previously mentioned, the effect of fatigue was only examined before and after, rather than as it progressed, and therefore, the timing of the change in symmetry is not known. A further limitation is that symmetry was examined only at maximum depth of the squat and not throughout the whole movement also, no assessment was made after a period of recovery so how long the effects of fatigue last cannot be determined.
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Conclusions This study showed that patients who had successfully returned to sport after ACLR surgery still performed a double-leg squat with weight-bearing and hip and knee joint asymmetry, but that this asymmetry significantly reduced after they were exposed to a generalised fatigue protocol.
References 1. Augustsson J, Thomee R, Linden C, Folkesson M, Tranberg R, Karlsson J (2006) Single-leg hop testing following fatiguing exercise: reliability and biomechanical analysis. Scand J Med Sci Sports 16(2):111–120 2. Borotikar BS, Newcomer R, Koppes R, McLean SG, Borotikar BS, Newcomer R, Koppes R, McLean SG (2008) Combined effects of fatigue and decision making on female lower limb landing postures: central and peripheral contributions to ACL injury risk. Clin Biomech 23(1):81–92 3. Castanharo R, da Luz BS, Bitar AC, D’Elia CO, Castropil W, Duarte M (2011) Males still have limb asymmetries in multijoint movement tasks more than 2 years following anterior cruciate ligament reconstruction. J Orthop Sci 16(5):531–535 4. Chmielewski TL (2011) Asymmetrical lower extremity loading after ACL reconstruction: more than meets the eye. J Orthop Sports Phys Ther 41(6):374–376 5. Chmielewski TL, Wilk KE, Snyder-Mackler L (2002) Changes in weight-bearing following injury or surgical reconstruction of the ACL: relationship to quadriceps strength and function. Gait Posture 16(1):87–95 6. Davis RB, Ounpuu S, Tyburski D, Gage JR (1991) A gait analysis data collection and reduction technique. Hum Mov Sci 10:575–587 7. Ernst GP, Saliba E, Diduch DR, Hurwitz SR, Ball DW (2000) Lower extremity compensations following anterior cruciate ligament reconstruction. Phys Ther 80(3):251–260 8. Escamilla RF (2001) Knee biomechanics of the dynamic squat exercise. Med Sci Sports Exerc 33(1):127–141 9. Fagenbaum R, Darling W (2003) Jump landing strategies in male and female college athletes and the implications of such strategies for anterior cruciate ligament injury. Am J Sports Med 31(2): 233–240 10. Hewett T, Myer G, Ford K, Heidt RJ, Colosimo A, McLean S, van den Bogert A, Paterno M, Succop P (2005) Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports Med 33(4):492–501 11. Hodges SJ, Patrick RJ, Reiser FR 2nd (2011) Effects of fatigue on bilateral ground reaction force asymmetries during the squat exercise. J Strength Cond Res 25(11):3107–3117 12. Kadaba MP, Ramakrishnan HK, Wootten ME (1990) Measurement of lower extremity kinematics during level walking. J Orthop Res 8(3):383–392 13. Kernozek T, Torry M, Iwasaki M (2008) Gender differences in lower extremity landing mechanics caused by neuromuscular fatigue. Am J Sports Med 36(3):554
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14. Kowalk DL, Duncan JA, McCue FC 3rd, Vaughan CL (1997) Anterior cruciate ligament reconstruction and joint dynamics during stair climbing. Med Sci Sports Exerc 29(11):1406–1413 15. Logerstedt D, Lynch A, Axe MJ, Snyder-Mackler L (2013) Symmetry restoration and functional recovery before and after anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 21(4):859–868 16. McClelland JA, Webster KE, Grant C, Feller J (2010) Alternative modelling procedures for pelvic marker occlusion during motion analysis. Gait Posture 31(4):415–419 17. McLean SG, Samorezov JE (2009) Fatigue-induced ACL injury risk stems from a degradation in central control. Med Sci Sports Exerc 41(8):1661–1672 18. Neitzel JA, Kernozek TW, Davies GJ (2002) Loading response following anterior cruciate ligament reconstruction during the parallel squat exercise. Clin Biomech 17(7):551–554 19. Noakes TD (2000) Physiological models to understand exercise fatigue and the adaptations that predict or enhance athletic performance. Scand J Med Sci Sports 10(3):123–145 20. Orishimo KF, Kremenic IJ, Mullaney MJ, McHugh MP, Nicholas SJ (2010) Adaptations in single-leg hop biomechanics following anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 18(11):1587–1593 21. Paterno MV, Schmitt LC, Ford KR, Rauh MJ, Myer GD, Huang B, Hewett TE (2010) Biomechanical measures during landing and postural stability predict second anterior cruciate ligament injury after anterior cruciate ligament reconstruction and return to sport. Am J Sports Med 38(10):1968–1978 22. Richards JG (1999) The measurement of human motion: a comparison of commercially available systems. Hum Mov Sci 18(5):589–602 23. Robinson RO, Herzog W, Nigg BM (1987) Use of force platform variables to quantify the effects of chiropractic manipulation on gait symmetry. J Manipulative Physiol Ther 10(4):172–176 24. Salem GJ, Salinas R, Harding FV (2003) Bilateral kinematic and kinetic analysis of the squat exercise after anterior cruciate ligament reconstruction. Arch Phys Med Rehabil 84(8):1211–1216 25. Thomee´ R, Neeter C, Gustavsson A, Thomee´ P, Augustsson J, Eriksson B, Karlsson J (2012) Variability in leg muscle power and hop performance after anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 20(6):1143–1151 26. Webster KE, Gonzalez-Adrio R, Feller JA (2004) Dynamic joint loading following hamstring and patellar tendon anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 12(1):15–21 27. Webster KE, Santamaria LJ, McClelland JA, Feller JA (2012) Effect of fatigue on landing biomechanics after anterior cruciate ligament reconstruction surgery. Med Sci Sports Exerc 44(5): 910–916 28. Webster KE, Wittwer JE, O’Brien J, Feller JA (2005) Gait patterns after anterior cruciate ligament reconstruction are related to graft type. Am J Sports Med 33(2):247–254 29. White K, Di Stasi S, Smith A, Snyder-Mackler L (2013) Anterior cruciate ligament-specialized post-operative return-to-sports (ACL-SPORTS) training: a randomized control trial. BMC Musculoskelet Disord 14:108 30. Windolfa M, Go¨tzen N, Morlockb M (2008) Systematic accuracy and precision analysis of video motion capturing systems— exemplified on the Vicon-460 system. J Biomech 41(12): 2776–2780
KNEE
Symmetry of squatting and the effect of fatigue following anterior cruciate ligament reconstruction Kate E. Webster • Darren C. Austin • Julian A. Feller • Ross A. Clark • Jodie A. McClelland
Received: 3 December 2013 / Accepted: 3 June 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose To evaluate weight-bearing and joint symmetry during the double-leg squat exercise at baseline and after fatigue in patients who have undergone anterior cruciate ligament reconstruction (ACLR) compared to uninjured controls. Methods Ten males who had returned to sport after primary ACLR participated along with ten uninjured male controls. Lower limb kinematic, kinetic and ground reaction force (GRF) data were collected during double-leg squats at baseline and after a generalised fatigue protocol. Symmetry indices were calculated for hip and knee external flexion moments and the vertical GRF (weight-bearing symmetry) at maximum squat depth. These were compared between ACLR and control groups before and after fatigue using ANOVA models. Results The ACL group preferentially unloaded the reconstructed limb at baseline, but changed to a more symmetrical load distribution to perform the squat exercise in the fatigue condition. This same loading pattern was seen at both the knee and hip joints. The control group did
K. E. Webster (&) J. A. Feller School of Allied Health, La Trobe University, Melbourne, VIC 3086, Australia e-mail: [email protected] D. C. Austin J. A. McClelland Department of Physiotherapy, La Trobe University, Melbourne, Australia J. A. Feller Epworth HealthCare, Richmond, Australia R. A. Clark School of Exercise Science, Australian Catholic University, Melbourne, Australia
not show any effect of fatigue. For both groups, symmetry indices were closer to zero (which indicated perfect symmetry) in the fatigue condition. Conclusions When prescribing squat exercises, it should be recognised that initially, patients with ACLR tend to unload the affected knee. More symmetrical loading patterns may be achieved by inducing bilateral fatigue. When fatigued, loading symmetry was similar between this patient group and controls. This is relevant information for those who implement rehabilitation training programmes. Level of evidence III. Keywords ACL Knee joint Rehabilitation Weightbearing asymmetry Biomechanics
Introduction The squat exercise is commonly used in anterior cruciate ligament reconstruction (ACLR) rehabilitation programmes to restore quadriceps muscle mass and strength. Biomechanical studies support the use of the squat for knee rehabilitation as anterior shear forces are generally low during this movement [8]. Double-leg squat exercises may also help to maintain and even improve strength in the unaffected limb. Studies have, however, shown that patients who undergo ACLR tend to have asymmetrical loading patterns during doubleleg activities, where usually load is reduced on the surgical side [5, 18]. During multiple joint exercises, such as the squat, patients may redistribute the effort from the targeted muscle group (the knee extensors) to another muscle group (i.e. hip extensors). Salem et al. [24] reported exactly this in a group of eight patients who were approximately 7–8 months post-ACLR surgery. In this
123
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study, the patient group increased the demand for muscular effort at the hip and reduced the effort at the knee on the operated side [24]. A later study by Castanharo et al. [3] also showed that deficits in knee power were partially substituted by the hip during loaded squats in a group of males who were more than 2 years following ACLR surgery. These results suggest that muscular imbalances persist after ACLR, despite rehabilitation efforts. These findings also suggest that it may be beneficial to investigate symmetry about individual joints, in addition to overall weight-bearing symmetry. Strengthening exercises are expected to induce some degree of fatigue in order for physiological adaption to occur [19]. There is, however, no data to show whether fatigue affects how the squat exercise is performed in people who have had an ACLR, as previous studies assessed the squat technique only in a non-fatigued state. It is important to understand how the body responds to fatigue as it may, for example, alter the effectiveness of strengthening exercises, or cause compensations to occur which may negatively affect performance or increase risk of injury. This study aimed to analyse weight-bearing symmetry along with hip and knee joint symmetry during a doubleleg squat in a group who had undergone ACLR surgery, both at baseline and following fatigue, and compare symmetry to an uninjured control group. As stated above, examining squatting in the fatigued state has not been done previously in this patient group and will therefore provide new information which may be of relevance for rehabilitation programmes. It was hypothesised that the ACLR group would unload the reconstructed knee joint and therefore perform the squat movement with greater asymmetry than uninjured controls and become more asymmetrical when fatigued.
the study when they attended the clinic for routine postoperative review. Participants in the control group were recruited via advertisement. ACLR procedures were performed arthroscopically by the same experienced knee surgeon. All patients received hamstring tendon grafts (doubled semitendinosus and doubled gracilis) which were secured with an Endobutton (Smith & Nephew, Memphis, TN) on the femoral side and an interference screw on the tibial side. Three patients had meniscal tears resected (one medial, two laterals). The post-operative rehabilitation protocol emphasised restoration of knee range of motion and quadriceps function, in particular that of the vastus medialis. Quadricepsstrengthening exercises were restricted to closed kinetic chain for the first 3 months. Running was typically permitted at 10 weeks with sports-specific drills from 3 months. Return to full competitive sports participation was usually permitted at 9 months as long as a full postoperative rehabilitation programme had been completed, and the patient had achieved full knee range of motion, adequate knee stability, functional quadriceps control, and there was no effusion. The inclusion criteria for both groups required the participants to be engaged in sports activities on a weekly basis. The exclusion criteria applicable to the ACLR group were any other surgeries to the lower limbs, including previous ACLR surgery to the same or contralateral knee. Exclusion criteria applicable to the control group were any previous history of lower limb surgery or any injury within 6 months prior to participation. At the time of participation, all in the ACLR group had returned to competitive sport and were part of a previous study investigating the effect of fatigue on landing biomechanics [27]. Instrumentation
Materials and methods Subjects Twenty male volunteers took part in the study. Ten participants had undergone primary ACLR surgery (mean age 23 ± 3 years, height 1.80 ± 0.09 m, and body mass 74 ± 8 kg) and were matched by gender, age (±4 years) and sport participation to ten healthy participants (control group: mean age 23 ± 2 years, height 1.82 ± 0.04 m and body mass 77 ± 7 kg). None of these demographic variables were significantly different between the groups. For all participants in the ACLR group, the surgical procedure had been performed a minimum 52 weeks prior to participation and the mean post-operative time was 69 ± 9 weeks. Participants in this group were invited into
123
Concurrent bilateral ground reaction force (GRF) and 3-dimensional kinematic data were collected using two floor-embedded force plates (Kistler Instruments AG, Winterthur, Switzerland; AMTI, Watertown, MA, USA) and 10 infrared-sensitive cameras (100 Hz) (Vicon-MX3, Oxford Metrics Ltd, Oxford, UK). Experimental procedures Although this was a kinetic study, inverse dynamics was used to calculate joint moment data from kinematic data; therefore, reflective markers were attached to each participant based on the Helen Hayes system [12]. A further two markers were placed bilaterally over the iliac crest, and an additional marker was placed over the tenth thoracic vertebra (T10). A reference point for the markers was obtained
Knee Surg Sports Traumatol Arthrosc
by taking a static trial with the subject in standing for approximately 5 s using knee alignment devices as previously described [28]. During testing, participants were asked to stand with one foot on each force plate with a comfortable stance width. Ten consecutive double limb squats were then performed at a steady pace (4 s/squat; 2 s descent, 2 s ascent). Participants were instructed to descend to a level where the thighs were parallel to the floor, whilst keeping the arms parallel to the ground and then ascend to a full upright position. Bilateral GRF and joint kinetic data were recorded during the squats, both before and after exposure to a generalised fatigue protocol. The fatigue protocol has been described previously and consisted of the subject performing a set of squats (910), two vertical jumps and 10 drop landings (5 left leg, 5 right leg) [27]. Jump height was measured from the two vertical jumps and used to determine maximal fatigue which was operationally defined as either the point where jump height reduced by 20 % of that of the highest previous attempt or when the subject could no longer complete the fatigue protocol [1, 2, 9, 13]. The series of 10 drop landings, 10 squats and 2 vertical jumps was repeated five times and then to expedite fatigue; the number of squats in each set was increased by multiples of 10 (i.e. 20, 30) until fatigue was reached. At no point during the fatigue protocol were participants permitted to rest and participants transitioned immediately between the drop landing, squat and vertical jump components. Data analysis Vicon Plug-in-Gait (Oxford Metrics) biomechanical modelling software was used to process and output kinetic profiles. This model uses inverse dynamic analysis as described by Kadaba et al. [12] and Davis et al. [6] to calculate joint kinetics, which were normalised to body mass. If the anterior superior iliac spine markers were occluded during the squat movement, markers from the iliac crest were used to reconstruct the pelvis based on a previously published procedure [16]. Previous reports have investigated measurement accuracy of three-dimensional motional analysis systems [22, 30]. Data from the first set of squats were not analysed and were considered a familiarisation trial. Data from the second and third set of ten squats were exported into LabView (LabView 8.5, National Instruments, Austin, TX, USA), and using a custom-made programme, a sinusoidal-type wave was constructed using the T10 marker’s Z plane (vertical) displacement. From this wave, the three consecutive squats that had the lowest coefficient of variation in vertical displacement were selected from each set. This totalled six squats (three from each set) which were averaged as the baseline (pre-fatigue) condition. Using the
same method, three consecutive squats were chosen from the final two sets of squats immediately prior to cessation of testing and these six squats were averaged as the fatigue condition. This analysis technique was implemented to identify and focus on the most consistent repetitions within each set. For each limb, sagittal plane external joint moments were recorded at the point of maximum squat depth for the hip and knee for both the baseline and fatigue conditions, along with the peak vertical GRF. From this data, symmetry indices were calculated according to the method of Robinson et al. [23] using the following equation: Vaffected Vunaffected 100 2 ðVaffected þ Vunaffected Þ
SI ¼ 1
where Vaffected is the biomechanical variable recorded for the ACLR limb and Vunaffected is the biomechanical variable recorded for the contralateral limb in the ACL group. As control subjects do not have an affected and unaffected side, for comparison purposes, they were matched to the ACL group in which there were seven right-sided reconstructions and three left. Therefore, for seven control subjects, the right side was designated as ‘affected’, and for three, the left was designated as ‘affected’. The symmetry index (SI) that was calculated from the vertical GRF was referred to as weight-bearing symmetry. The magnitude of the SI reflects the degree of symmetry (ranges from -200 to 200 %) and the sign (positive or negative) the pattern of symmetry. A value of zero represents perfect symmetry. A positive SI indicates that the magnitude of that variable is larger on the affected side, whilst a negative SI value indicates a larger magnitude on the unaffected side. Consent was obtained for all subjects, and the procedures were approved by the Faculty of Health Sciences Ethics Committee at La Trobe University (FHEC09/162). Statistical analysis The sample size was consistent with a previous study investigating squatting following ACLR [24] and with statistical significance set at a two-sided level of 0.05, a power of 0.8, and the correlation of repeated measures at 0.5, a minimum of ten subjects was required for each group in order to detect a medium–large (f = 0.3) effect size. Symmetry indices were compared between the ACLR and control groups using a two-factor ANOVA model with condition (baseline, fatigue) as a within-subjects factor and group (ACL, control) as a between-subjects factor. An alpha level p B 0.05 was used to represent statistical significance. In the case of significant main effects or interactions, planned comparisons were made using paired tests for within-group comparisons and independent t tests for
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between-group comparisons with an adjustment of 0.025 made to the alpha level due these multiple comparisons.
Baseline
A
10
Fatigue
ACLR control
5
Knee and hip joint moment symmetry There was a significant group by condition interaction for knee joint moment symmetry [F(1,18) = 4.3, p = 0.05, g2p = 0.19, power = 0.5]. In the ACLR group, the knee joint moment was higher about the unaffected knee in the baseline condition, but in the fatigue condition, the knee joint moment was higher about the affected knee (this change was significant at the 0.05 level, but not at the alpha adjusted level of 0.025), whereas there was no change in the control group. In both groups, knee joint moment symmetry values were closest to zero in the fatigue condition (Table 1; Fig. 1b). Table 1 Mean (SD) symmetry values for the ACL and control groups at baseline and after fatigue Baseline
Fatigue
0
-5
-10
-15
B
20 15
0 -5 -10 -15 -20 -25
C
15 10
Control
-9.1*
1.9
-0.6
1.7
(10.4)
(15.2)
(16.4)
(14.4)
Knee
-12.0 (14.7)
3.8 (25.5)
5.7 (23.4)
3.5 (21.9)
-15
Hip
-11.7*
-6.5
4.0
2.8
-20
(17.2)
(20.2)
(26.1)
(26.6)
123
control
0
ACL
* Significant difference between baseline and fatigue in ACL group p \ 0.025
ACLR
5
Control
Note that a value of 0 represents equal symmetry
control
5
ACL vGRF
ACLR
10
Symmetry Index
There was a significant group by condition interaction for weight-bearing symmetry [F(1,18) = 5.3, p = 0.03, g2p = 0.199, power = 0.6]. Examination of the interaction showed that the ACLR group changed significantly between baseline and fatigue conditions (p = 0.02). This patient group placed more weight over the unaffected limb at baseline, but became more symmetrical when fatigued, whereas there was no change in the control group which performed the squat task with an average SI of less than 2 in both conditions (Table 1; Fig. 1a). The preference to unload the affected limb at baseline was consistent within the ACL group with 9 out of the 10 participants placing greater weight on the unaffected limb.
Symmetry Index
Weight-bearing symmetry
Symmetry Index
Results
-5 -10
Fig. 1 Mean (±SEM) weight-bearing symmetry (a), knee joint moment symmetry (b) and hip joint moment symmetry (c) for ACL reconstruction (ACLR) and control groups at baseline and after fatigue
Knee Surg Sports Traumatol Arthrosc
There was a significant condition effect for hip joint moment symmetry [F(1,18) = 11.5, p = 0.003, g2p = 0.389, power = 0.9]. Post hoc testing showed that the ACLR group significantly changed from a higher moment on the unaffected side at baseline to a higher moment on the affected side when fatigued (p = 0.004). In the control group, the difference between conditions was not significant. Again, in both groups, average hip joint moment symmetry values were closest to zero in the fatigue condition (Table 1; Fig. 1c).
Discussion The most important finding of this study was that a group of male athletes who had ACLR surgery performed a double-leg squat movement with greater symmetry when fatigued compared to a baseline non-fatigued condition. That this patient group initially performed the squat movement with asymmetry is consistent with the limited amount of work that has previously been conducted [3, 24] and confirms that even after having undergone a rehabilitation programme and returning to sport, asymmetries still remain. The new finding from this study was that these baseline asymmetries are significantly reduced after inducing fatigue and the hypothesis that ACLR patients would become more asymmetrical when fatigued was therefore not supported. Asymmetrical lower extremity loading is a topic that has received significant attention and has been reported in a variety of activities following ACLR [7, 14, 15, 20, 25, 26]. Although loading asymmetry has been postulated to contribute to the development or progression of post-traumatic knee osteoarthritis [4], perhaps the most serious short-term consequence of loading asymmetry is the apparent increased risk of re-injury to either the reconstructed or contralateral knee [10, 21]. Therefore, the baseline asymmetry that was present in this group of patients that had made a successful return to sport is relevant as this further highlights the challenge of achieving symmetry. From a clinical and rehabilitation perspective, the finding that the ACLR group became more symmetrical when fatigued has implications as it suggests that the initial asymmetry and tendency to unload the affected limb can be overcome. The current data showed that if sufficient repetitions of the squat movement are performed as part of a generalised fatigue protocol, loading will be increased on the reconstructed side and also specifically at the knee. Whilst this is relevant information for those who implement rehabilitation training programmes, the current data are nonetheless preliminary and their clinical relevance is unclear. The current data raise a number of interesting
questions that future research should address, such as whether it is worthwhile fatiguing (be it to a partial level), the unaffected limb during rehabilitation before double limb activities are performed. Any such changes to rehabilitation protocols should always be closely monitored, and current trials are investigating return to sport-specific training following ACLR [29]. Whilst this study only assessed patients when they were at maximum fatigue, it is possible that the effects of fatigue are seen at a much earlier time point. Previous studies have, for example, shown changes in lower limb biomechanical parameters when participants were only partially fatigued (50 % fatigue) [2, 17, 27]. It should, however, be noted that whilst squatting is not a novel task, it is possible that it was the learning adaptation with the repetitions of the movement that caused changes in weight-bearing and joint symmetry rather than fatigue per se. However, if this were the case, some change might also have been expected in the control group, but this was not observed. This study cannot specifically determine the reason why the patient group became more symmetrical with fatigue. One possible explanation is that the unaffected limb can only sustain the greater load for a limited time, after which there is no choice, but to distribute some weight onto the affected side. In future studies, single-leg fatigue protocols could be employed to examine this possibility. These results cannot be readily compared to others as this is the only study we are aware of which has evaluated the effect of fatigue on double-leg squatting in this patient group. The current results are, however, consistent with a previous study which also showed that weight-bearing asymmetries were less during a fatigued condition when a group of uninjured athletes performed weighted barbell squats [11]. A strength of the current study is that a number of biomechanical outcome measures were used to assess squat symmetry. This allowed us to not only examine overall weight bearing, but also the degree of symmetry at both the knee and hip joints. As most settings are not equipped to conduct three-dimensional motion analysis of a squat exercise, it is noteworthy that data from the force plate alone was representative of the overall findings in this study. The current study has some limitations. Only males participated, and therefore, the findings are not generalisable to females. As previously mentioned, the effect of fatigue was only examined before and after, rather than as it progressed, and therefore, the timing of the change in symmetry is not known. A further limitation is that symmetry was examined only at maximum depth of the squat and not throughout the whole movement also, no assessment was made after a period of recovery so how long the effects of fatigue last cannot be determined.
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Conclusions This study showed that patients who had successfully returned to sport after ACLR surgery still performed a double-leg squat with weight-bearing and hip and knee joint asymmetry, but that this asymmetry significantly reduced after they were exposed to a generalised fatigue protocol.
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