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Journal of Back and Musculoskeletal Rehabilitation 28 (2015) 789–795 DOI 10.3233/BMR-150586 IOS Press

Selective recruitment of the thoracic erector spinae during prone trunk-extension exercise Kyung-Hee Parka , Min-Hyeok Kangb , Tae-Hoon Kimc , Duk-Hyun Anb and Jae-Seop Ohb,∗ a

Department of Physical Therapy, Inje University, Gimhae, Korea Department of Rehabilitation Science, Graduate School, Inje University, Gimhae, Korea c Department of Occupational Therapy, Dongseo University, Busan, Korea b

Abstract. OBJECTIVE: This study examined the effect of modified prone trunk-extension (PTE) exercises on selective activity of the thoracic erector spinae. METHODS: Thirty-nine healthy subjects performed four modified PTE exercises, involving location of the edge of the table (iliac crests [IC] vs. xiphoid process [XP]) and the degree of trunk extension (horizontal vs. hyperextension). Electromyography signals were collected bilaterally from the longissimus thoracis (LT), iliocostalis thoracis (ICT), and iliocostalis lumborum (ICL). Normalized LT:ICL and ICT:ICL ratios were calculated. The data were analyzed using a repeated measures two-way analysis of variance. RESULTS: The LT:ICL and ICT:ICL ratios were significantly higher under the XP than the IC condition (p < 0.05); however, the degree of trunk extension did not affect the ratio (p > 0.05). Activity in the lumbar erector spinae and left ICT muscles was greater when subjects were in the hyperextended position than in the horizontal position. Moreover, activity in the thoracic erector spinae was greater when the table edge was aligned with the IC compared with the XP (p < 0.05). CONCLUSIONS: Our findings suggest that PTE exercise with the XP aligned with the table edge increased the selective activation of the thoracic erector spinae muscles. Keywords: Electromyography, thoracic extensor muscles, thoracic spine

1. Introduction The Biering-Sørensen test evaluates prone trunkextension (PTE) endurance and is used to predict low back pain in the next year [1,2]. PTE exercises increase strength, motor control, and endurance of the back extensor muscles and are used to prevent or treat low back pain [3–6]. Conditioning exercise for the erector spinae muscle has been shown to increase spine stability and improve acute lower back pain (LBP) [7] and prevent the natural progression of thoracic kyphosis and thoracic flexion syndrome [6,8,9]. ∗ Corresponding author: Jae-Seop Oh, Department of Physical Therapy, College of Biomedical Science and Engineering, Inje University, 607 Obang-Dong, Gimhae-Si, Gyeongsangnam-Do, 621740, Korea. Tel.: +82 55 320 3679; Fax: +82 55 329 1678; E-mail: [email protected].

The erector spinae, the most important trunk extensor muscle, is not a homogeneous muscle [10–13]; consequently, anatomical and functional differences exist between the thoracic and lumbar regions [14]. The lumbar part that is attached relatively deeper tends to stabilize the spine and had greater endurance, whereas the superficial thoracic part tends to strengthen the spine and produce more force [15,16]. Good balance between the lumbar and thoracic extensor muscles is crucial [16]. Although several studies have investigated the electromyography (EMG) activity of the erector spinae during various PTE exercises [1, 2,15–18], few studies deal with the thoracic erector spinae [15,16]. The majority of those studies focused on endurance or muscle activity during exercise, not balance or relative muscle activity compared with the lumbar erector spinae. Weak, fatigable thoracic erector spinae resulting in a slouched spinal posture and selective strengthening of this muscle is needed.

c 2015 – IOS Press and the authors. All rights reserved ISSN 1053-8127/15/$35.00 

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K.-H. Park et al. / Selective recruitment of the thoracic erector spinae during prone trunk-extension exercise

PTE exercises are generally performed with the anterior-superior iliac spine or iliac crests (IC) aligned with the edge of a table [2,18–20]. This posture has a long lever arm from the fixed point, so a high load is imposed on the lumbar erector spinae (i.e., the head, arms, and trunk). A short lever arm reduces the burden on the lumbar erector spinae and selectively activates the thoracic erector spinae. Mobility in the lower thoracic and thoracolumbar region is relatively high in the sagittal plane because there are no supporting structures such as the rib cage and sternum, which stabilize the upper and middle thoracic region [21]. More neuromuscular control and recruitment of lumbar erector spinae are needed to maintain stability of the lower thoracic region. Thus, we aligned the xiphoid process (XP) with the table edge to shorten the lever arm and support the lower trunk in this study. Supporting the lower trunk during PTE is thought to minimize involvement of the lumbar spine and lumbar erector muscle, increasing selective activation of thoracic erector spinae muscle. Generally, PTE exercises are performed with the upper body in a horizontal position [2,18–20], and our literature search revealed no previous investigation of PTE performed with a hyperextended trunk. Hyperextending the trunk requires extension of the lumbar and thoracic spine and might facilitate recruitment of the erector spinae muscle. However, hyperextension of the lumbar spine confers a high compressive force on spine, and maintaining neutral lordosis of the lumbar spine is essential [16,22]. Therefore, it is not clear whether exercise in this position is effective. We examined the effect of two modified PTE exercises on thoracic and lumbar erector spinae activity. We compared the effect of aligning the table edge with the IC versus the XP, in which the abdomen contacts the table providing support for the lower trunk. Supporting the lower trunk during PTE exercise might minimize involvement of the lumbar spine and lumbar erector muscle, improving the selective recruitment of the thoracic erector spinae. In addition, we examined the effect of degree of trunk extension by comparing the muscle activity recruited in PTE exercises performed in the hyperextended and horizontal positions.

2. Methods 2.1. Subjects We recruited 39 healthy volunteers (17 males, 22 females) from three universities in Korea. The mean

age of the subjects was 27.54 ± 4.29 years (mean ± SD), mean height was 168.21 ± 8.03 cm, and mean body mass was 61.64 ± 9.18 kg. Participants with metabolic, neuromuscular, or musculoskeletal disorders, a history of spinal surgery, episodes of back pain, and any pain in the test posture were excluded. The Inje University Faculty of Health Science Human Ethics Committee granted approval for this study, and all subjects provided written informed consent prior to participation. 2.2. Instrumentation Surface EMG signals were recorded using eight pre-amplified (gain: 1000) active surface electrodes (Model DE-2.3, Delsys, Wellesley, MA, USA). EMG signals from the recording sites were band-pass filtered between 20 and 450 Hz, analog-to-digital converted at a sampling rate of 2048 Hz and stored on a computer hard disk for later analysis. The electrodes were positioned bilaterally on the iliocostalis lumborum (ICL) at the L3 level midway between the lateral palpable border of the erector spinae and a vertical line through the posterior superior iliac spine [4,14,15], the longissimus thoracis (LT) at the T10 level midway between a line through the spinous process and a vertical line through the posterior superior iliac spine approximately 5 cm laterally [4,14], and the iliocostalis thoracis (ICT) at the T10 level midway between the lateral palpable border of the erector spinae and a vertical line through the posterior superior iliac spine [5,7,13,15]. Skin impedance was reduced by shaving excess body hair, if necessary, and gently abrading the skin with fine-grade sandpaper and then wiping it with alcohol swabs. Two physical therapists with more than 5 years’ experience measured the angles of the 1st (T1) and 12th (T12) thoracic spines and the 5th lumbar spine (L5) using two gravity-dependent inclinometers (zebris Medical, Isny im Allgäu, Germany) to compare movement in the thoracic and lumbar spine in the various PTE positions [23]. The inclinometers were placed over the spinal processes thought to correspond to the T1, T12, and L5 spinous processes. These spinal levels were determined by palpation and identified with stickers. The L5 spinous process was identified above the sacrum; the T12 spinous process was determined by palpating superiorly from the L5 point; and the T1 spinous process was identified inferiorly from the 7th cervical vertebra, which is designated as the most prominent spinal process [23]. The intra- and inter-rater reliabil-

K.-H. Park et al. / Selective recruitment of the thoracic erector spinae during prone trunk-extension exercise

A: IC-PTE

B: IC-PTHE

C: XP-PTE

D: XP-PTHE

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Fig. 1. PTE exercise postures: (A) iliac crest-prone trunk extension (IC-PTE); (B) iliac crest-prone trunk hyperextension (IC-PTHE); (C) xyphoid process-prone trunk extension (XP-PTE); (D) xyphoid process-prone trunk hyperextension (XP-PTHE).

ity were established during IC-PTE exercise in 15 of the participants. The incline of each spinous process was measured twice by the first examiner and once by the second examiner. The intra-class correlation coefficient (ICC[2,1] ) was calculated from two measurements by the first examiner. The mean data obtained from the first trial first examiner is compared with the data from the second examiner to calculate the interclass correlation coefficients (ICC[1,2] ). The intra- and inter-rater reliabilities were both high (ICC[1,2] = 0.97, ICC[2,1] = 0.91). 2.3. Procedures The subjects were asked to perform four bodyweight-dependent isometric back extension exercises in the prone position with their arms crossed at the chest and lower limbs fixed with non-elastic straps at the hips, knees, and ankles (location of table edge: iliac crest or xiphoid process × the degree of trunk extension: horizontal or hyperextension; Fig. 1). The subjects were instructed to hold each position for 5 s. Exercise 1 was performed with the iliac crest aligned with the table edge and the subjects were instructed to hold their body (head, arms, and trunk) horizontal to the ground (IC-PTE) [2]. In Exercise 2, the iliac crest was aligned with the table edge, and the subjects were instructed to hyperextend their trunk 7 cm at the level of T8 (IC-PTHE). Exercise 3 was executed with the xiphoid process aligned with the table edge to support the lower trunk. The subjects were instructed to hold their body horizontal to the ground (XP-PTE). Exercise 4 was performed with the xiphoid process aligned

with the table edge and the subjects were instructed to hyperextend their trunk by 7 cm at the level of T8 (XPPTHE). The subjects were taught how to perform the exercises before data collection and were allowed two practice tries. A bar was placed at approximately T8 to indicate the horizontal and 7 cm hyperextension positions. A rest of at least 2 min was scheduled between exercises to prevent muscle fatigue. All exercises were repeated three times in a random order. Maximum voluntary isometric contraction (MVIC) was used to normalize the erector spine thoracic and lumborum muscle activity. The MVIC of the erector spinae thoracis and lumborum was measured simultaneously with the subjects in the prone position with their legs strapped to the table and their hands placed on their heads. They were instructed to extend their back using maximum isometric effort against resistance placed on the angular inferior scapulae by the experimenter [16,24]. The procedure was repeated three times with a 30-s rest between sessions. During the PTE task, the thoracic kyphosis and lumbar lordosis were measured using a dual inclinometer. Clockwise rotation of the indicator (in the direction of extension) was a positive value, and the opposite direction was a negative value. The angle of thoracic kyphosis was calculated as T1–T12, and the absolute value of T12–T1 was the thoracic extension angle. The lumbar lordosis angle was defined as L5–T12. 2.4. Data analysis The root mean square (RMS) was calculated over successive 250-ms (512 points) time-windows and

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K.-H. Park et al. / Selective recruitment of the thoracic erector spinae during prone trunk-extension exercise Table 1 The raw EMG data of each erector spinae in maximum voluntary isometric contraction (µV; mean ± SD) Muscle Right LT Left LT Right ICT Left ICT Right ICL Left ICL

1st MVIC 11.799 ± 8.225 11.771 ± 8.560 6.944 ± 5.064 6.671 ± 5.619 7.898 ± 3.695 7.818 ± 4.287

2nd MVIC 11.528 ± 8.694 11.648 ± 7.713 6.467 ± 4.963 6.786 ± 3.416 7.694 ± 3.706 7.511 ± 4.056

3rd MVIC 11.382 ± 8.740 11.587 ± 8.041 6.801 ± 5.137 6.484 ± 5.411 7.504 ± 4.037 7.716 ± 4.378

LT, longissimus thoracis; ICT, iliocostalis lumborum pars thoracis; ICL: iliocostalis lumborum pars lumborum.

2.5. Statistics The Kolmogorov-Smirnov test was used to assess the homogeneity of variance of the % MVIC of each muscle and LT:ICL and ICT:ICL ratios. Repeated measures two-way analysis of variance (ANOVA) was performed to investigate the effect of ‘location of the table edge (IC vs. XP) and ‘degree of trunk extension’ (horizontal vs. hyperextension) and interactions with the dependent variable; i.e., % MVIC of each erector spinae muscle, the ratio of thoracic erector spinae to lumborum, and movement of thoracic and lumbar spine. Where a significant difference was detected (p < 0.05), post hoc comparisons were performed using the Bonferroni correction. Significance for the post hoc analysis was set at α = 0.003 (0.05/16 comparisons). The Statistical Package for the Social Sciences ver. 18.0 (SPSS, Chicago, IL, USA) was used to conduct all statistical analyses.

3. Results Table 1 presents the raw EMG data for each erector spinae at MVIC. The % MVIC was greater in the hyperextended position than in the horizontal position for the left ICT and ICL bilaterally; however, we found no significant difference in the LT:ICL or ICT:ICL ratios. The % MVIC in all muscles was not significantly different between the XP and IC conditions; however, the LT:ICL and ICT:ICL ratios were significantly greater under the XP condition (Table 2; Fig. 2). The T1, T12, and thoracic extension angles were significantly greater during the hyperextended trunk

1.5

Muscle activation ratio to ICL

EMG values were calculated using the middle 3 s of the 5-s isometric contraction during the PTE exercise and MVIC testing. The data obtained were normalized (% MVIC) using the mean RMS value during MVIC testing. The normalized LT:ICL and ICT:ICL ratios were calculated to measure selective recruitment of the thoracic erector spinae.

**

**

**

**

1.0

0.5

0.0

rt LT

lt LT

Iliac crest

rt ICT

lt ICT

Muscles Xyphoid process

Fig. 2. Ratio of longissimus thoracis (LT) and iliocostalis lumborum pars thoracis (ICT) to iliocostalis lumborum pars lumborum (ICL) according to the location of the table edge. Rt, right; Lt, left.

exercises compared with those in the horizontal position. The angle of L5, thoracic kyphosis, and the lumbar lordosis angle were not significantly different between positions. We found no difference in the T1, L5 and angle of thoracic kyphosis, thoracic extension, or lumbar lordosis according to the location of the table edge; however, the angles at T12 were significantly greater under the XP condition compared with the IC condition (Table 3).

4. Discussion To our knowledge, our study is the first to investigate whether the position of the trunk and degree of trunk extension during PTE exercise affect thoracic erector spinae activity. The selective activation of the thoracic erector spinae was increased significantly under the XP condition compared to the IC condition. The activity of lumbar erector spinae was significantly greater in the hyperextended position than with the trunk horizontal;

K.-H. Park et al. / Selective recruitment of the thoracic erector spinae during prone trunk-extension exercise

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Table 2 The % MVIC values of the thoracic erector spinae muscles and the ratio of thoracic erector spinae to lumbar activity during prone trunk-extension exercises (%MVIC; mean ± SD) Variable % MVIC

Muscle Right LT

Ext

HO HE Left LT HO HE Right ICT HO HE Left ICT HO HE Right ICL HO HE Left ICL HO HE LT:ICL ratio Right LT HO HE Left LT HO HE ICT:ICL ratio Right ICT HO HE Left ICT HO HE

Location of table edge IC XP 35.59 ± 17.70 34.24 ± 18.77 41.30 ± 16.91 41.58 ± 20.02 36.61 ± 14.94 33.15 ± 13.75 42.70 ± 17.01 42.06 ± 18.61 43.66 ± 17.29 30.18 ± 18.55 52.38 ± 18.94 41.35 ± 16.51 41.09 ± 14.56 34.53 ± 15.28 48.72 ± 16.06 45.81 ± 18.80 61.29 ± 20.62 40.48 ± 15.86 71.46 ± 19.40 54.64 ± 18.12 63.58 ± 17.88 45.34 ± 14.95 72.71 ± 16.49 58.04 ± 17.46 0.62 ± 0.32 0.89 ± 0.44 0.60 ± 0.24 0.80 ± 0.37 0.60 ± 0.24 0.77 ± 0.31 0.60 ± 0.22 0.78 ± 0.42 0.75 ± 0.29 0.98 ± 0.35 0.76 ± 0.25 0.91 ± 0.31 0.66 ± 0.22 0.79 ± 0.30 0.68 ± 0.20 0.84 ± 0.43

F 4.911

EXT R2 0.031

p 0.028

F 0.033

IC/XP R2 0.000

p 0.857

Interaction F R2 p 0.077 0.001 0.782

8.362

0.052

0.004

0.626

0.004

0.430

0.296 0.002 0.587

4.236

0.027

0.041

6.429

0.041

0.012

0.064 0.002 0.801

13.173

0.080

0.000∗

3.309

0.021

0.071

0.492 0.003 0.484

16.708

0.099

0.000∗

0.208

39.979

0.000∗

0.448 0.003 0.504

16.607

0.098

0.000∗

37.728

0.199

0.000∗

0.443 0.003 0.507

0.951

0.006

0.331

18.035

0.106

0.000∗

0.329 0.002 0.567

0.003

0.000

0.960

13.017

0.079

0.000∗

0.004 0.000 0.953

0.370

0.002

0.544

15.451

0.092

0.000∗

0.455 0.003 0.501

0.460

0.003

0.499

8.986

0.056

0.003∗

0.153 0.001 0.696

EXT, extension; IC, iliac crests; XP, xiphoid process, LT, longissimus thoracis; ICT, iliocostalis thoracis; ICL, iliocostalis lumborum; HE, hyperextended; HO, horizontal. ∗ p < 0.003 (0.05/16). Table 3 Angle of the spine during PTE exercise (◦ ; mean ± SD) Variable T1

EXT

HO HE T12 HO HE L5 HO HE Thoracic kyphosis HO HE Thoracic extension HO HE Lumbar Lordosis HO HE

Location of table edge IC XP −1.01 ± 10.77 −0.50 ± 9.79 −8.45 ± 10.14 −9.92 ± 9.31 −10.21 ± 4.68 −12.21 ± 4.26 −14.86 ± 5.24 −16.73 ± 4.39 13.79 ± 6.61 11.31 ± 6.50 10.96 ± 6.66 8.24 ± 6.63 9.20 ± 11.84 11.70 ± 10.64 6.41 ± 11.12 6.80 ± 10.82 11.21 ± 11.65 12.71 ± 10.72 23.32 ± 11.70 26.65 ± 9.73 23.99 ± 7.04 23.51 ± 6.81 25.82 ± 7.31 24.97 ± 7.23

EXT F R2 27.621 0.154

p 0.000∗

IC/XP F R2 p 0.091 0.001 0.764

Interaction F R2 p 0.379 0.002 0.539

37.932 0.200

0.000∗

6.718 0.010

0.010∗

0.008 0.000 0.927

7.758 0.006

0.006

6.023 0.38

0.015

0.013 0.000 0.910

4.660 0.030

0.032

0.663 0.004

0.417

0.352 0.002 0.554

54.857 0.265

0.000∗

1.886 0.012

0.172

0.273 0.002 0.602

2.083 0.014

0.151

0.344 0.002

0.558

0.027 0.000 0.869

EXT, extension; IC, iliac crest; XP, xiphoid process; T1, 1st thoracic spine; T12, 12th thoracic spine; L5, 5th lumbar spine; HE, hyperextension; HO, horizontal. ∗ p < 0.003 (0.05/16).

however, we did not observe differential recruitment of the thoracic erector spinae. The degree of trunk movement was significantly greater in the hyperextended than the horizontal trunk position at the thoracic spine level (i.e., T1, T12, and thoracic extension angle), but thoracic kyphosis and lumbar lordosis were not significantly different. In addition, we found that the inclination of T12 was increased more in the extension direction in XP-PTE than in IC-PTE. The lordotic curve of the lumbar spine did not differ according to the location of the table edge or degree of extension.

Although the thoracic and lumbar erector spinae function synergistically during trunk extension, the functions of these muscles differ somewhat according to anatomical characteristics or posture [14]. When the XP was aligned with the table edge (XP-PTE), we observed that the selective activation of the thoracic erector spinae was increased compared with when the iliac crest was aligned (IC-PTE) because of a significant decrease in the bilateral activity of the lumbar erector spinae, while there was no change in the thoracic erector spinae activity. This finding may be explained as follows. First, in this position, the axis of the trunk ex-

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K.-H. Park et al. / Selective recruitment of the thoracic erector spinae during prone trunk-extension exercise

tension and the center of gravity (head, arms and upper and mid-thoracic) shifts in the cephalic direction decreasing the load on the back muscles and the length of the moment arm from the moveable vertebrae to the COM location. Second, the upper and mid-thoracic spine is more stable than the lower thoracic and lumbar regions because of the supporting rib cage and sternum; thus, greater neuromuscular activation is necessary to maintain the antigravity position in the lower thoracic and lumbar regions [21,25]. Third, when the table provided less support, L5 movement was decreased and T12 movement was extended, and the increased extension of the thoracic spine activated the thoracic erector spinae. Various studies have found it necessary to restrain the pelvis to effectively isolate the paraspinal musculature to the hip muscles during lumbar extension [17,26,27]. The XP-PTE exercise, which supports the lower trunk, effectively restricts lumbar spine movement and selectively activates the thoracic erector spinae. The lumbar erector spinae muscle activity was significantly greater in the hyperextended position than in the horizontal trunk position. A previous study showed that the strength of isometric lumbar extension was greatest in full flexion and decreased gradually as extension increased (from 317.0 to 174.5 Nm) [26], based on the length-tension relationship [11]. In the prone hyperextended trunk position, the tension of the trunk extensor is lowest because this muscle is the shortest, but the external torque does not change. Therefore, the lumbar erector spinae must increase its activity to maintain this position. In the hyperextended position, the T1, T12, and thoracic extension angles were significantly increased, whereas the L5 and thoracic kyphosis angles were decreased significantly. However, the lumbar lordotic curve did not differ between the PTE positions. PTE exercise is used to prevent thoracic hyperkyphosis [8] and to treat thoracic flexion syndrome [9]. PTE exercise with the thoracic spine hyperextended would be more effective for improving posture than general PTE exercise. However, maintaining a neutral curve of the lumbar spine during exercise was emphasized, because hyperlordosis of lumbar spine during exercise is related to a lower failure tolerance of the spine, joint compression, and anteroposterior joint shear forces [1,28]. PTE exercise in the hyperextended position appears to be safe, without causing hyperlordosis of the lumbar spine. No study participant complained of LBP in this position. However, this result was not generalized to the patients with LBP. We did

not find an effect of the degree of trunk extension on the thoracic erector spinae activity, perhaps because the muscle activity was increased similarly in the thoracic and lumbar erector spinae. This study had some limitations. First, the only difference in activation of the left ICT was seen in the hyperextended position, because the difference on the subject’s dominant side might affect the erector spinae. Second, as the electrodes for the ICT and LT were both attached at the T10 level, crosstalk cannot be ruled out. Also, we used MVIC EMG normalization methods in prone trunk extension posture such as BieringSorenson endurance test. Third, our results cannot be generalized to person with postural dysfunction in the thoracic spine and patients with LBP because our subjects were healthy young people. Fourth, we studied muscle activity during isometric contraction, and did not investigate dynamic exercises. Fifth, the physical properties of the spine, such as stability or flexibility, were not considered in this study.

5. Conclusion In conclusion, our results demonstrated the PTE exercise performed with the table edge aligned with the XP selectively activated the thoracic erector spinae over the lumbar erector spinae and facilitated the movement of T12. This suggests that XP-PTE exercise could be used to facilitate selective activation of the thoracic erector spinae. The PTE exercise performed in the hyperextended trunk position elicited a greater increase in the lumbar erector spinae muscle activities than did that in the horizontal position and the lumbar lordotic curve did not differ between the hyperextended and horizontal positions.

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Selective recruitment of the thoracic erector spinae during prone trunk-extension exercise.

This study examined the effect of modified prone trunk-extension (PTE) exercises on selective activity of the thoracic erector spinae...
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