an unstable support surface is not a sufficient condition for increases in muscle activity during...

7/27/2019 An Unstable Support Surface is Not a Sufficient Condition for Increases in Muscle Activity During Rehabilitation Exercise

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J Can Chiropr Assoc 2007; 51(3) 139

Commentary

An unstable support surface is not a sufficient condition

for increases in muscle activity during rehabilitation exerciseGregory J Lehman, MSc, DC*

IntroductionResistance training is commonly recognized as an essen-tial component to a rehabilitative treatment plan. Modifi-cations of various exercises are made on the belief that

different types of support surfaces and joint positions caninfluence muscle recruitment levels. One modification totraditional exercises is the addition of an unstable supportsurface (exercise balls, wobble boards, foam padding)under the assumption that this addition will result in anincrease in the neuromuscular activation of the involvedmuscles. Anecdotally, clinicians and rehabilitation spe-cialists are often heard to state that performing an exer-cise on an unstable surface causes the little muscles towork harder to stabilize the joints. It is a prevalent as-sumption that the mere addition of unstable surfaces tosimple tasks (e.g. sitting on an exercise) ball transformsthese simple tasks into exercises which stress the coremusculature (trunk and pelvis muscles).

The exercise biomechanics literature investigatingmodifications to surface stability has focused on the trunk muscles response during resistance exercise. Increases inthe amplitude of the EMG signal with the addition of anunstable support surface compared with a stable supportsurface have been documented during curl up exercises(external oblique), 1 trunk bridging exercises (rectus ab-dominis and external oblique) 24 and traditional strengthtraining exercises including the squat, shoulder press,dumbbell press (lower abdominal stabilizers, upper andlower erector spinae). 5,6 However, not every exercisemodified with the addition of an unstable support surfaceresults in a consistent response across participants 3 nor inan increase in the trunk muscles EMG amplitude. 36 This

lack of an increase has been exemplified in research dis-counting the common belief and assumption that replac-ing a chair with an exercise ball results in increases inmuscle activation. 7 Two studies have disproved this con-

tention demonstrating no difference in trunk muscle acti-vation levels between sitting on a Swiss ball and sittingon a chair. 8,9 There has even been evidence of decreasesin the amplitude of the abdominal muscles during trunk extension resistance exercises when performed on a sta-bility ball compared with floor. 10 The lack of increase inmuscle activation levels and inconsistent responsesacross participants during certain exercises and amongstspecific muscles with the addition of unstable surfacessuggest other factors (weight lifted, speed of movement,participant personal characteristics) may influence mus-cle recruitment to a greater extent than surface instability.

What is lost in the conclusions of the preceding re-search investigating support surface instability is thelarge variability in the inter-individual responses to theseexercises. While the mean activity levels for specific ex-ercises from some studies may suggest that the unstablesupport surface increases muscle activation levels thereare often individuals who do not respond in this manner.The same can be seen when there is no change in thegroup mean activation level between surface conditions(stable versus labile) yet a specific individual may begreatly affected by changes in surface stability. Thus, in-dividual factors may play a large role in how muscle acti-vation levels are affected by the addition of an unstablesurface. The intention of this proof of principle commen-tary is to illustrate through individual case studies, con-trasted with a population mean, examples where muscle

* Department of Graduate Studies, Canadian Memorial Chiropractic College, Toronto, ON, Canada.Address correspondence to: Gregory J Lehman, Assistant Professor, Canadian Memorial Chiropractic College, 6100 Leslie Ave, Toronto,Ontario, Canada, M2H 3J1, [email protected]


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Commentary

140 J Can Chiropr Assoc 2007; 51(3)

activation levels show unexpected responses to the exer-

cise demands when performed on unstable/labile surfacesversus stable surfaces. Sample case studies have been ex-tracted from our published research out of the CMCC Bi-omechanics Laboratory from research conducted over thepast 4 years.

Proof of Principle

Example #1: Triceps and Rectus Abdominis EMG during Push Up Exercises The following 4 cases are individual muscle responsestaken from a previously published paper. 11 Male partici-pants between the ages of 23 and 28 were recruited forthis study. Individuals were excluded if they had a currentor history of shoulder, upper limb or spine injury withinthe previous 6 months or had previous surgery on the re-spective areas. Individuals were required to perform pushup exercises with their hands either on a 65 cm diameterSwiss ball or with their hands supported on an exercisebench of a similar height. The surface electromyographicactivity was collected from the triceps, pectoralis major,latissimus dorsi, rectus abdominis and external obliquemuscles. The authors found no statistically significantdifference in average muscle activity between surfaces(bench vs. ball) in the pectoralis major, latissimus dorsiand external oblique muscles. The triceps and rectus abo-minis average activation level was increased when pushups were performed on a Swiss ball compared with anexercise bench. The average triceps muscle activity (ex-pressed as a percentage of a maximum voluntary contrac-tion MVC) increased from 22.2 % MVC (standarddeviation = 8.8) on an exercise bench to 43.1 % MVC(SD = 17.3) when push ups were performed on a Swissball. Figure 1 presents an example of an increase in tri-ceps EMG during three push ups when performed on an

exercise bench (average activity = 11.4% MVC) and on aSwiss ball (36.6 % MVC). The rectus abominis group av-erage activity increased from 13.46 %MVC (SD = 5.42)on an exercise bench to 22.63 %MVC (SD = 8.64) whenperformed on a Swiss ball. Figure 2 presents a typical in-crease in rectus abdominis activation level for one subjectduring three push ups performed on an exercise bench(average activity = 16.6 %MVC) versus a Swiss ball (av-erage activity = 22.2%MVC). However, not all individualresponded in a manner similar to the mean.

Instability is not a sufficient condition for increases in muscle activityLooking at the group mean activation levels and the sta-tistical analysis the reader (and author) is led to concludethat the instability of the Swiss ball causes increases inmuscle activation of the triceps and rectus abdominismuscle. While this can be supported as a general trend itis important to note that not every individual responds inthe same manner to changes in surface stability. Figure 3presents the EMG profiles of the triceps muscle for an in-

Figure 1 Triceps EMG increase during push upexercises on ball vs. bench.

Figure 2 Rectus Abdominis EMG increase during pushup exercises on ball vs. bench.

Triceps EMG during push ups on bench vs. ball

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GJ Lehman

J Can Chiropr Assoc 2007; 51(3) 141

dividual that showed a minimal change in average muscleactivation when a Swiss ball replaced an exercise benchduring push up exercises. This individuals activation lev-els were 17.1 % and 18.4% MVC on the exercise benchand Swiss ball, respectively. A similar non-effect can beseen in the rectus abominis muscle (Figure 4) for an indi-vidual whose average muscle activation levels were12.5% MVC and 13.75 % MVC for push ups when per-formed on a bench and Swiss ball, respectively. These

two examples illustrate that just as not all muscles in-

crease their activation levels during push ups when per-formed on an unstable surface (e.g. pectoralis major,external oblique) not all individuals respond in a similarmanner to changes in surface stability even in instanceswhere the group mean is significantly different betweensurface conditions. The reason for this outlier is un-known. An interesting finding in Figure 3 is the differ-ence in the shape of muscle activation profiles betweenthe two surfaces. The triceps activity on the benchpresents a bimodal shape corresponding to the eccentric(smaller hump) and concentric (larger hump) phase of themuscle action. These two humps do not exist during thepush ups on the ball. This finding was not a common oc-currence across participants and again suggests the varia-ble response across individuals. During the previousstudy, an attempt was made to visually ensure the sameperformance of the push up exercise (positions, speed of movement, height of support surfaces) however, this wasnot vigorously controlled with elaborate means to ensureidentical kinematics. The rationale for this lack of elabo-rate control was an attempt to mimic how these exerciseswould be instituted in a clinical environment since thesecontrols would not occur in practice. These outliers arerelevant to the evidenced based clinician because it sug-gests that merely adding an unstable surface to an exer-cise may not result in greater stress placed on a musclefor all individuals even when biomechanical researchmay support that belief. Clinicians wishing to incorporateprogressive resistance training principles into theirstrength and conditioning programs should therefore con-sider other means to increase the stress on the muscu-loskeletal system. A method of increasing the load on themusculoskeletal system during a push up would be tostagger the position of the arms (one forward and onebackward) and modify the speed of movement. 12

Example #2: Individual responses are highly variable The following two cases are individual responses takenfrom an unpublished CMCC resident research 13 piloproject ( n = 10) which quantified trunk muscle activityduring the performance of various exercises (squats,lunges, biceps curls, shoulder presses, triceps extensions)performed on a labile domed surface (similar to theBOSU Balance Trainer) and while standing on the

Figure 3 Triceps EMG demonstrates similar averageactivation levels across different surfaces (ball versusbench).

Figure 4 Rectus Abdominis EMG uninfluenced bysurface stability.

Triceps EMG during pushups on ball and bench

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Commentary

142 J Can Chiropr Assoc 2007; 51(3)

ground. We found that for all trunk muscles studied (up-per and lower erector spinae, rectus abdominis, externaloblique and lower abdominal stabilizers) there were nodifferences in muscle activation levels between surfaceconditions. However, individuals within the group re-sponded in highly variable and different manners. Thefollowing two cases will present how two different indi-viduals respond in different manners to modifications of surface stability during the double legged squat exercise.The group average EMG level for the lower erectorspinae was 16.98 %MVC (SD = 9.13) and 15.8 % MVC(SD = 9.67) for the squat exercise when performed on theground and the labile surface, respectively.

Figure 5 demonstrates a marked increase in averagemuscle activity in the lower erector spinae during squatswhen performed on the labile surface (26.58 %MVC)versus the ground (19.6 %MVC). Conversely, a differentindividual demonstrated a similar change in muscle activ-ity yet in the opposite direction. Figure 6 depicts lowererector spinae muscle during the squat on the ground(19.4% MVC) and on the labile surface (14.7 %MVC).

What is of interest in these findings is that the meanactivation level shows no response to changes in surfacestability. Yet individuals may be highly influenced. Forthis particular exercise (squat) and muscle (lower erectorspinae) studied half of participants increased their muscleactivity while half showed decreases.

Conclusion and clinical relevanceThe examples presented here and the findings from theliterature suggest that the mere addition of a labile sur-face is not a sufficient condition to achieve increasesin muscle activity for all muscles and for all individuals.Individuals can present markedly varied responses andcan respond in a manner significantly different from themean. This is relevant to the rehabilitation professionalwhen designing conditioning programs that attempt tocreate overload conditions over time. Merely adding la-bile surfaces may not increase the load on the neuromus-cular system for specific patients. An argument can evenbe made that adding Swiss balls to certain exercises (wallsquats 14 and spine extensor exercises 7) decreases thestress on the musculature due to decreases in muscle acti-vation following the incorporation of a Swiss ball.

References1 Vera-Garcia FJ, Grenier SG, McGill S. Abdominal muscle

response during curl-ups on both stable and labile surfaces.Physical Therapy 2000; 80:564569.

2 Behm D, Leonard A, Young W, MacKinnon, S. Trunk muscle electromyographic activity with unstable andunilateral exercises. J Strength Conditioning Research 2005; 19 (1):193201.

3 Lehman GJ, Gordon T, Langley J, Pemrose P, Tregaskis S.Replacing a Swiss ball for an exercise bench causesvariable changes in trunk muscle activity during upper limbstrength exercises. Dynamic Medicine 2005: Jun 3;4.

Figure 5 Lower Erector Spinae EMG increases when performed on a labile surface.

Figure 6 Lower Erector Spinae EMG increases when performed on the ground.

Lower Erector Spinae EMG during squats on stable/unstable

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