Effect of bilateral versus unilateral exertion tests on maximum voluntary activity and within-participant reproducibility in the shoulder

Normalized surface EMG requires known muscle activity from a well defined reference contraction, often a maximum voluntary isometric exertion (MVIE). One factor that may impact the activity obtained during a MVIE is the use of a bilateral or unilateral version of the test. The objective of this work was to investigate how performing MVIE tests unilaterally or bilaterally affects maximum voluntary electrical activity (MVE) for selected shoulder muscles. Further, we examined if within-participant reproducibility could be improved with the use of bilateral or unilateral MVIE tests.Sixteen participants completed five repetitions of a palm press, empty can, and external rotation MVIE tests both bilaterally and unilaterally. Muscle activity was recorded from seven muscles acting at the glenohumeral joint during the performance of each test. Repeated measures ANOVAs were used to determine within-participant differences on muscle specific MVEs and hand force measures between the two versions of the test. Gender was compared as a between participants factor. Within-participant reproducibility in muscle activity between bilateral and unilateral MVIEs was compared using coefficients of variation.Performing a bilateral empty can MVIE test elicited significantly higher MVEs for the upper trapezius (14%) and supraspinatus muscles (15%) than a unilateral test in this study population. MVEs during the palm press and external rotation were not affected by performing the tests bilaterally or unilaterally. Reliability was similar between unilateral and bilateral exertions across MVIE tests. Hence, we recommend that the empty can MVIE test be performed bilaterally, while either bilateral or unilateral configurations are equivalent during the palm press and external rotation MVIE tests.