An unstable support surface is not a sufficient condition for increases in muscle activity during rehabilitation exercise.

Introduction Resistance training is commonly recognized as an essential component to a rehabilitative treatment plan. Modifications of various exercises are made on the belief that different types of support surfaces and joint positions can influence muscle recruitment levels. One modification to traditional exercises is the addition of an unstable support surface (exercise balls, wobble boards, foam padding) under the assumption that this addition will result in an increase in the neuromuscular activation of the involved muscles. Anecdotally, clinicians and rehabilitation specialists are often heard to state that performing an exercise on an unstable surface causes “the little muscles to work harder to stabilize the joints.” It is a prevalent assumption that the mere addition of unstable surfaces to simple tasks (e.g. sitting on an exercise) ball transforms these simple tasks into exercises which stress the “core” musculature (trunk and pelvis muscles). The exercise biomechanics literature investigating modifications to surface stability has focused on the trunk muscle’s response during resistance exercise. Increases in the amplitude of the EMG signal with the addition of an unstable support surface compared with a stable support surface have been documented during curl up exercises (external oblique),1 trunk bridging exercises (rectus abdominis and external oblique)2–4 and traditional strength training exercises including the squat, shoulder press, dumbbell press (lower abdominal stabilizers, upper and lower erector spinae).5,6 However, not every exercise modified with the addition of an unstable support surface results in a consistent response across participants3 nor in an increase in the trunk muscle’s EMG amplitude.3–6 This lack of an increase has been exemplified in research discounting the common belief and assumption that replacing a chair with an exercise ball results in increases in muscle activation.7 Two studies have disproved this contention demonstrating no difference in trunk muscle activation levels between sitting on a Swiss ball and sitting on a chair.8,9 There has even been evidence of decreases in the amplitude of the abdominal muscles during trunk extension resistance exercises when performed on a stability ball compared with floor.10 The lack of increase in muscle activation levels and inconsistent responses across participants during certain exercises and amongst specific muscles with the addition of unstable surfaces suggest other factors (weight lifted, speed of movement, participant personal characteristics) may influence muscle recruitment to a greater extent than surface instability. What is lost in the conclusions of the preceding research investigating support surface instability is the large variability in the inter-individual responses to these exercises. While the mean activity levels for specific exercises from some studies may suggest that the unstable support surface increases muscle activation levels there are often individuals who do not respond in this manner. The same can be seen when there is no change in the group mean activation level between surface conditions (stable versus labile) yet a specific individual may be greatly affected by changes in surface stability. Thus, individual factors may play a large role in how muscle activation levels are affected by the addition of an unstable surface. The intention of this proof of principle commentary is to illustrate through individual case studies, contrasted with a population mean, examples where muscle