Why is force output decreased while sitting on an exercise ball? The core stability debate continues.

“Core Stability” has become a very popular term when discussing topics ranging from functional performance in athletes to rehabilitation for patients with low back pain. The exercise ball has long been used to promote core stability through specific exercises aimed at activating core muscles including the abdominals, low back, and pelvic stabilizers. The use of unstable surfaces such as a Thera-Band® exercise ball has been shown to increase muscle activity compared to a stable surface by providing additional challenge to postural stability. Researchers have shown, however, that exercises on an unstable surface reduce force output, suggesting that traditional resistance training exercises should not be performed on an unstable surface when the goal is to increase core activation. This lack of force output on unstable surfaces is thought to be related to a lack of core stability.

Researchers in New Zealand set out to determine if the loss in force output while performing an overhead press exercise on an exercise ball was related to core muscle endurance. They were interested in the ‘specificity vs. generality” of core stability; in other words, is core stability related to overall physical performance, or simply a measure of core muscle endurance?

30 resistance-trained, healthy young males participated in the study. The subjects were tested for their predicted 1RM with an overhead dumbbell press while seated on a stable exercise bench with back support, and again on an exercise ball. Next, subjects were tested for muscular endurance of their core muscles in 3 positions in random order: supine curl-up, back extension on a roman chair, and a side-bridge.

The researchers calculated the percent difference in predicted 1RM strength between the stable and unstable overhead press, described as the “instability strength level” (ISL). 2 sub-groups were analyzed: a higher ISL (>90%) and a lower ISL (<85%). The analysis revealed no relationship between the ISL and core muscle endurance, with the exception of the supine curl-up. The authors concluded that the 3 core muscle tests were not able to predict core stability with an overhead press, noting that these tests were likely task-specific to only the core muscles. Therefore, total body exercises replicating functional movements may be necessary to assess and improve core stability.

This study raised several issues. First, the measures of “core stability” were actually measurements of static structural muscle endurance. It can be argued that the 3 tests are in fact not valid measures of functional stability. Functional stability relates to the ability to efficiently transfer forces through the spine (not just the “core”) and into the extremities for purposeful and coordinated movement. Static trunk muscle endurance arguably is not a pre-requisite of dynamic movement, particularly for an overhead press. True pelvic stability should be quantified by the ability of the pelvic stabilizing muscles to turn on and off in a coordinated manner as suggested by Dr. Stu McGill. It would be interesting to see results of a different exercise such as a bench press or lower body movement.

This study suggests that core training by simply increasing muscle endurance is not specific to “core stability” in relation to functional tasks. This may lead to the conclusion that isolated core strengthening exercise such as sit-ups and side bridges are not effective at providing true functional “core stability;” instead, whole-body functional movements that challenge reflexive core stabilization may be more appropriate.

In conclusion, the decreased force output that subjects experienced during the overhead press while sitting on an exercise ball was not accounted for by structural muscle endurance; it must be related to another structure or functional component. Obviously, more research is needed on this topic.

Keogh JW, et al. Can common measures of core stability distinguish performance in a shoulder pressing task under stable and unstable conditions? J Strength Cond Res. 2010 Feb;24(2):422-9.

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