Unstable Surface and Force Production

If you are reading this article, it is a safe bet that you are a gym goer. If you have been going to your favorite gym regularly for a time period that goes back at least ten years, you have also likely noticed that a gym just doesn’t look the way it used to. For starters, they are bigger and brighter (well, some of them), but in particular many gyms have more toys or implements that their patrons can use for any number of reasons. Medicine balls, foam rollers, foam pads, elastic bands, kettlebells, swiss medicine balls, BOSU platforms, etc. are what I’m talking about. If you have no idea what any of these are you either don’t get to a gym, work out in your garage, work out a gym that could double as a garage, go to Curves or you are “old school” and only do push-ups, sit-ups and pull-ups.

Many of these implements offer excellent chances to train your body through multiple directions, which many strength coaches and exercise scientists will tell you is a good thing. A common theme amongst many of these “toys” and something which has exploded in popularity is training on an unstable surface. The reason for training on an unstable surface are simple as doing so forces many other parts of your body to get involved to help you maintain necessary posture to complete whatever exercise you may be performing. More muscle activation is thought to result in improved strength, stability and performance. But is this actually the case?

For starters, people who train on an unstable surface and even more so those people who are less experienced need to: 1) use common sense and 2) be careful. If you think stupid things can’t happen while trying out the latest and greatest instability device, just to go to YouTube and type in weight lighting accidents or injuries. Keep searching and you are certain a video of some person standing on a swiss medicine ball or doing squats on such a medicine ball with or without resistance. Not only is this dangerous, it’s largely useless. Therefore, use common sense and be careful.

To shift the discussion more towards how this type of training influences performance, the notion of the size principle comes into play. The size principle applies to everyone who picks up a weight and tells us that as we perform exercises that require higher and higher levels of force to be produced the type of muscle fibers that are recruited changes. For example, when using lighter weights, lower levels of force are required and as a result lesser numbers of muscle fibers are needed (to move the lighter weight). Of those fibers which are recruited they are typically smaller and have less potential for force production. Alternatively, as you complete activities that require greater levels of force, the number of muscle fibers recruited will be increased and larger muscle fibers will get involved as well.

A recent study had study participants complete two different bouts of resistance exercise. In targeting the upper-body, a traditional bench press exercise was completed on a stable surface and a separate workout was completed with the individual lying on a large swiss medicine ball. Therefore, one condition was stable while the other was unstable. The same comparison was made for the lower-body where a traditional barbell squat was completed both on a stable surface and an unstable surface (standing on a BOSU ball). Each exercise bout required the individual to lift a weight that corresponded to 70% of their maximum strength and complete six sets of eight repetitions.

The results are interesting. When upper-body power production was compared between a stable and unstable surface, power production was lower when an unstable surface was used and the reduction in power production got bigger as more sets were completed. A similar phenomenon was also discovered when squats were performed on an unstable vs. a stable surface. Interestingly, a greater difference in power production between a stable versus unstable condition was found with the upper body (Zemkova, Jelen et al. 2012).

Results from this study are very important! No one disputes the importance of force production and how it can impact improvements in strength, power and muscle mass so people need to carefully consider how much they train on an unstable surface. Why? Because research such as this tells us that it can reduce power production and a reduction in power may negatively impact your progress in areas related to strength, power and performance. For these reasons, many people feel like some but not all exercises should have an unstable component or to cycle through workout phases where you incorporate unstable conditions to strike a balance between improved balance and optimal force production.


Zemkova, E., M. Jelen, et al. (2012). “Power outputs in the concentric phase of resistance exercises performed in the interval mode on stable and unstable surfaces.” J Strength Cond Res 26(12): 3230-3236.