In the latest Strength and Conditioning Journal, there is a great writeup by Daniel Hubbard on whether or not unstable surface training is advisable for healthy adults. Looking at all of the research and a little bit of common sense, it is clearly not advisable at all. When a movement is performed on an unstable surface, some extra stabilizers are activated, yes, but the prime movers that are ever so important for real time functional activity are unable to produce the optimal force that they usually do, and have to contribute more to stabilization instead of doing their normal job. Why in the world would anybody want to inhibit the prime movers during an exercise? This makes one more susceptible to injury, hurts performance and can ruin movement patterns. It has also been shown that training on an unstable surface can alter neuromuscular recruitment that can conflict with normal training and activity on a regular surface. During exercise on an unstable surface, antagonists are actually activated more as the agonists produce less force. During normal exercise on a stable surface, antagonist activity usually stays the same or decreases. There is no logic or good reason to do this.
The one time that training on an unstable surface would be advised is to train proprioceptive awareness and balance, and reactive ability right after an injury or in certain individuals with specific balance or awareness issues.
I do like the stability ball for use in certain exercises such as stability ball leg curls and rollouts. However, this is a different story. I would never advise someone to stand on a stability ball to do an exercise as this can lead to injury and completely destroy proper motor control and firing patterns.
Don't believe me? Here are some published research studies to back this stuff up.
In a 2004 study in the Journal of Strength and Conditioning Research by Anderson and Behm, it was determined that neuromuscular recruitment was altered and that force output was diminished during unstable surface training. (1)
In a 2007 study by Cressey et al. it was shown that unstable surface training actually hurt performance improvements in young, healthy athletes. This is because there is no carry over from doing things on something like a bosu ball or inflatable disc (used in this study) to doing things in a real time activity. Like I said earlier, force output is less during this type of training which also hurts performance such as shown in this study. (2)
In a 2008 study by Nuzzo et al. it was shown that trunk muscle activity (or "core" activity) during stability ball exercise was less than progressive squat and deadlift exercises performed normally. The so called added "core" activity was supposed to be one of the magical benefits of doing these types of exercises. (3)
These are just a few studies. There are many many more that have consistently shown the same types of results. The fact of the matter is, progressing in the tried and true big bang exercises such as squatting variations, deadlift variations, chinups, etc. will make you stronger, hammer your core, improve proper neuromuscular control and movement patterns and improve performance and body composition better than any bosu ball, stability ball or disc will ever do.
1. Anderson KG and Behm DG. Maintenance of EMG activity and loss of force output with instability. Journal of Strength and Conditioning Research 18: 637-640, 2004.
2. Cressey EM, West CA, Tiberio DP, Kraemer WJ, and Maresh CM. The effects of ten weeks of lower body unstable surface training on markers of athletic performance. Journal of Strength and Conditioning Research 21: 561-567, 2007.
3. Nuzzo JL, McCaulley GO, Cormie P, Cavill MJ, and McBride JM. Trunk muscle activity during stability ball and free weight exercises. Journal of Strength and Conditioning Research 22: 95-102, 2006.