If you have promoted stability ball exercises for strength training, you may want to adjust your recommendation. A study published in the August 2002 issue of the Journal of Strength and Conditioning Research suggests that they may not be so effective in this regard.
Some have thought that the stability ball is useful for developing strength because it creates an unstable training environment requiring greater neuromuscular adaptation than do conventional resistance training methods using more stable benches and floors. They assert that the greater neuromuscular adaptations lead to increases in strength. The researchers tested this notion by having eight men perform unilateral leg extensor and plantar flexor contractions while seated on a bench, chair or stability ball. Isometric leg extensor and plantar flexor forces were 70.5 percent and 20.2 percent less, respectively, on the stability ball than on the bench or chair. Quadriceps activation and plantar flexor activation were 44.3 percent and 2.9 percent less, respectively.
The researchers conclude that the stability ball is useful in resistance training only under light to moderate instability. “If an individual is in a position whereby he or she cannot stay upright (attempting to stand or perform a squat maneuver on a [stability] ball), the amount of resistance that can be applied to the muscle will be negligible because all focus is on balance,” they contend.
Of course, by that reckoning, “the greatest contribution of instability training may be to improve core stability rather than limb strength...the preliminary purpose of the stability ball need not be significant strength gains but an attempt to improve balance, stability and proprioceptive capabilities,” the researchers propose.