If you've reached a plateau in your strength training, you may need to strengthen your stabilizers using exercise ball exercises to push past that sticking point. Excessive movement or instability in a joint stimulates receptors in the joints and muscles sending inhibitory messages (a negative feedback mechanism) to decrease the activity in the prime mover of the joint. This reduces power and strength.
By stabilizing the joints, the prime movers or "mobilizing muscles" can exert a stronger force across the joint while strength training without stimulating these inhibitory messages to the spinal cord, weakening the effort.
When you first start exercising with the ball you will not be able to handle as much weight. For instance, if you can bench press 100 lb for 10 reps, you may only be able to do 5 reps on the ball. Continue strength training with the bench press using the ball for six weeks until you reach the point where you can press 100lb for 10 reps on the ball. When you go back to the bench you will find you can press more than you could before. This is due to increased joint stabilizer strength and activity.
You are only as strong as your weakest link, and often that is the core.
for weight training with the exercise ball:
Muscle activation patterns differ when lifting on unstable and stable surfaces (1)
Muscle activity in trunk muscles and extremity muscles was measured through the use of surface electromyography during and overhead lift. 30 individuals that train regularly participated in a shoulder press for three sets of three reps with a barbell and dumbbells. Electrodes were placed over the trapezius, anterior deltoid, middle deltoid, rectus abdominis, triceps brachii, external obliques, and erector spinae to measure muscle activity on those muscles.
Results showed that as the instability of the surface (ie bench vs ball) increased the load the individual was capable of lifting decreased. As the instability of the weight increased (ie dumbbells vs barbell), also, the load lifted decreased.
The muscular activity in the erector spinae was greatest when performing the barbell and dumbbell presses on the exercise ball.
In essence it seems intuitive that if we are to substitute a ball for a bench we must lighten the weight lifted as we start to recruit muscles needed to stabilize our trunk.
1. Kohler JM, Flanagan SP, Whiting WC. Muscle activation patterns while lifting stable and unstable loads on stable and unstable surfaces. J Strength Cond Res. 2010 Feb;24(2):313-21.