One of my pet peeves these days is the number of new fangled ways to exercise. One perfect example is found here- Alternating Chest Press on the Ball.
I understand the concept; you are required to recruit some additional muscles for balance, and so on.
The bottom line for most people is to optimize their time strength training- maximize the benefits in the least amount of time. Fooling around with dumbells on an exercise ball to me is counterproductive- you are going to have a much more difficult time fatiguing those muscles if you have to focus on balance at the same time you are performing a strength exercise. Your weight will also have to be lighter relative to when you do this exercise on a bench.
The PushUpPowerRack is an example of one exercise prop that can help you maximize the efficiency of your workout- 10 repetitions at each grip with no fooling around will fatigue those muscles in no time, with no worries about position on the ball, balance, or which dumbells to use. As I mentioned earlier, you do achieve secondary benefits by recruiting several other muscle groups at the same time.
The bottom line is this- if you cannot work your muscles to fatigue with the prop that you are using in 15-20 reps, , then you are not maximizing the benefits of your workout. Bringing it too many other factors to worry about is inefficient and time consuming.
(c) Dave Elger 2007 All rights reserved