A couple of interesting new articles in Journal of Strength and
Conditioning Research:
It appears that the classic Paul Chek squat-standing-on-a-Swiss-ball,
compared to a regular back squat, "significantly reduces peak force,
rate of force development, and agonist muscle activity with no change
in antagonist or synergist muscle activity" and "[i]n terms of
providing a stimulus for strength gain no discernable benefit of
performing a resistance exercise in an unstable condition was
observed."
http://tinyurl.com/ybxas2
By the same token, a comparison of standard bench presses and bench
presses on a Swiss ball "showed that deltoid and abdominal muscle
activity was increased for repetitions performed using the swiss
ball."
http://tinyurl.com/yer58t
I never tried squats on a Swiss ball. I guess I always thought they
were stupid, with a high potential for embarrassing and injurious
results. I have done heavy dumbbell bench presses on a Swiss ball.
They're OK, but be careful how high you raise the dumbbells in
relation to your body; too high and you start to roll over head first.
I know that for a fact.