In a recent article describing how to do the jump squat it was stated that you should keep your shoulders down and abs tight as you straighten your legs and jump up. Is this good advice? Sadly the answer is no. The answer would have been obvious had the author looked at live digital film of qualified athletes doing the jump squat.
An additional training tip stated that you should keep your abs actively engaged to avoid arching your lower back as you begin the jump portion. There are two problems with this recommendation. First, there is nothing wrong with arching the back, unless of course you have something wrong with your back to begin with. Thus this recommendation has no basis for athletes.
Second, your abdominals have nothing to do with execution of the jump. Contracting them in the jump squat leads to disruption of the jump. Tight abs hold the torso in a position that does not allow it to function freely and as it should in jumping. Statements such as these only lead to more confusion in the field and impede progress toward greater sports mastery.
Keeping your abs tight on takeoff prevents you from fully extending the upper body to go as high as possible. To maintain the abdominal muscle contraction you have to concentrate on this action which detracts from proper execution of the main joint actions.
For maximum safety you should let nature dictate when the abdominals contract. If you do this on the jump takeoff, contraction of the lower back muscles will take place — not of the abdominals. Thus, arching your back on the takeoff and relaxation of the abdominals are desirable acts in the jump, not negatives.
See Build a Better Athlete, Biomechanics and Kinesiology of Exercise and Explosive Basketball Training for more information on jumping and jump exercises.