With one step during any race or runway being from static, you have to keep in mind what you are accomplishing with your jump squat. Heavy jumps to help block starts are ok, but I would rather keep my focus on the other 2- . . . steps. When the weight gets too heavy, the coupling time on the ground becomes far to difficult and slow in order to gain the slight eccentric load which one would want.
A jump squat, for max power, will be a slight load with sustained repeated efforts. The eccentric-iso-concentric contraction is what to look for, not just the concentric. Jump squats are a way to train elastic qualities that are close to GCT’s experienced in sprinting/jumping.
Should we only train that part of the force-time curve that we utilize during the event? If so, why do anything other than sprint? As CF likes to say, “Basically, advances ANYWHERE on the curve can have effect EVERYWHERE on the curve (though, of course, primarily on the specific area worked).” All cap emphasis are CF’s.
[I would contend that jump squats as a method of improving speed when running is wishful thinking. GCT’s in a jump squat would be far more than 300ms compared to 100ms needed for sprinting at speed. Normal plyometrics provides more than enough stimulus (too much load in fact) with better GCT’s than jump squatting.
Jump squats (30% of 1RM for maximal power) have only been shown to improve 5m times with no added improvement after that. [b]Jump squats with more weight (80% of 1RM) slows you down.[/b] (McBride et al. 2002, Wilson et al. 1992, Lyttle et al. 1996).
Sets of 6 reps seems to work better than sets of 8 reps.
You’re probably right that jump squats have a greater impact on starts and accel, I think because they are quad dominant, not necessarily because of slower GCT.