"Effects of muscle strengthening on vertical jump height: a simulation study." Bobbert et al, MSSE '94.
In this study, isolation training caused a 20% increase in knee extensor force, BUT caused a 9cm DECREASE in vertical jump height.
Orchard, J., Walt, S., McIntosh, A. and Garlick, D. (2002) Muscle activity during the drop punt kick. In: Science and Football IV. Eds: Sprinks, W., Reilly, T. and Murphy, A. London: Taylor and Francis. 32-43.
Australian footballers have been having a real tough go of it with hamstring strains. But its a lot more than a strength/flexibility thing. This study shows they (hamstrings) are not particularly active in the follow through of a kick, as you would think. They seem to have more of a timing sort of purpose.
I don't mean to beat this topic to death, just saying how tough it is to rock a rhyme thats right on time.