Muscle slack is a term I picked up from Frans Bosch this past summer at GAIN. Quite simply, it means the interval between initial muscle stimulation and contraction is too long. Don't confuse it with Coach Wilt's "amortization" term as it is applied to plyometrics. While Amortization is a proprioceptive term; muscle slack is more of a coordination term. Improper joint angles or posture at any given point in a skill results in a lack, or excessive pre stretch in any one given muscle.
I decided to do a post on it because it seems it is being misapplied on many internet sites. Exercises are singled out because the practitioners feel they contribute to muscle slack and should be avoided. I think the mistake they are making is applying it specifically to straight ahead running on a relatively flat surface. Once you get away from that, you need allot of tools in the toolbox. Please be prudent when applying sports specificity- you could be creating new problems.
So far I've only seen muscle slack applied to sprinting. It also happens in batting, tackling etc. I'll address it later posts.