P&P #22. The Sharp Distinction between Static and Dynamic Muscle Contraction May Be Misleading.
At the outset we must ask two questions: "At what point does a slowing dynamic movement become isometric?" or its converse: "At what point does an accelerating isometric movement become dynamic?". Possibly the answer is as trivial as one's initial reaction to state that isometric action is strictly action that occurs when the muscle complex is not changing its length or when there is no external movement of the joint involved. One must also be careful to distinguish between internal movement within the muscle complex and external movement of the joint.
Before one jumps to any conclusions it is relevant to recall work by Perrine and Edgerton which revealed that the Force-Velocity curve of muscle action displays a force peak, not when velocity is zero, but at a slow velocity of about 100 degrees per second. One must also examine any exercises that are executed with near maximal or maximal loads: in such cases the movement always slows down to what may be termed quasi-isometric action or even isometric action at a temporary 'sticking point'.
Moreover, all movement from rest is initiated by isometric muscle contraction. Even rebound plyometrics involves a moment of explosive isometric action. Discuss these and any other issues that may have a bearing on our current definitions of static versus dynamic muscle contraction.
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