During a muscle contraction by lower to medium motor unit groups, how do high threshold muscle fibers shorten? Passive? Active?
So if I bend my arm in the elbow without weight in my hand, how do high threshold muscle fibers related to high motor unit groups shorten? Do their actin and myosin overlap? Is the tension produced by the other MU groups enough to shorten the connective tissue in the other muscle fibers? Is there active overlap without cross-bridge cycling in the HMU groups and their fibers?
This is a very interesting question and I do not know any study dealing with this problem. Muscle fibres from different MUs (not their filaments) do overlap, so my guess is they shorten passively. However, since the mechanism of the crossbridge action during active muscle lenghtening is still unresolved, it is doubtful that there were any attempts to solve the problem you are asking for, although there may be some old, forgottened studies.
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