Tired of Fatigue? Factors Affecting the Force-Length Relationship of Muscle

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Biology (HMC)

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One of the fundamental properties of skeletal muscle, the force-length relationship (FLR), originally served as primary evidence for the sliding filament model of force generation (4). Past work has examined factors that may affect a muscle’s FLR, with evidence that the force depression of fatigued muscle correlates with a rightward shift in the muscle’s force-length (FL) behavior (3, 10). This would imply that fatigued muscles favor force generation at longer lengths, possibly due to stretch of series elastic elements resulting in sarcomeres contracting at a shorter length. In a recent study reported in this issue of the Journal of Applied Physiology, MacNaughton and MacIntosh (7) show that when the series elastic compliance of a whole muscle (rat medial gastrocnemius) and its aponeurosis and/or tendon is taken into account, force depression resulting from fatigue via repetitive in situ muscle stimulation does not result in a substantial rightward shift in the muscle’s FLR. The authors demonstrate this by using sonomicrometry (6) to measure directly muscle fascicle length in situ, finding no significant change in fascicle length before and after fatigue. The use of sonomicrometry allows the authors to show a potential problem with previous interpretations of the rightward shift in FLR due to fatigue. The current method of calculating active force uses the passive force at the actual fascicle length during contraction, instead of the passive force at the length of the inactive muscle.

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©2006 American Physiological Society

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