Length-Tension Relationship
The force of the contraction cycle in a given muscle fibre depends on:
The length of sarcomeres within a muscle before contraction begins
At the resting length of most muscles, 2.0-2.4 micrometres, the zone of overlap in each sarcomere is OPTIMAL and the muscle fibre can develop maximum tension (100%). Under maximum tension the zone of overlap between a thick and thin filament extends from the edge of the H-zone to one end of a thick filament. As sarcomeres continue to be stretched to a longer length, the zone of overlap shortens and as a result fewer myosin heads can make contact with thin filaments. Therefore, the tension the fibre can produce is DECREASED. At 170% of optimal length there is no overlap between thin and thick filaments and because no myosin heads can bind to thin filaments, the muscle fibre cannot contract, and tension is ZERO. In contrast, as sarcomere lengths become shorter than optimal, the tension that can develop also DECREASES because thick filaments crumple as they are compressed by the Z-discs, resulting in fewer myosin heads making contact with thin filaments.
Under normal conditions resting muscle fibre length is held very close to optimum.
Maximal Contraction (understretch): some myosin-binding sites are covered by overlap of actin filaments along the midline
Maximal Stretch (overstretch): some myosin-binding sites do not overlap with myosin
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