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Length–Tension Relationship

Experiments studying the relationship between muscle length and the tension produced at a fixed length during passive stretch and active isometric contraction have demonstrated that force generation depends on the length of the muscle.

The passive curve was determined by measuring the tension in muscle at a series of different lengths. The increasing tension with greater passive stretch is a result of the connective tissue and fascial components surrounding muscle fibers and, possibly, to some extent, even the myofibrillar proteins.

In fact, passive stretch tensions can exceed maximal isometric contraction tensions at the limits of mechanical failure of muscle. The active tension curve was determined by measuring the tension of isometric tetanic contractions at various fixed lengths. This curve demonstrates that an isometric contraction generates maximal force at a certain length (Lo) at which the maximal number of mechanical couplings of thick and thin filaments occurs.

The linear decrease in force production at greater lengths is proportional to the lesser number of actin–myosin cross-bridges that can be formed as the overlap of thick and thin filaments decreases with the increased sarcomere length. At muscle lengths shorter than Lo, tension development is not maximal because of excessive overlap of thick and thin filaments, resulting in disordered cytoskeletal geometry.

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