Muscletendon

Muscletendon, or musculotendinous unit (MTU), is the functional assembly of a skeletal muscle and its associated tendon. It transmits contractile force to bone and stores elastic energy during movement. The MTU includes the muscle belly, the tendon or aponeurotic extensions, and the musculotendinous junction, the zone where muscle fibers progressively merge into tendon tissue.

Structure and mechanics: The MTU consists of contractile muscle fibers arranged in fascicles connected in series with noncontractile tendinous tissue. Tendon tissue, rich in type I collagen, acts as a compliant but strong elastic element that buffers loads and enhances force transmission. The musculotendinous junction is a graded transition that helps distribute stresses and reduces the risk of focal injury.

Function and dynamics: During contraction, the MTU generates force that moves joints, while the tendon stores and returns elastic energy, improving movement efficiency, especially in running and jumping. The overall behavior depends on muscle architecture, tendon stiffness, and the length-tension relationship.

Adaptation and aging: Training can increase MTU strength and adjust tendon stiffness and collagen alignment. Eccentric and plyometric loading are particularly effective at conditioning tendons. Aging tends to reduce tendon elasticity and resilience, elevating injury risk.

Injury and pathology: MTU injuries include musculotendinous strains, tendinopathy, and ruptures, often at the musculotendinous junction or within the tendon. Risk factors include fatigue, rapid overload, poor tissue quality, and overuse. Common sites include the hamstrings, Achilles, and rotator cuff.

Diagnosis and management: Diagnosis relies on clinical assessment and imaging such as ultrasound or MRI. Treatment emphasizes gradual, progressive loading, with eccentric training and parameterized rehabilitation; severe ruptures may require surgical repair and longer return-to-use time.

Prevention: Warm-up, conditioning programs with progressive loading, and attention to recovery help maintain MTU integrity.