Force responses to controlled stretches of electrically stimulated human muscle-tendon complex.

Human first dorsal interosseus muscle was tetanized using percutaneous electrical stimulation. During the tetanus the muscle was subjected to constant velocity stretches. The stretch produced an enhancement of muscular force of up to 80% during the stretch. The size of the enhancement was dependent on both the amplitude and the velocity of the stretch. During an isometric hold phase after the stretch, the force decayed quickly for the first 100 ms and thereafter much more slowly, reaching a level 30% higher than the isometric force without pre-stretch. The force during this hold phase was dependent on amplitude of stretch but was independent of stretch velocity. The interaction of tendon elasticity and muscle fibre mechanics in producing these responses is discussed. Implications for normal human movements are also explored.