Muscle reaction to passive shortening in normal man.

We have studied in 32 healthy subjects the EMG activity of biceps and triceps brachii, wrist flexors and wrist extensors, triceps surae and tibialis anterior, sterno-cleido-mastoideus (SCM) and splenius, during their passive shortening. A shortening reaction was observed more frequently in wrist extensors, SCM and tibialis anterior than in biceps and triceps brachii. The response appeared only when the joint reached a specific angle regardless of the starting point or the speed used to reach this position. During a muscle ischaemia which abolished the tendon reflex the reaction was not modified. During a joint ischaemia produced distal to the examined muscle its amplitude was strongly reduced. Our results support the hypothesis that joint receptors could play a role in its origin, perhaps through a long-loop reflex.