Changes in illusory ankle movements induced by tendon vibrations during the delayed recovery phase of stretch-shortening cycle fatigue: an indirect study of muscle spindle sensitivity modifications.

This study examined the perceived movement velocity induced by tendon vibrations during the delayed recovery phase of a stretch-shortening cycle (SSC)-type exercise characterized by 2 to 4 days of neuromuscular and proprioceptive impairments. Seven subjects performed until exhaustion series of unilateral rebounds involving mostly the triceps surae muscle group. Fatigue effects were quantified for the exercised and non-exercised legs through muscle soreness and maximal voluntary plantarflexion test (MVC) performed immediately before (PRE) and after the SSC exercise, and repeated 2 days later (D2). At PRE and D2, mechanical vibrations at 40, 60, 80, 100, and 120 Hz were applied to distal tendons of the exercised ankle. For each vibration, the subjects had to reproduce the perceived movement velocity with the non-exercised ankle. According to previous studies, the sole exercised leg was characterized by a D2 peak of muscle soreness associated, in the MVC test, with significant decreases in maximal force and mean soleus muscle activity. As compared to the PRE test and in all subjects, the vibrations applied at D2 to the tendon of the fatigued ankle extensor muscles led to significant decreases in the perceived movement velocity at 80 and 100 Hz, but to an increased one at 40 Hz. In contrast, vibrations applied to the tendon of the non-fatigued ankle flexor muscle did not result in any significant change. These results suggest that the delayed recovery phase of SSC fatigue is characterized by changes in muscle proprioception, which may partly result from a decreased sensitivity of the primary endings.