Acute and chronic adaptations of muscle proprioceptors in response to increased use.

Acute adaptations to use have been shown to occur in both the muscle spindle and Golgi tendon organ pathways. This short term activation adaptation increases the excitability of the motoneuron pool, thereby potentially providing a nonvoluntary increase in the excitation of subsequent contractions. It is thought that the intrafusal muscle fibres reset to a higher gain after contraction and that the tendon organ pathway undergoes a brief desensitisation. These phenomena could be important clinically when trying to stretch muscles, i.e. a contraction before a stretch should make the stretch more difficult. Also, this could affect the amount of muscle force generated, thereby altering motor behaviours requiring fine accuracy. Research on the adaptations of proprioceptors during free movement, using locomotion as a model, has found that the hypothesis of consistent alpha-gamma coactivation during motor behaviours is much more complicated and adaptable, depending on the environmental circumstance and the specific motor task. These research findings support the use of selective training, i.e. training to the task, for optimal motor learning. The results of the relatively limited research on chronic adaptations of proprioceptors due to exercise has shown that on a microlevel, the intrafusal muscle fibres may show some metabolic changes but do not show any hypertrophy. However, on a more macro level, with extended training, the latency of the stretch reflex response is found to be decreased and the amplitude is found to be increased in both animals and humans. Through classical conditioning research, proprioceptors may also be necessary for motor learning. Lastly, both primates and humans have been shown to be able to up- or down-grade their stretch reflex responses using operant conditioning techniques and many practice sessions. These findings have important implications both for rehabilitation of persons with abnormal reflex activity and in the training of athletes. This review provides some consolidation of the recent research findings, but much more research needs to be done in order to fully understand the purpose and importance of the proprioceptive sensory system.