Age-dependent effects of muscle vibration and the Jendrassik maneuver on the patellar tendon reflex response

Objective: To explore possible effects of aging on the excitability of spinal reflexes. Design: Using a cross-sectional design, the influences of muscle vibration and the Jendrassik maneuver on patellar tendon reflex function were compared between 30 young adults and 15 older adults. Setting: Motor control research laboratory. Subjects: The young adults were volunteers of college age. The older adults (74.5 ± 4.14yr) were volunteers from the local community. All subjects were free of medications and neurological conditions that would affect normal neuromuscular responses. Main Outcome Measures: A force-time curve analysis of the patellar tendon reflex response was used to assess the inhibition and facilitation of spinal reflexes. In the experimental protocol to assess spinal reflex inhibition, 100Hz vibration was applied to the tight quadriceps muscle. In another experimental protocol, spinal reflex facilitation was assessed using the Jendrassik maneuver. To perform the Jendrassik maneuver, subjects were instructed to grasp their hands together and to pull as hard as possible while breathing normally. After a 2-second count, the tendon tap was delivered to the right leg and the subject was instructed to relax. In both experimental protocols, control patellar tendon reflexes were collected. Results: Analysis of variance for reflex peak force revealed a significant 30% reduction in the amount of vibration-induced reflex inhibition with increasing age, and a similar 33% reduction in the amount of Jendrassik maneuver facilitation observed for the older adults as compared with the younger adults. Conclusion: These results support the hypothesis that inhibitory and excitatory influences acting on the alpha motoneuron pool are different in young and older adults.