Aging-related neuromuscular changes characterized by tendon reflex system properties

Objective: To quantitatively evaluate changes in neuromuscular reflex system properties that are associated with aging. Design: Controlled, experimental. Setting: Research laboratory in a rehabilitation hospital. Participants: Fourteen elderly (age, 69.4±7.1y) and 18 young (age, 29.9±6.5y) healthy subjects. Interventions: Not applicable. Main Outcome Measures: With the subject relaxed, an instrumented reflex hammer was used to tap the Achilles' tendon and record the tapping force, whereas the ankle plantarflexion torque and plantarflexor muscle electromyographic activity were recorded isometrically as measures of the reflex responses. Tendon reflex system properties were evaluated by using system identification techniques. Tendon tapping force was designated as system input and reflex-mediated torque and electromyographic activity as outputs. The dynamic relations between input and output were characterized by the system parameters of reflex-mediated torque and electromyographic gains, contraction and excitation rate, reflex-mediated torque and electromyographic delays, and electromechanic delay. Results: Various aging-related changes were found in the tendon reflex system properties, including decreased tendon reflex gain (P=.029), slower contraction and half-relaxation rates (P≤.018), and longer electromyographic activation delay, electromechanical delay and overall torque reflex delay (P≤.025). In contrast, changes in the electromyographic properties were not significant, except for the longer electromyographic activation delay. Conclusions: Aging is associated with significant changes in the neuromuscular reflex system properties. The changes were mainly associated with weaker and slower muscle force generation but not with significant decrease in motoneuronal excitability.