The H reflex is a valuable tool for assessing acute or long-term changes in the neural component of the excitatory pathway originating from muscle spindles with ageing (Burke, 2016). Although the current results should be interpreted cautiously, they highlight several interesting features that can be useful in clinical practice. First, the results confirm that a long pulse duration (1 ms) is recommended to evoke the largest H-reflex amplitude while minimising the associated M wave, regardless of age. Second, this study indicates that variables other than the H_MAX and M_MAX can provide relevant information about age-related neural changes. In particular, the I_H50, H_M5% and the M_Hmax can indirectly inform on the neural sensory components of the H-reflex pathway. In old adults, greater I_H50 and M_Hmax but lesser H_M5% likely reflect greater changes in sensory than motor components of the reflex. Third, this study suggests that using different stimulus durations may provide indirect cues on structural (loss of large diameter Ia afferents) and functional (decrease in synaptic efficacy) alterations within the H-reflex pathway. This approach could then be relevant to document the effects of chronic neuromuscular conditions (spinal cord injury, stroke, etc.).

This study investigated age-related changes in the H-reflex pathway by using different pulse durations in young and old adults. The results indicate greater age effects on the sensory than the motor component of the H-reflex pathway and suggest that part of these effects relies on the loss of afferent axons and alteration in synaptic efficacy.

Conflict of Interest StatementNone of the authors have potential conflicts of interest to be disclosed.