Changes in neural modulation and motor control during voluntary movement of older individuals

Changes in the modulation of soleus alpha motoneuron excitability, as assessed by H reflexes, and temporal sequencing of the soleus and tibialis anterior muscles during voluntary ankle dorsiflexions and plantar flexions of young (24.7 +/- 11.5; n = 13) and older (68.7 +/- 5.4; n = 13) subjects were assessed to determine potential neural mechanisms that might contribute to motor control changes associated with aging.

A repetitive stimulation (5 Hz) soleus H-reflex testing protocol and surface electromyography (EMG) were used to assess the latencies of soleus H-reflex changes in relation to tibialis anterior and soleus EMG activations of standing subjects during voluntary ankle dorsiflexions and plantar flexions at self-selected speeds. The pattern and latency of H-reflex changes in relation to EMG activity were compared between young and old subjects.

There were no differences in the relative amount of antagonist muscle (soleus) inhibition during voluntary ankle dorsiflexions between young and old subjects (26.4% and 27.2% decrease from resting H-reflex values, respectively). Older subjects, however, required additional time to achieve these levels of inhibition. Delays in the activation of soleus H reflexes during the plantar flexion task were also observed in older subjects. Older subjects also had considerable intra- and intersubject variability in muscle temporal sequencing patterns during ankle plantar flexions.

Although older subjects exhibited similar relative levels of alpha motoneuron inhibition and excitation during voluntary movements, this modulation was delayed when compared to younger subjects. Temporal sequencing of distal muscle activations also appears to undergo change with aging.