Article

Motor unit firing frequencies are low during maximal voluntary
contractions (MVCs) of human thenar muscles impaired by cervical spinal cord
injury (SCI). This study aimed to examine whether thenar motor unit
firing frequencies increase when driven by both maximal voluntary drive and other
concurrent inputs compared with an MVC alone. METHODS: Motor unit firing rates,
force, and surface electromyographic activity (EMG) were compared across 2
contractions: (a) MVC alone and (b) MVC combined with another input (combination
contraction). Other inputs (conditions) included vibration, heat, or cold applied
to the anterior surface of the forearm, electrical stimulation delivered to the
anterior surface of the middle finger, a muscle spasm, or a voluntary contraction
of the contralateral thenar muscles against resistance. RESULTS: The maximal
firing frequency (n = 68 units), force, and electromyographic activity (n = 92
contraction pairs) were all significantly higher during the combined contractions
compared with MVCs alone. There was a 3-way interaction between contraction,
condition, and subject for maximal motor unit firing rates, force, and EMG. Thus,
combined contraction responses were different for conditions across subjects.
Some conditions (eg, a muscle spasm) resulted in more effective and more frequent
responses (increases in unit firing frequency, force, EMG in >50% contractions)
than others. Recruitment of new units also occurred in combined contractions.
CONCLUSIONS: Motoneurons are still responsive to additional afferent inputs from
various sources when rate modulation from voluntary drive is limited by SCI.
Individuals with SCI may be able to combine inputs to control functional tasks
they cannot perform with voluntary drive alone.

Langue : ANGLAIS