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1 Centre for Neuroscience, University of Alberta, Edmonton, Canada
2 Centre for Neuroscience, University of Alberta, Edmonton, Canada; Faculty of Physical Education and Recreation, University of Alberta, Edmonton, Canada
* To whom correspondence should be addressed. E-mail: dave.collins{at}ualberta.ca.
Electrical stimulation (1 ms pulses, 100 Hz) produces more torque than expected from motor axon activation alone (extra contractions). This experiment investigates the most effective method of delivering this stimulation for neuromuscular electrical stimulation (NMES). Surface stimulation (1 ms pulses; 20Hz for 2s, 100Hz for 2s, 20Hz for 3s) was delivered to triceps surae and wrist flexors (muscle stimulation), and median and tibial nerves (nerve stimulation) at two intensities. Contractions were evaluated for amplitude, consistency and stability. Surface EMG was collected to assess the how H-reflexes and M-waves contribute. In the triceps surae, muscle stimulation produced the largest absolute contractions (23% MVC), evoked the largest extra contractions as torque increased by 412% after the 100 Hz stimulation, and was more consistent and stable compared to tibial nerve stimulation. Absolute and extra contraction amplitude, consistency, and stability of evoked wrist flexor torques were similar between stimulation types: torques reached 11% MVC and extra contractions increased torque by 161%. Extra contractions were 10 times larger in plantar flexors compared to wrist flexors with muscle stimulation, but were similar with nerve stimulation. For triceps surae, H-reflexes were 3.4 times larger than M-waves during nerve stimulation, yet M-waves were 15 times larger than H-reflexes during muscle stimulation. M-waves in the wrist flexors were larger than H-reflexes during nerve (8.5 times) and muscle (18.5 times) stimulation. This is an initial step towards utilizing extra contractions for NMES and the first to demonstrate their presence in the wrist flexors.
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