Decreases in the excitability of motor axons contribute substantially to contraction fatigability during neuromuscular electrical stimulation

• Author / Creator

  Luu, Minh J

• Activity-dependent changes in axonal excitability are well-documented, yet the contribution to contraction fatigability during neuromuscular electrical stimulation (NMES) is unclear. The present study was designed to: 1) characterize the magnitude and time course of changes in motor axon excitability before, during and after contractions produced at three stimulation frequencies, and 2) determine the relationship between changes at the axon, neuromuscular junction and muscle to the decline in torque observed at each frequency. Eight neurologically-intact participants attended three sessions during which NMES was delivered to the common peroneal nerve at 20, 40, or 60 Hz for 8 min (0.3 s “on”, 0.7 s “off”). Decreases in axonal excitability were measured as increases in current needed to produce an M-wave of 30% maximal (threshold current). Supramaximal stimuli were delivered over the nerve trunk or muscle belly to assess neuromuscular transmission and the force-generating capacity of the muscle, respectively. Torque decreased by 51 and 64% during 40 and 60 Hz NMES, respectively, but did not decrease significantly during 20 Hz NMES. The current required to produce the target M-wave increased 14, 27, and 35% during, and returned to baseline 1, 3, and 5 min following, 20, 40 and 60 Hz NMES, respectively. There was no other evidence of impaired neuromuscular transmission. Reduction in the force-generating capacity of the muscle was evident at 40 and 60 Hz NMES. Regression analysis showed decreases in torque were best predicted by decreases in axonal excitability overall, but were equally dependent on changes in the axon and muscle at each frequency. The present study demonstrates that activity-dependent decreases in axonal excitability contributes substantially to contraction fatigability during NMES.

• Subjects / Keywords

  • Neuromuscular electrical stimulation
  • Threshold tracking
  • Contraction fatigability

• Graduation date

  Fall 2017

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3GH9BP78

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.