Involuntary sustained firing of plantar flexor motor neurones: effect of electrical stimulation parameters during tendon vibration

Journal article, 2021

Simultaneous application of tendon vibration and neuromuscular electrical stimulation (NMES) induces an invol-
untary sustained torque. We examined the effect of different NMES parameters (intensity, pattern of stimulation and pulse
width) on the magnitude of the evoked involuntary torque.
Plantar flexor torque was recorded during 33-s Achilles tendon vibration with simultaneous 20-Hz NMES bouts on
triceps surae (n = 20; 13 women). Intensity was set to elicit 10, 20 or 30% of maximal voluntary contraction torque (MVC),
pulse width was narrow (0.2 ms) or wide (1 ms), and the stimulus pattern varied (5 × 2-s or 10 × 1-s). Up to 12 different
trials were performed in a randomized order, and then repeated in those who produced a sustained involuntary torque after
the cessation of vibration.
Six of 7 men and 5 of 13 women produced a post-vibration sustained torque. Eight of 20 participants did not complete
the 30% trials, as they were perceived as painful. Torque during vibration at the end of NMES and the increase in torque
throughout the trial were significantly higher in 20 than 10% trials (n = 11; 9.7 ± 9.0 vs 7.1 ± 6.1% MVC and 4.3 ± 4.5 vs
3.6 ± 3.5% MVC, respectively). Post-vibration sustained torque was higher in wide pulse-width trials (5.4 ± 5.9 vs 4.1 ± 4.3%
MVC). Measures of involuntary torque were not different between 20 and 30% trials (n = 8).
Conclusion Bouts of 5 × 2-s NMES with wide pulse width eliciting 20% MVC provides the most robust responses and could
be used to maximise the production of involuntary torque in triceps surae.