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

Abstract

Purpose Simultaneous application of tendon vibration and neuromuscular electrical stimulation (NMES) induces an involuntary 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. Methods 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. Results 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.

Effects of reciprocal inhibition and whole‐body relaxation on persistent inward currents estimated by two different methods

“…An experienced researcher held the vibrator with a steady pressure on the posterior aspect of the tendon at the level of the medial malleolus. Ten seconds after vibration onset, 5 × 2‐s NMES rectangular bursts at 4‐s intervals (2 s on, 2 s off) with wide (1 ms) pulse width, 20‐Hz frequency, and intensity that evoked 20% of MVC torque were applied to the triceps surae whilst tendon vibration continued (Mesquita et al., 2021). The current required to evoke 20% of MVC torque was re‐assessed in each experimental session.…”

“…During VibStim tests, tendon vibration was combined with percutaneous NMES to evoke involuntary muscle contractions (Mesquita et al., 2021; Trajano et al., 2014). Achilles tendon vibration was applied at 115 Hz and 1 mm amplitude for 33 s using a hand‐held vibrator (Vibrasens, Techno Concepts, France).…”

“…Importantly, the evoked, ongoing contractions display characteristics consistent with PIC behaviour. Self‐sustained torque has been observed after cessation of both the vibration and NMES, which may be explained by PIC‐related bistable behaviour in some spinal motoneurons (Lee & Heckman, 1998), and the progressive increase in torque during the trials (Kirk et al., 2019; Magalhães & Kohn, 2010; Mesquita et al., 2021; Trajano et al., 2014) is consistent with the calcium‐dependent facilitation (i.e. warm‐up) effect seen during repetitive activation when PIC amplitude has been measured directly (Svirskis & Hounsgaard, 1997).…”

“…A potential alternative, or supplementary, method to estimate PIC activity is ongoing tendon vibration overlaid by short bursts of neuromuscular electrical stimulation (NMES) in an otherwise‐inactive target muscle (Bochkezanian et al., 2018; Espeit et al., 2021; Kirk et al., 2019; Magalhães & Kohn, 2010; Mesquita et al., 2021; Trajano et al., 2014). This method induces increasing involuntary muscle contraction forces with magnitudes that are larger than would be expected from the direct activation of motor axons.…”

“…The contribution of peripheral (i.e. within the muscle) mechanisms to this phenomenon is still a matter of debate (Frigon et al., 2011); however, there is increasing support for centrally mediated mechanisms, as recently discussed (Mesquita et al., 2021). Importantly, the evoked, ongoing contractions display characteristics consistent with PIC behaviour.…”