|Title of the paper:||Effect of vibration during neuromuscular electrical stimulation on occurrence and magnitude of extra force|
|Authors:||Espeit, L., Maffiuletti, N.A., Rozand, V., Millet, G.Y., Lapole, T.|
|Institution:||University Jean Monnet|
|Department:||Inter-university Laboratory of Human Movement Biology|
NMES-induced isometric contractions may arise from the direct activation of motor axons [i.e. efferent pathway; conventional (CONV) NMES] and/or from the recruitment of motoneurons in the spinal cord through the depolarization of sensory axons (i.e. afferent pathway) . This “extra” contraction, the so-called “extra force (EF)” is favoured by the use of low-intensity, wide-pulse-width and high-frequency stimulation (WPHF) . The combination of vibration applied to the Achilles tendon, known to strongly activate Ia afferents, and wide-pulse low-frequency (WPLF) NMES over the plantar flexor muscles has also been reported to elicit EF of considerable magnitude . Yet it may be hypothesized that combining tendon vibration and WPHF NMES would maximize EF. The aim of the present study was therefore to investigate and compare the occurrence and magnitude of EF during tendon vibration associated with CONV, WPLF or WPHF NMES.
NMES-induced isometric plantar flexion force was recorded in 20 healthy young subjects. Stimulation trains were delivered through electrodes placed over the triceps surae muscles. The protocol consisted in 3 stimulation trains of 20 s randomly delivered at different pulse widths and frequencies [CONV (0.05 ms, 20 Hz), WPLF (1 ms, 20 Hz) and WPHF (1 ms, 100 Hz)] either with or without Achilles tendon vibration (100 Hz, 1 mm in amplitude) applied from 2 s after electrical stimulation onset to the end of the stimulation period. Stimulation intensity was set to evoke an initial force level corresponding to 10% maximal voluntary contraction. Increase in force from beginning to the end of the stimulation was measured. EF was further calculated as the mean ratio between the real and the theoretical force-time integral. Subjects were arbitrarily characterized as responders to EF when this ratio was greater than 1.10.
For both CONV and WPLF NMES, a responder rate to EF of 20% was observed and this was increased to 25% when associated with vibration. For WPHF NMES, we found 65% of responders without vibration and 55% when adding vibration. The EF was significantly greater during WPHF (1.27±0.46) than CONV (1.05±0.10; p=0.007) and WPLF (1.10±0.23; p=0.043). In responders, force increased by an average of 5.5±4.0% (range: 1.1-19.8%) of the maximal voluntary contraction during WPHF NMES trains, regardless vibration application.
The occurrence and magnitude reported in this study for NMES only are in accordance with the literature . Both the occurrence and the magnitude of EF during NMES were maximized by the use of WPHF but EF was not potentiated by the application of vibration in any condition. The lack of beneficial effect of vibration on EF contrasts with previous findings , potentially due to saturation of the afferent pathways and/or differences in vibration parameters.
1) Collins, Exerc Sport Sci Rev, 2007
2) Dean et al., J Appl Physiol, 2007
3) Magalhães et al., J Neuroeng Rehabil, 2010
4) Wegrzyk et al., Clin Neurophysiol, 2015