Acute effects of neuromuscular electrical stimulation on contralateral plantar flexor neuromuscular function

Introduction Neuromuscular electrical stimulation (NMES) is used in athletes to enhance muscle strength (Filipovic et al., 2012) or in patients to restore muscle strength (Nussbaum et al., 2017). The increased maximal voluntary contraction (MVC) torque of one limb (e.g. right leg) while transcutaneous NMES is concomitantly applied to the contralateral limb (e.g. left leg) has been termed contralateral facilitation. This effect has previously been reported for the knee extensors (Cattagni et al., 2018; Minetto et al., 2018) but the underlying mechanisms remain to be investigated. It is also not known whether or not other muscle groups may show contralateral facilitation. The aim of this study was to compare plantar flexor contralateral facilitation between a submaximal voluntary contraction (~10% MVC torque) and two evoked contractions with presumably distinct motor unit recruitment patterns (conventional and wide-pulse high-frequency [WPHF] NMES; Bergquist et al., 2011) of the ipsilateral plantar flexors, with respect to a resting condition. Methods Twenty-two healthy volunteers (4 women: mean ± SD, 26 ±4 yrs, 162 ±7 cm, 65 ±9 kg and 18 men: 26 ±6 yrs, 180 ±5 cm, 76 ±6 kg) took part to the experiments. Contralateral MVC torque and maximal voluntary activation level were measured in 4 different conditions: while the ipsilateral plantar flexors were at rest, voluntarily contracted at 10% of MVC torque or stimulated with conventional (0.1 ms, 30 Hz) and wide-pulse high frequency (1 ms, 100 Hz) NMES for 15 s at a pre-determined intensity to evoke 10% of MVC torque. Additional neurophysiological parameters (soleus H-reflex and V-wave amplitude and tibialis anterior coactivation level) were quantified in a subgroup of 12 participants (4 women and 8 men). Results Conventional and WPHF NMES of the ipsilateral plantar flexors did not induce any contralateral facilitation of MVC torque with respect to the resting condition (respectively: mean ± SD, 133 ±44, 133 ±43 and 135 ±39 Nm) as well as maximal voluntary activation level, while an ipsilateral voluntary contraction of the same intensity resulted in lower contralateral strength than conventional NMES. Moreover, no alteration in the neurophysiological parameters was observed in the different conditions. Discussion/Conclusion The absence of contralateral facilitation contrasts with the results obtained on the knee extensors and can be attributed to the absence of neural changes observed on the contralateral side. These findings should be considered by clinicians/researchers in lower limb rehabilitation settings, as it seems easier to induce contralateral facilitation in proximal vs. distal lower limbs. References Bergquist, A. J., Clair, J. M., Lagerquist, O., Mang, C. S., Okuma, Y., & Collins, D. F. (2011). Neuromuscular electrical stimulation: Implications of the electrically evoked sensory volley. European Journal of Applied Physiology, 111, 2409–2426. https://doi.org/10.1007/s00421-011-2087-9 Cattagni, T., Lepers, R., & Maffiuletti, N. A. (2018). Effects of neuromuscular electrical stimulation on contralateral quadriceps function. Journal of Electromyography and Kinesiology, 38, 111-118. https://doi.org/10.1016/j.jelekin.2017.11.013 Filipovic, A., Kleinöder, H., Dörmann, U., & Mester, J. (2012). Electromyostimulation: A systematic review of the effects of different electromyostimulation methods on selected strength parameters in trained and elite athletes. Journal of Strength and Conditioning Research, 26(9), 2600-2614. https://doi.org/10.1519/JSC.0b013e31823f2cd1 Minetto, M., Botter, A., Gamerro, G., Varvello, I., Massazza, G., Bellomo, R., Maffiuletti, N., & Saggini, R. (2018). Contralateral effect of short-duration unilateral neuromuscular electrical stimulation and focal vibration in healthy subjects. European Journal of Physical and Rehabilitation Medicine, 54(6), 911-920. Nussbaum, E., Houghton, P., Anthony, J., Rennie, S., Shay, B., & Hoens, A. (2017). Neuromuscular electrical stimulation for treatment of muscle impairment: Critical review and recommendations for clinical practice. Physiotherapy Canada, 69(5), 1-76. https://doi.org/10.3138/ptc.2015-88