Assessment of plantar flexors activation capacity: nerve versus muscle stimulation by single versus double pulse

Abstract

This study was designed to investigate if the relationship between the interpolated twitch-torque (IT) and voluntary torque (VT) is affected by the number of electrical stimuli (single vs. double) and the stimulation site (nerve trunk vs. muscle). The results showed that the IT-VT relationship of the plantar flexors is appropriately described by a composite (linear + curvilinear) model. Indeed, whatever the stimulation method, the IT-VT relationship was linear between approximately 25 and 75% of the maximal voluntary torque (MVT) and curvilinear for higher contraction intensities. The four stimulation conditions are equivalent in assessing the maximal voluntary activation (VA% range 96.2 +/- 5.0 to 98.5 +/- 3.1%) as well as in determining the true maximal torque expected for total twitch occlusion (MT(exp) range 171.4 +/- 21.2 to 179.0 +/- 26.8 Nm). The gap between the MVT and MT(exp) should be viewed as an index of muscle inactivation. This gap was comparable for the four stimulation methods (2-6%) and close to the deficit in VA% (2-4%). No pulse-number effect was found on the IT-VT relationship when the nerve was stimulated but an effect on the concavity of the composite relationship was observed when the stimulation was applied over the muscle. Even though the four stimulation techniques are equivalent in assessing the maximal activation capacity our results demonstrate that the neural stimulation method is the most consistent as it guarantees the same motor pool recruitment independently from the number of pulses.

Neuromuscular fatigue induced by alternating isometric contractions of the ankle plantar and dorsiflexors

“…While the number of stimulations best suited for twitch interpolation has been disputed, single pulse stimulation is the least painful and has been shown to give a reliable measure of the participant's level of maximal voluntary activation (Behm et al, 1996;Scaglioni and Martin, 2009;Taylor, 2009). Similarly, the location of stimulation is debatable.…”

“…A conductive probe was used to find the specific nerve locations that provided the largest mechanical muscular response. When this location was found, 2 Â 2 cm surface electrodes (Compex, Annecy-le-vieux, France) were fixed over the tibial (Scaglioni and Martin, 2009) and common peroneal nerves (Burridge and McLellan, 2000). A constant current stimulator (Digitimer DS7A, Digitimer Ltd., Letchworth Garden City, UK) delivered single electrical pulses (pulse duration = 200 ls) through these electrodes.…”

Effect of stimulation intensity on assessment of voluntary activation

“…18 Previously, voluntary activation was found to be similar when it was assessed with maximal percutaneous or maximal nerve stimulation for the plantar flexors. 6 Recently, Place et al 16 showed that submaximal quadriceps muscle stimulation resulted in equal superimposed torques compared with maximal nerve stimulation, but VA was not calculated in that study. In this study, we elaborate on these observations by investigating the effects of stimulation type on actual VA, which in most studies that use superimposed stimulation is the primary parameter of interest.…”

“…It has become the standard technique to assess voluntary muscle activation (VA). [2][3][4] The quadriceps has been studied frequently with superimposed stimulation, [2][3][4][5][6][7] because it is a large muscle group with important contributions during sports and locomotion in daily life. The electrical stimulation is typically applied over either the nerve trunk (nerve stimulation) or the muscle belly (muscle stimulation).…”

“…While the relationship between voluntary torque and superimposed torque indeed was reported to be linear, 5,19 there is growing evidence that this relationship is curvilinear for the knee extensors 2,3,6,7 and also for other muscles. 6,20,21 It is time consuming and difficult to obtain a good and complete relationship between superimposed and voluntary torque. Therefore, in most studies VA has been calculated with the superimposed responses upon the highest of a few maximal voluntary contractions.…”