Intermuscle differences in activation

Abstract

The objective of this study was to investigate differences within individual subjects in the ability to activate the quadriceps, plantar flexors (PF), dorsiflexors (DF), and elbow flexors (EF) during isometric contractions. Twelve male subjects performed submaximal and maximal voluntary isometric contractions, and maximal tetanic contractions were also induced by electrical stimulation. The interpolated twitch technique was used to gauge the extent of muscle inactivation or inability to produce maximum force. Measurements included torque output, absolute and relative rate of force development (RFD), and percentage of muscle inactivation. The quadriceps exceeded all other muscle groups in voluntary and tetanic torque output, voluntary absolute RFD, and absolute and relative tetanic RFD. The quadriceps also exceeded the PF and DF in voluntary relative RFD and had greater muscle inactivation (15.5%) than the EF (5.0%), PF (5.0%), and DF (1.3%). Although the higher RFD may suggest a higher percentage of type II fibers in the quadriceps, their higher threshold of recruitment leads to greater difficulty in fully activating the quadriceps.

Assessing Voluntary Muscle Activation with the Twitch Interpolation Technique

“…Because of these limitations the estimates of true maximum force obtained via extrapolation are sometimes unrealistically large [33] or even less than the measured MVC forces [32] . Furthermore, stimulus-evoked increments in force are sometimes observed during MVCs that are equal to or greater than the predicted true maximums [17,32] . (Figure 2) and considerable disagreement exists as to which is the most appropriate [15,18,28,30,33] .…”

“…Accordingly, voluntary activation (VA) of the stimulated muscle(s) is frequently quantified with the linear equation, VA (%) = [1 -(Superimposed twitch / Control twitch)] * 100, where 3 the superimposed twitch is the force increment noted during a maximal contraction at the time of stimulation and the control twitch is that evoked in the relaxed muscle [14,15] . The control twitch is typically evoked 1.5 to 5 seconds after the MVC [14,15,17,19,27] and it is assumed that it and the superimposed twitch are equally potentiated by the voluntary effort.…”

“…Consequently, scaling superimposed twitches to control twitches will tend to overestimate voluntary activation but only to a small extent for healthy subjects who are typically capable of activating 85% or more of their muscles [14,15,17,20,21,23,25,28,29,32] .…”

Effect of stimulation intensity on assessment of voluntary activation

“…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).…”

Tracking upper limbs fatigue by means of electronic dynamometry

“…The lack of any difference between initial and final values for UT could be explained by the fact that less well-trained muscles have greater propensity for sub maximal activation, compared to trained muscles, resulting in a smaller number of fatigued fibres, and the ability to maintain a similar load at the end of the test (Behm, Whittle, Button, & Power, 2002;Bogdanis, 2012;Nordlund, Thorstensson, & Cresswell, 2004;Racinais et al, 2007). This behaviour could suggest that TR group, and especially the dominant limb, might be better adapted to load activities, rather than those requiring endurance when compared to the UT group.…”