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

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.