Spinal reflexes and coactivation of ankle muscles during a submaximal fatiguing contraction

Abstract

This study examined the involvement of spinal mechanisms in the control of coactivation during a sustained contraction of the ankle dorsiflexors at 50% of maximal voluntary contraction. Changes in the surface electromyogram (EMG) of the tibialis anterior and of two antagonist muscles, the soleus and lateral gastrocnemius, were investigated during and after the fatigue task. Concurrently, the compound action potential (M-wave) and the Hoffmann reflex of the soleus and lateral gastrocnemius were recorded. The results showed that the torque of the ankle dorsiflexors and the average EMG of the tibialis anterior during maximal voluntary contraction declined by 40.9 +/- 17.7% (mean +/- SD; P < 0.01) and 37.0 +/- 19.9% (P < 0.01), respectively, at task failure. During the submaximal fatiguing contraction, the average EMG of both the agonist and antagonist muscles increased, leading to a nearly constant ratio at the end of the contraction when normalized to postfatigue values. In contrast to the monotonic increase in average EMG of the antagonist muscles, the excitability of their spinal reflex pathways exhibited a biphasic modulation. The amplitude of the Hoffman reflexes in the soleus and lateral gastrocnemius increased to 147.5 +/- 52.9% (P < 0.05) and 166.7 +/- 74.9% (P < 0.01), respectively, during the first 20% of the contraction and then subsequently declined to 66.3 +/- 44.8 and 74.4 +/- 44.2% of their initial values. In conclusion, the results show that antagonist coactivation did not contribute to task failure. The different changes in voluntary EMG activity and spinal reflex excitability in the antagonist muscles during the fatiguing contraction support the concept that the level of coactivation is controlled by supraspinal rather than spinal mechanisms. The findings indicate, however, that antagonist coactivation cannot simply be mediated by a central descending "common drive" to the motor neuron pools of the agonist-antagonist muscle pairs. Rather, they suggest a more subtle regulation of the drive, possibly through presynaptic mechanisms, to the motoneurons that innervate the antagonist muscles.

The Effect of Motor Imagery on Spinal Motor Neuron Excitability and Its Clinical Use in Physical Therapy

“…Previous researchers interpreted these phenomena to additional recruitment of motor units, an increased firing rate, and synchronization of motor units' recruitment [43]. Furthermore, Levenez et al [44] demonstrated that sustained dorsiflexion under 50% MVC induced decline of soleus H-reflex amplitude. Rossi et al [45] demonstrated that sustained MVC of abductor digiti minimi induced decline of the F-wave amplitude, although the F-wave persistence was unchanged.…”

Bilateral motor unit synchronization is functionally organized

“…It is unlikely that this decrease in median frequency of the antagonist muscles was caused by increased extracellular metabolites because the cocontraction level was only about 10% of the agonist activity and peripheral fatigue was therefore not to be expected. Indeed, several studies have shown that the force generating capacity of the antagonist muscles remained equal after fatiguing contractions of the agonist muscles (e.g., Levenez et al 2005). Although the eVects on the EMG amplitude and median frequency of the antagonist muscle could, in principle, result from cross-talk, it is typically interpreted as an eVect of a common drive to the agonist-antagonist couple (Psek and Cafarelli 1993;Ebenbichler et al 1998;Mullany et al 2002;Levenez et al 2005).…”

“…Indeed, several studies have shown that the force generating capacity of the antagonist muscles remained equal after fatiguing contractions of the agonist muscles (e.g., Levenez et al 2005). Although the eVects on the EMG amplitude and median frequency of the antagonist muscle could, in principle, result from cross-talk, it is typically interpreted as an eVect of a common drive to the agonist-antagonist couple (Psek and Cafarelli 1993;Ebenbichler et al 1998;Mullany et al 2002;Levenez et al 2005). In support of this interpretation, an increase in amplitude and a decrease in median frequency have been found in the contralateral homologous muscle after unilateral muscle fatigue (Morrison et al 2005).…”

Comparison of neuromuscular adjustments associated with sustained isometric contractions of four different muscle groups

“…This intensity was then increased by 10% and kept constant throughout the experiment. Such supramaximal stimulation intensity, as used in previous studies (46,53,56), limits the discomfort associated with the stimulation compared with higher supramaximal levels. Pulse width was set at 0.1 ms for the upper limbs (68) and at 1 ms for the lower limbs (53,57).…”