Task-Specific Depression of the Soleus H-Reflex After Cocontraction Training of Antagonistic Ankle Muscles

Abstract

Perez MA, Lundbye-Jensen J, Nielsen JB. Task-specific depression of the soleus H-reflex after cocontraction training of antagonistic ankle muscles. J Neurophysiol 98: 3677-3687, 2007. First published October 17, 2007 doi:10.1152/jn.00988.2007. Ballet dancers have small soleus (SOL) H-reflex amplitudes, which may be related to frequent use of cocontraction of antagonistic ankle muscles. Indeed, SOL H-reflexes are depressed during cocontraction compared with plantarflexion at matched background EMG level. We investigated the effect of 30-min training of simultaneous activation of ankle dorsiand plantarflexor muscles (cocontraction task) on the SOL H-reflex in 10 healthy volunteers. Measurements were taken during cocontraction. After training, there was a significant improvement in the ability of the subjects to perform a stable cocontraction. SOL H-reflex recruitment curves and H-max/M-max ratios were decreased after cocontraction training but not after 30 min of static dorsi or plantarflexion. The decreased H-reflex size correlated with improved motor performance. No changes in SOL and tibialis anterior (TA) EMG activity or EMG power were observed, suggesting that increased presynaptic inhibition of Ia afferents is a likely mechanism for H-reflex depression. In different sessions we measured SOL and TA motor-evoked potentials (MEPs) by using transcranial magnetic stimulation (TMS), TMS-elicited suppression of SOL EMG, and coherence between electroencephalographic (EEG) activity (Cz) and TA and SOL EMG. SOL and TA MEPs were depressed, whereas TMSelicited suppression of SOL EMG and coherence were increased after training. Decreased excitability of corticospinal neurons due to increased intracortical inhibition seems a likely explanation of these observations. Our results indicate that the depression in H-reflex observed during a cocontraction task can be trained and that repeated performance of tasks involving cocontraction may lead to prolonged changes in reflex and corticospinal excitability.

The optimal neural strategy for a stable motor task requires a compromise between level of muscle cocontraction and synaptic gain of afferent feedback

“…The condition with low afferent gain implied an average reduction in the number of recruited motor neurons of 38.7% with respect to simulations with high gain. This corresponds well to the reduction in the H-reflex amplitude previously observed experimentally during cocontraction [approximated values: 10 -30% (Nielsen and Kagamihara 1993), 40 -60% (Nielsen and Kagamihara 1992), 40% (Perez et al 2007)]. In the antagonist (heteronymous) muscle, the Ia input generated a decrease in the baseline motor neuron activity (Fig.…”

“…This is reflected in the fact that even the background activity of stretch-sensitive muscle receptors contributes substantially to the neural drive to the muscles in steady, isometric contractions (Gandevia et al 1990;Macefield et al 1993). The functional relevance of both strategies is underlined by the fact that the level of cocontraction (Osu et al 2002;Shadmehr et al 1993) as well as the presynaptic inhibition of the Ia afferent input (Perez et al 2007;Roche et al 2011) are modulated during the process of learning a new motor task.…”

Reflex Inhibition in Human Biceps Brachii Decreases With Practice of a Fatiguing Contraction

“…A number of studies have demonstrated that the amplitude of reflex responses can change with practice Geertsen et al 2007;Perez et al 2007;Voigt et al 1998). For example, both healthy and spinal-cord injured subjects were able to decrease the amplitude of the stretch reflex in biceps brachii after 24 training sessions in which subjects were provided with feedback of the response and a reward for producing the desired adaptation (Segal and Wolf 1994;Wolf and Segal 1996).…”

“…In addition to the attenuation of reflex inhibition from brachioradialis to biceps brachii, the increase in time to failure may have involved a reduction in recurrent inhibition, which declines during a sustained contraction (Löscher et al 1996), and an increase in excitatory input to biceps brachii motor neurons from afferents arising from forearm muscles (Cavallari and Katz 1989). In contrast, the adaptation likely did not involve a change in disynaptic reciprocal inhibition because the low level of EMG activity in triceps brachii did not change across the practice sessions (Perez et al 2007). These pathways are difficult to test in biceps brachii, however, due to the challenges associated with measuring the H reflex in this muscle (Klass et al 2008;Miller et al 1995a,b).…”

“…Practice-related adaptations in the nervous system can include changes in corticospinal excitability (Jensen et al 2005;Perez et al 2007), somatosensory feedback to the motor neurons (Geertsen et al 2007;Zehr 2006), and the output of the motor neuron pool to muscle (Del Balso and Cafarelli 2007;Duchateau et al 2006). The sites of plasticity in the nervous system are specific to the task performed and can differ, for instance, between motor skill training and strength training (Jensen et al 2005;Semmler and Nordstrom 1998).…”