Sustained submaximal contraction yields biphasic modulation of soleus Post-activation depression in healthy humans.

The amplitude of the H-reflex during the development and progression of fatigue reflects a complex interplay between central and peripheral factors. The purpose of this study is to characterize H-reflex homosynaptic post-activation depression (PAD) in an online fashion during a sustained submaximal fatigue task. The task required a high motor output in order to increase the likelihood of creating partial muscle ischemia with accumulation of fatigue metabolites, an important potential inhibitory influence upon the H-reflex during the progression of fatigue. Eleven subjects without neurologic impairment maintained volitional, isometric plantar flexion at 60% of maximal voluntary contraction until exhaustion. A paired-pulse stimulus (2 Hz) was delivered to the tibial nerve to elicit paired H-reflexes before, during, and after the fatigue protocol. The normalized amplitude of the second H-reflex (depression ratio) served as an estimate of PAD. Depression ratio increased during the first half of the fatigue protocol (P < 0.001), indicating a diminution of PAD, and then returned as exhaustion approached. The biphasic behavior of homosynaptic H-reflex depression during fatigue to exhaustion suggests a role for metabolic mediators of post-activation depression during fatigue.

Mechanomyographic responses during recruitment curves in the soleus muscle.

INTRODUCTION: In this study we examined relationships among mechanomyographic (MMG), electromyographic (EMG), and peak twitch torque (PTT) responses as well as test-retest reliability when recorded during recruitment curves in the soleus muscle. METHODS: PTT, EMG (M-wave, H-reflex), and MMG responses were recorded during recruitment curves in 16 subjects (age 24 ± 2 years) on 2 separate days. The sum of the M-wave and H-reflex (M+H) was calculated. Correlations among variables and test-retest reliability were determined. RESULTS: MMG was correlated with PTT (mean r = 0.93, range r = 0.59-0.99), the M-wave (0.95, 0.04-0.98), and M+H (0.91, 0.42-0.97), but was unrelated to the H-reflex (-0.06, -0.56 to 0.47). Reliability was consistently high among most variables, but normalizing to the maximum value improved MMG reliability and the minimum detectable change. CONCLUSION: MMG responses predicted 86%-90% of the variability in PTT, M-wave, and M+H; thus, MMG may be a useful alternative for estimating twitch torque and maximal activation. Muscle Nerve 56: 107-116, 2017.

Modulation of H-Reflex Depression with Paired-Pulse Stimulation in Healthy Active Humans.

Depression of the Hoffman reflex (H-reflex) is used to examine spinal control mechanisms during exercise, fatigue, and vibration and in response to training. H-reflex depression protocols frequently use trains of stimuli; this is time-consuming and prevents instantaneous assessment of motor neuronal excitability. The purpose of this study was to determine if paired-pulse H-reflex depression is reproducible and whether paired-pulse stimulation adequately estimates the depression induced by the more traditional ten-pulse train. H-reflexes were elicited via ten-pulse trains at 0.1, 0.2, 1, 2, and 5 Hz in ten neurologically intact individuals on two separate days. We measured the depression elicited by the second pulse (H2) and the mean depression elicited by pulses 2-10 (Hmean). H2 was consistent at all frequencies on both days (r2 = 0.97, p < 0.05, and ICC(3,1) = 0.81). H2 did not differ from Hmean (p > 0.05). The results indicate that paired-pulse H-reflex depression has high between-day reliability and yields depression estimates that are comparable to those obtained via ten-pulse trains. Paired-pulse H-reflex depression may be especially useful for studies that require rapid assessment of motor neuronal excitability, such as during exercise, fatigue, and vibration, or to establish recovery curves following inhibition.

Dynamic Fatigue Does Not Alter Soleus H-Reflexes Conditioned by Homonymous or Heteronymous Pathways.

H-reflex depression (diminution of amplitude after a conditioning stimulus) is mediated presynaptically and therefore can help distinguish central versus peripheral mechanisms of fatigue. We examined the effects of a dynamic exercise protocol on H-reflex depression using two conditioning methods: homonymous conditioning (paired-pulse tibial nerve stimulation); and heteronymous conditioning (common peroneal nerve stimulation). Ten subjects performed dynamic contractions of the soleus muscle through 30° ankle range of motion. The concentric phase required a target force of 10% of maximum voluntary isometric contraction (MVIC) and the eccentric phase force target was 80% MVIC. Fatigue persisted for >20 min after cessation of the exercise. Compared with prefatigue values, the dynamic fatigue protocol did not increase presynaptic inhibition after either homonymous or heteronymous conditioning. Peak to peak amplitude of unconditioned H-reflexes was likewise unchanged despite a long term depression of muscle force (long duration fatigue). These results suggest that persistent fatigue after dynamic exercise is attributed to muscle changes and not altered spinal mechanisms.