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

Abstract

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.

Fatigue does not increase limb asymmetry or induce proximal joint power shift during sprinting in habitual, multi-speed runners

“…Small but non-statistical increases in NDL plantarflexor moment (~12%) and power (~9%) may have also contributed to the similarity in moment and positive work performed after fatigue between legs. The retention of ankle moment and work production during fatigue might speculatively be explained by the fatigue resistance of soleus, the largest muscle (by physiological cross-section) in the leg, due to its predominately slow, fatigue-resistant fibre type (Garland & McComas, 1990;Oza et al, 2017). This would have subsequently allowed energy storage in, and reuse from, the Achilles tendon when other muscle-tendon units spanning hip and knee joints lost capacity as they fatigued.…”

Myotonic dystrophy type 1 alters muscle twitch properties, spinal reflexes, and perturbation‐induced trans‐cortical reflexes

“…As has been recently emphasized the H‐reflex is often mistakenly presumed to be a monosynaptic response that provides a simple probe into excitability of the stretch reflex arc. In reality, the H‐reflex is modulated by a host of homonymous and heteronymous pathways that reveal facilitatory and inhibitory processes at the spinal level and in higher centers . Diminution of the H‐reflex in response to repetitive stimulation is an important window into homosynaptic depression of the spinal reflex system …”

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

“…H‐reflex amplitude is modifiable by preceding homosynaptic and heterosynaptic influences . Thus, a more complete understanding of the H‐reflex response to fatigue can be gained by examining of the effects of conditioning stimuli that modify the amplitude of subsequent H‐reflex responses.…”

“…H-reflex amplitude is modifiable by preceding homosynaptic and heterosynaptic influences. 8 Thus, a more complete understanding of the H-reflex response to fatigue can be gained by examining of the effects of conditioning stimuli that modify the amplitude of subsequent H-reflex responses. With repetitive stimulation of a muscle such as the soleus, H-reflex depression is attributed to homosynaptic depression at the pre-synaptic terminal of the Ia afferent fiber.…”

“…H‐reflex amplitude increases in conjunction with voluntary muscular contraction, then plateaus or declines at activation levels exceeding ~30% to 60% of maximum voluntary contraction (MVC) . With tasks that elicit fatigue, the amplitude of the H‐reflex has been observed to remain constant or to increase for protocols that use intermittent contractions. Among other potential mediating factors, this likely reflects a net enhancement of excitatory drive to the alpha motoneuronal pool during the progression of fatigue .…”