Modulation of homosynaptic depression during voluntary contraction and muscle fatigue with different test reflex size

Abstract

The Hoffmann (H-) reflex can be depressed when elicited repetitively at a frequency of ~1 Hz. This H-reflex depression is termed homosynaptic depression (HD) and is attributed to impaired neurotransmitter release from the presynaptic Ia terminal. In the present study, we systematically evaluated the extent to which HD in the soleus was modulated by the level of homonymous muscle contraction and the size of the test H-reflex. Changes in HD were also assessed while the target muscle was subjected to fatigue. The participants were 11 healthy male volunteers aged 20−25 years. HD was induced by delivering a percutaneous electrical stimulus at 1 Hz to the right posterior peroneal nerve. HD was proportional to the size of the test reflex size up to 60% of the maximum size of the direct motor response (M max), at which point values plateaued. HD decreased significantly during voluntary contraction of the homonymous muscle (range, <20% of maximal voluntary contraction [MVC]), irrespective of the degree of contraction. Within 1−5 minutes after completing 4 sets of the MVC for 60 s with ischemic arterial blockade, the degree of HD decreased significantly when the test reflex size was <60% of the Mmax. We conclude that test reflex size is a crucial factor when evaluating the nature of HD. Furthermore, HD reduction during voluntary contraction and muscle fatigue may represent an innate mechanism that serves to retain Ia transmission.

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

“…These studies offered the best prediction of the behavior of PAD that might be expected in the present study. Similar to our findings, Takahashi and coauthors reported a significant reduction in homosynaptic PAD in fatigued muscle . Our study extends these findings to reveal an apparent restoration of PAD as exhaustion approaches.…”

“…Stein and coauthors highlighted the important differences in PAD that occur between rest and voluntary contraction, concluding that "Measurements at rest do not accurately represent the H-reflex during motor tasks". 6 In their work and in other reports, maintenance of a voluntary contraction yields a reduction in PAD and a concomitant increase in conditioned H-reflex amplitude, 9,25,31,32 similar to what we observed at F1 and F2. None of these reports examined fatigue to exhaustion.…”

“…Of the several previous Groups who have assessed H‐reflexes in an online fashion during fatiguing contractions, rather than in an intermittent rest cycle, none examined homosynaptic PAD. Two studies examined post‐fatigue PAD after an intermittent MVC protocol, with the subjects maintaining a low‐level contraction (20%‐30% MVC) for acquisition of conditioned H‐reflexes . These studies offered the best prediction of the behavior of PAD that might be expected in the present study.…”

“…To examine homosynaptic PAD, we capitalized on a doublet stimulation protocol 30 instead of a more traditional 10-pulse protocol. 25 This approach enabled us to reliably assess PAD with the fewest pulses possible, reducing an important source of discomfort, distraction, and force variability as the subjects attempted to maintain a relatively strong contraction (60% MVC). During the sustained contraction, we observed an enhancement of H2 that reached statistical significance at the midpoint of the fatigue task (F2).…”

“…Some insights may be gained by fatigue studies that employed intermittent contractions and very brief pauses. Two such studies indicated that PAD may be expected to decline during the development of fatigue . However, the introduction of brief pauses into a strong contraction could allow for resolution of ischemia and removal of fatigue metabolites, making the role of Group III/IV afferent input uncertain.…”