Acceleration dependence and task-specific modulation of short- and medium-latency reflexes in the ankle extensors

Abstract

Involuntary responses to muscle stretch are often composed of a short-latency reflex (SLR) and more variable responses at longer latencies such as the medium-latency (MLR) and long-latency stretch reflex (LLR). Although longer latency reflexes are enhanced in the upper limb during stabilization of external loads, it remains unknown if they have a similar role in the lower limb. This uncertainty results in part from the inconsistency with which longer latency reflexes have been observed in the lower limb. A review of the literature suggests that studies that only observe SLRs have used perturbations with large accelerations, possibly causing a synchronization of motoneuron refractory periods or an activation of force-dependent inhibition. We therefore hypothesized that the amplitude of longer latency reflexes would vary with perturbation acceleration. We further hypothesized that if longer latency reflexes were elicited, they would increase in amplitude during control of an unstable load, as has been observed in the upper limb. These hypotheses were tested at the ankle while subjects performed a torque or position control task. SLR and MLR reflex components were elicited by ankle flexion perturbations with a fixed peak velocity and variable acceleration. Both reflex components initially scaled with acceleration, however, while the SLR continued to increase at high accelerations, the MLR weakened. At accelerations that reliably elicited MLRs, both the SLR and MLR were reduced during control of the unstable load. These findings clarify the conditions required to elicit MLRs in the ankle extensors and provide additional evidence that rapid feedback pathways are downregulated when stability is compromised in the lower limb.

Unilateral Eccentric Contraction of the Plantarflexors Leads to Bilateral Alterations in Leg Dexterity

“…Given that the spinal stretch reflex pathway involves transmission delays which have the potential to induce instabilities, reducing feedback gains could be interpreted as an intelligent strategy to limit delay-induced instability, particularly in tasks that require rapid adjustments. Consistent with this idea, previous studies have shown that short- and medium-latency reflex responses of the ankle extensors are attenuated during control of unstable loads (Finley et al, 2012, 2013). Although those studies suggested that increased levels of cocontraction were a potential mechanism responsible for reductions in the gain of spinal stretch reflex pathway, it seems reasonable to speculate that the attenuation of spinal stretch reflex gain due to increased compliance of muscle-tendon complex would have helped to reduce the potentially detrimental effects of delayed feedback on control of dynamic foot-ground interactions.…”

“…Among these, muscle proprioception likely plays a significant role in the feedback control of dexterity. For example, previous studies have shown that short- and medium-latency reflex responses are modulated according to nature of the dynamic interface between the foot and environment (i.e., rigid vs. compliant vs. unstable) (Finley et al, 2012, 2013). These results suggest that tuning of sensorimotor function through modulation of proprioception may be critical for controlling dexterity in different environments.…”

Estimation of Human Ankle Impedance During the Stance Phase of Walking

“…We believe that the estimated ankle impedance is likely to be dominated by the intrinsic mechanics of the muscles and passive tissues crossing the ankle joint, rather than reflex contributions. The shortest latency reflexes occur at approximately 40 ms following an externally imposed movement [54, 55] and peak muscle force in the triceps surae muscles occurs approximately 60 ms later [56, 57]. Since our analysis window was restricted to 100 ms following perturbation onset, any existing reflex contribution would have a limited impact on the measured torque considered in this study.…”

Mechanisms contributing to reduced knee stiffness during movement

“…For the perturbation-elicited responses, background EMG was defined as the average EMG activity from 40 ms prior to the perturbation and reflex activity was defined as the average of activity from 30–60 ms following the onset of the perturbation. This time was selected since longer latency responses were not observed, as previously reported for perturbations with high accelerations, as used in this study (Finley et al 2013). …”