Increased human stretch reflex dynamic sensitivity with height‐induced postural threat

Abstract

Postural threat increases soleus tendon-tap (t-) reflexes. However, it is not known whether t-reflex changes are a result of central modulation, altered muscle spindle dynamic sensitivity or combined spindle static and dynamic sensitization. Ramp-and-hold dorsiflexion stretches of varying velocities and amplitudes were used to examine velocity- and amplitude-dependent scaling of short- (SLR) and medium-latency (MLR) stretch reflexes as an indirect indicator of spindle sensitivity. t-reflexes were also performed to replicate previous work. In the present study, we examined the effects of postural threat on SLR, MLR and t-reflex amplitude, as well as SLR-stretch velocity scaling. Forty young-healthy adults stood with one foot on a servo-controlled tilting platform and the other on a stable surface. The platform was positioned on a hydraulic lift. Threat was manipulated by having participants stand in low (height 1.1 m; away from edge) then high (height 3.5 m; at the edge) threat conditions. Soleus stretch reflexes were recorded with surface electromyography and SLRs and MLRs were probed with fixed-amplitude variable-velocity stretches. t-reflexes were evoked with Achilles tendon taps using a linear motor. SLR, MLR and t-reflexes were 11%, 9.5% and 16.9% larger, respectively, in the high compared to low threat condition. In 22 out of 40 participants, SLR amplitude was correlated to stretch velocity at both threat levels. In these participants, the gain of the SLR-velocity relationship was increased by 36.1% with high postural threat. These findings provide new supportive evidence for increased muscle spindle dynamic sensitivity with postural threat and provide further support for the context-dependent modulation of human somatosensory pathways.

New Insights on Emotional Contributions to Human Postural Control

“…Neuro-anatomical evidence for direct influences of emotion onto balance control systems has been well-established in animal models ( 83 , 85 , 86 ). Supporting evidence has been established in standing humans, with threat-induced increases observed in: (1) muscle-spindle sensitivity ( 56 , 57 , 87 ); (2) 1b reflex gain ( 65 ); and (3) vestibular gain of balance, head and eye-reflexes ( 60 – 63 , 66 ). While early cortical potentials seem unaffected by threat ( 55 , 56 ), later cortical changes thought to be responsible for processing sensory information are significantly influenced with threat ( 44 , 55 , 88 ).…”

Reward-Based Improvements in Motor Control Are Driven by Multiple Error-Reducing Mechanisms

“…Another possibility is that the stretch reflex increases, leading to stronger counter-acting forces produced against the perturbation. For instance, the stretch reflex is sensitive to cognitive factors such as standing next to a void (Horslen et al, 2018).…”

Adaptation of emotional state and standing balance parameters following repeated exposure to height-induced postural threat

“…Lastly, some threat-induced changes in behaviour may be associated with state-related changes not measured in the present study. For example, it has been shown that there are broad, multi-sensory changes when individuals are exposed to height-induced threat, whereby balance relevant proprioceptive 41–44 and vestibular 45–47 reflexes are facilitated. It is unclear if these changes in sensory processing scale linearly with individuals’ emotional response to threat.…”