title: Precise coding of ankle angle and velocity by human calf muscle spindles.
abstract: Human standing balance control requires the integration of sensory feedback to produce anticipatory, stabilizing ankle torques. However, the ability of human triceps surae muscle spindles to provide reliable sensory feedback regarding the small, slow ankle movements that occur during upright standing has recently come under question. We performed microneurography to directly record axon potentials from single muscle spindle afferents in the human triceps surae during servo-controlled movement of the ankle joint. To simulate movements of the ankle while standing, we delivered random 90-s dorsiflexion/plantar flexion oscillations of the ankle joint, with a peak-to-peak amplitude of 0.7° and frequency content below 0.5Hz. In roughly half of the trials (46%), participants held a low-level, near-isometric contraction of the triceps surae muscles. We demonstrate that afferent activity in a population of muscle spindles closely reflects ankle movements at frequencies and amplitudes characteristic of human standing. Four out of five soleus spindles, and three out of seven gastrocnemius spindles coded for at least a single frequency component of anteroposterior ankle rotation. Concatenating within muscles, coherence was significantly greater for soleus spindles at all stimulus frequencies. Voluntary contraction of the parent muscle reduced spindle sensitivity, but only significantly near the mean power frequency of the stimulus (∼0.3Hz). In conclusion, these results provide direct evidence that triceps surae muscle spindles are potentially capable of providing important sensory feedback for the control of human standing balance.
authors.affiliation: School of Kinesiology, University of British Columbia, Vancouver, BC V6T 1Z1, Canada. Electronic address: ryan.peters@ubc.ca.
authors.forename: Ryan M
authors.initials: RM
authors.lastname: Peters
authors.affiliation: School of Health and Exercise Sciences, University of British Columbia - Okanagan, Kelowna, BC V1V 1V7, Canada.
authors.forename: Brian H
authors.initials: BH
authors.lastname: Dalton
authors.affiliation: School of Kinesiology, University of British Columbia, Vancouver, BC V6T 1Z1, Canada; Djavad Mowafaghian Centre for Brain Health, University of British Columbia, Vancouver, BC V6T 1Z3, Canada; Institute for Computing, Information, and Cognitive Systems, University of British Columbia, Vancouver, BC V6T 1Z4, Canada.
authors.forename: Jean-Sébastien
authors.initials: JS
authors.lastname: Blouin
authors.affiliation: School of Kinesiology, University of British Columbia, Vancouver, BC V6T 1Z1, Canada; Djavad Mowafaghian Centre for Brain Health, University of British Columbia, Vancouver, BC V6T 1Z3, Canada; International Collaboration on Repair Discoveries, University of British Columbia, Vancouver, BC V5Z 1M9, Canada.
authors.forename: J Timothy
authors.initials: JT
authors.lastname: Inglis
mesh_terms: D000328:Adult; D000843:Ankle Joint / Q000502:physiology*; D056228:Feedback, Sensory / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009068:Movement / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D009470:Muscle Spindles / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology; D055815:Young Adult
keywords: human; microneurography; muscle spindles; posture; sensory coding; triceps surae