title: Vestibular contribution to balance control in the medial gastrocnemius and soleus.
abstract: The soleus (Sol) and medial gastrocnemius (mGas) muscles have different patterns of activity during standing balance and may have distinct functional roles. Using surface electromyography we previously observed larger responses to galvanic vestibular stimulation (GVS) in the mGas compared with the Sol muscle. However, it is unclear whether this difference is an artifact that reflects limitations associated with surface electromyography recordings or whether a compensatory balance response to a vestibular error signal activates the mGas to a greater extent than the Sol. In the present study, we compared the effect of GVS on the discharge behavior of 9 Sol and 21 mGas motor units from freely standing subjects. In both Sol and mGas motor units, vestibular stimulation induced biphasic responses in measures of discharge timing [11 ± 5.0 (mGas) and 5.6 ± 3.8 (Sol) counts relative to the sham (mean ± SD)], and frequency [0.86 ± 0.6 Hz (mGas), 0.34 ± 0.2 Hz (Sol) change relative to the sham]. Peak-to-trough response amplitudes were significantly larger in the mGas (62% in the probability-based measure and 160% in the frequency-based measure) compared with the Sol (multiple P < 0.05). Our results provide direct evidence that vestibular signals have a larger influence on the discharge activity of motor units in the mGas compared with the Sol. More tentatively, these results indicate the mGas plays a greater role in vestibular-driven balance corrections during standing balance.
authors.affiliation: School of Kinesiology, University of British Columbia, Vancouver, Canada; Institute of Neurology, University College London, London, United Kingdom;
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Dakin
authors.affiliation: School of Kinesiology, University of British Columbia, Vancouver, Canada; Neuroscience Research Australia, Sydney, Australia;
authors.forename: Martin E
authors.initials: ME
authors.lastname: Héroux
authors.affiliation: School of Kinesiology, University of British Columbia, Vancouver, Canada; Neuroscience Research Australia, Sydney, Australia;
authors.forename: Billy L
authors.initials: BL
authors.lastname: Luu
authors.affiliation: School of Kinesiology, University of British Columbia, Vancouver, Canada; David Mowafaghian Center for Brain Health, University of British Columbia, Vancouver, Canada; International Collaboration for Repair Discoveries, University of British Columbia, Vancouver, Canada; and.
authors.forename: John Timothy
authors.initials: JT
authors.lastname: Inglis
authors.affiliation: School of Kinesiology, University of British Columbia, Vancouver, Canada; David Mowafaghian Center for Brain Health, University of British Columbia, Vancouver, Canada; Institute for Computing Information and Cognitive Systems, University of British Columbia, Vancouver, Canada jsblouin@mail.ubc.ca.
authors.forename: Jean-Sébastien
authors.initials: JS
authors.lastname: Blouin
mesh_terms: D000328:Adult; D019054:Evoked Potentials, Motor; D006801:Humans; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000502:physiology*; D004856:Postural Balance; D014722:Vestibule, Labyrinth / Q000502:physiology*
keywords: balance; galvanic vestibular stimulation; motor units; vestibular system