title: Implicit learning and generalization of stretch response modulation in humans.
abstract: Adaptation of neural responses to repeated muscle stretching likely represents implicit learning to minimize muscle resistance to perturbations. To test this hypothesis, the forearm was placed on a horizontal manipulandum. Elbow flexors or extensors compensated an external load and were stretched by 20° or 70° rotations. Participants were instructed not to intervene by intentionally modifying the muscle resistance elicited by stretching. In addition to phasic stretch reflexes (SRs), muscle stretching was associated with inhibitory periods (IPs) in the ongoing muscle activity starting at minimal latencies of ∼35 ms. The SR amplitude decreased dramatically across 5-12 trials and was not restored after a resting period of 3-5 min, despite the increase in stretch amplitude from 20° to 70°, but IPs remained present. When SRs were suppressed, stretching of originally nonstretched, antagonist muscles initiated after the rest period showed immediate SR suppression while IPs remained present in the first and subsequent trials. Adaptation to muscle stretching thus includes features characteristic of implicit learning such as memory consolidation and generalization. Adaptation may be achieved by central shifts in the threshold positions at which muscles begin to be activated. Shifts are thought to be prepared in advance and triggered with stretch onset. Threshold position resetting provides a comprehensive explanation of the results in the broader context of the control of posture, movement, and motor learning in the healthy and damaged nervous system.
authors.affiliation: Department of Neurosciences, University of Montreal, Montreal, Quebec, Canada; Institute of Biomedical Engineering, University of Montreal, Montreal, Quebec, Canada; Center for Interdisciplinary Research in Rehabilitation (CRIR) of Greater Montreal, Montreal, Quebec, Canada.
authors.forename: Nicolas A
authors.initials: NA
authors.lastname: Turpin
authors.affiliation: School of Physical and Occupational Therapy, McGill University, Montreal, Quebec, Canada; and Center for Interdisciplinary Research in Rehabilitation (CRIR) of Greater Montreal, Montreal, Quebec, Canada.
authors.forename: Mindy F
authors.initials: MF
authors.lastname: Levin
authors.affiliation: Department of Neurosciences, University of Montreal, Montreal, Quebec, Canada; Institute of Biomedical Engineering, University of Montreal, Montreal, Quebec, Canada; Center for Interdisciplinary Research in Rehabilitation (CRIR) of Greater Montreal, Montreal, Quebec, Canada feldman@med.umontreal.ca.
authors.forename: Anatol G
authors.initials: AG
authors.lastname: Feldman
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D000328:Adult; D000704:Analysis of Variance; D001696:Biomechanical Phenomena; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D006801:Humans; D007858:Learning / Q000502:physiology*; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D011187:Posture; D012026:Reflex, Stretch / Q000502:physiology*; D055815:Young Adult
keywords: EMG; clasp-knife phenomenon; habituation; inhibitory reaction; motor control; spasticity; stretch reflex; threshold position resetting