RAPID COMMUNICATION Regional Variability of Stretch Reflex Amplitude in the Cat Medial Gastrocnemius Muscle During a Postural Task

Eng, Janice J. and J. A. Hoffer. Regional variability of stretch length (Hoffer et al. 1989, 1992). During active movements, reflex amplitude in the cat medial gastrocnemius muscle during a extrafusal fibers can shorten while the muscle is lengthening postural task. J. Neurophysiol. 78: 1150–1154, 1997. The relationand vice-versa, and thus muscle spindle lengths cannot be ship between local fibre stretch velocity (mechanical input) and the expected to mirror the whole muscle length profile. The corresponding local reflex electromyographic (EMG) amplitude pinnation angle of local fibers, the compliance of aponeurotic (a measure of the neural output) was assessed to determine the tissues, and location of a spindle within the muscle also contribution of muscle spindle feedback in postural control. We contribute to the spindle response. hypothesized that traditionally measured input variables (e.g., the Unlike other studies, we did not vary the external perturbavelocity of an external movement or whole muscle velocity) may tion. Instead, we compared the actual changes in cat medial not accurately represent the mechanical input to the muscle spindles, especially when the background forces are small. Three cats gastrocnemius (MG) muscle length and muscle fiber length were trained to stand on pedestals while ankle rotations were apat two locations in response to similar ankle rotations. The plied to the left hindlimb. EMG and fiber movement in both proxiactual extent of the muscle stretch varied slightly from trial mal and distal regions of the muscle were recorded in addition to to trial because of variations in posture and in levels of muscle length and tendon force. We found that local muscle velocmuscle activation. We examined the relationship between ity was correlated poorly with whole muscle velocity, demonstralocal fiber stretch velocity (input variable) and the correting that internal and external muscle movements are often dissimisponding local reflex electromyographic (EMG) amplitude lar, particularly during tasks that involve modest levels of muscle (a measure of the neural output) to determine how muscle activation. Local EMG reflex amplitudes were correlated well with spindle feedback participates in postural control. Preliminary the corresponding local fiber stretch velocities (R values ranging findings appeared in abstract form (Eng et al. 1996). from 0.5 to 0.8) but not with muscle stretch velocity. The lack of