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Publication AbstractsStretch- and H-Reflexes of the Lower Leg During Whole Body Cooling and Local WarmingJ. Oksa, Ph.D., H. Rintamäki, Ph.D., S. Rissanen, Ph.D., S. Rytky, M.D., U. Tolonen, Ph.D., and P. V. Komi, Ph.D.Aviat Space Environ Med 2000; 71:156-61 AbstractBackground: This study was undertaken to evaluate if possible changes in stretch- and H-reflexes could be related to the changes in the EMG activity of the cooled lower leg muscles observed during a stretch-shortening cycle exercise. Methods: Eight subjects wearing shorts and jogging shoes were exposed once to 27°C and twice to 10°C for 60 min each. During the second exposure to 10°C, the subject's lower legs were kept warm (10°Clw) with electrical pillows. After the exposures Achilles tendon reflex (stretch reflex) was induced and the EMG activity of the triceps surae was measured. Immediately after reflex measurements the EMG activity of the triceps surae and tibialis anterior during a drop-jump (stretch-shortening cycle) was measured. After similar thermal exposures electrically induced H-reflex from the calf was measured. Results: During the preactivity and stretch phases the EMG activity of the triceps surae increased after the exposure to 10°C, whereas during the shortening phase it decreased. During the shortening phase cooling, on the contrary, increased the activity of tibialis surae anterior. These changes disappeared at 10°Clw. At 10°C the maximum EMG-amplitude of triceps surae during stretch reflex decreased (p < 0.05), reflecting suppressed muscle spindle activity. Suppressed spindle activity causes the agonist to be unfacilitated and the antagonist muscle contraction to be uninhibited, which was seen in the present study as decreased agonist and increased antagonist EMG activity during the shortening phase at 10°C. The Hmax/Mmax-ratio, H-reflex latency and amplitude increased at 10°C (p < 0.05), reflecting increased motoneuron pool excitability. This in part may explain the increased EMG activity during the preactivity and stretch phases. Conclusion: Cooling-induced increase in the excitability of the motoneuron pool and suppression of muscle spindle activity seem to be responsible of the EMG activity changes during the stretch-shortening cycle, consequently decreasing muscular performance.Keywords: motor control, cooling, stretch-shortening cycle, stretch reflex, H-reflex. Information on subscribing, and on obtaining copies of an article or of an entire issue. Table of Contents for Volume 71, Number 2 of the ASME journal.
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