Stretch- and H-reflexes of the lower leg during whole body cooling and local warming

Background: This study was undertaken to evaluate if possible changes in st retch- and H-reflexes could be related to the changes in the EMG activity o f the cooled lower leg muscles observed during a stretch-shortening cycle e xercise. Methods: Eight subjects wearing shorts and jogging shoes were expo sed once to 27 degrees C and twice to 10 degrees C for 60 min each. During the second exposure to 10 degrees C, the subject's lower legs were kept war m (10 degrees Clw) with electrical pillows. After the exposures Achilles te ndon reflex (stretch reflex) was induced and the EMG activity of the tricep s surae was measured. Immediately after reflex measurements the EMC activit y of the triceps surae and tibialis anterior during a drop-jump (stretch-sh ortening cycle) was measured. After similar thermal exposures electrically induced H-reflex from the calf was measured. Results: During the preactivit y and stretch phases the EMG activity of the triceps surae increased after the exposure to 10 degrees C, whereas during the shortening phase it decrea sed. During the shortening phase cooling, on the contrary, increased the ac tivity of tibialis surae anterior. These changes disappeared at 10 degrees Clw. At 10 degrees C the maximum EMG-amplitude of triceps surae during stre tch reflex decreased (p < 0.05), reflecting suppressed muscle spindle activ ity. 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 EMC activity d uring the shortening phase at 10 degrees C. The Hmax/Mmax-ratio, H-reflex l atency and amplitude increased at 10 degrees C (p < 0.05), reflecting incre ased motoneuron pool excitability. This in part may explain the increased E MC 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 chan ges during the stretch-shortening cycle, consequently decreasing muscular p erformance.