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Righting response OF rats following chronic hindlimb-suspension.  F. Kawano¹, T. Nomura², T. Wakatsuki¹, A. Ishihara³, and Y. Ohira¹.  ¹ School Health Sport Sci., Osaka Univ., Osaka, ² Res. Ctr. Health, Phys. Fit. Sports, Nagoya Univ., Nagoya, and ³Fac. Integrat. Human Studies, Kyoto Univ., Kyoto, Japan  

     Seven weeks old male Wistar rats were hindlimb-unloaded by tail-suspension for 9 wk and allowed to ambulate for 8 wk thereafter.  Analyses of righting responses, as well as electromyogram (EMG) of neck and back muscles, were performed immediately (R+0-hr), 2 (R+2-wk) and 8 (R+8-wk) weeks after the termination of suspension.  Righting motions were videotaped at 60 frames per sec to measure hindlimb righting time when rats were dropped from supine position at ~30 cm height.  The EMG patterns were synchronized with motions.  The EMG activity of back muscle during suspension at 9th wk was less than the control level (~29.6 vs. ~43.7 V·sec per hr, p>0.05).  The righting time in unloaded rats was longer than controls (~220 vs. ~120 millisec).  The unloading-related change in righting time was accompanied by lowered activities of EMG, at a specific stage of falling, in both neck and back muscles.  After 8 wk of reambulation, righting time recovered toward control level but still slower than in the age-matched control.  However, the EMG activity of back muscle at R+8-wk was still less than controls.  In contrast, EMG of neck muscle was even increased to above the control level.  It is suggested that suspension-related inhibition of righting response of rat, which could be irreversible, may be closely associated with a decreased recruitment of back muscle fibers.