No abstract

The air-righting reaction is a vestibular input-triggered reflex inducing head and lumbar righting in response to a drop from supine position [1]. It is also believed that the mid-brain plays a key role for the initiation of the reflex [2]. The development of airrighting in growing new-born rats was clearly suppressed when their neck and/or back rotations were prevented [3], suggesting that the neck and back muscles play important roles for a quick righting. Further, Skorina et al. [4] reported that an inhibition of a quick righting in response to drop from supine position was induced after hindlimb suspension of rats during neonatal period including a critical developing period of the central vestibular system.We previously reported that the patterns of landing and posture adjustment in response to head-down drop from a height of ~30 cm were inhibited following 9-week hindlimb suspension of adult rats [5]. Although all of the control rats were able to land smoothly by using the four limbs as the shock absorber, the unloaded rats landed by hitting their abdomen. The hindlimb-suspended, but not control, rats dorsiflexed their trunk during fall. Even though the trunk angle recovered significantly within 2 days, it was not normalized completely even after 8 weeks. Such phenomena were associated with inhibited recruitment (mobilization) of neck (rhomboideus capitis) and back muscles (internal oblique). The detrimental effects on landing performance could be also due to the lowered force development caused by muscle fiber atrophy. However, it is still unclear how the air-righting performance in adult rats is influenced by hindlimb suspension, which is often used to unload the hindlimb muscles as one of the simulation model for the expo- Correspondence should be addressed to: Yoshinobu Ohira, School of Health and Sport Sciences, Osaka University, Toyonaka City, Osaka, 560-0043 Japan. Phone/Fax: +81-6-6850-6032, E-mail: ohira@space.hss.osaka-u.ac.jp Abstract: Effects of 9-week hindlimb suspension and 8-week recovery on air-righting reaction in response to drop from a supine position were studied in adult rats. The righting time in rats at the end of suspension (~220 ms) was longer than the age-matched controls (~120 ms, p < 0.05). The unloading-related change in righting time was accompanied by lowered activities of electromyogram (EMG) and altered recruitment of both neck and back muscles at a specific stage of drop. After 8 weeks of reambulation, righting time recovered toward the control level (~153 ms, p < 0.05), but the EMG activity of back muscle was still less than controls. In contrast, the EMG of neck muscle during fall was even increased. The differences in the characteristics of the muscle fibers between two groups were minor. It is suggested that inhibition of recruitment, rather than the changes in the fiber characteristics, of neck and back muscles is one of the major causes of the slow air-righting.