Effect of acute head-down tilt on skeletal muscle cross-sectional area and proton transverse relaxation time.

This study investigated changes in skeletal muscle cross-sectional area (CSA) evoked by fluid shifts that accompany short-term 6 degrees head-down tilt (HDT) or horizontal bed rest, the time course of the resolution of these changes after resumption of upright posture, and the effect of altered muscle CSA, in the absence of increased contractile activity, on proton transverse relaxation time (T2). Average muscle (CSA and T2 were determined by standard spin-echo magnetic resonance imaging. Analyses were performed on contiguous transaxial images of the neck and calf. After a day of normal activity, 24 h of HDT increased neck muscle CSA 19 +/- 4(SE)% (P < 0.05) while calf muscle CSA decreased 14 +/- 3% (P < 0.05). The horizontal posture (12 h) induced about one-half of these responses: an 11 +/- 2% (P < 0.05) in the neck muscle CSA and an 8 +/- 2% decrease (P < 0.05) in the calf. Within 2 h after resumption of upright posture, neck and calf muscle CSA returned to within 0.5% (P > 0.05) of the values assessed after a day of normal activity, with most of the change occurring within the first 30 min. No further change in muscle CSA was observed through 6 h of upright posture. Despite these large alterations in muscle CSA, T2 was not altered by more than 1.1 +/- 0.6% (P > 0.05) and did not relate to muscle size. These results suggest that postural manipulations and subsequent fluid shifts modeling micro-gravity elicit marked changes in muscle size. Because these responses were not associated with alterations in muscle T2, it does not appear that simple movement of water into muscle can explain the contrast shift observed after exercise.