Effects of gravitational unloading on back muscles tone

The study involved 12 healthy volunteers who were exposed to 6-h or 3-day dry immersion (DI). The back muscle tone was recorded by resonance vibrography using parameters of transverse stiffness of the muscles under study. The measurements in 3-day DI were performed twice before DI, daily in the course of DI, and twice after its completion; in the short-term (6-h) DI, the testing was carried out twice before DI, 1 and 4 h after the beginning of DI, and during the first hour after DI completion. It has been shown that the absence of support is followed by a sharp decrease in the back extensor muscles tone, which has the maximal values during the first hours and days of DI. The possible role of back muscle atony in the development of well-known phenomena of the spine length increase and back pain appearance observed at the beginning of space flights and in the first days after landing, as well as under the conditions of simulated microgravity (DI and head-down bed rest), is discussed.