Artificial gravity as a partially effective musculoskeletal countermeasure

Abstract

Physical de-conditioning is a well-known consequence of spaceflight and its ground based analog of experimental bed rest. Human centrifugation, also called artificial gravity (AG) has been proposed as a unique way to replace gravitational acceleration and to thus provide a therapeutic strategy against all known (and potentially un-known) side-effects of microgravity exposure. The European Space Agency therefore commissioned the BRAG-1 study in order to assess countermeasure efficacy of its short arm human centrifuge. Eleven healthy male subjects with a mean age of 34 (sd 7) years participated in a cross-over study designed to compare the effects of 5-days head-down tilt bed rest when done without countermeasures, or when subjects performed AG with 1 g at the center of mass for either 6 x 5 minutes or for 1 x 30 minutes per day (AG6x5 and AG1x30, respectively). The study was carried out at MEDES in Toulouse, France. Blood and urine samples were obtained, and jump and muscle strength tests were performed during baseline data collection (BDC), head down tilt (HDT) and recovery (REC). Bone formation responses were equivocal, with increases in bone alkaline phosphatase during HDT and REC (P < 0.001), but decreases in P1NP during REC (P = 0.013). None of the two depicted a time*AG interaction (P > 0.3). Urinary markers of bone resorption showed the expected increases for CTX (+25%, P < 0.001) and NTX (+25% P < 0.001), with a trend for an increase in NTX excretion by AG( P = 0.093). Excretion of DPD, a less specific resorption marker, suggested the contrary, namely a complete sparing (P = 0.014) of bed rest-induced bone resorption by AG1x30. Nitrogen balance became negative during HDT (P < 0.001). This effect was equalized by AG6x5 on day HDT 4 (P < 0.001). Neuromuscular performance tests yielded equivocal results for knee extensor and plantar flexor muscle strength, but clear-cut HDT-related decreases in vertical jump performance in terms of height, power and peak acceleration (P values between < 0.001 and 0.015). Of note, AG effectively maintained jump height and power (P = 0.039 and P = 0.034, respectively). Whilst the 5-day bed rest format was probably too short to pick up HDT effects in muscle strength and bone turn-over, it yielded characteristic changes in all other endpoints. AG6x5 was better tolerated thanAG1x30, but both AG modalities seem to have positively affected nitrogen balance and vertical jump performance.