GROUND REACTION FORCES DURING COUNTERMOVEMENT JUMPS IN SIMULATED AND TRUE MICROGRAVITY

Abstract

1407 The evidence of bone loss, particularly in the lower extremities, is well documented during space travel. Current exercise countermeasures, however, have been ineffective in eliminating space flight induced osteoporosis due in part to the absence of axial loading. Jumping has been studied as a means to alleviate bone loss. Such exercises performed in a ground-based zero gravity simulator produced loads large enough to yield calcaneal bone strain in the osteogenic range (greater than 0.1%). The purpose of this investigation, therefore, was to determine if the same magnitude of load and loading rates could be achieved in true microgravity. A zero gravity simulator (ZGS) was constructed using latex cord and rope to suspend subjects from the ceiling in a supine position. A gravity replacement system consisting of two steel springs was used to tether the subject to the wall at varied levels of body weight. Twelve subjects (6 female, 6 male) performed countermovement jumps in the ZGS with three different landingsû two foot toe-heel (Type I), two foot flat-footed (Type II) and one foot two-heel (Type III). Peak loads and loading rates were determined for the right foot contacting the force plate. The protocol for the ZGS was duplicated on the KC-135 microgravity aircraft for four subjects. In the ZGS, loads ranged from 432 to 4274 N. Type II landings (2631±663 N) were significantly higher than either of the toe-heel landings (Type I 1902±607 N and Type III 2394±754 N). The mean loading rates ranged between 221±140 and 427±153 kN/s and was significantly different for landing type (Type II>Type III>Type I). A similar pattern was found in the KC-135 with peak loads of 1688±793, 2697±1156 and 2206±912 N and loading rates of 270±202, 382±198 and 242±141 kN/s for landing Types I, II and III, respectively. There was no significant difference between the ZGS and KC-135 data sets (p=0.168 for peak load and p=0.466 for loading rate) indicating that the peak loads generated by jumping in microgravity are likely sufficient to counteract space flight induced osteoporosis. Supported by NASA grant NAG-4086