Exercise throughout 6° head-down tilt bed rest preserves thermoregulatory responses

Spaceflight and its bed rest analog [6° head-down tilt (HDT)] decrease plasma and blood volume and aerobic capacity. These responses may be associated with impaired thermoregulatory responses observed during exercise and passive heating after HDT exposure. This project tested the hypothesis that dynamic exercise during 13 days of HDT bed rest preserves thermoregulatory responses. Throughout HDT bed rest, 10 subjects exercised for 90 min/day (75% of pre-HDT maximum heart rate; supine). Before and after HDT bed rest, each subject exercised in the supine position at the same workload in a 28°C room. The internal temperature (T_core) threshold for the onset of sweating and cutaneous vasodilation, as well as the slope of the relationship between the elevation in T_core relative to the elevation in sweat rate (SR) and cutaneous vascular conductance (CVC; normalized to local heating maximum), were quantified pre- and post-HDT. T_core thresholds for the onset of cutaneous vasodilation on the chest and forearm (chest: 36.79 ± 0.12 to 36.94 ± 0.13°C, P = 0.28; forearm: 36.76 ± 0.12 to 36.91 ± 0.11°C, P = 0.16) and slope of the elevation in CVC relative to T_core (chest: 77.9 ± 14.2 to 80.6 ± 17.2%max/°C; P = 0.75; forearm: 76.3 ± 11.8 to 67.5 ± 14.3%max/°C, P = 0.39) were preserved post-HDT. Moreover, the T_core threshold for the onset of SR (36.66 ± 0.12 to 36.74 ± 0.10°C; P = 0.36) and the slope of the relationship between the elevation in SR and the elevation in T_core (1.23 ± 0.19 to 1.01 ± 0.14 mg·cm ^-2·min^-1·°C^-1; P = 0.16) were also maintained. Finally, after HDT bed rest, peak oxygen uptake and plasma and blood volumes were not different relative to pre-HDT bed rest values. These data suggest that dynamic exercise during this short period of HDT bed rest preserves thermoregulatory responses.