Heat production and heat loss responses to cold water immersion after 35 days horizontal bed rest

Introduction: Bed rest is a terrestrial experimental analogue of unloading experienced during exposure to microgravity. Such unloading causes atrophy predominantly of the postural muscles, especially those of the lower limbs. Methods: We tested the hypothesis that 35 d horizontal bed rest alters thermoregulatory responses of subjects (N = 10) immersed in 15oC water, particularly the heat produced by the shivering tremor of the skeletal muscles. Before and after bed rest we measured the thickness of the gastrocnemius medialis (GM), vastus lateralis (VL), tibialis anterior (TA), and biceps brachii (BB) muscles by ultrasonography. During the immersions, we monitored rectal and skin temperatures, heat fl ux, heart rate, and oxygen uptake. Results: After bed rest, muscle thickness decreased signifi cantly by 12.2 ± 8.8% and 8.0 ± 9.1% in the GM and VL, respectively. No changes were observed in the TA and BB muscles. The 35-d bed rest caused a signifi cant reduction in aerobic power, as refl ected in maximal oxygen uptake. There were no signifi cant differences in any of the observed thermoregulatory responses between the pre- and post-bed rest immersions. Conclusions: Cardiovascular and muscular deconditioning had no effect on the heat production and heat loss responses. Due to the signifi cant reduction in the mass of the muscles in the lower limbs, concomitant with no change in heat production, we conclude that leg muscles do not play a signifi cant role in shivering thermogenesis.