Impact of heat exposure during immobilization-induced de-training and re-training on aerobic capacity and haemoglobin mass

The aim of this work was to assess the effect of heat exposure on cardiorespiratory and haematological responses during de-training and re-training. Nineteen men (33.8 ± 2.7 years; 182 ± 5.7 cm, 84.4 ± 9.3 kg) completed 4 weeks of pre-training followed by heat exposure (HEAT; n = 9) or control (CON; n = 10). Both groups then de-trained for 2 weeks with lower-limb immobilization followed by 2 weeks of re-training. Cardiorespiratory fitness and total haemoglobin mass (Hb_mass) were measured at baseline (BASE), prior to (IMMO_pre) and following (IMMO_post) immobilization and after a ‘return-to-sport’ (RTS_post) training phase. Compared with IMMO_pre, (Formula presented.) at the gas exchange threshold (GET) was reduced (d = −0.53; P < 0.005) at IMMO_post, whereas maximal oxygen uptake ((Formula presented.)) did not change significantly (d = 0.14; P = 0.07). The reduction in GET (Formula presented.) was more pronounced for HEAT than CON (d = −0.77; P = 0.001). At IMMO_post, GET and peak power output were lower than IMMO_pre (d = −1.4, P < 0.005 and d = 0.43, P = 0.01, respectively), however there was no difference between groups. At RTS_post, the (Formula presented.) at GET increased again in both groups yet remained lower than IMMO_pre although the reduction in HEAT (d = −0.42; P = 0.43) was less than CON (d = −0.8; P = 0.05). At RTS_post, HEAT fully recovered GET power losses compared to IMMO_post (d = 1.1; P = 0.001), while CON only showed a trivial change (d = 0.07; P = 0.1). No significant changes were observed in Hb_mass, haematocrit or plasma volume throughout the study (P ≥ 0.763). Heat exposure did not attenuate a decline in cardiorespiratory fitness during immobilization-induced de-training, but could potentiate its recovery upon re-commencement of training.