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 , at the gas exchange threshold (GET) was reduced ( d = −0.53; P < 0.005) at IMMO post , whereas maximal oxygen uptake () did not change significantly ( d = 0.14; P = 0.07). The reduction in GET 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 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.