Table 2. Meta-analysis of heat acclimation included studies. Study HA / C group HA activity Intervention group: HA frequency and duration Control group: frequency and duration Measure (Outcome) Performance test Chalmers 2016 Study 1: 9 / 9 Study 2: 12 / 11 Perceptually regulated treadmill exercise training Study 1: 5 days × 38 min in 35 °C and 30% RH Study 2: 4 days × 58 min in 35 °C and 30% RH Study 1: 5 days × 38 min in 19 °C and 30% RH Study 2: 4 days × 58 min in 19 °C and 30% RH HRE, TC, VO2max, PV, Blood Lactate, TComf, RPE - Chen 2013 7 / 7 Cycling at 10% below VT to 10% above VT 5 days × 35 min (mean) in 38.4°C ± 0.4°C, 52.0% ± 4.6% RH 5 days × 35 min (mean) in 24.1°C ± 0.3°C, 51.5% ± 4.5% RH VO2max, Performance, TS, HRM Maximal leg cycle exercise test (GXT): time to reach exhaustion Guy 2016 8 / 8 Cycling at 55 % VO2max Cycling at 50, 60, and 70 % VO2 max 7 days × 40 min at 35°C and 70% RH 3 HST tests × 3 sets × 10 min at 35 °C and 70 % RH 7 days × 40 min at 20°C, 45% RH 3 HST tests × 3 sets × 10 min at 35 °C and 70 % RH TC, Performance, HRTT, RPE, TComf, HRE 5 km TT performance Karlsen 2015 9 / 9 Cycling at 80–100% HRMAX Cycling at moderate intensity 14 days × 38 min in 35 °C 14 days × 90 min in 35 °C 14 days × 38 min in 5–13 °C 14 days × 90 min in 5–13 °C VO2max, PV - Kelly 2016 7 / 7 High Intensity Interval Training at 90% and 30% VO2 5 sessions × 27 min in 38.7 ± 0.5 °C; 34.4 ± 1.3 % RH 5 sessions × 27 min in 22.3 ± 0.2 °C; 35.8 ± 0.6 % RH HRE, TS, RPE, TComf - Lorenzo 2010 12 / 8 Cycling at 50 % VO2max 10 days × 2 bouts × 45 min with 10 min rest in 40°C and 30% RH 10 days × 2 bouts × 45 min with 10 min rest in 13°C and 30% RH TC, V̇O2max, TS, HRM, PV, Blood Lactate, - Nielsen 1993 8 / 5 Cycling at 60 % VO2max 9-12 days × 90 min in 40-42°C and 10-15% RH 9-12 days × 90 min in 18-20°C and 10-15% RH TC, TS, PV - Petersen 2010 6 / 6 High intensity cycling 4 days × 38 min (mean) in 30°C, 60% RH 4 days × 38 min (mean) in 20°C, 60% RH HRE, TC, TS, Blood Lactate, TComf - Schmit 2017 19 / 10 Low intensity training or High intensity training 5 days × 60 min in 30°C, 50% RH Undertook some training sessions in the lab at 21°C, 50% RH Performance, TC, HRTT, RPE 20-km cycling TT in 35°C, 50% RH Sunderland 2007 12 / 5 Loughborough Intermittent Shuttle Test 4 sessions × 38 min (mean) in 30°C, 24% RH 4 sessions × 38 min (mean) in 18°C, 41% RH TC, HRE, PV, Blood lactate, RPE, TComf - Willmott 2016 14 / 7 Cycling at 50 % VO2max SDHA: 4 days × 45 min in 35.2 ±0.5°C, 60±2% RH TDHA: 2 days × 2 sessions × 45 min in 35.4±0.8, 61±3% RH 4 days × 45 min in 21.7±0.6°C, 39±5 % RH Performance, HRE, TC, PV, HRTT, RPE 3 km TT performance in 30 °C, 60% RH HA = Heat Acclimation, HR = Heart Rate, RH = Relative Humidity, PV = Plasma Volume, VT = Ventilator Threshold, HST = Heat Stress Tests, HRTT = Heart Rate Time Trial, TC = Core Temperature, TS = Skin Temperature, HRE = Exercising Heart Rate, RPE = Rate of Perceived Exertion, TComf = Thermal Comfort, HRM = Maximum Heart Rate, HIIT = High Intensity Interval Training, SDHA = Single Session Per Day Heat Acclimation, TDHA = Twice Daily Heat Acclimation.