Purpose: To investigate the effects of short-term, high-intensity interval-training (HIIT) heat acclimation (HA). Methods: Male cyclists/triathletes were assigned into either an HA (n = 13) or a comparison (COMP, n = 10) group. HA completed 3 cycling heat stress tests (HSTs) to exhaustion (60% Wmax; HST1, pre-HA; HST2, post-HA; HST3, 7 d post-HA). HA consisted of 30-min bouts of HIIT cycling (6 min at 50% Wmax, then 12 × 1-min 100%-Wmax bouts with 1-min rests between bouts) on 5 consecutive days. COMP completed HST1 and HST2 only. HST and HA trials were conducted in 35°C/50% relative humidity. Cycling capacity and physiological and perceptual data were recorded. Results: Cycling capacity was impaired after HIIT HA (77.2 [34.2] min vs 56.2 [24.4] min, P = .03) and did not return to baseline after 7 d of no HA (59.2 [37.4] min). Capacity in HST1 and HST2 was similar in COMP (43.5 [8.3] min vs 46.8 [15.7] min, P = .54). HIIT HA lowered resting rectal (37.0°C [0.3°C] vs 36.8°C [0.2°C], P = .05) and body temperature (36.0°C [0.3°C] vs 35.8°C [0.3°C], P = .03) in HST2 compared with HST1 and lowered mean skin temperature (35.4°C [0.5°C] vs 35.1°C [0.3°C], P = .02) and perceived strain on day 5 compared with day 1 of HA. All other data were unaffected. Conclusions: Cycling capacity was impaired in the heat after 5 d of consecutive HIIT HA despite some heat adaptation. Based on data, this approach is not recommended for athletes preparing to compete in the heat; however, it is possible that it may be beneficial if a state of overreaching is avoided.

Objectives: To investigate the effect of progressive whole-body hyperthermia on maximal, and rapid voluntary torque production, and their neuromuscular determinants. Design: Repeated measures, randomised. Methods: Nine participants performed sets of neuromuscular assessments in HOT conditions (∼50 • C, ∼35% relative humidity) at rectal temperatures (T re ) of 37, 38.5 and 39.5 • C and in CON conditions (∼22 • C, ∼35% relative humidity) at a T re of ∼37 • C and pre-determined comparative time-points. Electrically evoked twitch (single impulse) and octet (8 impulses at 300 Hz) responses were measured at rest. Maximum voluntary torque (MVT), surface electromyography (EMG) normalised to maximal M-wave, and voluntary activation (VA) were measured during 3−5 s isometric maximal voluntary contractions. Rate of torque development (RTD) and normalised EMG were measured during rapid voluntary isometric contractions from rest. Results: All neuromuscular variables were unaffected by time in CON. In HOT, MVT, normalised EMG at MVT and VA were lower at 39.5 • C compared to 37 • C (p < 0.05). Early-(0−50 ms) and middle-(50−100 ms) phase voluntary RTD were unaffected by increased T re (p > 0.05), despite lower normalised EMG at T re 39.5 • C (p < 0.05) in rapid contractions. In contrast, late-phase (100−150 ms) voluntary RTD was lower at 38.5 • C and 39.5 • C compared to 37 • C (p < 0.05) in HOT. Evoked twitch and octet RTD increased with increased T re (p < 0.05). Conclusions: Hyperthermia reduced late-phase voluntary RTD, likely due to reduced neural drive and the reduction in MVT. In contrast, early-and middle-phase voluntary RTD were unaffected by hyperthermia, likely due to the conflicting effects of reduced neural drive but faster intrinsic contractile properties.

The effect of localised head and neck per-cooling on central and peripheral fatigue during high thermal strain was investigated. Fourteen participants cycled for 60 min at 50% V̇O2peak on three occasions: CON (18°C), HOT (35°C) and HOT with cooling (HOTcooling). Maximal voluntary force (MVF) and central activation ratio (CAR) of the knee extensors were measured every 30s during a sustained maximal voluntary contraction (MVC). Triplet peak force was measured following cycling, pre-and post the MVC. Rectal temperatures were higher in HOTcooling (39.2 ± 0.6°C) and HOT (39.3 ± 0.5°C) than CON (38.1 ± 0.3°C; P < 0.05). Head and neck thermal sensation was similar in HOTcooling (4.2 ± 1.4) and CON (4.4 ± 0.9; P > 0.05) but lower than HOT (5.9 ± 1.5; P < 0.05). MVF and CAR were lower in HOT than CON throughout the MVC (P < 0.05). MVF and CAR were also lower in HOTcooling than CON at 5, 60, and 120s, but similar at 30 and 90s into the MVC (P > 0.05). Furthermore, they were greater in HOTcooling than HOT at 30s, whilst triplet peak force was preserved in HOT post-MVC. These results provide evidence that central fatigue following exercise in the heat is partially attenuated with head and neck cooling, which may be at the expense of greater peripheral fatigue. Novelty • Central fatigue was greatest during hyperthermia • Head and neck cooling partially attenuated the greater central fatigue in the heat • Per-cooling led to more voluntary force production and more peripheral fatigue

Purpose This study investigated the effects of acute hyperthermia and heat acclimation (HA) on maximal and rapid voluntary torque production, and their neuromuscular determinants. Methods Ten participants completed 10 days of isothermic HA (50 °C, 50% rh) and had their knee-extensor neuromuscular function assessed in normothermic and hyperthermic conditions, pre-, after 5 and after 10 days of HA. Electrically evoked twitch and octet (300 Hz) contractions were delivered at rest. Maximum voluntary torque (MVT), surface electromyography (EMG) normalised to maximal M-wave, and voluntary activation (VA) were assessed during brief maximal isometric voluntary contractions. Rate of torque development (RTD) and normalised EMG were measured during rapid voluntary contractions. Results Acute hyperthermia reduced neural drive (EMG at MVT and during rapid voluntary contractions; P < 0.05), increased evoked torques (P < 0.05), and shortened contraction and relaxation rates (P < 0.05). HA lowered resting rectal temperature and heart rate after 10 days (P < 0.05), and increased sweating rate after 5 and 10 days (P < 0.05), no differences were observed between 5 and 10 days. The hyperthermia-induced reduction in twitch half-relaxation was attenuated after 5 and 10 days of HA, but there were no other effects on neuromuscular function either in normothermic or hyperthermic conditions. Conclusion HA-induced favourable adaptations to the heat after 5 and 10 days of exposure, but there was no measurable benefit on voluntary neuromuscular function in normothermic or hyperthermic conditions. HA did reduce the hyperthermic-induced reduction in twitch half-relaxation time, which may benefit twitch force summation and thus help preserve voluntary torque in hot environmental conditions.

New Findings What is the central question of this study?Hyperthermia reduces the human capacity to produce muscular force, which is associated with decreased neural drive: does mitigating a reduction in neural drive by altering localised thermal sensation help to preserve voluntary force output? What is the main finding and its importance?Altering thermal sensation by cooling and heating the head independent of core temperature did not change neural drive or benefit voluntary force production. Head cooling did slow the rate of rise in core temperature during heating, which may have practical applications in passive settings. Abstract This study investigated altered local head and neck thermal sensation on maximal and rapid torque production during voluntary contractions. Nine participants completed four visits in two environmental conditions: at rectal temperatures ∼39.5°C in hot (HOT; ∼50°C, ∼39% relative humidity) and ∼37°C in thermoneutral (NEU; ∼22°C, ∼46% relative humidity) conditions. Local thermal sensation was manipulated by heating in thermoneutral conditions and cooling in hot conditions. Evoked twitches and octets were delivered at rest. Maximum voluntary torque (MVT), normalised surface electromyography (EMG) and voluntary activation (VA) were assessed during brief maximal isometric voluntary contractions of the knee extensors. Rate of torque development (RTD) and EMG were measured during rapid voluntary contractions. MVT (P = 0.463) and RTD (P = 0.061) were similar between environmental conditions despite reduced VA (−6%; P = 0.047) and EMG at MVT (−31%; P = 0.019). EMG in the rapid voluntary contractions was also lower in HOT versus NEU during the initial 100 ms (−24%; P = 0.035) and 150 ms (−26%; P = 0.035). Evoked twitch (+70%; P < 0.001) and octet (+27%; P < 0.001) RTD during the initial 50 ms were greater in the HOT compared to NEU conditions, in addition to a faster relaxation rate of the muscle (−33%; P < 0.001). In conclusion, hyperthermia reduced neural drive without affecting voluntary torque, likely due to the compensatory effects of improved intrinsic contractile function and faster contraction and relaxation rates of the knee extensors. Changes in local thermal perception of the head and neck whilst hyperthermic or normothermic did not affect voluntary torque.