Combining heat stress and moderate hypoxia reduces cycling time to exhaustion without modifying neuromuscular fatigue characteristics.

PURPOSE: This study investigated the isolated and combined effects of heat [temperate (22 °C/30 % rH) vs. hot (35 °C/40 % rH)] and hypoxia [sea level (FiO2 0.21) vs. moderate altitude (FiO2 0.15)] on exercise capacity and neuromuscular fatigue characteristics. METHODS: Eleven physically active subjects cycled to exhaustion at constant workload (66 % of the power output associated with their maximal oxygen uptake in temperate conditions) in four different environmental conditions [temperate/sea level (control), hot/sea level (hot), temperate/moderate altitude (hypoxia) and hot/moderate altitude (hot + hypoxia)]. Torque and electromyography (EMG) responses following electrical stimulation of the tibial nerve (plantar-flexion; soleus) were recorded before and 5 min after exercise. RESULTS: Time to exhaustion was reduced (P < 0.05) in hot (-35 ± 15 %) or hypoxia (-36 ± 14 %) compared to control (61 ± 28 min), while hot + hypoxia (-51 ± 20 %) further compromised exercise capacity (P < 0.05). However, the effect of temperature or altitude on end-exercise core temperature (P = 0.089 and P = 0.070, respectively) and rating of perceived exertion (P > 0.05) did not reach significance. Maximal voluntary contraction torque, voluntary activation (twitch interpolation) and peak twitch torque decreased from pre- to post-exercise (-9 ± 1, -4 ± 1 and -6 ± 1 % all trials compounded, respectively; P < 0.05), with no effect of the temperature or altitude. M-wave amplitude and root mean square activity were reduced (P < 0.05) in hot compared to temperate conditions, while normalized maximal EMG activity did not change. Altitude had no effect on any measured parameters. CONCLUSION: Moderate hypoxia in combination with heat stress reduces cycling time to exhaustion without modifying neuromuscular fatigue characteristics. Impaired oxygen delivery or increased cardiovascular strain, increasing relative exercise intensity, may have also contributed to earlier exercise cessation

Breakpoints in ventilation, cerebral and muscle oxygenation, and muscle activity during an incremental cycling exercise.

The aim of this study was to locate the breakpoints of cerebral and muscle oxygenation and muscle electrical activity during a ramp exercise in reference to the first and second ventilatory thresholds. Twenty-five cyclists completed a maximal ramp test on an electromagnetically braked cycle-ergometer with a rate of increment of 25 W/min. Expired gazes (breath-by-breath), prefrontal cortex and vastus lateralis (VL) oxygenation [Near-infrared spectroscopy (NIRS)] together with electromyographic (EMG) Root Mean Square (RMS) activity for the VL, rectus femoris (RF), and biceps femoris (BF) muscles were continuously assessed. There was a non-linear increase in both cerebral deoxyhemoglobin (at 56 ± 13% of the exercise) and oxyhemoglobin (56 ± 8% of exercise) concomitantly to the first ventilatory threshold (57 ± 6% of exercise, p > 0.86, Cohen's d < 0.1). Cerebral deoxyhemoglobin further increased (87 ± 10% of exercise) while oxyhemoglobin reached a plateau/decreased (86 ± 8% of exercise) after the second ventilatory threshold (81 ± 6% of exercise, p < 0.05, d > 0.8). We identified one threshold only for muscle parameters with a non-linear decrease in muscle oxyhemoglobin (78 ± 9% of exercise), attenuation in muscle deoxyhemoglobin (80 ± 8% of exercise), and increase in EMG activity of VL (89 ± 5% of exercise), RF (82 ± 14% of exercise), and BF (85 ± 9% of exercise). The thresholds in BF and VL EMG activity occurred after the second ventilatory threshold (p < 0.05, d > 0.6). Our results suggest that the metabolic and ventilatory events characterizing this latter cardiopulmonary threshold may affect both cerebral and muscle oxygenation levels, and in turn, muscle recruitment responses

Cardiovascular adaptations supporting human exercise-heat acclimation

AbstractThis review examines the cardiovascular adaptations along with total body water and plasma volume adjustments that occur in parallel with improved heat loss responses during exercise-heat acclimation. The cardiovascular system is well recognized as an important contributor to exercise-heat acclimation that acts to minimize physiological strain, reduce the risk of serious heat illness and better sustain exercise capacity. The upright posture adopted by humans during most physical activities and the large skin surface area contribute to the circulatory and blood pressure regulation challenge of simultaneously supporting skeletal muscle blood flow and dissipating heat via increased skin blood flow and sweat secretion during exercise-heat stress. Although it was traditionally held that cardiac output increased during exercise-heat stress to primarily support elevated skin blood flow requirements, recent evidence suggests that temperature-sensitive mechanisms may also mediate an elevation in skeletal muscle blood flow. The cardiovascular adaptations supporting this challenge include an increase in total body water, plasma volume expansion, better sustainment and/or elevation of stroke volume, reduction in heart rate, improvement in ventricular filling and myocardial efficiency, and enhanced skin blood flow and sweating responses. The magnitude of these adaptations is variable and dependent on several factors such as exercise intensity, duration of exposure, frequency and total number of exposures, as well as the environmental conditions (i.e. dry or humid heat) in which acclimation occurs

Validation of an ingestible temperature data logging and telemetry system during exercise in the heat

Aim: Intestinal temperature telemetry systems are promising monitoring and research tools in athletes. However, the additional equipment that must be carried to continuously record temperature data limits their use to training. The purpose of this study was to assess the validity and reliability of a new gastrointestinal temperature data logging and telemetry system (e-Celsius™) during water bath experimentation and exercise trials. Materials and Methods: Temperature readings of 23 pairs of e-Celsius (T(eC)) and VitalSense (T(VS)) ingestible capsules were compared to rectal thermistor responses (T(rec)) at 35, 38.5 and 42°C in a water bath. Devices were also assessed in vivo during steady-state cycling (n = 11) and intermittent running (n = 11) in hot conditions. Results: The water bath experiment showed T(VS) and T(eC) under-reported T(rec) (P<0.001). This underestimation of T(rec) also occurred during both cycling (mean bias vs T(VS): 0.21°C, ICC: 0.84, 95% CI: 0.66–0.91; mean bias vs. T(eC): 0.44°C, ICC: 0.68, 95% CI: 0.07–0.86, P<0.05) and running trials (mean bias vs. T(VS): 0.15°C, ICC: 0.92, 95% CI: 0.83–0.96; mean bias vs. T(eC): 0.25, ICC: 0.86, 95% CI: 0.61–0.94, P<0.05). However, calibrating the devices attenuated this difference during cycling and eliminated it during running. During recovery following cycling exercise, T(eC) and T(VS) were significantly lower than T(rec) despite calibration (P<0.01). Conclusion: These results indicate that both T(eC) and T(VS) under-report T(rec) during steady-state and intermittent exercise in the heat, with T(eC) predicting T(rec) with the least accuracy of the telemetry devices. It is therefore recommended to calibrate these devices at multiple temperatures prior to use

Heat acclimation has a protective effect on the central but not peripheral nervous system

This study aimed to clarify the pathway mediating hyperthermia-induced alterations in neural drive transmission, and determine if heat acclimation protects voluntary muscle activation and cognitive function in hyperthermic humans. Electrically evoked potentials (H-reflex and M-wave), executive function (special planning and working memory) and maximal voluntary isometric contractions (120 s) were assessed in fourteen participants in control condition (CON, 24ºC, 40% RH) and hyperthermic states (HYP, 44-50ºC, 50% RH), on consecutive days in a counterbalanced order. Thereafter, Participants were passively heat acclimated for 11 days (1 h per day, 48-50ºC, 50% RH) before repeating the initial assessments. Heat acclimation decreased rectal temperature in CON (-0.2ºC, p0.05), heat acclimation restored executive function, whilst protecting the ability to sustain voluntary activation and torque production during a prolonged contraction in hyperthermia (p<0.05). Ultimately, heat acclimation induces beneficial central but not peripheral neural adaptations

Prediction of Critical Power and W′ in Hypoxia: Application to Work-Balance Modelling

Purpose: Develop a prediction equation for critical power (CP) and work above CP (W′) in hypoxia for use in the work-balance ([Formula: see text]) model. Methods: Nine trained male cyclists completed cycling time trials (TT; 12, 7, and 3 min) to determine CP and W′ at five altitudes (250, 1,250, 2,250, 3,250, and 4,250 m). Least squares regression was used to predict CP and W′ at altitude. A high-intensity intermittent test (HIIT) was performed at 250 and 2,250 m. Actual and predicted CP and W′ were used to compute W′ during HIIT using differential ([Formula: see text]) and integral ([Formula: see text]) forms of the [Formula: see text] model. Results: CP decreased at altitude (P < 0.001) as described by 3rd order polynomial function (R(2) = 0.99). W′ decreased at 4,250 m only (P < 0.001). A double-linear function characterized the effect of altitude on W′ (R(2) = 0.99). There was no significant effect of parameter input (actual vs. predicted CP and W′) on modelled [Formula: see text] at 2,250 m (P = 0.24). [Formula: see text] returned higher values than [Formula: see text] throughout HIIT (P < 0.001). During HIIT, [Formula: see text] was not different to 0 kJ at completion, at 250 m (0.7 ± 2.0 kJ; P = 0.33) and 2,250 m (−1.3 ± 3.5 kJ; P = 0.30). However, [Formula: see text] was lower than 0 kJ at 250 m (−0.9 ± 1.3 kJ; P = 0.058) and 2,250 m (−2.8 ± 2.8 kJ; P = 0.02). Conclusion: The altitude prediction equations for CP and W′ developed in this study are suitable for use with the [Formula: see text] model in acute hypoxia. This enables the application of [Formula: see text] modelling to training prescription and competition analysis at altitude

Physiological Responses and Physical Performance during Football in the Heat

PURPOSE: To examine the impact of hot ambient conditions on physical performance and physiological responses during football match-play. METHODS: Two experimental games were completed in temperate (∼ 21°C; CON) and hot ambient conditions (∼ 43°C; HOT). Physical performance was assessed by match analysis in 17 male elite players during the games and a repeated sprint test was conducted after the two game trials. Core and muscle temperature were measured and blood samples were obtained, before and after the games. RESULTS: Muscle and core temperatures were ∼ 1°C higher (P<0.05) in HOT (40.3 ± 0.1 and 39.5 ± 0.1°C, respectively) compared to CON (39.2 ± 0.1 and 38.3 ± 0.1°C). Average heart rate, plasma lactate concentration, body weight loss as well as post-game sprint performance were similar between the two conditions. Total game distance declined (P<0.05) by 7% and high intensity running (>14 km ⋅ h(-1)) by 26% in HOT compared to CON), but peak sprint speed was 4% higher (P<0.05) in HOT than in CON, while there were no differences in the quantity or length of sprints (>24 km ⋅ h(-1)) between CON and HOT. In HOT, success rates for passes and crosses were 8 and 9% higher (P<0.05), respectively, compared to CON. Delta increase in core temperature and absolute core temperature in HOT were correlated to total game distance in the heat (r = 0.85 and r = 0.53, respectively; P<0.05), whereas, total and high intensity distance deficit between CON and HOT were not correlated to absolute or delta changes in muscle or core temperature. CONCLUSION: Total game distance and especially high intensity running were lower during a football game in the heat, but these changes were not directly related to the absolute or relative changes in core or muscle temperature. However, peak sprinting speed and execution of successful passes and crosses were improved in the HOT condition

Hypoxia and fatigue impair rapid torque development of knee extensors in elite alpine skiers

This study examined the effects of acute hypoxia on maximal and explosive torque and fatigability in knee extensors of skiers. Twenty-two elite male alpine skiers performed 35 maximal, repeated isokinetic knee extensions at 180∘s-1 (total exercise duration 61.25 s) in normoxia (NOR, FiO2 0.21) and normobaric hypoxia (HYP, FiO2 0.13) in a randomized, single-blind design. Peak torque and rate of torque development (RTD) from 0 to 100 ms and associated Vastus Lateralis peak EMG activity and rate of EMG rise (RER) were determined for each contraction. Relative changes in deoxyhemoglobin concentration of the VL muscle were monitored by near-infrared spectroscopy. Peak torque and peak EMG activity did not differ between conditions and decreased similarly with fatigue (p < 0.001), with peak torque decreasing continuously but EMG activity decreasing significantly after 30 contractions only. Compared to NOR, RTD, and RER values were lower in HYP during the first 12 and 9 contractions, respectively (both p < 0.05). Deoxyhemoglobin concentration during the last five contractions was higher in HYP than NOR (p = 0.050) but the delta between maximal and minimal deoxyhemoglobin for each contraction was similar in HYP and NOR suggesting a similar muscle O2 utilization. Post-exercise heart rate (138 ± 24 bpm) and blood lactate concentration (5.8 ± 3.1 mmol.l-1) did not differ between conditions. Arterial oxygen saturation was significantly lower (84 ± 4 vs. 98 ± 1%, p < 0.001) and ratings of perceived exertion higher (6 ± 1 vs. 5 ± 1, p < 0.001) in HYP than NOR. In summary, hypoxia limits RTD via a decrease in neural drive in elite alpine skiers undertaking maximal repeated isokinetic knee extensions, but the effect of hypoxic exposure is negated as fatigue develops. Isokinetic testing protocols for elite alpine skiers should incorporate RTD and RER measurements as they display a higher sensitivity than peak torque and EMG activity

Sleep and psychological factors are associated with meeting discharge criteria to return to sport following ACL reconstruction in athletes

INTRODUCTION: This study aimed to determine if sleep quality and psychological factors were associated with time to meet the discharge criteria to return to sport (RTS) following anterior cruciate ligament reconstruction (ACL-R) among athletes experiencing better quality of sleep and psychological responses returning faster to full activity. METHOD: A cohort-study design included 89 athletes following ACL-R. Each participant completed a battery of questionnaires at 6 different time points: within 3 days of injury occurrence and at post-surgery (1.5m, 3m, 4.5m, 6m and when discharge criteria were met). Assessment included sleep quality and quantity, symptoms of depression, anxiety, stress, psychological readiness to RTS and fear of re-injury. The primary outcome was the time needed to meet all discharge criteria to RTS. RESULTS: Sleep parameters and psychological factors were not associated with time to meet the discharge criteria to RTS. However, athletes that had low anxiety and insomnia scores at baseline and better sleep quality at 3m, 4.5m, 6m and at discharge were more adherent to the rehabilitation program and more likely to meet the RTS discharge criteria OR 1.2 (95% CI 1.0-1.34), 1.3 (95% CI 1.1, 1.7) and 2.0 (95% CI 1.1-3.4) respectively. CONCLUSIONS: Sleep quality and psychological factors were not associated with time to meet the discharge criteria to RTS but impacted whether athletes adhered and completed their rehabilitation program or not. Monitoring sleep quality and psychological factors of athletes before and following ACL-R surgery is important to identify athletes who could have difficulties in adhering to and completing their rehabilitation program to RTS

Hyperthermia impairs short-term memory and peripheral motor drive transmission

The aims of this study were to determine (i) the effect of passive hyperthermia on motor drive and cognitive function, and (ii) whether head cooling can limit the hyperthermia-induced alterations. Sixteen subjects were randomly exposed for 2 h to three different conditions: control (Con, 20°C), hot (Hot, 50°C) and hot head cool (HHC – where cold packs were applied to the head under Hot conditions). Three cognitive tests measuring attention and two measuring memory were performed. Neuromuscular testing included electrically evoked muscle action potentials (M-waves) and reflex waves (H-reflex) at rest and during brief (4–5 s) and sustained (120 s) maximal voluntary contractions (MVC) of the plantar flexors. All the tests were performed in the environmental room. During brief MVC, torque was significantly lower in both Hot and HHC as compared to Con (P < 0.05). The decrease in muscle activation was significant in Hot (P < 0.05) but not in HBC (P= 0.07). This was accompanied by peripheral failures in the transmission of the neural drive at both spinal (significant decrements in H-reflexes and V-waves, P < 0.05) and neuromuscular junction (significant decrements in M-waves, P < 0.05) levels. During sustained MVC, muscle activation was further depressed (P < 0.05) without any concomitant failures in M-waves, suggesting neural activation adjustments occurring probably at the supraspinal level. Cerebral perturbations were confirmed by significant decrements in both memory tests in Hot as compared with Con (P < 0.05) but not in simple tests (attention tests) that were not affected by hyperthermia. The decrement in memory capacity suggested the existence of frontal lobe activity impairments. Thus, HHC preserved memory capacity but not the visual memory