Journal of applied physiology最新文献

Short duration heat acclimation (HA) (≤5 daily heat exposures) elicits incomplete adaptation compared with longer interventions, possibly due to the lower accumulated thermal "dose." It is unknown if matching thermal "dose" over a shorter timescale elicits comparable adaptation to a longer intervention. Using a parallel-groups design, we compared: 1) "condensed" HA (CHA; n = 17 males) consisting of 4 × 75 min·day^-1 heat exposures [target rectal temperature (T_rec) = 38.5 °C] for two consecutive days, with 2) "traditional" HA (THA; n = 15 males) consisting of 1 × 75 min·day^-1 heat exposure (target T_rec = 38.5°C) for eight consecutive days. Physiological responses to exercise heat stress, hypoxia, and normoxic exercise performance were evaluated pre- and postintervention. Thermal (T_rec over final 45 min: CHA = 38.45 ± 0.17°C, THA = 38.53 ± 0.13°C, P = 0.126) and cardiovascular strain were not different during interventions, indicating similar thermal "dose," although CHA had lower sweating rate, higher starting T_rec, and greater inflammation, gastrointestinal permeability, and renal stress (P < 0.05). However, CHA elicited an array of thermophysiological adaptations that did not differ from THA [reduced indices of peak thermal (e.g., Δ peak T_rec CHA = -0.28 ± 0.26°C, THA = -0.36 ± 0.17°C, P = 0.303) and cardiovascular strain, inflammation, and renal stress; blood and plasma volume expansion; improved perceptual indices], although improvements in resting thermal strain (e.g., Δ resting T_rec CHA = -0.14 ± 0.21°C, THA = -0.35 ± 0.29°C, P = 0.027) and sweating rate were less with CHA. Both interventions improved aspects of hypoxic tolerance, but effects on temperate normoxic exercise indices were limited. The diminished thermal strain was well-maintained over a 22-day decay period. In conclusion, CHA could represent a viable acclimation option for time-restricted young healthy males preparing for a hot, and possibly high-altitude, environment.NEW & NOTEWORTHY This study has shown, for the first time, that a novel condensed heat acclimation program can elicit an array of thermophysiological adaptations, many of which do not differ from traditional heat acclimation. These findings suggest that accumulated thermal "dose" is an important factor contributing to the adaptive responses to heat stress and that condensed heat acclimation may represent a viable option for time-restricted individuals (e.g., military personnel, firefighters, and athletes) preparing to enter a hot environment.

Exertional heat stroke is characterized by profound central nervous system dysfunction and core (rectal) temperature typically >40°C. With prompt recognition and response, the probability of survival is excellent; however, there are limited cases with T_c >43.3°C associated with good outcomes. A 23-yr-old male soldier was conducting land navigation training and was found unresponsive by a nonmedical cadre. Emergency medical services personnel obtained a rectal temperature of 44.3°C, which is the highest-ever body core temperature recorded in a patient with exertional heat stroke who survived without significant sequelae. In this case, we report numerous key decisions that contributed to the good outcome. Among those were the use of a GPS-enabled tracking device that enabled the location of the patient on the land navigation course, and the rapid recognition and response by nonmedical personnel at the point of injury. In addition, prioritizing airway, breathing, and circulation over the choice of cooling modality was important in the setting of a patient in acute respiratory distress. Finally, the careful selection of pharmaceutical agents in the Emergency Department minimized additional stress, primarily on the liver and kidneys, which were already significantly stressed. After transfer to a higher level of care due to developing heat-induced disseminated intravascular coagulation and liver failure, the patient was transferred to inpatient rehabilitation 3 wk postinjury. He recovered by 14 mo postinjury, has been medically cleared to return to active duty without limitations, and is continuing his military service.NEW & NOTEWORTHY We present the details surrounding an exertional heat stroke casualty who had the highest-ever body core temperature, 44.3°C, and survived without significant sequelae. Critical decisions that contributed to this outcome, from the point of injury through the first 24 h, are detailed. Treatment considerations included rapid cooling, maintaining the patient's airway, and hemodynamic stability, and minimizing further physiological strain due to the choice of pharmaceutical agents.

Exposure to elevated inspired carbon dioxide (CO₂) levels, an environmental threat in several occupational settings, is known to induce systemic hypercapnia and provoke headache. However, the impact of CO₂ exposure dose on headache severity has not been determined, and countermeasures to mitigate systemic hypercapnia and headache during CO₂ exposure are lacking. In this study, we first characterized respiratory responses and headache with graded CO₂ exposure (sequential 12-minute stages of 0%, 2%, 4%, 6%, and 8% inspired CO₂, all with 21% oxygen) during seated rest in 24 healthy males and females. As expected, graded CO₂ exposure resulted in stepwise increases (41±3, 43±2, 46±2, 53±2, 65±1 mmHg; p<0.001) in end-tidal CO₂ across the spectrum from normocapnia to severe hypercapnia. Headache increased (p<0.05) beginning at 4% inspired CO₂ (1±2, 2±3, 8±8, 16±13, 32±20 mm on a 100 mm visual analog scale). Participants then completed the same graded CO₂ exposure 1 hour following either caffeine (400 mg) or placebo supplementation in a randomized, double-blind, crossover manner (n=23). Caffeine increased ventilation and lowered end-tidal CO₂ at inspired CO₂ levels between 0% and 6% (p<0.05), corresponding with a leftward shift in the end-tidal CO₂-ventilation response curve with unchanged slope. Caffeine substantially reduced headache during graded CO₂ exposure, an effect that was most pronounced at 8% inspired CO₂ (placebo: 25±15 mm, caffeine: 13±12 mm; p<0.05). Our novel findings establish prophylactic caffeine supplementation as a translational countermeasure to mitigate systemic hypercapnia and headache during CO₂ exposure.

The objective is to investigate the intersegmental coordination during landing from a countermovement jump in different gravitational environments to gain insight on how humans perceive gravity and coordinate complex motor tasks. Eight participants performed countermovement jumps on Earth and while submitted to four simulated gravity levels (from 0.2 to 1 g) generated by a downward pull-down force in weightlessness induced by parabolic flights. The orientation of body segments (i.e., elevation angles) was recorded using a high-speed camera. A Principal Component Analysis was performed on the elevation angles of the foot, shank, thigh and trunk segments together with a correlation analysis. Regardless the environment, the movements of the four body segments are tuned through a law of intersegmental coordination; the vertical position of the centre of mass of the body being identified as the parameter controlled. The movement of the foot seems an independent factor, given its minimal contribution to the intersegmental coordination and the poor correlation with the shank segment. In weightlessness, the intersegmental coordination is less unidimensional and more variable compared to Earth's gravity. In addition, the lower the gravity level simulated in weightlessness, the greater the contribution of the foot and of the shank, and the lower the contribution of the thigh, suggesting an adjustment of the intersegmental coordination through a reweighting of altered sensory inputs. In conclusion, the intersegmental coordination remains better optimised for Earth gravity, but the unidimensional synergy is preserved in weightlessness when using a downward pull-down force to simulate gravity.

There is speculation that oral contraceptive pill (OCP) use affects skeletal muscle biology and protein turnover in response to resistance exercise; however, research in this area is scarce. We aimed to assess, using stable isotope tracers and skeletal muscle biopsies, how second-generation OCP phase affected muscle protein synthesis and whole-body proteolysis. Participants (n=12) completed two 6-day study phases in a randomized order: an active pill phase (Active; week two of a monthly active OCP cycle) and an inactive pill phase (Inactive; final week of a monthly OCP cycle). Participants performed unilateral resistance exercise in each study phase, exercising the contralateral leg in the opposite phase in a randomized, counterbalanced order. The Active phase myofibrillar protein synthesis (MPS) rates were 1.44 ± 0.14 %•d^-1 in the control leg and 1.64 ± 0.15 %•d^-1 in the exercise leg (p < 0.001). The Inactive phase MPS rates were 1.49 ± 0.12 %•d-1 %/d in the control leg and 1.71 ± 0.16 %•d^-1 in the exercise leg (p < 0.001), with no interaction between phases (p = 0.63). There was no significant effect of OCP phase on whole-body myofibrillar proteolytic rate (active phase k = 0.018 ± 0.01; inactive phase k = 0.018 ± 0.006; p = 0.55). Skeletal muscle remains equally as responsive, in terms of stimulation of MPS, during Active and Inactive OCP phases; hence, our data does not support a pro-anabolic or catabolic, based on myofibrillar proteolysis, effect of OCP phase on skeletal muscle in females.

Despite consistent evidence that face masks (FMs) increase dyspnea during exercise, few studies have examined the sex differences in the physiological and perceptual responses to FMs. In a randomized, cross-over design, 32 healthy individuals (16 female;23±3yr) completed incremental cycling tests on two visits with either no mask or a surgical FM. Dyspnea intensity and unpleasantness were assessed using the 0-10 category-ratio Borg scale. Diaphragmatic electromyography, esophageal pressure (P_eso), and transdiaphragmatic pressure (P_di) were measured using a nasogastric catheter to estimate neural inspiratory drive and respiratory muscle effort. Surface EMG was measured on the sternocleidomastoid and scalene muscles. FMs resulted in a steeper increase in dyspnea unpleasantness in males compared to females across the 0-100% work rate (estimate=1.47CR10, P=0.005), with no significant difference in the effect of the FM on dyspnea intensity between sexes (P>0.05). Males had a greater increase in P_eso and P_di with FMs compared to females across work rate (3.77cmH₂O,P=0.009;4.74cmH₂O,P=0.011, respectively) and a greater increase in sternocleidomastoid activation from 40-60% WR (all P<0.05). Additionally, moisture accumulation pre- vs. post-exercise (P=0.01) was significantly greater in males compared to females. FMs resulted in a similar absolute reduction in exercise time in both sexes although it was only statistically significant in females (P=0.006). This study highlights that males likely experience greater dyspnea unpleasantness with FMs due to higher flows and ventilations, which increase mask resistance and, combined with greater moisture accumulation, elevate respiratory muscle effort and accessory muscle activation. Nonetheless, the absolute impact on exercise duration appears similar between sexes.

Hypoxia stimulates glucose uptake independently from the action of insulin. The purpose of this study was to determine the effect of intermittent hypoxia, consisting of alternating short bouts of breathing hypoxic and room air, on glucose concentration, insulin concentration, and insulin sensitivity during an oral glucose tolerance test in adults with type 2 diabetes and adults with normal glycemic control. Nine adults with type 2 diabetes (two women, HbA1c: 7.3±1.5%, age: 52±13 years) and nine adults with normal glycemic control (four women, HbA1c: 5.4±0.1%, age: 24±4 years) performed a 2-hour oral glucose tolerance test on two separate visits to the laboratory. Following ingestion of the glucose drink, participants were exposed to either an intermittent hypoxia protocol, consisting of eight 4-min hypoxic cycles at a targeted oxygen saturation of 80% interspersed with breathing room air to resaturation, or a sham protocol consisting of eight 4-min normoxic cycles interspersed with breathing room air. Intermittent hypoxia did not attenuate the increase in glucose concentration but attenuated the increase in insulin concentration in response to an oral glucose tolerance test in comparison with the sham protocol in adults with type 2 diabetes. Insulin sensitivity was greater during intermittent hypoxia in comparison with the sham protocol in adults with type 2 diabetes (0.043±0.036 vs. 0.032±0.046 μmol/kg/min/pmol, p=0.01), but did not change in the control group (0.122±0.015 vs. 0.128±0.008 μmol/kg/min/pmol, p=0.12). In conclusion, intermittent hypoxia improved insulin sensitivity in adults with type 2 diabetes.

Recent investigations of middle cerebral artery blood velocity (MCAv) kinetics at the onset of exercise have not accounted for potential dynamic changes in arterial partial pressure of carbon dioxide (P_aCO₂) during the transient phase of exercise transitions when modeling MCAv kinetics, despite P_aCO₂ having known effects on cerebrovascular tone. The purpose of our study was to determine the independent effects of mean arterial pressure (MAP) and estimated P_aCO₂ (eP_aCO₂) on mean MCAv during repeated moderate-intensity exercise transitions. We hypothesized that cerebral autoregulation would minimize the effect of sustained exercise-induced changes in MAP on mean MCAv, and that dynamic changes in eP_aCO₂ would contribute to changes in mean MCAv. Eighteen young healthy adults (7 women, age: 28±5 yr) performed three exercise transitions from 25 W to 90% of the ventilatory threshold in sequence with 5 min stages. Mean MCAv increased (p<0.001) from 25 W (60.5±14.0 cmꞏs^-1) to 90% of ventilatory threshold (68.8±15.1 cmꞏs^-1). MAP_MCA (Δ = 14±8 mmHg, p<0.001) and eP_aCO₂ (Δ = 2.7±1.8 mmHg, p<0.001) also increased with exercise intensity. Autoregressive moving average analysis isolated the independent effects of dynamic changes in MAP_MCA and eP_aCO₂ on MCAv, with low prediction error (mean absolute error = 1.12±0.25 cmꞏs^-1). Calculated steady-states of the ARMA step responses were 0.13±0.15 cmꞏs^-1ꞏmmHg^-1 for Δmean MCAv/ΔMAP_MCA and 1.95±0.83 cmꞏs^-1ꞏmmHg^-1 for Δmean MCAv/ΔeP_aCO₂. These data demonstrate that the combination of dynamic changes MAP and eP_aCO₂ largely explain the MCAv response during transitions in exercise intensity.

High-intensity interval training (HIIT) has shown to improve exercise capacity, symptom burden, and quality of life in COPD patients, but it remains to be investigated if HIIT can counteract limb muscle dysfunction. Therefore, we examined the impact of a 12-week supervised HIIT protocol on muscle oxygen conductance. Eight patients with mild to moderate COPD and eight age-, sex- and BMI-matched controls underwent a 12-week HIIT intervention. Leg blood flow (Q̇_leg) and arterio-venous blood samples were collected at rest and during active single-leg knee extensions (KEE) at unloaded (0 watts) and 20% of peak workload (W_Lpeak) to estimate leg muscle oxygen conductance pre- and post-HIIT. Pre-HIIT, Q̇_leg was similar between groups during unloaded KEE (p=0.108) but lower at 20% W_Lpeak in the COPD group, compared to control group. Q̇_leg responses were higher during unloaded KEE (28%, p=0.012) and 20% W_Lpeak (40%, p<0.001) post-HIIT in the COPD group, whereas no change occurred in the control group. Flow adjusted skeletal muscle O2 conductance was higher in the COPD pre-HIIT but only increased in the control group. Thus, there was no difference in diffusive or convective capacity between groups post- HIIT at submaximal KEE. COPD assessment score decreased by 2.8[1;4] (p=0.003) in the COPD group and V̇O_2peak increased in both groups (COPD 192 mL O2/min, p=0.032, control 257 mL O2/min, p=0.004) with no time/group interaction. A 12-week HIIT intervention may improve peripheral exercise capacity in COPD by increasing the vasodilatory function in working muscle, while concurrently improving whole-body exercise capacity and symptom burden.