BACKGROUND:Skeletal muscle recovery is improved with immediate postexercise carbohydrate feeding. Little is known regarding muscle recovery and performance when feeding is delayed. The purpose of this study was to examine the effects of varying exogenous carbohydrates on cycling performance with delayed feedings and low skeletal muscle glycogen content. METHODS:Following 60 min of cycling and an overnight fast (12.1 ± 0.4 h), the participants consumed 2.10 ± 0.13 g·kg-1 bodyweight carbohydrate of one of the following: whole potatoes (POT), cooked pasta (PAS), energy gel (GEL) or remained unfed (control, CON), then rested for 120 min. The participants then cycled for 60 min at 65% Wmax and completed a 19.4 km time trial. Muscle and blood samples were collected prefeeding, 120 min postfeeding, and after cycling for glycogen, glucose, and insulin analyses. RESULTS:The time trial mean power output was higher in the PAS (213 ± 56W, p = 0.006) and GEL (209 ± 71W, p = 0.011) compared to CON (179 ± 68W), but POT (196 ± 51W, p = 0.199) was not different from CON. Power was similar between POT, PAS, and GEL (p > 0.05). Time trial finish time trended towards significance (p = 0.088) with carbohydrate trials averaging 3 min faster than CON. Muscle glycogen was similar between trials (p = 0.446) and did not change due to feeding (prefeeding: 44 ± 21 mmol·kg-1, postfeeding: 47 ± 23 mmol·kg-1, p = 0.120). Glycogen declined after cycling for 60 min (26 ± 16 mmol·kg-1, p < 0.001) compared to pre-feeding and post-feeding samples. Glucose and insulin were elevated in carbohydrate trials over CON 0-30 min post-feeding (p < 0.05). CONCLUSIONS:Varied pre-exercise exogenous carbohydrate sources effectively improve cycling time trial performance in a glycogen compromised state.

Abstract Determine how matched duration but varied exposure scheduling impacts heat acclimation in male and female adults. Thirty males and thirty females walked daily (7 days, 38°C, 60% RH, 6.1 METs) in one of four groups (sustained males/females [SM/SF], periodic males/females [PM/PF]). SM/SF performed 90‐min exposures; PM/PF completed three 30‐min exposures 3 h apart. Females had similar ovarian‐hormone fluctuation. Acclimation markers were assessed within the first 30‐min exposure on days 1, 4, and 7. SM/SF rectal temperature decreased from day 1 to days 4 and 7 (37.5 ± 0.3°C, 37.3 ± 0.3°C, 37.2 ± 0.3°C, p < 0.001) and further decreased from day 4 to 7 (p = 0.011). PM/PF rectal temperature was unchanged between days 1, 4, and 7 (37.4 ± 0.3°C, 37.4 ± 0.3°C, 37.3 ± 0.3°C, p > 0.05). SM/SF 3‐site surface temperature decreased from day 1 to days 4 and 7 (37.1 ± 0.5°C, 36.9 ± 0.4°C, 36.8 ± 0.4°C, p < 0.001) but was unchanged from day 4 to 7 (p = 0.090). PM/PF 3‐site surface temperature was unchanged from day 1 (37.0 ± 0.4°C) to days 4 (37.0 ± 0.4°C, p = 0.726) and 7 (36.9 ± 0.4°C, p = 0.109) but decreased from day 4 to 7 (p = 0.013). Females had higher rectal (p < 0.001) and 3‐site surface (p = 0.036) temperatures than males throughout acclimation. Thrice‐daily exposures are not as effective at inducing heat adaptations compared to once‐daily exposures. Sex differences persisted throughout acclimation without altering adaptations.

We validated a wearable accelerometer and running watch for estimating peak vertical ground reaction force (vGRF) and cumulative weighted damage (CWD). First, 18 well-trained runners of equal sex distribution (3.3 ± 0.3 m/s habitual training pace) ran a range of speeds (2.7-4.0 m/s, plus training pace) on an instrumented treadmill as reference standard vGRF and wearable data were collected. Wearable data were inputted into a published equation to estimate peak vGRF. The wearable and equation systematically underestimated peak vGRF compared with the instrumented treadmill (mean bias: -0.22 BWs with LOA: -0.44 to 0.01; 9.4-10.1%RMSE). A linear regression model was used to derive a corrected equation to improve the estimate of peak vGRF. In the validation stage, the wearable and corrected equation were validated in 10 runners in an independent sample of equal sex distribution (3.2 ± 0.3 m/s habitual training pace) yielding excellent relationships and trivial errors for peak vGRF versus the reference standard (ICC3,1 = 0.87-0.97; p < 0.001; mean bias -0.008 BWs with LOA: -0.15 to 0.14 BWs; 1.5%-4.0% RMSE). To illustrate how these methods can be applied to runners at risk for a bone stress injury, CWD per km was estimated, resulting in trivial errors by the wearable versus the instrumented treadmill (ICC3,1 = 0.85-0.97; p < 0.001; 1.5-4.1%RMSE). In summary, a commercial wearable and corrected equation were valid estimates of peak vGRF and CWD. These methods have potential for monitoring training loads in runners at risk for bone stress injuries. An online calculator is provided to estimate peak vGRF and CWD from the running watch and wearable.

The upper limits for total energy expenditure (TEE) and water turnover (rH2O) in humans have been reported during several continuous single-day ultraendurance races (running, cycling, and triathlon). Currently, the upper limits for TEE and rH2O during continuous single-day activity in cold weather (<0 °C) remain unknown. The Arrowhead Ultra is one of the coldest ultraendurance races in North America and provides a unique opportunity to answer these questions. Racers select a bicycle, cross-country skis, or foot travel to traverse a 214-km snow-covered trail (altitude range: 345-426 m; 2,030-m elevation gain). Historically, approximately one-half of the racers complete the event. In this case study, we assessed TEE and rH2O from the racer [cyclist: age: 22 yr; height: 1.84 m; body mass: 87.7 kg; and maximal oxygen consumption (V̇o2max): 5.0 LO2·min-1] who won the 2025 Arrowhead Ultra (17.9 h; -13 to -1 °C) using the doubly labeled water method. Total energy and fluid intake were recorded to assess energy and fluid balance. Mean heart rate was 141 beats·min-1 (71% of maximum heart rate). TEE was 63.9 MJ (15,273 kcal; 9.6 times basal metabolic rate) while total energy intake was 33.2 MJ (7,941 kcal). Mean carbohydrate intake was 88 g·h-1. Water turnover was 17.7 L, yielding a rH2O/TEE ratio of 0.28 L·MJ-1 for the race. The cyclist demonstrated high TEE and rH2O that were comparable to values from other ultraendurance athletes competing in a range of temperatures (3-34°C). Notably, rH2O from this cyclist was higher compared to athletes performing other ultratype endeavors in cold weather conditions (-25 to -19 °C). Our observations shed light on energy and fluid demands during continuous single-day activity in the cold and have endurance training and performance implications.NEW & NOTEWORTHY In this short report, we detail new data on the upper limits of human energy expenditure (9.6 times basal metabolic rate) and water turnover (10.5 L·12 h-1) during continuous single-day exercise in the cold. Our results indicated elevated energy use, and impressively, water turnover comparable to exercise in warmer conditions. These findings have implications for training and endurance performance of strenuous exercise in cold weather environments.

Purpose:The Alaska Mountain Wilderness Ski Classic (AMWSC) is a self-supported and self-oriented winter expedition that occurs in the remote North American Brooks Range, ∼200 km north of the Arctic Circle. Few investigations have evaluated sex-specific physiological responses under extreme cold and isolated circumstances. Our study examined sex-specific differences in total energy expenditure (TEE), water turnover (WT), and changes in body composition during the expedition. Methods:Twenty adult participants (8 females, age: 41 ± 6 years, body mass index: 22.8 ± 1.9 kg/m2 and 12 males, age: 38 ± 4 years, body mass index: 22.7 ± 1.6 kg/m2) enrolled in and completed the study. TEE and WT were examined during the expedition using the doubly labeled water (DLW) method. Body composition was measured using multi-frequency bioelectrical impedance. Results:The duration of the expedition was similar in females (8.1 ± 1.6 days) and males (7.5 ± 0.9 days). Absolute rates of TEE were lower in females (20.8 ± 4.7 MJ/day) compared to males (31.1 ± 7.5 MJ/day). However, when expressed relative to fat free mass (FFM), rates of TEE were similar in females (0.42 ± 0.07 MJ/FFM/day) and males (0.45 ± 0.10 MJ/FFM/day). TEE/body mass plus pack weight (i.e., total load carriage) was lower in females compared to males. WT was reduced compared to previous reports of athletes exercising in thermoneutral and hot environments. Conclusion:Absolute rates of TEE were lower in females compared to males, but there was no difference when TEE was expressed relative to fat free mass. Estimates of TEE/total load carriage were lower in females than males, modestly suggesting greater functional efficiency in females during this expedition. Compared to other ultra-endurance events in warm environments, WT may have been reduced by lack of water availability, self-selected reductions in exercise intensity, and limited sweat loss.

Valid surrogates are needed to monitor tibial forces during tactical load carriage training. We assessed (a) effects of load carriage on internal tibial forces, average vertical loading rate (AVLR), and peak tibial accelerations; and (b) the relationships among these variables. Walking (1.35 m/s: 0-kg, 20.4-kg, and 34.0-kg) and running (2.7 m/s: 0-kg and 20.4-kg) biomechanics were sampled (n = 34). Peak tibial force increased with load carriage during walking and running (all p < 0.001; F:25.79-371.54). AVLR increased during walking with load carriage (p < 0.001; F = 131.83), with no difference (p = 0.61; F = 0.27) when running with load carriage. Peak tibial accelerations during walking increased with 20.4-kg of load carriage (p < 0.001-0.02). When load carriage was increased to 34.0-kg, small increases were observed for peak positive acceleration (+0.1 g, p = 0.02) only, with no change in peak resultant acceleration (p = 0.99). During running, peak tibial accelerometry surprisingly decreased (p = 0.01-0.04; F = 4.69-7.35) with load carriage. Peak tibial resultant accelerometry was only weakly to moderately associated with peak tibial force during running (r = 0.38-0.52; p = 0.002-0.04); with no other associations between peak tibial force, AVLR, and peak positive tibial acceleration (r = -0.23 - 0.13; p = 0.20-0.92). Thus, AVLR and tibial acceleration are questionable surrogates for internal tibial forces during walking or running, with or without load carriage.

Load carriage training is universal during military training, regardless of sex or physical characteristics, and may contribute to the 1.3-2.2× higher incidence of patellofemoral pain (PFP) in female versus male recruits. This study aimed to assess sex differences in patellofemoral joint (PFJ) stress during load carriage, controlling for anthropometrics and quadriceps strength. Twenty males and 20 females walked (1.35 m/s) on an instrumented treadmill with 0-kg, 20.4-kg, and 34.0-kg of load carriage. An inverse-dynamics musculoskeletal model estimated peak, impulse, and cumulative PFJ stress. To assess quadriceps strength, peak isometric knee extensor torque normalized to body mass was measured via dynamometry. Analyses of covariance (ANCOVA) adjusting for body mass, height, and quadriceps strength assessed the effects of load (0-kg, 20.4-kg, and 34.0-kg) and biological sex (male, female) on PFJ stress and gait parameters. Females were shorter, had lower mass, and lower quadriceps strength (all p < 0.001; d = 1.50-1.54, indicating large effect sizes). Peak, impulse per step, and cumulative PFJ stress increased with load carriage, with greater increases in females compared to males (sex × load interactions p = 0.002-0.005; ηp 2 = 0.12-0.13, indicating moderate effect sizes) after controlling for body mass, height, and quadriceps strength. These data indicate that anthropometrics and quadriceps strength do not explain the substantially greater increases in per step and cumulative PFJ stress in females versus males with load carriage. Female recruits may benefit from targeted prevention efforts, such as slower progressions of load carriage training, either in amount carried or distance trained, to reduce their risk of PFP.

This study sought to investigate the effect of cold ambient temperature on subcutaneous abdominal adipose tissue (SCAAT) lipolysis and blood flow during steady-state endurance exercise in endurance-trained cyclists. Ten males (age: 23 ± 3 years; peak oxygen consumption: 60.60 ± 4.84 ml·kg−1·min−1; body fat: 18.4% ± 3.5%) participated in baseline lactate threshold (LT) and peak oxygen consumption testing, two familiarization trials, and two experimental trials. Experimental trials consisted of cycling in COLD (3 °C; 42% relative humidity) and neutral (NEU; 19 °C; 39% relative humidity) temperatures. Exercise consisted of 25 min cycling at 70% LT and 25 min at 90% LT. In situ SCAAT lipolysis and blood flow were measured via microdialysis. Heart rate, core temperature, carbohydrate and fat oxidation, blood glucose, and blood lactate were also measured. Heart rate, core temperature, oxygen consumption, and blood lactate increased with exercise but were not different between COLD and NEU. SCAAT blood flow did not change from rest to exercise or between COLD and NEU. Interstitial glycerol increased during exercise (p < .001) with no difference between COLD and NEU. Fat oxidation increased (p < .001) at the onset of exercise and remained elevated thereafter with no difference between COLD and NEU. Carbohydrate oxidation increased with increasing exercise intensity and was greater at 70% LT in COLD compared to NEU (p = .030). No differences were observed between conditions for any other variable. Cycling exercise increased SCAAT lipolysis but not blood flow. Ambient temperature did not alter SCAAT metabolism, SCAAT blood flow, or fat oxidation in well-trained cyclists, though cold exposure increased whole-body carbohydrate oxidation at lower exercise intensities.

Nocturnal oxygen enrichment improves sleep at high altitudes but may impair acclimatization. Our purpose was to determine if nocturnal oxygen enrichment impacts acclimatization. A 7-day acclimatization protocol occurred at a field-based research site between 0 and 4,200 m. Participants were housed at 2,800 m and slept with ([Formula: see text], 32.3 ± 2.5% O2) or without ([Formula: see text], 20.8 ± 0.1% O2) nocturnal oxygen enrichment. Resting and steady-state cycling (5-min, 1.75 W·kg-1) tests occurred on Day 0 (0 m) and Days 1, 4, and 7 (2,800 m). Sleep, vastus lateralis muscle oxygenation [oxygenated hemoglobin (O2Hb), deoxygenated hemoglobin (HHb)], arterial blood oxygen saturation ([Formula: see text]), heart rate (HR), and expired gases were measured. Five daily hikes from 2,800 to 4,200 m were also completed. Sleep was longer (P = 0.028) and overnight [Formula: see text] higher (P < 0.001) in the [Formula: see text] (452 ± 63 min, 96 ± 1%) than the [Formula: see text] group (427 ± 63 min, 91 ± 2%). The [Formula: see text] and [Formula: see text] groups did not differ at rest in ΔO2Hb (-1.47 ± 0.99, -1.46 ± 1.30 A.U., P = 0.901), ΔHHb (0.78 ± 0.84, 0.51 ± 0.96 A.U., P = 0.202), [Formula: see text] (93 ± 3, 93 ± 3%, P = 1.000), HR (59 ± 6, 64 ± 13 beats·min-1, P = 0.229), respiratory exchange ratio (RER, 0.81 ± 0.07, 0.79 ± 0.06, P = 0.274), and ventilation body temperature pressure saturated (BTPS) (10.56 ± 2.12, 10.80 ± 1.96 L·min-1, P = 0.717). The [Formula: see text] and [Formula: see text] groups also did not differ while cycling in ΔO2Hb (-2.96 ± 3.03, -1.70 ± 3.46 A.U., P = 0.278), ΔHHb (7.59 ± 4.65, 6.34 ± 3.21 A.U., P = 0.451), [Formula: see text] (90 ± 6, 89 ± 6%, P = 0.875), HR (113 ± 10, 118 ± 16 beats·min-1, P = 0.408), RER (0.89 ± 0.06, 0.89 ± 0.07, P = 0.756), and ventilation BTPS (54.00 ± 15.42, 60.18 ± 18.42 L·min-1, P = 0.371). [Formula: see text] while cycling returned toward Day 0 (0 m) values by Day 7 (2,800 m) in both groups (P < 0.001) indicating short-term acclimatization. Nocturnal oxygen enrichment improves sleep but does not impair short-term acclimatization when completing daily prolonged exercise.NEW & NOTEWORTHY This work examined the impact of nocturnal oxygen enrichment on short-term high-altitude acclimatization to 2,800 m while completing daily hikes to 4,200 m. Recurrently dampening the required hypoxic stimulus for acclimatization via nocturnal oxygen enrichment improved sleep but did not impair short-term high-altitude acclimatization. This was evinced through ventilatory and cardiovascular adjustments that improved arterial blood oxygen saturation after 7 days.

Introduction We have previously described negative energy balance (ie, −9.7±3.4 MJ/d) and weight loss (Δ−1.5 ± 0.7 kg) influenced by high levels of energy expenditure (ie, 17.4±2.6 MJ/d) during remote expeditionary hunting in Alaska. Despite negative energy balance, participants retained skeletal muscle. The purpose of this pilot study was to measure skeletal muscle protein synthesis and examine molecular markers of skeletal muscle protein metabolism under similar conditions of physical and nutrient stress. Methods The “virtual biopsy method” was used to evaluate integrated fractional synthetic rates (FSRs) of muscle protein from blood samples in 4 participants. Muscle biopsies were taken to measure molecular markers of muscle protein kinetics (ie, FSTL1, MEF2, MYOD1, B2M, and miR-1-3p, -206, -208b, 23a, and 499a) using real-time polymerase chain reaction. Results Our findings in 4 participants (2 females [28 and 62 y of age; 66.2 and 71.8 kg body weight; 25.5 and 26.7 kg/m2 body mass index] and 2 males [47 and 56 y of age; 87.5 and 91.4 kg body weight; 26.1 and 28.3 kg/m2 body mass index]) describe mean muscle FSRs of serum carbonic anhydrase (2.4%) and creatine kinase M-type (4.0%) and positive increments in molecular regulation. Conclusions Preservation of skeletal muscle under conditions of physical and nutrient stress seems to be supported by positive inflection of skeletal muscle FSR and molecular activation.

Environmental temperature can impact exercise-induced blood oxidative stress; however, the effects of heat acclimation on this response have not been fully elucidated. The purpose of the study was to investigate the effects of hot (33°C) and room temperature (20°C) environments on post-exercise blood oxidative stress responses following 15 temperature acclimation sessions. Untrained participants (n = 38, 26 ± 7 years, VO2peak = 38.0 ± 7.2 years) completed 15 temperature acclimation sessions of a cycling bout at an intensity perceived as "hard" in either a hot (33°C) or room temperature (20°C) environment. Pre and post acclimation exercise tolerance trials were conducted, which involved cycling at 50% Wpeak for one hour. Blood sampling occurred before exercise, immediately after, two hours, and four hours after the exercise tolerance trials. Blood samples were analyzed for oxidative stress markers including lipid hydroperoxides, 8-isoprostanes, protein carbonyls, 3-nitrotyrosine, ferric-reducing ability of plasma, and Trolox-equivalent antioxidant capacity. Exercise-dependent increases were observed in lipid hydroperoxides, Trolox-equivalent antioxidant capacity, and ferric-reducing ability of plasma (p < 0.001). Considering exercise-induced elevations in markers of blood oxidative stress, there were no differences observed between environmental temperatures before or after the acclimation training period.