Effect of Physical Load on Aerobic Exercise Performance during Heat Stress

Heat stress increases carbohydrate oxidation rates and oxygen uptake during prolonged load carriage exercise

Military missions are conducted in a multitude of environments including heat and may involve walking under load following severe exertion, the metabolic demands of which may have nutritional implications for fueling and recovery planning. Ten males equipped a military pack loaded to 30% of their body mass and walked in 20°C/40% relative humidity (RH) (TEMP) or 37°C/20% RH (HOT) either continuously (CW) for 90 min at the first ventilatory threshold or mixed walking (MW) with unloaded running intervals above the second ventilatory threshold between min 35 and 55 of the 90 min bout. Pulmonary gas, thermoregulatory, and cardiovascular variables were analyzed following running intervals. Final rectal temperature (MW: p < 0.001, g = 3.81, CW: p < 0.001, g = 4.04), oxygen uptake, cardiovascular strain, and energy expenditure were higher during HOT trials (p ≤ 0.05) regardless of exercise type. Both HOT trials elicited higher final carbohydrate oxidation (CHOox) than TEMP CW at min 90 (HOT MW: p < 0.001, g = 1.45, HOT CW: p = 0.009, g = 0.67) and HOT MW CHOox exceeded TEMP MW at min 80 and 90 (p = 0.049, g = 0.60 and p = 0.024, g = 0.73, respectively). There were no within-environment differences in substrate oxidation indicating that severe exertion work cycles did not produce a carryover effect during subsequent loaded walking. The rate of CHOox during 90 minutes of load carriage in the heat appears to be primarily affected by accumulated thermal load.

Beating the heat: military training and operations in the era of global warming

Global climate change has resulted in an increase in the number and intensity of environmental heat waves, both in areas traditionally associated with hot temperatures and in areas where heat waves did not previously occur. For military communities around the world, these changes pose progressively increasing risks of heat-related illnesses and interference with training sessions. This is a significant and persistent "noncombat threat" to both training and operational activities of military personnel. In addition to these important health and safety concerns, there are broader implications in terms of the ability of worldwide security forces to effectively do their job (particularly in areas that historically already have high ambient temperatures). In the present review, we attempt to quantify the impact of climate change on various aspects of military training and performance. We also summarize ongoing research efforts designed to minimize and/or prevent heat injuries and illness. In terms of future approaches, we propose the need to "think outside the box" for a more effective training/schedule paradigm. One approach may be to investigate potential impacts of a reversal of sleep-wake cycles during basic training during the hot months of the year, to minimize the usual increase in heat-related injuries, and to enhance the capacity for physical training and combat performance. Regardless of which approaches are taken, a central feature of successful present and future interventions will be that they are rigorously tested using integrative physiological approaches.

Load carriage aerobic exercise stimulates a transient rise in biochemical markers of bone formation and resorption

Exercise can be both anabolic and catabolic for bone tissue. The temporal response of both bone formation and resorption following an acute bout of exercise is not well described. We assayed biochemical markers of bone and calcium metabolism for up to 3 days after military-relevant exercise. In randomized order, male (n = 18) and female (n = 2) Soldiers (means ± SD; 21.2 ± 4.1 years) performed a 60-min bout of load carriage (30% body mass; 22.4 ± 3.7 kg) treadmill exercise (EXER) or a resting control trial (REST). Blood samples were collected following provision of a standardized breakfast before (PRE), after (POST) exercise/rest, 1 h, 2 h, and 4 h into recovery. Fasted samples were also collected at 0630 on EXER and REST and for the next three mornings after EXER. Parathyroid hormone and phosphorus were elevated (208% and 128% of PRE, respectively, P < 0.05), and ionized calcium reduced (88% of PRE, P < 0.05) after EXER. N-terminal propeptide of type 1 collagen was elevated at POST (111% of PRE, P < 0.05), and the resorption marker, C-terminal propeptide of type 1 collagen was elevated at 1 h (153% of PRE, P < 0.05). Osteocalcin was higher than PRE at 1 through 4 h post EXER (119%-120% of PRE, P < 0.05). Sclerostin and Dickkopf-related protein-1 were elevated only at POST (132% and 121% of PRE, respectively, P < 0.05) during EXER. Trivial changes in biomarkers during successive recovery days were observed. These results suggest that 60 min of load carriage exercise elicits transient increases in bone formation and resorption that return to pre-exercise concentrations within 24 h post-exercise.NEW & NOTEWORTHY In this study, we demonstrated evidence for increases in both bone formation and resorption in the first 4 h after a bout of load carriage exercise. However, these changes largely disappear by 24 h after exercise. Acute formation and resorption of bone following exercise may reflect distinct physiological mechanoadaptive responses. Future work is needed to identify ways to promote acute post-exercise bone formation and minimize post-exercise resorption to optimize bone adaptation to exercise.

Aerobic Exercise Performance During Load Carriage and Acute Altitude Exposure

Coffman, KE, Luippold, AJ, Salgado, RM, Heavens, KR, Caruso, EM, Fulco, CS, and Kenefick, RW. Aerobic exercise performance during load carriage and acute altitude exposure. J Strength Cond Res 34(4): 946-951, 2020-This study quantified the impact of combined load carriage and acute altitude exposure on 5-km running time-trial (TT) performance and self-selected pacing strategy. Furthermore, this study developed a velocity prediction tool (nomogram) for similar aerobic exercise tasks performed under various combinations of altitude and load stress. Nine volunteers (6M/3F, age: 24 ± 7 years, height: 171 ± 6 cm, body mass: 72 ± 7 kg, and V[Combining Dot Above]O2peak: 50.5 ± 5.2 ml·min·kg) completed a randomized, repeated-measures design protocol. Volunteers performed 3 familiarization (FAM) trials at sea level (SL; 250 m) with no-load carriage. Experimental testing included 3 self-paced, blinded 5-km running TT on a treadmill while carrying a 30% body mass external load at SL, moderate altitude (MA; 2000 m), and high altitude (HA; 3000 m). At SL, load carriage resulted in a 36% decrement in 5-km exercise performance in comparison with FAM trials (43 ± 7 vs. 32 ± 3 minutes; p < 0.001). Time required to complete the 5-km distance while carrying an external load was increased by 11% when performed at HA vs. SL (48 ± 7 vs. 43 ± 7 minutes; p = 0.001). TT pace was not different among experimental conditions (load carriage at SL, MA, and HA) until after 1 km of the running distance had been completed. Heart rate was not different among experimental conditions throughout the entire TT (170 ± 17 b·min). These data quantify the anticipated reduction in aerobic exercise performance under various combinations of acute altitude exposure and load carriage conditions. The self-paced running TT approach used presently allowed for development of an altitude-load nomogram for use in recreational, occupational, or military settings.