Comparison of isometric mid-thigh pull strength and power across different work-rest cycles in heat

During deployments in hot environments, military personnel sustain long ruck marches while concurrently performing tasks requiring lower body strength and power. Considering that neuromuscular performance degradation due to heat stress can be detrimental to mission success, the purpose of this study was to determine a work/rest (W/R) cycle structure that mitigates neuromuscular performance decrements during ruck marching in the heat. We hypothesized that strength and rate of force development would decrease progressively across all W/R cycles. Eleven young, healthy participants (5 females; age: 25±4 y; height: 172±8 cm; body mass 69.6±11.8 kg; maximal oxygen consumption: 40.8±6.0 mL/kg/min) completed experimental sessions that each included 180 min work and 60 min rest to one of three W/R cycles: 1) 40/30 min (N = 11), 2) 30/20 min (N = 9), or 3) 20/12 min (N = 9). These W/R cycles occurred in a hot (40.1±1.0°C, 31±1% relative humidity) environment. Lower body strength and power were assessed using an Isometric Mid-Thigh Pull (IMTP). Measures were taken at baseline (BL) and the end of the first and last work periods. Absolute peak force, peak force relative to body mass, and peak rate of force development (RFD) were calculated. The peak RFD value was the highest RFD during a specific 20 ms time sampling window. Participants were familiarized to the IMTP in two familiarization sessions prior to the trials. Mixed model analysis of variance and a priori pairwise comparison t-tests were used to compare across trials and time points. Absolute peak force decreased from baseline (1646±415 N) through the end of the last work bout (1477±388 N, p< 0.01) in 30/20 while absolute peak force was maintained in 40/30 (1773±667 N, 1686±545 N, p=0.20) and 20/12 (1680±382 N, 1621±436 N, p=0.30), equating to a 10.5±8.6% (p< 0.01), 3.7±9.7% (p=0.24), and 9.2±13.9% (p=0.08) decrease in absolute peak force in 30/20, 40/30, and 20/12, respectively. There were no differences in absolute peak force among W/R cycles at any time point (all p>0.05). This relationship among trials and times was maintained when calculated relative to body mass (all p>0.05). Peak RFD was maintained from baseline through the first work bout and last work bout for 30/20 (29,331±14,089 N/sec; 29,587±14,212 N/sec, p=0.44; 27,748±11,948 N/sec, p=0.36), 40/30 (28,216±14,684 N/sec; 29,491±13,981 N/sec, p=0.10; 30,457±10,660 N/sec, p=0.14), and 20/12 (28,777±13,444 N/sec; 25,969±13,033 N/sec, p=0.26; 25,999±13,033 N/sec, p=0.22), respectively, with no differences among W/R cycles at any time point (all p>0.05). In contrast to our hypothesis, three hours of ruck marching in the heat resulted in the maintenance of lower body strength in the 40/30 and 20/12 W/R cycles and a reduction in strength in the 30/20 cycle, while lower body power was maintained throughout all three W/R cycles. This project was funded by the Department of Defense (HT94252410330). This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.