Blood pressure and core temperature responses to load carriage across work/rest cycles in the heat

Shorter work/rest (W/R) cycles may limit heat strain during sustained physical work. However, the accompanying blood pressure responses to equal total work and rest duration are not well understood, limiting evidence-based guidance for W/R planning in occupational and military settings. Therefore, the purpose of this study was to compare blood pressure and thermoregulatory responses across different W/R cycles during prolonged load carriage exercise in the heat. We hypothesized that blood pressure and thermoregulatory responses would differ across W/R cycles despite matched total work and rest duration. Eight participants (5 females; age=26±4 y; height=171.2±8.4 cm; body mass=67.5±12.6 kg; body fat=24±8%; maximal oxygen consumption=40.6±7.0 ml·kg - ¹·min - ¹) completed three 180 min load carriage trials (20/12 min, 30/20 min, 40/30 min W/R cycles) wearing a backpack with a 30% body mass load. Both work and seated rest occurred in a hot room (air temperature=40±1°C, relative humidity=31±1%), and a carbohydrate-electrolyte beverage was consumed ad libitum. Core temperature (Tc), heart rate (HR), and brachial blood pressure (BP) were recorded. BP was measured every 5 min and averaged over a work bout. Repeated measures ANOVA and paired t-tests assessed the effects of time and W/R cycle, with Tc and HR analyzed as pre-to-post changes and BP based on averaged values from the first and last work bouts. Effect sizes were calculated and reported as partial η 2 for ANOVA and Cohen’s d for pairwise comparisons. Tc increased across all W/R cycles (p< 0.001, η 2 =0.94). 40/30 showed a greater increase in Tc (ΔTc=0.73±0.29°C) compared to 30/20 (0.48±0.30°C, p< 0.01, d=1.24), but not compared to 20/12 (0.50±0.31°C, p=0.21, d=0.49). HR increased during work (ΔHR:20/12=47±17 bpm; 30/20=45±14 bpm; 40/30=44±21 bpm, p< 0.01, η 2 =0.96) but not different across cycles (p=0.84). Mean arterial pressure (MAP) increased from the first to the last work bout in 20/12 (ΔMAP: 11±5 mmHg), which was greater than 30/20 (0±4 mmHg, p=0.016, d=1.62) and tended to be greater than 40/30 (3±4 mmHg, p=0.065, d=1.00). Diastolic blood pressure (DBP) showed a similar first-to-last work bout increase in 20/12 compared to 30/20 and 40/30 (both p< 0.05, d=1.67 and 0.91, respectively), while systolic blood pressure remained unchanged (p=0.34). Total fluid intake did not differ among W/R cycles (20/12=1.65±0.52 L; 30/20=1.44±0.39 L; 40/30=1.37±0.52 L, p=0.053). Although HR responses were not different across W/R cycles, the larger MAP and DBP changes observed in 20/12 indicate greater cardiovascular responses during shorter cycles, without exacerbating heat stress, while 40/30 elicited the highest Tc rise. Collectively, these results show that W/R cycle selection meaningfully alters both thermal and cardiovascular responses during prolonged load carriage in the heat. Practically, a 30/20 W/R cycle provides a balanced approach by limiting heat accumulation and BP changes. 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.