Resting heart rates reflect thermal load during intermittent exercise

Abstract

Background Assessment of thermal load during exercise is important in preventing heat-related injury. To evaluate the workload during exercise, heat rates (HR) have been widely used. In addition, we could get reliable and continuous monitoring of HR with several kinds of the wearable sensors. HR also increased with thermal load (i.e., increase in core body temperature, Tcore and/or skin temperature, Tsk). Therefore, researchers have been tried to assess the thermal load by measuring HR. However, they seem to fail, because the increase in HR is largely influenced by several factors besides Tcore such as relative workload for individuals, clothes, age, and sex, etc. In the present study, we hypothesized that, when individuals conduct intermittent exercise, resting HR between the exercise bouts increases with body temperature. Methods Young males (n = 12; age, 23.7 ± 2.4; body weight, 64.34 ± 8.86 kg) participated in the present study, consisting of two trials conducted on different days. In both trials, each participant conducted 63-min intermittent treadmill exercise, which consisted of 5-bout treadmill run with 2-min rest in between, of which speed was gradually increased. One trial was conducted at ambient temperature (Ta) of 35ºC with relative humidity (RH) of 65% (HH), and the other 25ºC and 30% RH (CON). Rectal temperature (Trec), skin temperature (Tsk) at 4 sites (chest, upper arm, thigh, and lower leg) and HR were measured. Changes in Trec and HR from the baseline values before the exercise were obtained (ΔTrec and ΔHR, respectively). Mean body temperature (T_b) was calculated by Trec and Tsk. The minimal value of ∆HR between the exercise bouts was assessed as HR recovery (HRRmin). The ratio of the maximal ∆HR during the exercise to HRRmin was estimated as HRRslope. Results ∆T_b and HRRmin in HH were higher than in CON in each period between the exercise bouts (P < 0.001). There were significant linear correlation HRRmin and T_b in both CON and HH (CON, y = 0.02 × – 0.09, P < 0.001; HH, y = 0.02 × – 0.01, P < 0.001; each equation denotes that for the regression line). No difference in the regression slope was observed between the values of the regression slope (P = 0.613). Conclusions HRRmin increased with ∆Tb in both CON and HH. In addition, the relationship between the two values was similar in both trials, which may suggest that resting HR during the intermittent exercise may reflect the thermal load per se (increases in core body and skin temperature). These results may indicate that, if we assess changes in resting HR during exercise and/or labor, the thermal load of individuals can be estimated. We also need to verify if the relationship between HRRmin and ∆Tb is similar among individuals with different physiological backgrounds.