Changes in calf skin temperature during ramp-incremental cycling exercise and their association with gas exchange and respiratory compensation thresholds.

Objective.We tested the hypothesis that the oxygen uptake (O2) associated with systematic changes in skin temperature (Tsk) during ramp-incremental exercise would coincide with the respiratory compensation point (RCP) and not the gas exchange threshold (GET).Approach.Thirty-eight healthy adults (16 females; 24 ± 5 years, body mass 73.0 ± 11.4 kg, height 174.8 ± 8.5 cm) completed a ramp-incremental exercise test on a cycle ergometer (20-45 W min-1depending on self-reported aerobic fitness; selected to induce task disengagement within 10-15 min) in a climate-controlled laboratory. Throughout the protocol, ventilatory and gas exchange variables were acquired breath-by-breath using a metabolic cart andTskof the posterior calf was recorded revolution-by-revolution using an infrared camera.Main results.TheTsk-time relationship exhibited either an abrupt or gradual deceleration as exercise time progressed. TheO2associated with this change-point was identified visually or by finding the local maximum of the second derivative of a 6th-order polynomial fit to the data. TheO2at GET and RCP were determined visually. Overall, theO2at GET, RCP andTskbreakpoint identified visually or modelled were 1884 ± 466, 2633 ± 747, 1969 ± 462, and 1769 ± 508 ml∙min-1, respectively. Compared to theO2at GET, higher values were reported whenTskbreakpoint was identified visually (p= 0.04, ESg = 0.3, 95%CI[1, 170 ml min-1]) and lower by the second derivative (p< 0.01, ESg = 0.4, 95%CI[30, 199 ml min-1]). Larger effect sizes were observed for RCP (visually:p< 0.001, ESg = 1.3, 95%CI[524, 803 ml min-1]; second derivative:p< 0.001, ESg = 1.7, 95%CI[724, 1003 ml min-1]). Intraclass correlation ofO2betweenTskand GET ranged from very good-to-excellent (0.67-0.92) but were not significant for RCP. For GET, mean absolute percentage error was 12.5% (95%CI: 9.4%-15.7%) and 14.5% (95%CI: 11.1%-17.9%) for the visual and second-derivative methods, respectively, and for RCP, 23.2% (95% CI: 18.6%-27.8%) and 31.4% (95% CI: 26.7%-36.2%).Significance.Findings suggest a statistical association between intensity-dependent changes in metabolism and theTskresponse of the calf.