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 (V̇O2) 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 depending on self-reported aerobic fitness; selected to induce task disengagement within 10-15 minutes) in a climate-controlled laboratory. Throughout the protocol, ventilatory and gas exchange variables were acquired breath-by-breath using a metabolic cart and Tsk of the posterior calf was recorded revolution-by-revolution using an infrared camera. MAIN RESULTS: The Tsk-time relationship exhibited either an abrupt or gradual deceleration as exercise time progressed. The V̇O2 associated 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. The V̇O2 at GET and RCP were determined visually. Overall, the V̇O2 at GET, RCP and Tsk breakpoint identified visually or modelled were 1884±466, 2633±747, 1969±462, and 1769±508 mL∙min-1, respectively. Compared to the V̇O2 at GET, higher values were reported when Tsk breakpoint was identified visually (p=0.04, ESg=0.3, 95%CI[1, 170 ml/min]) and lower by the second derivative (p<0.01, ESg=0.4, 95%CI[30, 199 ml/min]). Larger effect sizes were observed for RCP (visually: p<0.001, ESg=1.3, 95%CI[524, 803 ml/min]; second derivative: p<0.001, ESg=1.7, 95%CI[724, 1003 ml/min]). Intraclass correlation of V̇O2 between Tsk and 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 the Tsk response of the calf.