Noninvasive assessment of muscle temperature during rest, exercise, and postexercise recovery in different environments.

We introduced noninvasive and accurate techniques to estimate muscle temperature (Tm) of vastus lateralis (VL), triceps brachii (TB), and trapezius (TRAP) during rest, exercise, and postexercise recovery using the insulation disk (iDISK) technique. Thirty-six volunteers (24 men, 12 women; 73.0 ± 12.2 kg; 1.75 ± 0.07 m; 24.4 ± 5.5 yr; 49.2 ± 6.8 ml·kg(-1)·min(-1) peak oxygen uptake) underwent periods of rest, cycling exercise at 40% of peak oxygen uptake, and postexercise recovery in three environments: Normal (24°C, 56% relative humidity), Hot-Humid (30°C, 60% relative humidity), and Hot-Dry (40°C, 24% relative humidity). Participants were randomly allocated into the "model" and the "validation" groups. Results in the model group demonstrated that Tm (VL: 36.65 ± 1.27°C; TB: 35.76 ± 1.73°C; TRAP: 36.53 ± 0.96°C) was increased compared with iDISK (VL: 35.67 ± 1.71°C; TB: 34.77 ± 2.27°C; TRAP: 35.98 ± 1.34°C) across all environments (P < 0.001). Stepwise regression analysis generated models that accurately predicted Tm (predTm) of VL (R(2) = 0.73-0.91), TB (R(2) = 0.85-0.93), and TRAP (R(2) = 0.84-0.86) using iDISK and the difference between the current iDISK temperature and that recorded between 1 and 4 min before. Cross-validation analyses in the validation group demonstrated small differences (P < 0.05) of no physiological significance, small effect size of the differences, and strong associations (r = 0.85-0.97; P < 0.001) between Tm and predTm. Moreover, narrow 95% limits of agreement and low percent coefficient of variation were observed between Tm and predTm. It is concluded that the developed noninvasive, practical, and inexpensive techniques provide accurate estimations of VL, TB, and TRAP Tm during rest, cycling exercise, and postexercise recovery.