Tc 2000

Core temperature (T_c) was measured via the ingestion of a wireless telemetry pill (HQInc, Palmetto, FL; Study 1) or the self-insertion of a rectal thermocouple 15 cm beyond the anal sphincter (PhysiTemp, Clifton, NJ; Study 2). Mean body skin temperature of the whole body minus the cold leg (T̄_sk) was calculated as a weighted mean using wireless temperature sensors (iButtons, Maxim, CA) attached to the arm, chest, thigh, and calf, with relative contributions calculated according to the formula T̄_sk = 0.3(T_chest + T_arm) + 0.2(T_thigh + T_calf). Blood temperature (T_b) in Study 1 was measured using the fine-wire thermocouples mentioned previously. Methodological limitations prevented the measurements of T_b in Study 2. However, measurements of muscle temperature (T_m) obtained at a tissue depth of 2 to 3 cm allowed the assessment of the relationship between local tissue temperatures and blood flow. Leg skin temperature (T_{sk leg}) was recorded via type-t thermocouples (PhysiTemp, Clifton, NJ) and calculated as the average of thigh and calf measurements. All temperature inputs were fed through a thermocouple meter (TC-2000; Sable Systems) for continuous measurement throughout the protocol.

For the first protocol, skin temperature (T_sk) was assessed using skin thermistors placed on two sites on each arm (type‐t thermocouples; Grant Instruments, Cambridge, UK) and securely held in place throughout the protocol by the use of medical tape. Core temperature (T_c) was assessed using a commercially available rectal probe (Thermalert; Physitemp), inserted 10 cm past the sphincter muscle using a commercially available rectal probe (Thermalert; Physitemp). Skin, rectal and venous thermistors were connected to two four‐channel amplifiers (TC‐2000; Sable Systems, Las Vegas, NV, USA) and a data acquisition system (Powerlab 16/30 ML 880/P; ADInstruments, Bella Vista, NSW, Australia).

In the knee extensor study, intestinal temperature was measured with a wireless telemetry temperature sensor (HQInc, Palmetto, US) ingested 2–3 h prior the experimental trial. Thigh skin temperatures were obtained from each leg using wireless thermistors interfaced with data loggers (iButtons, Maxim). During the cycling study, core body temperature was estimated from the measurement of blood temperature in the common femoral vein using a thermistor connected to a thermocouple meter (TC‐2000, Sable Systems) and a data acquisition board (Powerlab, ADInstruments, Australia) and analyzed using LabChart software (version 8, ADInstruments, UK). Mean skin temperature was calculated as the weighted mean temperatures obtained from thermistors placed on the chest, arm, thigh, and calf (Ramanathan 1964), enabling the acquisition of systemic and leg skin temperatures.

Core temperature (T_c) was measured using a rectal probe (Ret‐1 Special, Physitemp, Clifton, NJ, USA) which was self‐inserted 15 cm past the sphincter muscle. Skin temperature (T_sk) in the quadriceps, hamstrings and calf for both legs was measured using commercially available thermistors (IT‐18, Physitemp) which were securely held in place using medical tape. T_c and T_sk were recorded online using a commercially available thermocouple metre (TC‐2000, Sable Systems International, Las Vegas, NV, USA) connected to a data acquisition system (PowerLab 26T, ADInstruments, Dunedin, New Zealand). Foot T_sk was collected via wireless temperature loggers (DS1922L iButton Thermochron, Measurement Systems Ltd, Newbury, UK). Following the protocol, foot temperature was exported from the wireless temperature loggers in 30‐s bins using a specialist logging software (iButtons, Measurement Systems Ltd). Data were then imported and analysed in Microsoft Excel software, reported as 2‐min averages. In addition, mean skin leg temperature (T¯Leg) was calculated as an unweighted average of quadriceps, hamstrings, calf and foot T_sk. Similarly, mean skin upper‐leg temperature (T¯Upper−Leg) was calculated as the unweighted average of quadriceps and hamstrings T_sk, and mean skin lower‐leg temperature (T¯Lower−Leg) was calculated as the unweighted average of calf and foot T_sk.

HR was monitored via a three‐lead electrocardiogram. Arterial blood pressure was measured non‐invasively using finger photoplethysmography (Finometer, Finapres Medical Systems). The monitoring cuff was placed around the middle finger of the right hand, with the forearm and hand supported so that the cuff was at the vertical level of the heart. Core temperature (T_c) was assessed using a commercially available rectal probe (RET‐1, Physitemp Instruments) inserted 15 cm past the sphincter muscle and connected to a thermocouple meter (TC‐2000, Sable Systems). Mean skin temperature (T_sk) from four sites (standard weightings of chest, arm, thigh, and calf, (Ramanathan, 1964)) was obtained using a wireless monitoring system (iButton^®, Maxim Integrated). Analog signals of the electrocardiogram, blood pressure waveform, and T_c were sampled at 1,000 Hz using a data acquisition unit (Powerlab 16/30, ADInstruments) and analyzed using an off‐line data analysis software (LabChart 8, ADInstruments).

Heart rate was obtained by telemetry (Polar Electro, Kempele, Finland). Arterial and femoral venous pressure waveforms were recorded using pressure transducers (Pressure Monitoring Kit, TruWave, Edwards Lifesciences, Germany) zeroed at the level of the right atrium in the mid‐axillary line (arterial) and at the level of the tip of the catheter (femoral venous). Pressure waveforms were amplified (BP amp, ADIstruments) and sampled at 1000 Hz using a data acquisition unit (Powerlab 16/30, ADInstruments, Oxfordshire, UK) for offline analysis. For measurements of femoral venous blood temperature (T_B), a thermistor (T204a, PhysiTemp, Clifton, NJ) was inserted through the femoral venous catheter and connected to a thermocouple meter (TC‐2000, Sable Systems, NV) and routed through the data acquisition system. In the control trial, esophageal temperature (T_Oes) was measured using a thermistor (Physitemp, New England), inserted pernasally into the esophagus at a depth of ¼ standing height. Increases in core temperature during cycling exercise reflect the rise in femoral venous blood temperature, as T_B and T_Oes have been shown to be within ~0.1°C (González‐Alonso et al. 1999). Mean skin temperature (T_sk) from four sites (standard weightings of chest, arm, thigh, and calf; (Ramanathan 1964) was obtained using a wireless monitoring system (iButton^®, Maxim Integrated, San José, CA).