It is well established that muscle temperature (Tm) has a significant effect on muscle function. (2,7,10,13,21,25). Much of the available literature suggests that events that are of short duration (<5 min) and are more heavily reliant on high levels of power production tend to benefit from increases in muscle temperature (2,17). For example, it has been demonstrated that there is a ∼4%-per-degree-centigrade improvement in instantaneous power output during a vertical jump as Tm increased (7). In addition, during short-term (<30 s) maximal sprint cycling, peak power output increases between 2% and 10% per degree centigrade as Tm was elevated (25). In contrast to short-term “sprint”-based activities, performance during endurance events may be impaired if the warm-up results in depleted glycogen stores (8,15) and an increase in thermoregulatory strain (12,19). Therefore, it appears that the use of extensive warm-ups may be of most benefit for short-duration activities that require high levels of short-term power output.
Muscle temperature in humans may be elevated in one of two ways. First, it can be achieved via an active warm-up, involving exercise that induces increases in metabolic activity and heat production within the body. Active warm-ups are commonly completed using a method that mimics the activity for which the warm-up is done, such as running, swimming, or cycling (9). After 15 min of active warm-up, Tm has been shown to increase by approximately 3°C–4°C at depth of 2–4 cm (9). Second, passive heating, which involves the use of an external heat source to add heat to the desired muscles, can be used to elicit a rise in Tm. This is commonly done using water immersion (16,25), warm air exposure (27), or heating pads (16). Gray et al. (16) have demonstrated that, with a combination of 30-min warm water immersion followed by passive heating using electric blankets, Tm can be elevated by approximately 3°C at a depth of 3 cm.
Although most athletes perform some sort of warm-up technique in preparation for their specific athletic event, it is common for athletes to experience delays between completion of the warm-up and the start of competition or where there are delays between repeated efforts/rounds of competition. These periods of time may be sufficient for Tm to drop below an optimal level that may have a detrimental effect on performance, possibly because of the temperature-related effect on power output. Currently, there is very little information about the exact effect a standardized sprint-based warm-up has on muscle temperature, and what the rate of decline in Tm is after warm-up cessation.
It is also logical to suggest that, if Tm can be artificially maintained after a warm-up by the use of insulative clothing or clothing incorporating a form of external heating, it may yield a performance benefit in subsequent activities that require high levels of power output. Therefore, the aim of this study was to examine how different ways of externally insulating the thigh muscles with either an insulated pant or a combined heated and insulated pant would influence Tm during a recovery period after a standardized warm-up procedure and, subsequently, whether power output during a 30-s maximal sprint test would be affected. To our knowledge, this is the first study to directly assess the effect of an extended delay between warm-up completion and sprint performance on muscle temperature, its effect on performance, and, in particular, the use of an applied method of attenuating any apparent Tm drop. It was hypothesized that added insulation, and moreover the combination of heating and insulation, would result in better maintenance of Tm after a warm-up and contribute to elevated power output in the subsequent trial.
METHODS
Participants
Eleven male competitive cyclists and triathletes (age = 24.7 ± 4.2 yr, height = 1.82 ± 0.72 m, body mass = 77.9 ± 9.8 kg; mean ± SD) volunteered to participate in this study. All participants performed at least three cycle-based training sessions per week. Participants completed a general health-screening questionnaire and were nonsmokers and free from injury. They were informed of the requirements of the study before giving written informed consent. A repeated-measures study design was used, with each participant tested in all three interventions in an order-balanced manner. All procedures were approved by the Loughborough University Ethical Advisory Committee.
Study Overview
Before the main experimental trials, participants were familiarized with the nature of the exercise testing and general measurement procedures. For this, participants completed at least four 30-s maximal sprint tests separated by approximately 10 min of passive recovery. Trials were continued until the time to peak power output and peak power output showed minimal deviation, with peak power values within 5%–10%, and time to peak power within 0.2 s. Participants visited the laboratory on three further occasions at the same time of day to minimize circadian variation. On each occasion, they completed a 15-min standardized cycle sprint-based warm-up followed by 30 min passive recovery. During the 30-min recovery period, participants underwent one of three experimental interventions (details below) before completing a 30-s maximal sprint performance test on a cycle ergometer. All trials were completed in a balanced order, separated by a minimum of 72 h. In the 24 h before each trial, participants were asked to refrain from caffeine and alcohol ingestion and any strenuous exercise. They were also asked to keep a record of their food intake and replicate this before the subsequent visits. Experiments were performed in an environmental chamber maintained at 15.9°C ± 0.3°C and 54.0% ± 4.0% relative humidity.
Experimental Protocol
Participants attended the laboratory after a minimum of a 4-h fast, during which time they were allowed to consume water only. On arrival, participants voided and had their height (Seca, Birmingham, United Kingdom) and body mass (ID1 Multi Range; Sartorius, Goettingen, Germany) recorded. Participants then inserted a rectal probe (Grant Instruments Ltd., United Kingdom) 10 cm beyond the anal sphincter to allow for monitoring of core temperature (Tc).
Participants entered the environmental chamber and remained seated for 30 min wearing a standard tracksuit to allow for stabilization of muscle and core temperature. After the stabilization period, baseline measures for thermal sensation, thermal comfort, core temperature (Tc), HR, blood lactate concentration, and muscle temperature (Tm) were taken. Participants then mounted an electronically braked cycle ergometer (Lode Excalibur Sport, Groningen, The Netherlands) and performed the standardized warm-up exercise that consisted of 5 min of cycling at an external power output of 100 W, followed by five 10-s maximal sprints, with each sprint separated by 1 min 50 s of cycling at 75 W. Participants were instructed to maintain a cadence of 85 rpm, until the start of each sprint. The maximal sprints were performed at a frictional load equivalent to 10% body mass, which has been shown to be optimal for well trained athletes (5,11). Immediately on completion of the warm-up exercise, participants dismounted from the ergometer and Tm was measured. In addition, measurements for Tc, HR, blood lactate concentration, and thermal sensation and comfort of the legs and the whole body were made. Participants then donned a standard tracksuit top (adidas Clima365; adidas AG, Germany) and one of the three types of pant that made up the intervention: 1) control (CONT) where participants wore commercially available tracksuit pants (adidas Clima365; adidas AG); 2) insulation only (INS), where participants wore a pair of insulated athletic pants; and 3) insulation plus heating (HEAT), where participants wore the same insulated pant with the addition of a heating element located around the thigh. The element was designed to cover the vastus medialis, rectus femoris, vastus latralis, biceps femoris, semitendinosus, and semimembranosus of both legs. There was no coverage directly over the adductor muscles to allow for some variation in leg size between subjects, but the heating element remained in close contact with both thighs in all participants. The heating element was capable of reaching temperatures of 40°C–42°C and was powered by a 14.8-V battery that generated 7.5 W to each heating pad. Once donned, the participant remained in the environmental chamber in a seated position for 30 min. During this period, measurements of thermal comfort, thermal sensation, HR, and Tc were made every 5 min. At the end of the 30-min recovery, the tracksuit top and pants were removed and a further Tm measurement was made, along with measures of thermal sensation, thermal comfort, core temperature, HR, and capillary blood lactate concentration. Each participant then remounted the cycle ergometer and performed a 30-s maximal sprint test with a frictional load equivalent to 10% body mass. The sprint test was preceded by a 15-s period of cycling against 75 W at a pedal cadence of 85 rpm. A further blood sample was taken within 20 s of completion of the sprint.
Measurements
Power output and pedal cadence
Power output and pedal frequency were recorded continuously (at a sampling rate of 10 Hz) throughout the warm-up and sprint tests (Wingate Test Plus Module; Lode). Peak, minimum and mean power output, time to peak power, peak pedal cadence, and time to peak pedal cadence were determined. Peak power was determined as the highest power output recorded at any individual time point within the 30-s maximal sprint test. Mean power was calculated using the average of all collected samples. Fatigue index was calculated as the change in power output divided by peak power multiplied by 100.
Muscle temperature
Muscle temperature of the vastus lateralis of the left leg was measured at depths of 3, 2, and 1 cm using a needle thermistor probe with a temperature measurement range of 15°–45° ± 0.1°C and response time of 0.3 s (MKA08050A275T; Ellab, Copenhagen, Denmark). The needle probe was inserted to an initial depth of 3 cm beyond the muscle fascia where the temperature was allowed to visually stabilize for 5 s at each depth before the probe was withdrawn to 2-cm and then 1-cm depths, with the temperature recorded at each depth.
HR, thermal sensation, and blood lactate concentration
HR was recorded using a wireless HR monitor (RS800; Polar, Kempele, Finland). Thermal sensation (1) and thermal comfort (14) were recorded. Whole blood and capillary blood samples (∼5 μL) were taken from the fingertip and analyzed immediately using an automated blood lactate analyzer (Lactate Pro; Arkray, Shiga, Japan). The use of which has been demonstrated to closely correlate with larger, laboratory constrained lactate analyzers (4,20).
Calculations
The temperature dependence (temperature coefficient, Q10) of both absolute and relative power output is presented as temperature coefficients (Q10), calculated as follows (6):
;)
where R2 and R1 represent rate processes for changes in power output, respectively, at temperatures T2 and T1, with T2 being greater than T1 after the warm-up. A Q10 value of > 1.0 indicates a positive thermal dependence, a Q10 = 1 indicates a thermal independence, and Q10 < 1.0 indicates a negative thermal dependence.
Clothing insulation
Insulation values for the heated pants for the legs and hips and whole body were determined using a thermal manikin (Newton; MTNW, Seattle, WA) with a uniform skin temperature of 34°C and environmental temperature of 21°C. For dry heat insulation, the following equation was used (18):
where αi = (surface area of segment i) / (total surface area of manikin); IT = total insulation of complete ensemble including enclosed and surface air layers (m2·K·W−1); Ta = ambient temperature;
= mean skin temperature; Hi = heat loss of segment i (W).
Statistics
Performance data were analyzed using repeated-measures ANOVA. Tm, thermal sensation, thermal comfort, Tc, and HR data were analyzed using two-way repeated-measures ANOVA (condition × time). Where significant effects were identified, post hoc pairwise comparisons with a Bonferroni correction were conducted. The accepted level of significance was P < 0.05. Data are presented as mean ± SD.
RESULTS
Clothing insulation
When the heating element was inactive, the insulation value of the insulated pants alone was 0.438 m2·K·W−1 (2.8 clo), with whole body insulation, composed of the tracksuit top and insulated pants being 0.241 m2·K·W−1 (1.6 clo). When the heating element was activated, insulation of the legs and hips increased by 0.378 m2·K·W−1 (2.4 clo) to give a total insulation for the pants of 0.815 m2·K·W−1 (5.3 clo), with whole body insulation increasing to 0.263 m2·K·W−1 (1.7 clo). Effective insulation values for the heated pants will be even higher than those reported here (18). As the heating input from the pants reduces the manikin heat loss to zero, and even warms up the manikin, the measured values underestimate the full effect of the heating. Nevertheless, the observed values provide an indication of the lowest estimate of the effect size of the added heating.
Muscle temperature
There were no differences in Tm at any depths before the warm-up in any condition. Immediately after the completion of the warm-up, Tm increased at all depths and was the same between conditions (Fig. 1). After the recovery period, Tm declined in all conditions; however, it remained higher during HEAT at all depths compared with that during CONT and INS (P < 0.0005).
FIGURE 1: Muscle temperature at 3, 2, and 1 cm depth measured before the warm-up (0WUP), immediately after the warm-up (15WUP), and after 30 min seated recovery (30REC) in the control (CONT), insulation only (INS), and insulation + heating (HEAT). *P < 0.0005, HEAT > CONT. †P < 0.0005, HEAT > INS.
The difference in Tm at the three depths between conditions after the recovery period is further highlighted in Figure 2, which shows the Tm gradient for all three conditions at 30REC.
FIGURE 2: Muscle temperature measured at 3, 2, and 1 cm depth after 30 min of seated recovery (30REC) in the control (CONT), insulation only (INS), and insulation + heating (HEAT). **P < 0.005, HEAT > CONT. ††P < 0.01, HEAT > INS. *P < 0.0005, HEAT > CONT. †P< 0.01, HEAT > INS.
Power output and pedal cadence
Absolute and power output per kilogram of bodyweight (relative peak power output, W·kg−1) were higher in HEAT compared to that in CONT (P < 0.05) by 9.6% and 9.1%, respectively (Table 1), with 9 of the 11 participants showing a clear improvement in peak power output in the HEAT condition and the remaining 2 participants showing no difference in peak power output compared to that in the CONT condition. For INS versus CONT, 2 of the 11 participants showed slight improvements in power output and the remaining 9 showed no improvement or lower peak power. Absolute peak power output had a strong positive thermal dependence (Q10 = 2.4 at 1 cm, 2.5 at 2 and 3 cm) as did relative peak power output (Q10 = 2.2 at all depths). There were no significant differences in absolute or relative peak power output between HEAT and INS or between INS and CONT (Table 1). There was no effect of condition on mean power output, minimum power output, time to peak power output, peak cadence, or time to peak cadence (Table 1). There was an overall effect of condition on fatigue index (P < 0.05), but there were no significant differences between individual conditions (Table 1).
TABLE 1: Peak power output (PPO), relative peak power output (rPPO), mean power output (Mean PO), minimum power output (Min PO), peak cadence (Peak RPM), time to peak cadence and fatigue index for control (CONT), insulation only (INS), and insulation + heating (HEAT).
Blood lactate concentration
After the warm-up, blood lactate concentration increased (P < 0.05) and was the same in all conditions. After the recovery period, blood lactate concentration declined in all conditions but remained higher (P < 0.01) in INS compared to that in HEAT (Fig. 3). After the maximal sprint test, blood lactate concentration increased (P < 0.05) in all conditions and was higher in HEAT compared to that in CONT (P < 0.05). Moreover, the change in blood lactate concentration during the sprint test was greater (P < 0.05) in HEAT (Δ = 6.3 ± 1.8 mmol·L−1) compared to that in CONT (Δ = 4.1 ± 1.9 mmol·L−1).
FIGURE 3: Blood lactate concentrations at baseline (0WUP), after warm-up (15WUP), after passive recovery (30REC), and immediately after the maximal sprint test (POST WIN) for control (CONT), insulation only (INS), and insulation + heating (HEAT). *P < 0.01, INS > HEAT. **P < 0.05, HEAT > CONT.
HR and rectal temperature
There was no effect of condition on either HR or core temperature during the trials (Table 2).
TABLE 2: Core temperature (T c), HRmax after warm-up (15WUP), after 30 min passive recovery (30REC), and immediately after the maximal sprint test (POST WIN) for control (CONT), insulation only (INS), and insulation + heating (HEAT) conditions.
Thermal sensation and comfort
Thermal sensation of the legs was higher toward “warm” in the legs for HEAT compared to that for CONT at the following time points: 10REC (P < 0.05), 15REC (P < 0.005), 20REC (P < 0.001), 25REC (P < 0.05), and 30REC (P < 0.0005). There were also elevations in thermal sensation for INS versus CONT at 20REC (P < 0.001) and for INS versus HEAT at 25REC (P < 0.05). Thermal comfort for the whole body or legs between conditions was the same at all time points (Fig. 4).
FIGURE 4: Subjective measures of (A) thermal sensation of the legs and (B) whole body thermal comfort for control (CONT), insulation only (INS), and insulation + heating (HEAT). *P < 0.005, HEAT > CONT. **P < 0.05, INS > CONT. †P < 0.05, HEAT > INS.
DISCUSSION
This study has demonstrated that the use of an insulated garment combined with internal heating elements around the thigh attenuated the decline in muscle temperature that occurs after a sprint-specific cycling warm-up, whereas an insulated garment alone did not. The heated garment resulted in a muscle temperature that was approximately 1°C higher at a depth of 1 cm and 0.4°C higher at 3 cm 30 min after the warm-up compared to when no additional insulation or insulation only was provided. This was associated with a greater peak power output (∼9%) during a 30-s maximal sprint when muscle temperature was maintained with HEAT compared to CONT. The use of frictional load equivalent to 10% of body mass during the 30-s sprint test has been shown to be optimal for well trained athletes (5,11), thus leading to an increased sensitivity of the test to detect changes in overall performance. Furthermore, the completion of several familiarization sprint tests will have reduced any practice effects gained from repetition of the tests. Taken together, this allows us to be confident that the results presented here are a true reflection of performance changes that would be mirrored in actual sprint competition. In support of this, it has been shown that a 30-s maximal sprint test on a cycle ergometer is moderately correlated to field-based measures of sprint performance (3), suggesting that the performance benefit reported in the present study will translate to improved field-based sprint performance.
It is acknowledged that the Tm probe used in the present study was unable to measure Tm at all depth simultaneously. The time it took to take measurements at all three depths (∼15 s) is unlikely to significantly affect the Tm measure recorded. It is possible that the external heating procedure used here could be applied in situations where athletes experience delays between warm-up and competition or where there are repeated efforts/rounds separated by periods of low to moderate activity, which are not sufficient to maintain Tm via metabolic heat production alone. Such scenarios are evident in many track and field athletics and track cycling competitions.
The present data demonstrate that there is a 9% improvement in peak power output per-degree-centigrade elevation in Tm when measured at depths of between 1 and 3 cm. This supports the notion that, at a depth of between 1 and 4 cm with every 1°C rise in muscle temperature, there is an approximate 4%–10% improvement in peak power output (7,25), although this is dependent on pedal cadence (25). The relationship between muscle temperature and skeletal muscle function has been well documented (7,10,22,23,25), with increasing muscle temperature affecting the force–velocity relationship in both fast- and slow-twitch muscle fibers (10,23). This interaction causes a shift in the force–velocity curve, resulting in increased peak power output with a temperature coefficient (Q10) of 2–2.5 (24). This is similar to the Q10 reported here (Q10 2.4), indicating that the in vivo thermal dependence of peak power output mirrors that documented in single-fiber and animal models.
The increase in power output after HEAT is probably because of an elevated rate of ATP turnover as a consequence of the higher muscle temperature (16). This is reflected by the higher concentration of blood lactate after the maximal sprint in the HEAT condition. However, blood lactate concentration is dependent on many other factors including its rate of appearance and disappearance. Nevertheless, direct measurements of ATP turnover have shown ATP turnover to be elevated with increased muscle temperature. Gray et al. (16) have shown that, when muscle temperature is artificially elevated using water immersion of the legs, there is an increase in anaerobic ATP turnover during a 6-s maximal sprint cycling, as derived from direct measurement of ATP turnover via muscle biopsy analysis. Furthermore, in partial agreement with the present results, they report an increase in both mean and maximal power output and peak pedal cadence after passive heating. During short-term maximal exercise, anaerobic metabolic pathways account for approximately 70%–80% of ATP turnover (28,29). Therefore, it would be expected that there was a higher concentration of blood lactate after the maximal sprint test when power output is elevated, as demonstrated here. However, it is possible that the differences in blood lactate concentration reported in the present study represent an increased peripheral vasodilatation in the HEAT condition resulting in faster blood lactate appearance and may not necessarily directly indicate a greater anaerobic component of the 30-s sprint test, which can only reliably be gained from direct measures of muscle metabolites.
It can be seen as positive that the only significant difference in perceptual measures reported was a change in thermal sensation of the legs. These differences are likely due to the increased insulation and Tsk in the HEAT condition. Had there been associated negative changes in thermal comfort as a result of the heating protocol, it is possible that performance would have been adversely affected. Despite the inherent difficulties in isolating thermal comfort from other physiological and thermal perceptions and their effects on exercise performance, Schlader et al. (26) have demonstrated that elevations in preexercise thermal discomfort were associated with impaired cycling performance. Importantly, in the present study, there were no differences in ratings of overall thermal comfort or thermal sensation, which indicates that the degree of heating was not perceived as more uncomfortable than when no heat was applied and is therefore less likely to have a negative effect on performance.
CONCLUSIONS
This study has demonstrated that external heating of the quadriceps and hamstrings during periods of inactivity between exercise bouts can attenuate the reduction in Tm. Importantly, this leads to a significant and relevant improvement in peak power output compared to CONT and INS after 30 min of inactivity. This may have important practical implications for power athletes who may experience delays in their competition performance after the completion of a warm-up or experience muscle cooling because of the intermittent nature of their sport or competition.
The research presented was co-funded by the Adidas Innovation Team, Germany, and the Environmental Ergonomics Research Centre, Loughborough University. The authors were fully responsible for the conduct of the trial and the data.
The authors would like to acknowledge the continued support from the Adidas Innovation Team during this study, with special thanks to Dr. Berthold Krabbe. Further thanks must also go to Bryan Roberts and the Sports Technology Institute at Loughborough University for the loan of equipment.
The authors declare that there are no conflicts of interest. The results from the present study do not constitute endorsement by the American College of Sports Medicine.
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