Hot and humid conditions compound the physiological strain of increased metabolic heat production associated with exercise while reducing avenues for heat dissipation to the surrounding environment (30). Accordingly, both physiological and behavioral responses control the rise in thermoregulatory load, often to the detriment of exercise performance (39). These noted responses may be particularly pertinent for team-sport athletes competing in warm environments, owing to increased thermal loads associated with intermittent-sprint activity compared to continuous modes at matched intensities (5,16). Given the observed exacerbated loads and reduced performances in the heat, the popularity of precooling has increased as team-sport athletes seek to counter the reduction of exercise performance in the heat.
A large quantity of laboratory-based research supports the efficacy of precooling for endurance exercise performance in the heat (11,26,33). Regardless of traditionally equivocal findings (11,26,33), more recent studies of prolonged intermittent-sprint exercise after precooling demonstrate ergogenic benefits (8), particularly in the maintenance of self-paced submaximal work (12,14). Typically, whole-body methods using cold microclimates, specifically water immersion or cold air, can result in improved exercise performance and/or reductions in thermoregulatory strain (11,26,33). Despite wide support (11), whole-body cold water immersion may provide environmental and logistical concerns surrounding their field-based application, resulting in problematic (water access) and/or impractical (cooling a full team) implementation (26). Recently, the dosage effect demonstrated when comparing whole- versus part-body cooling methods (8,9,12) has provoked interest in combining multiple part-body techniques, thus maintaining surface area coverage and enhancing the practicality of precooling (14,34). However, the explicit effect of the volume of the imposed cooling stimulus necessary for optimal physiological, metabolic, and performance outcomes remains equivocal.
Augmented heat storage capacity after precooling aids in the suppression of increased thermoregulatory load associated with exercise in the heat (26). Although related reductions in HR along with skin and/or core temperature may indicate the maintenance of central blood volume (19), the mechanisms of precooling may relate to the role of higher central regulation (31). Elevated core temperature impairs CNS motor drive, consequently reducing neuromuscular recruitment, force output, and voluntary activation (VA) (21,28,31,37). Hence, blunting the rise in core temperature by precooling may lead to better pacing during self-paced exercise (13,21,22). Further, the reduction in thermoregulatory and/or physiological load may also ease generic stress responses relating to alterations in metabolic processes (17) and elevated damage and inflammatory markers after exercise in the heat (2). As such, the cause-effect relationship and underlying mechanisms of the imposed precooling stimulus requires further attention to provide insight as to the mechanisms on neuromuscular function, which might improve exercise in the heat and also allow for the development of ecologically valid, evidence-based cooling techniques.
Although the rationale for precooling athletes for the protection of acute exercise performance in the heat is accepted (11,26,33), practical limitations may restrict the application of whole-body immersion techniques in the field. Mixed-method approaches to precooling demonstrate advantageous effects on exercise performance, physiological loads, and perceptual state (14,34). However, a lack of data exists to demonstrate the optimal volume of cooling stimulus necessary for ergogenic benefit. Moreover, apparent influences of precooling on CNS activity and subsequent self-selected work output require further investigation. Therefore, the purpose of this study was to determine the effects of precooling volume on free-paced intermittent-sprint performance and physiological responses in heat stress. A complementary aim was to determine the effects of precooling on voluntary force and evoked twitch properties and their relationship to exercise performance in the heat.
Ten, well-trained, male team-sport athletes (mean ± SD: age = 20.9 ± 2.6 yr, height = 182.1 ± 8.8 cm, body mass = 77.8 ± 6.7 kg, body surface area = 1.98 ± 0.12 m2) volunteered to participate in this study. Participants regularly competed in regional-level team-sport competitions (cricket, rugby union) and reported three or more training days per week including sports-specific skill-based strength and conditioning sessions. After disclosure of all risks and benefits, all participants provided verbal and written consent before the commencement of all testing procedures. Experimentation was approved by the ethics in human research committee of the university.
A randomized, repeated-measures crossover design was used to determine the effects of precooling volume on performance, physiological, biochemical, and perceptual responses to an intermittent-sprint exercise protocol. The study was conducted as part of a series of cricket-related studies, and hence, the exercise protocol was based on fast-bowling-specific intermittent-sprint exercise (32) but was adapted to increase the volume of work required to allow reporting of a more generic protocol for team-sport activity as previously suggested (12). An initial equipment and procedural familiarization session was performed before commencing data collection. This included completion of the entire exercise protocol in hot conditions and application of neuromuscular assessments and cooling techniques. Testing sessions were standardized for each participant and separated by 5-7 d to allow full recovery. Physical activity, diet, and fluid intake were all documented in food and physical activity diaries in the 24 h before the first testing session and replicated for the following sessions. Participants were educated on keeping dietary and workload records, with standardization compliance qualitatively assessed before commencement of each session. Sessions were conducted on an enclosed 20-m synthetic running track in hot conditions (33.0°C ± 0.7°C and 33.3% ± 3.9% relative humidity). Environmental temperatures were controlled by a customized gas heating system and four electronic 2000-W room heaters (Kambrook, Port Melbourne, Australia) positioned at 5-m increments alongside the running track. All testing sessions were identical so that precooling volume was the only manipulated variable throughout. Participants performed five sessions including a familiarization session, control session (no precooling), head cooling session (precooling with an iced towel), head and hand cooling session (precooling with an iced towel and container of cold water), and a mixed-method whole-body session (precooling with iced towel, container of cold water, ice vest, and ice packs applied to the quadriceps). Participants were required to abstain from strenuous exercise and alcohol 24 h before and all caffeine and food substances 3 h before each testing session. A standardized volume of water (500 mL) was consumed 1 h before exercise to regulate hydration status.
Participants performed an intermittent-sprint running protocol comprising of 2 × 35-min spells of exercise (spells 1 and 2) separated by a 15-min recovery period on the enclosed 20-m running track. Before commencement, a 5-min warm-up period was completed in hot conditions involving 20-m shuttle running with increments in running speed each minute and six repeated 15-m maximal sprints. The 2 × 35-min exercise protocol consisted of 10 (2 × 5) bouts of 6 × 15-m maximal sprint efforts separated by 5-min bouts of self-paced activity at different intensities, designed to incorporate the movement requirements for cricket fast bowlers (32). Participants performed a set of 6 × 15-m sprints commencing at 30-s intervals, emulating a six-ball cricket over. During the 5-min interval between sets of sprints, participants performed 1-min periods of self-paced, submaximal exercise, including walking, jogging, and hard running (12). Participants were informed of the required intensity on a minute-by-minute basis. Self-paced submaximal activity was performed on the 15-m running track in a shuttle run fashion, and participants were requested to return to the starting position at 50 s of each self-paced minute to then commence the subsequent intensity. Each data collection session involved 10 sets of six sprints and eight periods of 5-min self-paced submaximal running bouts, thus incorporating the demands of 2 × 5 over spells of fast bowling. Participants were offered verbal support and encouragement throughout to cover the greatest distance during the hard running bouts, jog, or walk at a self-selected pace during the respective jogging and walking bouts. All fluid consumption was restricted throughout the exercise protocol. Previously collected but unpublished reliability data demonstrate that the intraclass correlation (ICC) of mean sprint times, self-paced distances, and hard running distances covered was r = 0.94-0.98, whereas the technical error of measurement was 0.5%-2% and coefficient of variation was 0.6%-2%.
After all resting measures, a precooling intervention was performed for 20 min before exercise and for 5 min during the 15-min midsession recovery period. To induce a volume effect, a stepwise approach to part-body cooling was used, with precooling sessions involving no cooling (control), head cooling (H), head + hand cooling (HH), or mixed-method whole-body cooling (WB). During the H cooling session, participants were cooled using an iced towel soaked in water (5.0°C ± 0.5°C) before being placed over the head and neck. HH cooling was achieved using an iced towel (5.0°C ± 0.5°C) covering the head and neck, while each hand immersed up to the wrist in separate containers of cold water was actively maintained at 9.0°C ± 0.5°C. During the WB cooling session, participants were cooled with an iced towel over the head and neck, hands were immersed to the wrist in cold water, an ice vest covered the torso (Arctic Heat, Brisbane, Australia), and frozen ice packs were applied to the quadriceps (Techni Ice, Frankston, Australia). Ice vests and icepacks were stored at −20°C before and after use. The mixed-method approach was selected on the basis of the practical ease and portability of equipment compared with cold water immersion (13). No cooling stimulus was applied during the control condition (CONT). Standardized warm-up procedures commenced immediately after neuromuscular assessment, approximately 5 min after intervention completion. Reapplication of cooling methods during the midexercise recovery period occurred within approximately 10 min to accommodate the collection of neuromuscular and biochemical measures. All treatments were performed in a seated position within controlled laboratory conditions of 33°C and 33% relative humidity. Percentage surface area volumes covered during precooling were estimated at 10%, 15%, and 35% for H, HH, and WB trials, respectively (23).
Exercise performance was determined by 15-m sprint time and distance covered throughout submaximal exercise bouts. An infrared timing system (Speed-Light, Swift, Australia) was used to record sprint times, whereas percent decrement was calculated according to Dawson et al. (10). Incremental 1-m markings along the 15-m synthetic running track allowed for calculation of distances covered during each individual submaximal exercise bout. Participants wore personal athletic training attire (T-shirt, shorts, socks, and running shoes) that were standardized throughout. Data were reported as mean or total for individual exercise modes (walk, jog, hard run).
Measures of voluntary force and evoked twitch properties of the right knee extensors were assessed before intervention, after intervention, during exercise, and after exercise using an isokinetic dynamometer (Kin-Com, Model 125; Chattanooga Group, Inc., Hixon, TN) connected to a host computer and customized software (v8.0, LabVIEW; National Instruments, North Ryde, NSW, Australia). Participants were seated in an upright position, with the axis of rotation of the dynamometer visually aligned with the lateral femoral epicondyle and the lower leg attached to the lever arm 1 cm above lateral malleolus on the right leg. Seating posture was standardized with the knee and hip positioned at 90° flexion (0° represents full extension) and securely fastened to the dynamometer using conventional waist and shoulder straps. Supramaximal transcutaneous electrical stimulation of the femoral nerve was administered via a reusable gel adhesive electrode (diameter = 10 mm; MEDI-TRACE™ Mini 100 Pediatric Foam Electrodes; Covidien, Mansfield, MA) located on the anterior thigh 3 cm below the inguinal fold. An additional reusable gel adhesive electrode (90 × 50 mm; Verity Medical, Ltd., Stockbridge, Hampshire, England, UK) positioned on the medioposterior aspect of the upper thigh below the gluteal fold acted as the anode. A single square-wave pulse with a width of 200 μs (400 V with a current of 100-450 mA) was delivered by a Digitimer DS7 stimulator (Digitimer, Ltd., Welwyn Garden City, Hertfordshire, UK) connected to a BNC2100 terminal block and signal acquisition system (PXI1024; National Instruments, Austin, TX). Peak twitch force was detected with incremental increases in stimulus intensity, and final levels were amplified by 10% to ensure attainment of supramaximal stimulation. Five pulses separated by 20 s were delivered in a rested state to assess resting evoked twitch properties. Subsequently, participants completed a maximal voluntary contraction (MVC) protocol involving 5 × 5-s isometric trials using a work-to-rest ratio of 1:6. The MVC was defined as the mean peak torque value attained during voluntary contractions. A superimposed twitch was manually triggered during each MVC within 1-2 s after commencement to coincide with an observed plateau in peak torque. An additional stimulus was delivered to the resting muscle immediately after contraction to calculate potentiated twitch properties. VA was determined using the twitch interpolation technique (1). Time-to-peak torque (TPt) was calculated using the time from evoke force onset to peak potentiated twitch torque. Analyses of neuromuscular data were performed using MATLAB software (R2009b 188.8.131.529; The MathWorks Inc., Natick, MA).
Preexercise and postexercise towel-dried measures of nude body mass were recorded using calibrated scales (HW 150 K; A&D, Adelaide, Australia) to estimate sweat loss. A midstream urine sample was collected before exercise to assess urine specific gravity (Refractometer 503; Nippon Optical Works, Co., Tokyo, Japan). HR was measured using a chest transmitter and wristwatch receiver (FS1; Polar Electro Oy, Kempele, Finland) at 5-min intervals during respective intervention and exercise protocols. Core temperature (Tc) was measured using a telemetric capsule (VitalSense; Mini Mitter, Bend, OR), ingested at a standardized 5-h preexercise to ensure passing into the gastrointestinal tract. The Tc was measured before intervention and at 5-min intervals throughout the cooling intervention and exercise protocol, respectively. For a detailed discussion of the reliability (ICC = 0.99) and validity (r = 0.98) of ingestible Tc capsules, see Gant et al. (18). Skin temperatures (Tsk) were assessed at the sternum, midforearm, midquadriceps, and medial calf by an infrared thermometer (ThermoScan 3000; Braun, Kronberg, Germany) at 5-min intervals during the precooling intervention, midprotocol break, and after exercise. This technique has been reported to be a reliable (ICC = 0.96) and valid (r = 0.92) measure of Tsk (4). Mean Tsk was calculated according to Ramanathan (35).
Blood collection and biochemical analysis.
A 100-μL preexercise sample of capillary blood was collected from a hyperemic earlobe to measure pH, glucose, lactate [La−], and bicarbonate (HCO3) (ABL825 Radiometer, Copenhagen, Denmark). Additional capillary blood samples were collected midexercise and immediately after exercise. To determine the effects of precooling on muscle damage, inflammation, and generic stress responses, venous blood was collected from an antecubital vein before and 30 min after exercise and was analyzed for creatine kinase (CK), C-reactive protein (CRP), testosterone (TEST), cortisol (CORT), and insulin (INS). Using an evacuated venipuncture system and serum separator tubes (Monovette; Sarstedt, Numbrecht, Germany), samples were allowed to clot at room temperature before centrifugation for 10 min at 4000 rpm. Supernatant was then extracted and stored at −20°C until analysis. Before analysis, the serum was allowed to reach room temperature and mixed via gentle inversion. CK and CRP were analyzed according to the manufacturer's instructions provided in the respective assay kits (Dimension Xpand spectrophotometer; Dade Behring, Atlanta, GA). CK concentrations were determined using an enzymatic method and bichromatic rate technique. CRP samples were manually diluted according to the manufacturer's instructions and analyzed with the particle-enhanced turbidimetric immunoassay technique. INS, TEST, and CORT levels were detected using a solid-phase, competitive chemiluminescent enzyme immunoassay (Immulite 2000; Diagnostic Products Corp., Los Angeles, CA). To avoid interassay variations, all samples for each subject were analyzed in the same assay run. Intra-assay coefficients of variance were <5% for all venous blood analyses. Serum hormone concentrations were not corrected for plasma volume shifts; thus, all statistical analyses were performed on hormone values on the basis of actual measured circulating concentrations.
RPE and thermal sensation scale (TSS) were recorded at 5-min intervals throughout precooling and exercise protocols. RPE was determined according to the Borg CR-10 scale, where ranking ranged from 0 (nothing at all) to 10 (maximal). TSS was assessed using an eight-point Likert scale, ranging from 0 (unbearably cold) to 8 (unbearably hot).
Data are reported as mean ± SD. A repeated-measures ANOVA was performed to detect within treatment differences. Unprotected pairwise comparisons (protected Fisher LSD) were applied to determine the source of significance, which was accepted when P ≤ 0.05. Analysis was performed using the Statistical Package for Social Sciences (SPSS v 16.0, Chicago, IL). Standardized effect sizes (ES; Cohen d) analyses were used in interpreting the magnitude of differences between conditions. An ES was classified as trivial (<0.20), small (0.20-0.49), moderate (0.50-0.79), or large (>0.80).
Mean and peak ± SD speed and percent decline in speed for spells 1 and 2, respectively, are presented in Table 1, whereas mean ± SD individual sprint times are presented in Figure 1A. No significant differences were detected between cooling procedures for mean sprint times or peak sprint speeds during spell 1 or 2, respectively (P = 0.08-0.91). Large ES indicated faster mean sprint times with WB (d = 0.94) and HH cooling (d = 1.07) compared with CONT during spell 2. The percent decline during spell 2 was attenuated with WB cooling compared with control (P = 0.04). Large ES were apparent for a smaller percent decline during spell 1 in H (d = 0.92), HH (d = 1.26), and WB (d = 0.87) compared with CONT conditions.
Results for mean and total distance covered during submaximal exercise bouts including hard running, jogging, and walking are presented in Table 1. Mean ± SD individual self-paced hard running efforts are presented in Figure 1B. Overall, total distances accumulated were significantly higher with WB (4833 ± 380 m) and HH cooling (4644 ± 360 m) compared with H (4602 ± 448 m) and CONT conditions (4413 ± 545 m), respectively (P = 0.001-0.04, d = 0.70-1.26). No significant differences and moderate ES continued a dose-response effect between the remaining total distance comparisons (P = 0.06-0.12, d = 0.53-0.72). Significant differences and large ES data indicated greater mean and total distances covered in the WB condition for hard running compared with CONT (P = 0.001, d = 1.49), H (P = 0.001, d = 0.86), and HH cooling (P = 0.02, d = 0.83) conditions. There were also significant increases in hard running distances in a dose-response fashion after H (P = 0.02, d = 0.62) and HH cooling (P = 0.001, d = 0.81) compared with the CONT. Moreover, a significantly increased mean and total hard running completed in spell 1 was observed with HH cooling data compared with CONT (P = 0.01). Similarly, spell 2 H (P = 0.04) and HH sessions (P = 0.00, d = 0.87) resulted in significantly higher mean and total hard running work completed compared with CONT although still less than WB cooling.
There were significant increases in mean and total jogging distances covered during spells 1 and 2 in WB (P = 0.03 and 0.02) and HH sessions (P = 0.01 and 0.01) compared with CONT. Greater mean and total values for jogging distances covered were evident between WB and H cooling sessions during spell 1 (P = 0.02). No other significant differences were noted among conditions for all remaining mean and total distances for jogging or walking variables during spell 1 and 2 (P = 0.08-0.88).
Significant reductions and large ES were apparent for Tc (Fig. 2A) at the end of the intervention period for WB (P = 0.03, d = 1.62) and HH cooling sessions (P = 0.04, d = 1.37) compared with H. Similarly, Tc was reduced (Fig. 2A) at the end of the intervention period for WB (P = 0.003, d = 1.72) and HH cooling sessions (P = 0.04, d = 1.46) compared with CONT. This trend was maintained throughout, with large ES (d = 0.82-1.41) indicative of a reduced Tc after WB cooling. Significant reductions in Tc with HH cooling compared with CONT were also noted during the protocol (P = 0.01, d = 0.85). In comparison with the WB method, Tc response for H cooling and CONT was elevated during the exercise protocol (d = 0.85-1.41). Finally, HH cooling did not differ from the H cooling condition, rather it was increased compared with the WB cooling specifically between the 10th and 35th minute (d = 0.81-1.19) and between the 80th and 85th minute (d = 0.81-0.85).
Significant differences and large ES indicated a lower Tsk throughout the WB intervention period (P = 0.001, d = 2.54-4.62) compared with all other conditions (Fig. 2B). Large reductions in Tsk were also apparent for H (d = 1.00-1.85) and HH cooling sessions (d = 0.99-1.74) during the intervention period compared with CONT. During the exercise protocol, WB cooling resulted in a reduced Tsk compared with all other conditions (P = 0.001-0.03, d = 1.02-5.71); however, Tsk did not differ between any other conditions.
Significant differences and large ES data indicated a lower HR after intervention within WB (P = 0.03, d = 1.83) and HH cooling sessions (P = 0.02, d = 1.75) compared with CONT conditions (Fig. 2C). Moreover, reduced HR values with WB (d = 1.07) and HH cooling (d = 1.01) over H cooling interventions were evident after intervention. HR responses after WB cooling were significantly reduced (P = 0.05, d = 1.07) compared with CONT during the exercise protocol. Large ES indicated reduced HR values after WB cooling compared with H (d = 1.25) and HH cooling (d = 1.34) during the exercise protocol.
Preexercise urine specific gravity was not significantly different between WB cooling (1.018 ± 0.006), HH cooling (1.017 ± 0.005), H (1.017 ± 0.008), or CONT conditions (1.017 ± 0.008, P = 0.70-0.84, d = 0.06-0.22). Body mass changes from nonurine fluid loss after exercise were significantly less with WB cooling compared with CONT (1.8 ± 0.2 vs 2.2 ± 0.4, P = 0.01, d = 1.58). Large ES data indicated less sweat losses from changes in body mass with WB cooling compared with HH (2.0 ± 0.4, d = 0.82) and H cooling (2.1 ± 0.5, d = 1.00), respectively.
No significant differences and trivial to moderate ES (Table 2; P = 0.07-0.87, d = 0.03-0.68) were observed between respective cooling conditions for mean MVC at all time points. Mean MVC was significantly reduced from before to after exercise in H cooling (P = 0.04, d = 1.09) and CONT conditions (P = 0.01, d = 1.46). Pre- to postexercise differences in mean MVC were not significantly different, with trivial to small ES data after WB and HH cooling (P = 0.432-0.925, d = 0.04-0.35). No significant differences were evident between respective cooling volumes for postintervention TPt (Table 2; P = 0.52-0.92). However, large ES were evident for faster TPt during exercise with faster responses evident in the WB cooling compared with H (d = 1.13) and CONT sessions (d = 1.07). Changes in pre- to postexercise TPt were not significant, although moderate ES were detected for reduced TPt in CONT (P = 0.39, d = 0.79), H (P = 0.34, d = 0.74), and HH (P = 0.47, d = 0.65). Conversely, a large ES depicts slower TPt from before to after exercise for WB (P = 0.11, d = 1.16). No significant differences and trivial to moderate ES (Table 2; d = 0.03-0.45) were detected between VA at all time points. Finally, no significant change, although moderate to large ES were detected before to after exercise for reduced VA (P = 0.06-0.47, d = 0.72-2.03).
No significant differences (P = 0.06-1.00) were evident between conditions in pH, glucose, [La−], or HCO3 markers before or after exercise (Fig. 3). Significant differences and large ES indicated reduced [La−] concentrations midexercise with WB cooling compared with CONT (P = 0.02, d = 1.64). Further, lower [La−] values were evident midexercise with WB compared with H cooling conditions (d = 1.36). This trend is continued postexercise, with [La−] largely reduced following all precooling compared to CONT (d = 0.95-1.37). Large ES indicated reduced glucose concentrations after exercise between HH cooling with H cooling (d = 0.82) and CONT sessions (d = 1.03). No significant differences and trivial to moderate trends (P = 0.25-1.00, d = 0.00-0.59) were determined for all preexercise CK, CRP, TEST, INS, and CORT concentrations (Table 3). Large ES indicated reduced postexercise CK measures with WB (d = 0.82) and H cooling (d = 0.83) compared with CONT conditions. Large ES data were detected for CORT levels within the WB cooling condition and were increased compared with CONT (d = 1.06) after exercise. No significant change and trivial to moderate ES were detected for all remaining postexercise venous blood variables of CK, CRP, TEST, INS, and CORT (P = 0.07-0.99, d = 0.00-0.58).
Significantly reduced mean RPE values were evident for WB (4.2 ± 0.8, P = 0.03, d = 1.17), HH (4.6 ± 0.7, P = 0.04), and H cooling conditions (4.6 ± 0.8, P = 0.02) compared with CONT condition (4.9 ± 1.0). Significant differences and large ES data indicate reduced mean TSS with WB cooling (5.2 ± 0.5) compared with HH (5.7 ± 0.4, P = 0.00, d = 1.69), H (5.8 ± 0.5, P = 0.01, d = 1.95) and CONT conditions (6.3 ± 0.6, P = 0.00, d = 2.93). Moreover, significantly reduced TSS values during HH (P = 0.01, d = 1.87) and H cooling (P = 0.00, d = 1.28) were evident compared with CONT sessions.
The aim of this investigation was to determine the effects of volume-dependent precooling on physiological and performance outcomes for self-paced intermittent-sprint exercise in the heat. In addition, we attempted to determine the related volume effect on neuromuscular function as related to intermittent-sprint exercise performance in the heat. The novel finding of this study is that performance appears to be affected by the dose of precooling, in that larger surface area coverage resulted in increased work capacity, with greater suppression of physiological load. Most salient was that precooling resulted in the maintenance of MVC, although there was increased work completed across the respective cooling interventions. These findings indicate that enhanced thermoregulatory control after precooling may negate the down-regulation of exercise performance in the heat and hence allows for the maintenance of skeletal muscle recruitment and work output.
Exercise and environmentally induced heat stress may severely impair exercise capacity through reduced time to fatigue and an inability to maintain desired intensities (20,31). Results from the current investigation are consistent with previous precooling studies of attenuated thermoregulatory strain and improved intermittent-sprint exercise performance in the heat (8,12,14). A common feature of the present study and previous work (12) is the precooling-induced improvements in self-paced, submaximal distances covered between maximal-sprint exercise bouts. The stepwise response to cooling resulted in the reduction in distance covered as the volume of cooling was reduced. However, with WB cooling, participants covered greater hard running and jogging distances in spells 1 and 2, respectively (Table 1 and Fig. 1B). Similarly, larger cooling application was associated with greater maintenance of maximal sprint times (Table 1 and Fig. 1A). Hence, the current study corroborates previous research, which shows performance benefits for free-paced intermittent-sprint exercise in the heat (8,12,14).
Unique to the current intermittent-sprint precooling study was the collection of neuromuscular data at set time points during the testing session rather than purely before and after exercise. The attenuated thermoregulatory loads observed with WB cooling maintained MVC during and after exercise (Table 2). These changes are apparent despite the absence of condition-specific differences in percent VA (Table 2). Exercise- and environmentally induced hyperthermia seemingly reduce skeletal muscle force output through the impairment of VA (28,31,36,37). However, an inverse relationship between muscle function and Tc is apparent as isometric MVC and percent VA returns to baseline after the resumption of normative Tc values (28,36). Hence, it is postulated that neuromuscular responses to exercise in the heat are directly linked to elevated thermal load in addition to any protocol-specific fatigue (28,36). Accordingly, the blunting of Tc during exercise and subsequent preservation of MVC and VA with more extensive cooling (Fig. 2A) may account for the relationship between cooling volumes and self-paced running workloads. That said, an inability to directly assess muscle function during the exercise protocol means factors relating to physiological strain or anticipatory regulation of CNS responses cannot be overlooked (13,27).
Given the lack of difference in VA between cooling volumes, the increased distances covered during self-paced exercise bouts may not be completely accounted for solely by peripheral or central factors. Rather, it is plausible that an interaction of afferent and efferent stimuli, combined with a reduced perceptual strain after precooling may augment muscle recruitment during exercise, which manifests as a sustained MVC during and after exercise, despite the increased workloads performed (13,38). Although EMG responses were not measured in the present study, a faster TPt during exercise was evident with WB cooling. The faster TPt may highlight the protective properties of precooling for exercise in the heat (11). Accordingly, precooling may improve the adopted pacing strategy during self-selected work in the heat (13,22), essentially because of a greater preservation of MVC via maintenance of muscle recruitment (21) and subsequent curbing in the down-regulation of exercise intensity in the heat (as noted in the control condition).
Although WB cooling is often reported as the gold standard of cooling interventions, part-body cooling techniques may also have favorable physiological and performance benefits (3,8). Results from the present study demonstrate cooling volume to seemingly influence both the physiological response and ensuing intermittent-sprint exercise performance. Maintenance of repeated sprint time was evident in all precooling conditions in spell 1, although this trend was only continued in the WB condition during spell 2 after the reapplication of cooling procedures (Table 1; Fig. 1A). Further, increased hard running and jogging distances covered during self-paced exercise bouts imply the presence of improved maintenance of exercise intensities (13,22) after WB in contrast to part-body or no cooling conditions (Table 1; Fig. 1B). Further, as with the maintenance of MVC, a stepwise interaction seemed evident, in that the use of HH cooling indicated some performance benefits over the CONT condition. Conversely, reduced surface area coverage with H cooling and CONT provided little or no performance improvement, resulting in either the slowest sprint performances or least distance covered. These findings support the existence of a dose-dependent response to cooling application and performance outcomes (8).
The attenuation of thermal load, through the application of precooling, and subsequent improvements in heat storage and dissipation capacities increases the period for optimal exercise performance before the attainment of critical Tc in laboratory settings (20). Accordingly, WB cooling was most effective in reducing Tc and Tsk before exercise and continuing to blunt the rise in body temperature for the duration of the exercise protocol. Increased cooling volumes may result in greater physiological perturbations or the prolonged suppression of increased physiological loads (9,12); hence, the volume effect of cooling observed in the present investigation is not surprising. Nevertheless, an attenuated thermoregulatory response during exercise in the heat may delay redistribution of cardiac output to the periphery and sweat responses for heat dissipation (6). Moreover, the maintenance of central blood volume protects against reduced stroke volume and associated cardiovascular strain (19). Precooling-induced reductions in cardiovascular strain are observed during set-paced exercise protocols (3,24), yet higher workloads during self-paced exercise seem to conceal this effect (12,14,22).
In contrast, marked reductions in HR were observed after WB cooling despite higher workloads completed. Given the 20- to 30-min time frame during which the effects of precooling remain evident (40), the length of spells and cooling reapplication midsession might overstate the volume effects in this case. However, limited cardiovascular drift may reflect improved blood volume status owing to decreased sweating demands (29). Restricted fluid intake and sweat loss alterations in nude body mass in the present study between 2.4% and 2.9% exceed levels contributing to heightened thermal strain and aerobic performance decrements (7). Nevertheless, differences in sweat rate (∼400 mL) between WB cooling and CONT conditions may not adequately explain HR variance or augmented shuttle-running performance (19). Thus, it is likely that multiple afferent and efferent indicators owing to reduced physiological (sweat loss, Tc, and HR) and perceptual loads (RPE and TSS) promote continued maintenance of exercise intensity, muscle recruitment, and voluntary force production (13,38).
In accordance with previous reports, analysis of blood markers of anaerobic metabolites and muscle damage demonstrated minimal differences between conditions (8,12,15,25). Given the well-documented alterations in metabolism associated with exercise in the heat (17) and the reduced physiological responses to exercise with precooling and/or augmented performance outcomes (13,26,34), the lack of differences in metabolic markers is not unexpected. Yet reduced [La−] during and after exercise with greater cooling volumes are inconsistent with elevated self-paced running distances and maintenance of maximal sprints speeds. Similarly, reduced CK concentrations after exercise in WB and HH cooling despite higher workloads may be a result of reduced levels of heat stress experienced during these conditions (2). Nevertheless, WB cooling seems to have stimulated sympathoadrenal activity and subsequently increased CORT secretion as detected after exercise. Although reduced sweat rates with more extensive precooling may have assisted in explaining reduced [La−] and CK concentrations, elevated CORT may be attributed to higher workloads completed or the larger cooling stimulus. As such, despite the reduced physiological strain evident during the (larger volume) cooling conditions, the increased work performed seems to still invoke greater generic stress responses.
In conclusion, this investigation highlights the dose-response relationship evident between precooling volume and ensuing physiological, perceptual, and performance outcomes in hot conditions (8,9,12). Improved performance responses may result from the maintenance of exercise intensities during self-paced exercise bouts and potentially relate to the maintenance of MVC. Moreover, despite WB methods proving most effective, part-body techniques also offer a blunted thermoregulatory response, albeit to a lesser extent. These responses may owe to precooling-induced suppression of thermoregulatory responses to exercise in the heat, allowing for enhanced physiological control and reduced perceptual efforts. Reduced levels of heat stress with precooling may facilitate the maintenance of MVC through the maintenance of muscle recruitment, thereby permitting higher work output. From a practical perspective, field-based practitioners should be aware of the inverse relationship between volume of cooling and performance benefits. In addition, practitioners should select interventions that allow for sufficient volume, yet within logistical constraints of their individual team contexts.
This research undertaking was solely funded by a Cricket Australia Sport Science Sport Medicine Research Grant and not any of the following organizations: the National Institutes of Health, the Wellcome Trust, or the Howard Hughes Medical Institute.
The authors sincerely thank Wayne Everingham at the School of Biomedical Sciences, Charles Sturt University, Bathurst, NSW, Australia, and Brian Heffernan at the Central West Pathology Service, Bathurst Base Hospital, NSW, Australia, for their technical assistance with immunoassay procedures.
M.P. is an employee of Cricket Australia. There are no conflicts of interest for any of the authors.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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