Of concern for many team-sports coaches and conditioning specialists is the observed reduction in performance when training or competing in high ambient temperatures (27). In line with this observation, a number of research studies have reported a reduction in exercise performance in warm/hot conditions or with high core body temperatures (2,12,20,21). As such, training for intermittent-sprint sports in the heat may result in a reduction of the acute training quality of a session. Accordingly, methods to counter the ergolytic effects of exercise in warm conditions are sought after in many sports. One such method is the process of pre-cooling, whereby cold substances or mediums are applied to the body to create a cooler microenvironment, thus increasing the thermal gradient between core and periphery and decreasing the thermoregulatory load of exercise (4,9,18).
The use of pre-cooling interventions prior to exercise in the heat have been demonstrated to be effective in improving exercise performance, particularly during prolonged, continuous endurance exercise (3,16). However, until recently the benefits of pre-cooling for team-sport type exercise were unclear, with multiple studies reporting no improvements in intermittent-sprint exercise following pre-cooling (6,8,11). More recent research has reported that pre-cooling prior to prolonged, intermittent-sprint exercise can maintain performance during repeat-sprint and submaximal modes of simulated team-sport exercise in the heat (5,9). In particular, when free-paced exercise protocols are used, similar ergogenic benefits are elicited to those observed in continuous exercise protocols (4,15,16). Accordingly, a growing volume of laboratory evidence exists to indicate the performance benefits of pre-cooling for intermittent-sprint exercise in the heat (18). However, 2 issues restrict the applicability of these data to field settings. First, laboratory intermittent-sprint protocols often do not mimic the demands of team-sports and second, the logistics of laboratory-based cooling often preclude or constrain the use of these methods in the field.
The most effective laboratory-based cooling protocols typically involve whole-body immersion in cold environments, such as cold rooms, water baths, and showers (18). However, these methods of cooling are often cumbersome and provide logistical difficulties for use in the field, where issues such as increased number of athletes, inadequate facilities, or travel may restrict effective implementation. Conversely, smaller and more applicable cooling interventions (ice vests) have had mixed results in eliciting performance or thermoregulatory benefits compared to larger interventions (9). Although individual smaller cooling methods may lack the sufficient volume of cooling, a combination of a variety of smaller, more practical methods may provide a greater cooling exposure, ensuring both performance and physiological benefits. To date, the use of a mixture of part-body cooling interventions to reduce the thermoregulatory load and improve intermittent-sprint performance in a field-based scenario remains untested.
Finally, previous research indicates the suppression of a range of physiological responses to exercise, such as heart rate and core temperature, following pre-cooling (4,15,18). Despite the reported acute performance benefits of pre-cooling in the heat (5,9), the potential blunting of exercise-induced physiological responses as a result of pre-cooling may diminish the physiological adaptation resulting from the exercise bout. Accordingly, in a training environment, the short-term gain of an improved individual session at the expense of physiological adaptation may not be ideal. To date, it is unknown as to whether the suppression of exercise-induced physiological responses limits the potential physiological adaptation to the session.
Therefore, based on both the type of intermittent-exercise protocols and cooling interventions used, there are limitations in the effective transference of laboratory pre-cooling interventions to field-based scenarios for team sports. Additionally, it is unknown as to whether the use of pre-cooling blunts the potential acute adaptive response to an exercise bout. The aim of this study was to investigate the effects of a practical, mixed-method, part-body pre-cooling procedure on exercise performance and physiological responses to field-based, team-sport exercise in the heat.
Experimental Approach to the Problem
Following familiarization with all equipment, procedures, and the exercise protocol, subjects performed 2 identical testing sessions, either with or without the cooling intervention, in a counterbalanced, crossover design. The respective testing sessions were conducted in mean ± SD environmental conditions of 32.4 ± 0.5°C and 44.0 ± 4.4% relative humidity (RH). The testing sessions consisted of an initial 20 minutes of either pre-cooling or control (no cooling) procedures while seated in a shaded thermoneutral environment (25°C), followed by a 10-minute warm-up and a 30-minute conditioning session. The conditioning session consisted of 4 × 5-minute bouts (20 minutes total) of free-paced, intermittent-sprint exercise, each separated by 2 minutes of recovery. Prior to each session it was made clear to all subjects that the aim of the conditioning session was to cover as much distance as possible at all times. To determine the effects of pre-cooling on exercise performance, participants were monitored for distance and velocity via global positioning system (GPS) devices. Additionally, measures of core temperature, heart rate, and perceptual exertion were recorded throughout the cooling intervention and exercise to determine the physiological and perceptual effects of pre-cooling. Finally, body mass, urine-specific gravity, and venous blood markers of inflammation and stress were recorded prior to and following the session to determine the effect of pre-cooling on sweat loss, hydration status, and markers of physiological stress and adaptation to the session. All testing sessions were performed at the same time of day, separated by at least 7 days of recovery. Subjects abstained from strenuous physical activity and the ingestion of alcohol for 12 hours prior to testing and all caffeine and food substances 3 hours prior to testing. Additionally, subjects recorded all food consumed and activity performed in the 24 hours prior to the first testing session and were required to replicate these for the ensuing session. During each session, subjects were provided with 1 L of water, which was checked post-session to confirm standardized fluid consumption.
Seven male, National Collegiate Athletic Association (NCAA) Division 1 lacrosse players volunteered to participate in the present study. Subject characteristics included age of 19.9 ± 1.4 years, mass of 83.8 ± 5.8 kg, and height of 177.0 ± 4.7 cm. Subjects were trained lacrosse players who were part of the current NCAA Division 1 championship team, trained 6-8 times per week, and competed every weekend during the season. Testing was conducted during the pre-season training phase. All subjects were informed of the experimental risks and procedures and provided written informed consent prior to the investigation. The investigation was approved by the Syracuse University Institutional Review Board for Use of Human Subjects.
Prior to the exercise protocol, subjects performed a standardized 5-minute warm-up consisting of 3 jogging laps of a lacrosse field, 5 × 20-m run throughs at increasing speeds, and an additional 5 minutes of static stretching. Subjects then performed a 20-minute exercise protocol that consisted of 4 × 5-minute bouts of intermittent-sprint exercise, separated by 2 minutes of passive recovery, adapted from a previously published protocol (9). The protocol consisted of a 5-second sprint each minute, separated by free-paced bouts of hard running, jogging, and walking, which were performed 1 bout at a time for the rest of the minute and rotated each minute. The intraclass correlation coefficient (ICC) and coefficient of variation (CV) for distance covered in the various intensities of the protocol were r = 0.89 to 0.96 and <3%, respectively. Subjects performed the protocol in a shuttle-run fashion between the end and halfway lines of an outdoor, synthetic grass lacrosse field. The 5-second sprint was performed at the start of each minute, followed immediately by the respective free-paced bout until 10 seconds from the start of the next minute, where subjects made their way to the nearest line to start the next sprint. Subjects performed the respective sessions in 2 separate smaller groups, who all performed the same condition at the same time in a counterbalanced order. During each session subjects were spaced across the field to ensure separation by at least 10 m to avoid the direct influence of external pacing stimuli.
During each testing session, players wore the same portable GPS unit fitted between the scapulae of the shoulders in a custom-made harness (SPI elite and SPI 10, GPSports Systems, Australia), which recorded distance and velocity every second (1 Hz). After each testing session, data were downloaded to a portable computer and analyzed using specialized software (GPSports Analysis and Team AMS, GPSports Systems, Australia). The technical CV for this device for measuring distance has previously been reported to be 5%, whereas velocity measures were 5 to 20% depending on the speed zone (7). GPS data were time-aligned to include each of the 4 respective 5-minute exercise bouts. The GPS data for distance travelled and mean velocities collected during each bout and the total session were then categorized into 4 predefined zones (10):
- Low-intensity activity (LIA; speeds <7.0 km/h−1)
- Moderate-intensity activity (MIA; speeds 7.0-14.4 km/h−1)
- High-intensity running (HIR; speeds >14.5 km/h−1)
- Very-high-intensity running (VHIR; speeds >20 km/h−1)
Following all resting measures, subjects performed either the pre-cooling or control procedures while seated in a shaded environment. The pre-cooling intervention involved a 20-minute part-body, mixed-method cooling procedure that included wearing a cooling vest on the torso (Arctic Heat, Brisbane, Australia), cold towels around the neck and trapezius muscles that had previously been soaking in 3°C ice water, and 0.6 kg plastic bags of ice on the quadriceps muscle of each leg. Cooling interventions were kept in place during the 20-minute intervention and warm-up (except ice bags, which were removed during the warm-up). During the control condition, subjects were quietly seated for 20 minutes and received no cooling prior to or during the warm-up.
Core Temperature, Heart Rate, and Hydration Status
On arrival and following each session, subjects voided their bladder to collect a urine sample for analysis of urine-specific gravity (USG) as a measure of hydration status (Pocket Refractometer, Atago, Japan). Further, nude mass was measured on a set of calibrated weigh scales (WSI-600, Mettler Toledo, Columbus, Ohio, USA) prior to and following each session to determine the change in mass as a representative measure of sweat loss. Five hours prior to each session, subjects swallowed an ingestible telemetric capsule (VitalSense, Mini Mitter, Philips Respironics, Bend, Oregon, USA) to ensure that it had passed into the gastrointestinal tract and would be insensitive to temperature changes resulting from fluid intake during testing. Core temperature was recorded on a hand-held monitor (VitalSense) that received measures transmitted from the ingestible capsules (ICC = 0.98, CV = 0.33). Heart rate was measured via a chest strap transmitter and wrist watch receiver (RS100, Polar Electro Oy, Kempele, Finland). Core temperature and heart rate were recorded prior to and every 10 minutes during the cooling intervention, following each 5 minutes of the exercise protocol and 10 minutes following exercise. Finally, environmental temperature and %RH were measured during both testing sessions (WMR1000 wireless weather station, Oregon Scientific Inc., Tualatin, Oregon, USA).
Serum IL-6, IGF-1, and IGFBP-3 Measures
To provide insight into the effect of pre-cooling on inflammatory and growth or adaptive responses to the session, measures of interleukin-6 (IL-6), insulin like growth factor 1 (IGF-1), and insulin-like growth factor binding protein 3 (IGFBP-3) respectively, were obtained. Blood samples (5 mL) were drawn from the antecubital vein prior to and 1 hour following each session and collected into pyrogen/endotoxin free tubes for measurement of IL-6, IGF-1, and IGFBP-3. Samples were stored on ice and allowed to clot (within 1 hour after collection) prior to centrifugation (1,000 × g for 15 minutes). Serum was then transferred to 2-mL Eppendorf tubes and stored at -80°C until analysis. Prior to analysis, the serum was allowed to reach room temperature while being mixed using gentle inversion. All assays were conducted in duplicate in accordance with manufacturer's instructions. Serum IL-6 was measured using an enzyme-linked immunosorbent assay (ELISA; Abcam Inc., Napa Valley, California, USA), which has a minimum detectable dose of less than 0.033 pmol/L and an interassay coefficient of variation (CV) of 1.4%. Serum IGF-1 and IGFBP-3 were measured using ELISAs (Immunodiagnostic Systems, Scottsdale, Arizona, USA), which have a minimum detection limit of 0.641 and 3.186 nmol/L, respectively, and an interassay CV of 7 and 11.1%, respectively.
Rate of perceived exertion (RPE) and a rating from the thermal sensation scale (TSS) were obtained based on 20-point (Borg scale) and 8-point Likert scales, respectively. RPE and TSS measures were recorded prior to cooling, following the warm-up, following each respective 5-minute bout of the exercise protocol, and 10 minutes following the end of the protocol.
Data are reported as mean ± standard deviation. A 2-way (condition × time) repeated-measures analysis of variance (ANOVA) was used to determine differences between the 2 conditions (cooling vs. control). Post hoc paired t-test analyses were performed to determine the location of significant differences. Significance was set a priori at p < 0.05. Finally, effect size data (ES) were calculated (Cohen's d) to determine the magnitude of effect of the cooling intervention on performance and physiology with an ES of <0.2 classified as “trivial,” 0.2 to 0.4 as “small,” 0.4 to 0.7 as “moderate,” and >0.8 as “large” effects.
Pre-cooling resulted in subjects increasing the total distance covered during the intermittent-sprint protocol, specifically via an increase in the distance covered in the MIA zone (Figure 1; p = 0.05). Table 1 shows the respective distances and speeds for each zone during each of the 4 respective intermittent-sprint bouts (ISB). During ISB 1, total and MIA distance were increased in the cooling condition (p < 0.05; ES = 1.1), whereas LIA was greater in the control condition (p < 0.05; ES = 0.9). Further, MIA distance was also increased in the pre-cooling condition during ISB 2, 3, and 4, respectively (p < 0.05; ES > 0.7), without differences in VHIR or HIR distance (p > 0.05; ES < 0.4), Moreover, the distance covered in the LIA zone showed large effects for lower values in the cooling condition for all ISBs (ES > 0.8). There were no significant differences between each condition for any bout for the distance or velocity of HIR or VHIR or for MIA and LIA mean velocities (p > 0.05). However, mean MIA velocity for the whole session showed a large effect for an increased velocity in the cooling condition (ES = 0.9).
There were no differences between conditions for pre- or post-exercise USG (pre: 1.017 ± 0.008 vs. 1.017 ± 0.004; post: 1.020 ± 0.004 vs. 1.020 ± 0.004 for cooling vs. control, respectively; p > 0.05). However, there was a significantly lower change in nude mass, representing sweat loss, in the pre-cooling condition (1.0 ± 0.4 vs. 1.8 ± 0.7 kg for cooling and control, respectively; p < 0.05). The pre-cooling intervention was successful in blunting the increase in core temperature following the warm-up and until after ISB 3 (Figure 2; p < 0.05; ES = 1.0-2.0). However, there were no significant differences between conditions at any time for heart rate (Figure 2; p > 0.05; ES < 0.3). Pre-exercise IL-6 values were 0.054 ± 0.062 vs. 0.033 ± 0.05 pmol/L, whereas pre-exercise values for IGF-1 were 24.97 ± 4.58 vs. 21.31 ± 4.58 nmol/L for pre-cooling and control, respectively. The percentage change in these blood variables is reported in Figure 3, with both conditions showing a significant (p < 0.05) increase from pre-exercise values, without differences between the respective conditions (p > 0.05). Further, the IGF-1:IGFBP-3 ratio for pre-cooling (pre: 0.026 ± 0.014; post: 0.027 ± 0.012) and control (pre: 0.021 ± 0.005; post: 0.026 ± 0.004) did not differ (p > 0.05) between conditions in response to the exercise bout. Finally, environmental conditions were not significantly different between respective conditions (32.8 ± 1.0 vs. 32.0 ± 1.0°C and 47 ± 9 vs. 42 ± 10% RH for pre-cooling and control conditions, respectively; p > 0.05).
Results for RPE and TSS are presented in Figure 4. RPE was not significantly different between conditions following the warm-up or at any stage during the exercise protocol (p > 0.05; ES < 0.6). Similarly, TSS was also not significantly different between conditions during either the intervention or exercise protocol (p > 0.05; ES < 0.6).
The present study demonstrated that a combination of smaller, field-based cooling interventions could reduce the thermoregulatory load and increase the distance covered during conditioning training for team sports in the heat. These results represent the extension of laboratory-based research studies to a more practical and applied sporting environment. Previously, laboratory-based evidence has reported mixed findings for intermittent-sprint performance in the heat following pre-cooling (6,8,11). Studies involving the use of externally set exercise patterns have tended to result in minimal differences in work completed between conditions (6,8,11). However, recent evidence from studies using prolonged and/or free-paced intermittent-sprint protocols report an alteration in pacing strategies that results in an improved exercise performance compared to exercise without cooling (5,9).
The free-paced exercise protocol used in the current study was an adaptation of a previously published protocol (9) to a training environment and resulted in a similar improvement in performance. Previously, Castle et al. (5) reported that cooling methods maintained power output during the later stages of a 40-minute repeat sprint cycling protocol. More relevant, pre-cooling has been shown to improve the distance of free-paced running during 15-m shuttle run efforts without changes in maximal sprinting (9). Similarly, the current study indicated that pre-cooling improved distance covered during prolonged, intermittent submaximal efforts, particularly within the speed zone of 7 to 15 km·h−1, without alterations to very-high-intensity work. These results represent a self-selected increase in exercise volume/intensity following cooling, without an augmentation in physiological responses. Speeds in the range of 12 to 16 km·h−1 represent exercise intensities that are likely to be associated with the lactate threshold and hence intensities that are predominant and common in many team-sport training programs and competition demands (28). Further, the reduction in MIA and greater LIA distance noted in the control condition highlight that the reduction in total distance during the control condition was a result of a selection of lower speed zones. Accordingly, the prevention of the heat-induced reduction of exercise intensity during any training bout, particularly in the maintenance of threshold running speeds, represents a potential benefit for the quality of the training stimulus used.
Alongside the noted performance improvements, the part-body, mixed-method cooling procedure resulted in a reduction in the increase in core temperature. Moreover, rather than reducing pre-exercise core temperature, the use of multiple simple cooling procedures provided a sufficient heat sink that slowed the rate of increase in core temperature. The ∼0.3°C lower core temperature in the pre-cooled condition is similar to results reported in full-body cooling studies (9,16,19). Given that often there is a lack of reduction in core temperature in single-mode cooling interventions (9,16,19), the combination of methods seems as effective as full-body cooling, yet without the limitations of cumbersome equipment or logistics. Further, the lowered core temperature seems to provide an improved thermoregulatory efficiency (18,27), and may relate to the observed increase in total distance covered during the session. Added benefits of a blunted increase in core temperature in the field may also relate to a reduction in the possible risk of heat-induced illness (2,18,19) and, thus, practical mixed methods may also provide some protective health benefits. As such, the combination of a range of smaller cooling exposures offer a practical way to provide field-based pre-cooling and reduce the thermoregulatory load of an exercise bout in the heat.
In addition to lowered core temperatures, previous research has highlighted that pre-cooling can also reduce sweat loss and heart rate for any given exercise bout (15,18). The present study also indicated that an effective pre-cooling intervention can reduce core temperature and sweat loss and result in more work for the same physiological (heart rate) and perceptual (RPE) load (18,26). It is surprising that there were no differences in TSS; however, the similarity of the perceptual responses between conditions given a smaller volume of work in the control condition indicates that pre-cooling can lower the perceptual load of an exercise bout in the heat, even when a higher work output is selected (30). Accordingly, a reduced physiological (including thermoregulatory) or perceptual load for any given exercise bout may provide benefits for athlete acclimation and recovery when exposed to hot conditions, either within individual sessions or over the duration of a training phase. That said, the reduction in the physiological load may have the potential to reduce the likelihood of physiological adaptation occurring, because although the training output may be increased, the reduced physiological stress may act to inhibit the process of physiological adaptation (14).
Although the small sample size and number of time points restricts the conclusions made from the results of venous blood analyses, there were no significant differences in post-exercise levels of IL-6, IGF-1, or IGF-1:IGFBP-3 responses between conditions. Although the 60-minute post-exercise increase in IL-6 was somewhat blunted when compared to previous studies adapting pure running protocols (25), the kinetics of the IL-6 response to a field-based training session have, to the authors' knowledge, not previously been reported. The post-exercise increase in IL-6 most likely represents an increase in the acute inflammatory processes, either as a result of the prevalence of muscle contractile stress or a reduction in glycogen availability (25). The post-exercise increase in IGF-1 is in line with previous research and likely represents an increase in the signaling pathway to promote cell growth and adaptation (1). The majority of circulating IGF-1 is bound to plasma IGFBPs, particularly IGFBP-3 (1), prolonging the metabolic half-life of IGF-1 in the circulation. However, this binding also alters IGF-1 binding to cell surface receptors, and the presence of IGFBPs may modulate the bioavailability of IGF-1 (1). As such the ratio of IGF-1:IGFBP-3 is an important criteria to consider when assessing IGF-1 activity, and this was shown not to differ between conditions. Thus, while the use of pre-cooling interventions improved exercise performance and reduced the physiological and perceptual load, generic markers related to acute inflammation and adaptation did not differ. However, these measures respectively are a small and preliminary representation of a much larger and more complex system of stress and adaptation responses (1,25). Accordingly, practitioners should be aware that, although pre-cooling may suppress physiological responses to exercise in the heat, these effects may not necessarily result in a reduction in the processes related to training adaptation. Indeed, the chronic physiological and performance adaptations arising from constant training with the use of pre-cooling in the heat is an area that warrants further investigation.
Finally, while the ergogenic properties of pre-cooling are well documented, the mechanisms responsible for these improvements remain equivocal (18). Current evidence indicates that the reduction in exercise intensity in the heat may result from a reduction in recruitment of exercising musculature, possibly as a protective mechanism from the increasing endogenous heat stress (19,23). However, whether the earlier reduction in exercise intensity in the heat is a result of afferent feedback from any number of external parameters (13,22) or an anticipatory response based on projection of the completion of the bout (19,29) remains the topic of some debate (24). Previous match-based intermittent-sprint data highlight that the excessive increase of core temperature above elevated yet tolerable core temperatures (39.3-39.5°C) seems to be controlled via the reduction in moderate intensity activity (10-16 km·h−1) more so than the reduction in very-high intensities (+20 km·h−1) (10). These findings highlight the potential use of pacing in the context of match demands based on thermoregulatory responses and maintenance of physiological responses within homeostatic ranges (29,30). Accordingly, in the context of the current study, the cooling-induced blunting in the increase in core temperature may allow for the self-selection of higher exercise intensities, possibly via the continued recruitment of active musculature (17).
In conclusion, a mixed-method, part-body cooling intervention prior to exercise in the heat improved intermittent-sprint performance common to conditioning training for many team sports. The improved performance may result from an alteration in pacing strategies, with a maintenance or increase in free-paced, self-selected exercise intensities, particularly around the lactate threshold. Moreover, a series of smaller and more practical cooling procedures, utilized in a combined intervention, can assist to reduce the physiological and perceptual load of an exercise bout performed in warm conditions. As such, based on both laboratory and field-based evidence, it seems evident that pre-cooling may be a practical ergogenic aid for team sports as long as the cooling exposure is sufficient.
The results of the current study indicate that the use of mixed-method, part-body pre-cooling interventions can benefit exercise performance during team sport conditioning sessions in the heat. The use of such interventions may assist in improving training (or competition) performance and also prevent the excessive increase in body temperature during training sessions conducted in the heat, particularly when prior exposure or acclimation is lacking. Moreover, although traditional, and often more cumbersome, full body cooling exposures, such as cold-water immersion, are often more effective, the use of field-based, mixed-method, part-body techniques can also offer an effective and sufficient cooling intervention. The actual logistics of training/competition environments may dictate the appropriate cooling intervention to use. However, the ease, cost, and availability of a mixture of smaller methods such as ice vests, cold towels, and ice packs increase the total cooling exposure and can blunt the excessive increase in core temperature observed with exercise in the heat and may lead to improved training performance for athletes.
The authors wish to acknowledge the funding of this project by a Charles Sturt University Special Studies Program Grant. Further, the authors to thank the coaches and players of the Syracuse University Men's Lacrosse program for their participation in this study.
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