Effect of precooling and acclimation on repeat-sprint performance in heat

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1 This article was downloaded by: [Karen Wallman] On: 11 December 2012, At: 14:57 Publisher: Routledge Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Journal of Sports Sciences Publication details, including instructions for authors and subscription information: Effect of precooling and acclimation on repeat-sprint performance in heat Carly Brade a, Brian Dawson a & Karen Wallman a a University of Western Australia, School of Sport Science, Exercise and Health, Crawley, Australia Version of record first published: 10 Dec To cite this article: Carly Brade, Brian Dawson & Karen Wallman (2012): Effect of precooling and acclimation on repeatsprint performance in heat, Journal of Sports Sciences, DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Journal of Sports Sciences, 2012; 1 8, ifirst article Effect of precooling and acclimation on repeat-sprint performance in heat CARLY BRADE, BRIAN DAWSON, & KAREN WALLMAN University of Western Australia, School of Sport Science, Exercise and Health, Crawley, Australia (Accepted 12 November 2012) Downloaded by [Karen Wallman] at 14:57 11 December 2012 Abstract This study determined whether precooling would have an additive effect on repeat-sprint cycling performance in heat following partial acclimation. Ten males completed three trials; Pre Acclimation (Pre Acc) and two Post Acclimation trials, one with precooling (ice jacket and slushy; Post Acc þpc) and another without (Post Acc). Trials consisted of a 30-min baseline period followed by a 70-min repeat-sprint protocol in *358C and 60% relative humidity. Separating pre and post trials were five heat acclimation sessions. Although no significant differences were found for performance variables, inferential statistical analysis resulted in moderate effect sizes, which suggested more work (J kg 71 ) was performed in Post Acc compared with Pre Acc. Further, possible and very likely benefits were found for every performance variable for Post Acc compared with Pre Acc, while possible benefits were found for Post Acc, compared with Post Acc þpc, for peak power output (W and W kg 71 ). Moderate to strong effect sizes suggested lower core temperatures in both post acclimation trials compared with Pre Acc. Sweat loss was significantly higher (P ; 23.1%) in Post Acc þpc compared to other trials. In conclusion, no additional performance enhancement was seen when partially acclimated individuals precooled prior to repeat-sprint performance in heat. Keywords: core temperature, peak power, work, sweat loss Introduction Hot and humid environmental conditions have consistently resulted in detrimental effects on prolonged, repeat-sprint exercise. Both Morris, Nevill, Lakomy, Nicholas, and Williams (1998) and Morris, Nevill, and Williams (2000) reported reduced total distance completed during prolonged, intermittent high-intensity exercise performed in hot (308C) compared to thermoneutral (16 208C) conditions. Similarly, mean power output during repeated-sprints in heat (408C) was lower compared to normal (208C) conditions (Drust, Rasmussen,Mohr,Nielsen,&Nybo,2005). Impaired exercise performance in heat has consistently been attributed to increases in heat load that result in critically high core temperatures (T C ; Gonzalez-Alonso et al., 1999; MacDougall, Reddan, Layton, & Dempsey, 1974; Nielsen et al., 1993). Elevations in T C have been reported to affect metabolic (Febbraio, Snow, Stathis, Hargreaves, & Carey, 1994), central nervous system (Drust et al., 2005), cardiovascular (Gonzalez-Alonso, Mora-Rodriguez, Below, & Coyle, 1995), and physiological (Brooks, Hittelman, Faulkner, & Beyer, 1971) responses to exercise. The most reputable and well-studied technique used to counteract the negative effects of heat on exercise performance is heat acclimatisation/acclimation (Marino, 2002). Improved exercise performance (time to exhaustion) after heat acclimation has been reported by Nielsen et al. (1993) and Nielsen, Strange, Christensen, Warberg, and Saltin (1997) after 8 13 days of exercise training sessions in the heat, in conjunction with the classical indicators of acclimation; lower heart rate (HR) and T C and increased sweating. These types of acclimation protocols have commonly involved low-intensity (50 60% maximal oxygen uptake; _V O 2max ) exercise performed daily for a prolonged duration (greater than 45 min). However, this form of exercise is very different to that performed by team sport athletes. Recently, an acclimation protocol designed for team sports, which differs from conventional acclimation protocols by using fewer and shorter sessions of high-intensity Correspondence: Carly Brade, University of Western Australia, School of Sport Science, Exercise and Health, Crawley, Australia. bradec01@student.uwa.edu.au ISSN print/issn X online Ó 2012 Taylor & Francis

3 Downloaded by [Karen Wallman] at 14:57 11 December C. Brade et al. exercise, was tested (Sunderland, Morris, & Nevill, 2008). Here, participants performed only four acclimation sessions (over 10 days) involving highintensity intermittent running for min in 308C. Following this partial acclimation process, total running distance covered (before volitional fatigue) during the Loughborough Intermittent Shuttle Test was 33% greater compared to control, with this improvement attributed to increases in thermal comfort during exercise and lower T C values at the start of exercise (Sunderland et al., 2008). Petersen et al. (2010) also noted that similar (partial) acclimation resulted in decreases in HR and sweat electrolyte concentrations following four high-intensity acclimation sessions performed in 308C. Potentially, short-term (i.e. partial), high-intensity, intermittent acclimation protocols may be more appropriate for team sport athletes who require improved heat tolerance and who do not have the time nor the financial resources to fully acclimate. In addition to heat acclimation, an acute (preexercise) method of preparing for exercise in heat is precooling, which can enhance prolonged repeatedsprint performance by lowering pre-exercise T C and allowing greater heat storage capacity (Brade, Dawson, & Wallman, 2012; Castle et al., 2006; Duffield & Marino, 2007; Minett, Duffield, Marino, & Portus, 2011). These studies used a mixture of precooling methods for min prior to exercise; ice vest and cold bath immersion (Duffield & Marino, 2007), plus a mixed method whole-body cooling technique (ice towels on head and neck, hands immersed in cold water, cooling jacket and ice packs on thighs; Minett et al., 2011), just ice packs on thighs (Castle et al., 2006) and cooling jacket and slushy (Brade et al., 2012). Specifically, Brade et al. (2012) demonstrated that the combination of a cooling jacket and ice slushy resulted in improved prolonged (70-min) repeat-sprint performance in the heat, compared to ice slushy, cooling jacket and a control condition alone. Of interest is whether precooling could further improve exercise-heat performance if an athlete had previously acclimated. To date, only Castle, Mackenzie, Maxwell, Webborn, and Watt (2011) have reported that a traditional (low intensity endurance exercise) 10 day acclimation period resulted in a 2% increase in peak power output during a 40-min intermittent sprint protocol performed in heat. However, when participants were precooled (ice packs on thighs) prior to exercise, no further performance benefits were observed. They proposed that precooling was only beneficial when heat and exercise strain were high, with this benefit being negated by the effects of full heat acclimation (which reduced heat strain). They also speculated that precooling may improve exercise performance when individuals were only partially heat acclimated, as heat strain during exercise would still be high. These findings require further confirmation before any effect of precooling on heat acclimated exercise performance can be concluded. In addition, the effect of precooling on repeat-sprint performance following a short-term, high-intensity (rather than submaximal aerobic exercise) acclimation protocol has yet to be investigated. Therefore, the aim of this study was to determine if partial heat acclimation would improve prolonged repeat-sprint performance in heat and if further benefits would occur if precooling was then practised, both prior to and during exercise. Methods Participants Ten moderately trained males (mean + s: age years, height cm, body-mass kg, _V O 2peak ml kg 71 min 71, sum of seven skin-folds mm and body surface area m 2 ), who were all currently involved in team sports and trained at least twice a week, were recruited as participants. All provided informed consent and ethical approval was granted by the Human Research Ethics Committee of the University of Western Australia. Overview Participants completed two familiarisation sessions (5 7 days apart) at least five days prior to performing the first (pre acclimation; Pre Acc) of three experimental trials, consisting of a 30-min baseline period (rest) followed by a 70-min repeat-sprint cycling protocol performed in heat (Figure 1). This protocol comprised 2 x 30-min exercise periods separated by a 10-min half-time break (rest for Pre Acc). Participants then completed five acclimation sessions spread over 10 days (one day between), and were then randomly assigned to two post acclimation trials, which consisted of either cooling performed during the baseline period and at the half-time break (Post Acc þpc), or without cooling (Post Acc) (Figure 1). These trials (one day between) were completed within four days of the last acclimation session, with all trials performed at thesametimeoftheday+ 2 h. Participants replicated food and fluid intake for 24 h prior to each session, and abstained from alcohol and vigorous activity for 24 h and caffeine for 3 h prior to testing. Familiarisation sessions In the first familiarisation session, anthropometric measures including height (cm), body-mass (BM;

4 Repeat-sprints after acclimation and precooling 3 Figure 1. Study design. Downloaded by [Karen Wallman] at 14:57 11 December 2012 kg), sum of seven skinfolds (Harpenden callipers; mm; triceps, biceps, subscapular, abdominal, suprailiac, thigh and calf) and body surface area (m 2 : Dubois nomogram; McArdle, Katch, & Katch, 2001) were recorded. In addition, participants completed a _V O 2peak test on a calibrated, front access cycle ergometer (Model EX-10, Repco, Australia) to ensure that individuals exercised at similar relative intensities during the acclimation sessions. The _V O 2peak test began with a starting intensity of 100 W, which increased every 3 min by 50 W until volitional exhaustion. During the second familiarisation session, participants completed one half (30-min) of the repeat-sprint test and four blocks (16 min) of the acclimation exercise in the climate chamber for familiarisation to these protocols. Baseline and repeat-sprint exercise protocol Trials began with a 30-min baseline period completed under normal laboratory conditions ( C; %; relative humidity; RH) whilst seated. During the Post Acc þpc trial participants precooled during this time by ingesting 7gkg 71 BM (Ihsan, Landers, Brearley, & Peeling, 2010) of plain ice (0.68C) and wearing a cooling jacket containing PC25 (PCP Australia, West Perth, WA) simultaneously. To ensure consistency across trials, the ice slushy was consumed at a rate of 2.3 g kg 71 BM every 10 min. When frozen, PC25 appears as a white, crystalline solid substance that has a melting point of 258C and the ability to transfer 3.5 Watts (W) of heat per square cm from the body (manufacturer s details). The jacket, designed by the Australian Institute of Sport (Canberra, Australia), is a vest with four anterior and posterior pockets. Sealed packets (140 mm x 140 mm, 120 g) of frozen PC25 were fitted into these pockets. During the halftime recovery period, participants in the Post Acc þpc used these precooling methods again for *8 min. The amount of ice ingested was 2.3 g kg 71 BM. The jacket was retrieved from the refrigerator where it was stored during the first half. To control for fluid intake between trials, participants in the Pre Acc and Post Acc trials consumed identical amounts of tap water (*238C) to ice ingested during the Post Acc þpc trials in both the precooling and half-time periods. Following baseline, participants entered the climate chamber (* C and * % RH) and completed a 5-min cycling warm-up at varying intensities ( W) for 30-s periods and performed 2 x 4-s maximal sprints at 3.5 and 4.5 min. The repeat-sprint protocol was then commenced. Each half comprised 30 x 4-s maximal sprints, interspersed by 56 s of light exercise performed at intensities of 25, 50, 75 and 100 W. In addition, to replicate the unpredictable nature of team sports, six extra maximal sprints were performed in each half at 2.5, 7.5, 12.5, 17.5, 22.5 and 27.5 min (Duffield, Dawson, Bishop, Fitzsimons, & Lawrence, 2003). Participants ingested 100 ml of tap water (*238C) at the 15th min of both halves, while 100 ml of a commercial sports drink (8% carbohydrate content) was consumed during half-time. Cycling exercise was performed on the same ergometer used during the _V O 2peak test. Nude BM was measured prior to baseline and then after exercise (towel dried) using a digital platform scale (model ED3300; Sauter Multi-Range, Ebingen, West Germany + 10 g) for the purpose of calculating sweat loss (pre - post nude BM þ fluid ingested). During the repeat-sprint protocol, HR (Polar F1 TM HR monitor, Kempele, Finland) was recorded every 5 min. An ingestible radiotelemetry capsule (VitalSense, Mini Mitter, USA) swallowed 8 h prior to testing was used to measure T C. Skin temperature (T Sk ) was measured by dermal patches (VitalSense, Mini Mitter, USA) placed on the sternal notch, mid forearm, mid quadriceps and medial calf. Temperature measurements were made every 5 min for T C and 10 min for T Sk throughout the entire trial. Mean T Sk [¼ (0.3 x sternum temperature) þ (0.3 x forearm temperature) þ (0.2 x quadriceps temperature) þ (0.2 x calf temperature)] was calculated (Ramanathan, 1964). Ratings of perceived exertion (RPE; Borg, 1970; 6 20 scale) and thermal sensation (TS; 0 ¼ unbearably cold to 8 ¼ unbearably hot) were measured at the 15th and 30th min of both halves of exercise. Performance variables measured for each sprint included peak power (W), peak power per kilogram BM (W kg 71 ), mean power (W), work (kj) and work per kilogram BM (J kg 71 ). These variables were measured using a customised computer program (Cyclemax version 6.3, School of Sport Science,

5 Downloaded by [Karen Wallman] at 14:57 11 December C. Brade et al. Exercise and Health, UWA). Performance variables were not measured during the extra sprints. Acclimation The five acclimation sessions (358C and 60% RH), involved repeated cycling for 3-min at 80% maximum power output (group mean + s: W), as determined from the _V O 2peak test, with 1-min of passive rest between. During the first session, this protocol was repeated eight times (32 min), with an additional bout added to each session until a total of 12 repeats were performed (48 min). Participants drank ad libitum in these sessions, with total water ingested recorded for the purpose of calculating sweat loss. Core temperature was assessed during the first and last acclimation session using ingestible capsules (as described earlier), whilst tympanic temperature (Braun, Thermoscan 3000, Australia) was measured during the other sessions (for participants safety). Heart rate, RPE and TS were recorded during each minute of passive rest. Statistical analysis A two-way, repeated measures (trials x time) analysis of variance (ANOVA) tested for significant differences in T C, mean T Sk, HR, RPE, TS and performance variables (first and second half). Oneway, repeated measures ANOVAs were used to determine significance between trials for sweat loss, sweat sensitivity and overall mean power and work. Where appropriate, post hoc comparisons using least significant difference adjustments were calculated. Data was analysed using SPSS (Version 17.0 for Windows; SPSS Inc, Chicago, IL) with significance set at P Cohen s d effect sizes (ES) were calculated (50.5, small; , moderate; 0.8, large) to identify the magnitude of difference between trial scores (Cohen, 1988). Smallest worthwhile effects were also calculated for all performance variables. Where the chance of benefit or harm were both calculated to be 45%, the true effect was deemed unclear (Batterham & Hopkins, 2005). Otherwise, chances of benefit or harm were assessed as follows: 51%, almost certainly not; 1 5%, very unlikely; 5 25%, unlikely; 25 75%, possible; 75 95%, likely; 95 99%, very likely; 499%, almost certain. All values are expressed as mean + s. Results The first and last acclimation sessions resulted in changes (D) in T C of C and C, respectively. Sweat loss was kg over the first trial and kg over the last. Mean HR ( vs beats min 71 ), RPE ( vs ) and TS (6 + 1 vs ) were similar between the first and last trials respectively. While performance results were higher for both post acclimation trials (highest for Post Acc) versus Pre Acc for every variable assessed, these differences were not significant (P 40.05; Table I). However, higher performance scores following Post Acc, compared with Pre Acc, were supported by moderate ES (d ¼ ) for work (J kg 71 ; first and second half), as well as possible to very likely benefits for every performance variable assessed. Further, possible benefits were found for mean power (W; first half) and work (kj; first half) for Post Acc þpc compared with Pre Acc. Finally, possible benefits were found for peak power output (W; second half), peak power (W kg 71 ; first and second half) and work (J kg 71 ; second half) for Post Acc compared with Post Acc þpc. Mean T C decreased by 0.58C in Post Acc þpc over the precooling period, but stayed relatively stable in both other trials. Following precooling, moderate ES suggested lower T C in Post Acc þpc compared with Pre Acc (d ¼ 0.67). The D (as can be determined from Table II) in T C over the precooling period was significantly greater during Post Acc þpc ( C) compared with Post Acc ( C; P 0.05). No significant differences (P 40.05) were recorded for mean T Sk between any trials over the precooling period. Core temperature increased by C over the first half of exercise, while mean T Sk increased by C across all trials. Moderate ES (d ¼ 0.57) suggested a lower starting T C (post warm-up) in Post Acc þpc compared with Pre Acc, whilst at the end of the first half, Post Acc T C was lower compared with Post Acc þpc (d ¼ 0.67) and Pre Acc (d ¼ 0.85; Table II). Moderate to strong ES (d ¼ ) also suggested lower starting mean T Sk values in Post Acc þpc (post warm-up) compared with both other trials (Table II). In addition, both post acclimation trials had lower mean T Sk values at the end of the first half compared with Pre Acc as suggested by moderate ES (d ¼ ). Over half-time T C decreased by C inall trials. After half-time, a strong ES (d ¼ 0.85) suggested a lower T C in Post Acc þpc and Post Acc compared with Pre Acc (see Table II). Across all trials, mean T Sk decreased by C over halftime. A moderate ES (d ¼ 0.72) calculated after halftime indicated a lower absolute mean T Sk in Post Acc þpc compared with Pre Acc (Table II). Over the second half, mean T C increased by C, whilst mean T Sk increased by C inall trials. Moderate ES at the end (d ¼ ) of the second half suggested a lower T C in both Post Acc and Post Acc þpc compared with Pre Acc. Mean

6 Repeat-sprints after acclimation and precooling 5 Downloaded by [Karen Wallman] at 14:57 11 December 2012 Table I. Mean + s (n ¼ 10) performance data for each half for the Pre Acclimation (Pre Acc), Post Acclimation Precooling (Pre Acc þpc) and Post Acclimation (Post Acc) trials. Mean + s Cohen s d Effect Size / Mean change (%) + 90 % confidence limits / Percentage chance that effect is beneficial (trivial/harmful) # Pre Acc Post Acc þpc Post Acc Post Acc þpc vs. Pre Acc Post Acc vs. Pre Acc Post Acc vs. Post Acc þpc Peak Power Output (W) 1 st Half / /27 (61/12) 0.24/ /61 (37/2) 0.17/ /42 (57/1) 2 nd Half / /27 (63/10) 0.32/ /88 (12/0) 0.22/ /54 (45/1) Peak Power (W kg 71 ) 1 st Half / /35 (51/14) 0.31/ /68 (28/4) 0.22/ /58 (37/5) 2 nd Half / /41 (45/14) 0.46/ /95 (5/0) 0.32/ /70 (27/3) Mean Power (W) 1 st Half / /53 (43/4) 0.33/ /73 (26/1) 0.10/ /20 (78/2) 2 nd Half / /38 (56/6) 0.33/ /77 (23/0) 0.16/ /32 (67/1) Overall / /46 (49/5) 0.33/ /78 (22/0) 0.13/ /25 (74/1) Work (kj) 1 st Half / /53 (43/4) 0.34/ /74 (25/1) 0.10/ /20 (78/2) 2 nd Half / /38 (56/6) 0.32/ /76 (24/0) 0.16/ /32 (67/1) Overall / /47 (48/5) 0.33/ /78 (22/0) 0.13/ /25 (74/1) Work (J kg 71 ) 1 st Half / /67 (27/6) 0.56/ /84 (14/2) 0.18/ /44 (47/9) 2 nd Half / /58 (31/11) 0.73/ /90 (9/1) 0.35/ /63 (33/4) 1 ¼ moderate effect size with Pre Acc (d ¼ ) # Where the chance of benefit or harm were both calculated to be 4 5%, the true effect was deemed unclear. Otherwise, chances of benefit or harm were assessed as follows: 5 1%, almost certainly not; 1 5%, very unlikely; 5 25%, unlikely; 25 75%, possible; 75 95%, likely; 95 99%, very likely; 4 99%, almost certain.

7 6 C. Brade et al. Table II. Mean + s (n ¼ 10) Core (T C ) and mean skin (mean T Sk ) temperature at the start and finish of each phase (precooling, first half, half-time and second half) for the Pre Acclimation (Pre Acc), Post Acclimation Precooling (Post Acc þpc) and Post Acclimation (Post Acc) trials. Precooling Period (30 min) First Half of Exercise Half-Time (10 min) Second Half of Exercise Start Finish Start Finish Start Finish Start Finish T C Pre Acc Post Acc þpc a a a a a Post Acc a,b a,b a,b a a a Mean T Sk Pre Acc Post Acc þpc a a a a a a Post Acc a b a a a ¼ moderate to large effect size with Pre Acc (d ) b ¼ moderate to large effect size with Post Acc þpc (d ) Downloaded by [Karen Wallman] at 14:57 11 December 2012 T Sk for Post Acc þpc was lower (d ¼ 0.72) at the start of the second half compared with Pre Acc, while all final values were similar. After warm-up, mean HR was beats min 71, with this value increasing to beats min 71 (across all trials) by the end of the first half of exercise. At the end of the second half of exercise, mean HR in Pre Acc was significantly higher (P 0.05) than in Post Acc þpc and Post Acc ( vs and ). While RPE and TS increased over the course of the exercise protocol, there were no significant differences between trials at any time point (P 40.05). At the end of exercise, RPE for Pre Acc, Post Acc þpc and Post Acc was , and respectively, and for TS 6 + 0, and Sweat loss was significantly greater (P 0.05) in Post Acc þpc ( kg) compared with Pre Acc ( kg) and Post Acc ( kg). In addition, sweat sensitivity (ml of sweat per 18C rise in T C ) was significantly higher (P 0.05) in both Post Acc þpc ( ) and Post Acc ( ) compared with Pre Acc ( ). Finally, there were no significant order effects (first and second half) for work (kj; P ¼ 0.30, P ¼ 0.18), work (kj kg 71 ;P¼ 0.30, P ¼ 0.15), mean power (W; P ¼ 0.31, P ¼ 0.17), peak power (W; P ¼ 0.75, P ¼ 0.73), and peak power (W kg 71 ; P¼ 0.71, P ¼ 0.70). Discussion This study aimed to determine whether partial heat acclimation would improve prolonged repeat-sprint performance and if further benefit would occur if precooling was practised prior to and during exercise performance. To the authors knowledge, this is the first study to assess the combined effects of precooling and partial acclimation on repeat-sprint performance. The main finding was that a shortterm, high-intensity exercise acclimation protocol resulted in improved exercise performance (work and power output) compared with non-acclimation, as determined by qualitative analyses and moderate to large ES. Further, precooling provided no additional benefit to exercise performance in the heat after partial acclimation. Evidence that partial acclimation occurred in our participants is provided by lower exercise HR, T C and T Sk values, as well as greater sweat loss and sweat sensitivity following the partial acclimation protocol. Our study supports the work of Sunderland et al. (2008), who also found a benefit of partial heat acclimation on subsequent exercise in team-sport athletes, although this related to exercise capacity rather than performance. Of note, the acclimation sessions used by Sunderland et al. (2008) were similar to those used in the current study in relation to the number of exercise sessions performed and the duration and intensity of exercise undertaken (i.e. 4 x min sessions of repeat-sprints compared to 5 x min sessions of repeatsprints, respectively). Further, Castle et al. (2011) reported that full heat acclimation resulted in an increased peak power output during intermittent sprint cycling (compared to a non-acclimation trial) in the heat (*338C and 50% RH). They speculated that improved exercise performance after heat acclimation might have been due to reduced reliance on muscle glycogen, as a result of lower epinephrine concentrations and reduced muscle temperatures, which resulted in a higher power output over time (King, Costill, Fink, Hargreaves, & Fielding, 1985). While these variables were not measured in the current study (or in the study by Castle et al., 2011), it is possible that some glycogen sparing effects might have occurred in our participants. Further studies are needed to

8 Downloaded by [Karen Wallman] at 14:57 11 December 2012 confirm or refute the role of these variables on exercise performance in heat acclimated individuals. Heat acclimation has been reported to delay the attainment of a critical Tc, which has consistently been linked to numerous effects that impair exercise performance (see introduction). A critical T C has been reported to be between C for prolonged intermittent exercise (Drust et al., 2005; Morris et al., 1998). While T C did not reach this critical level during exercise performance in any trial, T C was lower during both post acclimation trials, compared to the pre-acclimation trial (supported by numerous moderate to large ES), suggesting that at least partial adaptation to heat had occurred. Our study provides further support to the findings of Sunderland et al. (2008), in that partial acclimation (as well as full acclimation), can improve high intensity intermittent exercise performance in the heat. Lack of benefit associated with precooling on exercise performance in heat after acclimation was surprising, considering the number of studies that have reported improved exercise performance following this procedure. For example, Brade et al. (2012) found a benefit of precooling, using the same method employed in the current study, on similar intermittent sprint cycle exercise performance (significantly higher overall work and mean power). Similarly, Castle et al. (2006) found a 4% increase in peak power during 40-min of intermittent sprint exercise following 20-min of precooling the thighs. Whilst Duffield and Marino (2007) and Minett et al. (2011) found no significant improvements in sprint performance, they did find significant increases in the distance run during submaximal bouts of exercise following precooling. Similarly, Castle et al. (2011) reported a lack of benefit of precooling (cold packs placed around the thighs for 20-min) on intermittent sprint cycle performance in heat acclimated participants. They suggested the similar performance results between a precooling (post acclimation) and a control trial were related to the effects of a full acclimation protocol performed, which resulted in maximal physiological adaptation to heat reducing the heat strain associated with exercise. According to Duffield and Marino (2007), precooling is only of benefit when heat strain is high. In our study, reduced heat strain was supported by lower T C and mean T Sk values recorded after both acclimation trials during the entire exercise protocol (compared to the preacclimation trial), with many time points assessed supported by moderate to large ES. These results suggest that partial acclimation, similar to full acclimation, may reduce heat strain to a point where precooling confers no further advantage. Repeat-sprints after acclimation and precooling 7 Another factor that may have reduced the role of precooling in improving exercise performance in the heat may be related to the BM of acclimated participants. According to Marino, Lambert, and Noakes (2004), heavier athletes produce heat at a faster rate than lighter athletes to achieve the same work due to a higher level of metabolically active, heat-producing muscle. Based on this premise, Castle et al. (2011) suggested that precooling might be better suited to heavier athletes. This observation was based on their earlier study (Castle et al., 2006) that found a benefit of precooling (thigh cooling) on peak power output during intermittent sprint cycle performance in the heat (348C and 52% RH), where these athletes were kg heavier (yet similar skin folds) compared to athletes in their later study, where no benefit of precooling was found. However, participants in their later study were heat acclimated, (those in the earlier study were not) leaving the influence of precooling and BM on exercise-heat strain in acclimated participants unclear. As participants in the current study had a similar average BM ( kg) to participants in the later study by Castle et al. (2011; kg), it is possible that this factor might have resulted in reduced heat strain and a consequent lack of effect of precooling. Further investigation is needed to clarify the role and effect of precooling and BM on heat strain in acclimated individuals. It should also be acknowledged that participants may have paced themselves during the prolonged exercise tests (Bishop & Claudius, 2004; Bishop & Maxwell, 2009) and this may not have allowed the benefits associated with acclimation and precooling to be fully realised. Sweat loss in the current study was significantly greater (P 0.05) following acclimation and precooling compared with Pre Acc and Post Acc. This finding is consistent with our earlier work, where higher sweat loss was seen in a combined jacket and ice slushy (precooling) condition compared to a control (Brade et al., 2012). The reason for this is still unclear as generally exercise sweat losses are lower after precooling (Duffield et al., 2003; Duffield & Marino, 2007). It is possible that higher sweat rates are seen when ingesting an ice slushy as cooler fluid moves through the body at a faster rate than fluid of a higher temperature (McArdle et al., 2001). Finally, the sweat sensitivity results here are consistent with those usually seen post acclimation as an individual s sweating efficiency increases (Cohen & Gisolfi, 1982; Nielsen et al., 1993). Conclusion Partial heat acclimation improved prolonged repeatsprint performance in the heat; however precooling

9 Downloaded by [Karen Wallman] at 14:57 11 December C. Brade et al. provided no additional benefit. This would suggest that heat acclimation alone is a more powerful method for improving exercise performance in the heat than acute (pre-exercise) precooling. Of practical importance, our results suggest that if team-sport athletes are partially acclimated, precooling is then not necessary in order to enhance subsequent repeat-sprint performance in heat. References Batterham, A., & Hopkins, W. (2005). Making meaningful inferences about magnitudes. Sport Science, 9, Bishop, D., & Claudius, B. (2004). The effects of warm up on intermittent-sprint performance. Acta Kinesiologiae Universitatis Tartuensis, 9, Bishop, D., & Maxwell, N. (2009). Effects of active warm up on thermoregulation and intermittent-sprint performance in hot conditions. Journal of Science & Medicine in Sport, 12(1), Borg, G. (1970). Perceived exertion as an indicator of somatic stress. 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