- Email: [email protected]
Supplemental intermittent-day heat training and the lactate threshold
Journal of Thermal Biology 65 (2017) 16–20
Contents lists available at ScienceDirect
Journal of Thermal Biology journal homepage: www.elsevier.com/locate/jtherbio
Supplemental intermittent-day heat training and the lactate threshold a,⁎
a,b
Stuart Gollan , Samuel Chalmers
a
, Stephen Alderton , Kevin Norton
MARK
a
a
Alliance for Research in Exercise, Nutrition, and Activity (ARENA), Sansom Institute for Health Research, University of South Australia, GPO Box 2471, Adelaide, SA 5001, Australia b Sport and Exercise Science, School of Science and Health, Western Sydney University, Australia
A R T I C L E I N F O
A BS T RAC T
Keywords: Performance Hot Acclimation Intermittent Running
Heat acclimation over consecutive days has been shown to improve aerobic-based performance. Recently, it has been suggested that heat training can improve performance in a temperate environment. However, due to the multifactorial training demands of athletes, consecutive-day heat training may not be suitable. The current study aimed to investigate the effect of brief (8×30 min) intermittent (every 3–4 days) supplemental heat training on the second lactate threshold point (LT2) in temperate and hot conditions. 21 participants undertook eight intermittent-day mixed-intensity treadmill exercise training sessions in hot (30 °C; 50% relative humidity [RH]) or temperate (18 °C; 30% RH) conditions. A pre- and post-incremental exercise test occurred in temperate (18 °C; 30% RH) and hot conditions (30 °C; 50% RH) to determine the change in LT2. The heat training protocol did not improve LT2 in temperate (Effect Size [ES] ± 90 confidence interval=0.10 ± 0.16) or hot (ES=0.26 ± 0.26) conditions. The primary finding was that although the intervention group had a change greater than the SWC, no statistically significant improvements were observed following an intermittent eight day supplemental heat training protocol comparable to a control group training only in temperate conditions. This is likely due to the brief length of each heat training session and/or the long duration between each heat exposure.
1. Introduction Repeated exercise in hot conditions elicits physiological and perceptual changes, termed heat acclimation/heat acclimatization (HA), that act as an ergogenic aid for aerobic performance in hot conditions. (Garrett et al., 2012; Lorenzo et al., 2010; Sunderland et al., 2008) Previous HA research has focused on physiological adaptations in thermally challenging environments, and subsequent changes of performance in similar conditions (Périard et al., 2016; Taylor, 2014). Less research has investigated performance changes in temperate conditions following HA training (Chalmers et al., 2014; Corbett et al., 2014). The potential to improve physical output by using an environmental stimulus different to the target environment has been demonstrated with altitude training and subsequent performance at sea-level (Bonetti and Hopkins, 2009). Improved shuttle running distance in temperate conditions (i.e. < 23 °C) has been reported following a seven (7%; Buchheit et al., 2011) and 14 (44%; Racinais et al., 2014) day training camp in a hot climate. Lorenzo et al. (2010) demonstrated that 10 consecutive days of heat training (100 min sessions per day) improved VO2max (5%), lactate threshold (6%), and time trial (5%) performance
⁎
in a mild environment. Interestingly, Neal et al. (2015) reported an enhanced lactate threshold in temperate conditions after just five days (90 min sessions per day) of heat training. The aforementioned studies highlight the benefit of a consecutive day heat training stimulus for improving performance, For athletes it can be impractical during many parts of the competitive season to undertake consecutive days of heat training due to the multifactorial demands (fitness, strength, skill, and tactical) required on a weekly basis. Hence, a pragmatic alternative may be employing an intermittent-day (i.e. every other day) HA protocol to supplement existing conditioning sessions, which may comparatively elicit less fatigue than consecutive day heat training whilst potentially stimulating an ergogenic effect. A separation of no more than 72 h between heat training sessions has been suggested to still retain the positive changes induced by HA (Chalmers et al., 2014), whereas seven days may be too long (Barnett and Maughan, 1993). Heat acclimation research involving heat training on intermittent days has been explored less than consecutive day programs. Whilst continuous heat training is desirable for effective adaptation, intermittent-day training may also promote adaptation, but to a lesser extent (Gill and Sleivert, 2001). Various intermittent training protocols have been employed, including; training on alternate
Corresponding author. E-mail address: [email protected] (S. Gollan).
http://dx.doi.org/10.1016/j.jtherbio.2017.01.011 Received 15 September 2016; Received in revised form 23 January 2017; Accepted 23 January 2017 Available online 26 January 2017 0306-4565/ © 2017 Elsevier Ltd. All rights reserved.
Journal of Thermal Biology 65 (2017) 16–20
S. Gollan et al.
took four testing sessions (pre- and post-testing) and eight treadmillbased training sessions in an environmentally controlled laboratory (Munters Pty Ltd., Albury, Australia) as outlined in Fig. 1. Each participant maintained habitual training and competition volume throughout the duration of the study. Participants completed all sessions at the same time of day ( ± 2 h) to minimize changes in performance caused by circadian variance (Rajaratnam and Arendt, 2001).
days (Gill and Sleivert, 2001; Weinman et al.,, 1967), every third day (Fein et al., 1975), and four times a week (Hodge et al., 2013). Sunderland et al. (2008) reported as little as four sessions of heat training (total of 150 min heat training) over 10 days was sufficient to improve the running performance of well-trained team-sport athletes in hot conditions. Kelly et al. (2016) described a brief in-season heat training program undertaken by professional Australian Football players (total of 135 min heat training), reporting that five sessions over nine days induced partial HA. This included a significant reduction in blood lactate concentration (Δ1.5 mmol/l) for the heat training group during sub-maximal exercise in hot conditions. Work produced at the lactate threshold is a strong predictor of endurance performance and sensitive to changes in aerobic-based performance (Bassett and Howley, 2000). The changes associated with HA appear to positively influence the lactate threshold (Chalmers et al., 2014; Corbett et al., 2014; Lorenzo et al., 2010). It is conceivable that adaptations from HA can impact the blood lactate response to exercise, and thus, influence the lactate threshold. Briefly, for example, an increase in plasma volume can lead to a diluted blood lactate concentration (Corbett et al., 2014), and a reduced reliance on carbohydrate as a primary fuel source during sub-maximal exercise can have significant implications for enhancing aerobic performance (Young et al., 1985) However, it remains unknown if an intermittentday HA protocol can translate to improved aerobic-based performance for athletes competing in a less thermally challenging environment. This is pertinent for sport scientists implementing heat training protocols because consecutive and intermittent-day protocols may promote different adaptation processes (Gill and Sleivert, 2001). Rather than provide a known dose of heat exposure that might improve performance (i.e. 1000 min) (Lorenzo et al., 2010), the current research aims to further expand knowledge surrounding brief and pragmatic supplemental heat training protocols for athletes. Therefore, the study investigated whether eight intermittent-day heat training sessions that supplemented habitual training was sufficient to improve running speed at the second lactate threshold (LT2) in both temperate and hot conditions for trained athletes.
2.3. Incremental exercise test (pre- and post-testing) Each participant was instructed about the procedures and risks of the study. Height (SECA stadiometer, Germany) and body mass (Soehnle scales [ ± 0.1 g] or FV-150KA1, A & D Co. LTD, Japan [ ± 0.05 g]) were measured. A laboratory-based test was chosen to ensure testing occurred in a controlled environment. Pre-testing occurred in a randomised order between hot (30 °C; 50% RH) and temperate (18 °C; 30% RH) conditions (Fig. 1). Two post-intervention incremental tests occurred in both temperate and hot conditions within five days of completing the training sessions (Fig. 1). The incremental exercise tests occurred on a calibrated motorized treadmill (TMX425CP, Trackmaster, Full Vision Inc., Newton, USA) set with no incline. The protocol of the incremental exercise test was in accordance with the guidelines of Chalmers et al. (2015), using four-minute incremental stages. Fingertip blood samples were collected during each incremental bout to calculate blood lactate concentration using a Lactate ProTM analyser (Arkray, Kyoto, Japan). Participants were not permitted to consume fluid during testing and were instructed to avoid strenuous exercise in the 24 h prior to each incremental exercise test. Running speed at LT2 was calculated using the paired standardised lactate threshold method (LTSDp) (Chalmers et al., 2015). The calculation of LT2 is modified from the method first described by Cheng et al. (1992) and later altered in the modified method (Bishop et al., 1998). 2.4. Training sessions A minimum of five days passed from pre-testing to the first training session to negate any cumulative effects and adaptations from the pretest in hot conditions (Fig. 1). Participants were randomly assigned to one of two groups: an intervention training group (30 °C; 50% RH) or a control training group (18 °C; 30% RH), and undertook eight training sessions within a 28 day period (Fig. 1). Each session involved 30 min of treadmill running at both high and moderate speeds (Fig. 1). The combined exercise time of the training sessions totalled 240 min. Participants from each training group completed the same relative intensity of exercise, since speeds were determined according to a target percentage of LT2 measured during pre-testing in the corresponding environment (Fig. 1). Participants were not made aware of their running speed at LT2 until completion of the study. Each session was separated by 72–96 h. Participants were able to consume water ad libitum and use a towel.
2. Methods 2.1. Participants Unacclimatized and healthy volunteers were recruited to participate in the study. Participants reported four or more aerobic-based sessions prior to the study beginning (no historical data pertaining to duration, distance or RPE could be obtained). Participants maintained a training diary (duration, distance) throughout the study and reported multiple aerobic-based training sessions (≥4 sessions per week). Five participants withdrew from the study due to training commitments, and injury or illness. Nine males and three females (n=12) completed the intervention (hot group) training (age 36 ± 11 y, height 1.75 ± 0.07 m, weight 71.9 ± 9.9 kg). Seven males and two females (n=9) completed the control (temperate group) training (age 39 ± 13 y, height 1.77 ± 0.07 m, weight 71.9 ± 9.9 kg). Participants wore athletic clothing and were instructed to consistently maintain a diet they considered typical before competitive exercise. Participants were considered unacclimatized because they were residents of the study location (Adelaide, South Australia) and confirmed that they had lived consecutively in a mild environment for the preceding ≥30 days. All participants provided written informed consent and the study was approved by the University of South Australia Human Ethics Committee.
3. Statistical analysis Changes in LT2 were analyzed using repeated measures analysis of variance (ANOVA) (SPSS). Independent samples t-tests compared the initial demographic data (height, mass, and age) of each group. Data were also analyzed using magnitude based inferences, as these indicate the magnitude of an effect, which may be considered more relevant to the practical detection of changes in LT2. Data were presented as mean ± standard deviation (SD), and also effect sizes (ES) with 90% confidence intervals. All data were log transformed before analysis to reduce bias arising from non-uniformity of error (Hopkins et al., 2009). LT2 data were reported as raw values for ease of interpretation. A modified statistical spreadsheet (Hopkins, 2006) was used to calculate ES between trials using pooled standard deviation. Chances of higher or lower differences were qualitatively evaluated as follows: < 1%,
2.2. Experimental design All sessions occurred in the winter/early spring months to minimize participants acquiring natural acclimatization (mean maximum temperature ~17 °C and relative humidity [RH] ~70%). Participants under17
Journal of Thermal Biology 65 (2017) 16–20
S. Gollan et al.
Fig. 1. Timeline representing the study design. LT2 Second lactate threshold. RH relative humidity.
limited that identifies the ergogenic potential of pragmatic intermittent-day training protocols aimed at supplementing habitual training. This is important since the adaptation process may differ between consecutive and intermittent day protocols (Gill and Sleivert, 2001). The current study adds to the body of research by suggesting that greater than 240 min of moderate-high intensity heat training over intermittent days may be required to substantially improve performance in temperate conditions for trained athletes. Despite no substantial improvements occurring to LT2, a small improvement beyond the typical error (1.5%; Chalmers et al., 2015) for the LTSDp method and SWC was observed for the intervention group in temperate and hot conditions. Similarly, Chalmers et al. (2016) observed that < 240 min of heat training did not significantly improve LT2 in mild conditions, but the athletes did demonstrate a small trend for improvement. Combined, these results support the suggestion that the duration of the heat exposures was inadequate and/or the regularity of training was insufficient to promote substantial adaptations within these athlete populations. The variable individual response to brief heat training observed in the current study (Fig. 2) has also been found in previous research (Racinais et al., 2012), indicating that some athletes may positively benefit from intermittent-day heat training (Sunderland et al., 2008) whereas others may not change (Kelly et al., 2016). While this suggests a possible genetic basis for response patterns, the overall extent of change is largely a function of environmental conditions, training protocol, test type, and hydration allowances during exercise. The break between heat exposures (3–4 days) during the current study is longer than commonly reported by other research (Gill and Sleivert, 2001; Sunderland et al., 2008; Kelly et al., 2016) and it is likely this may have influenced performance changes. Additionally, the brief nature of the training sessions (30 min) may have influenced adaptation potential by restricting time spent at an elevated core temperature (typically > 38.5 C) that is considered a critical threshold for the development of acclimation (Taylor, 2014). A degree of heat tolerance to exercise can develop in response to regular aerobic training in a temperate climate (Périard et al., 2015). The participants within the current study may have a limited adaptation potential due to their already-trained status, and the brief training program was insufficient to significantly improve performance.
almost certainly not; 1–5%, very unlikely; 5–25%, unlikely; 25–75%, possibly; 75–95%, likely; 95–99%, very likely; and > 99%, almost certain. If the chance of higher and lower differences was > 5%, the true difference was assessed as unclear. Probabilities to establish whether the true (unknown) differences were lower, similar, or higher than the smallest worthwhile change (as calculated by 0.2 multiplied by between subject standard deviation) were also calculated. Chances of higher or lower differences were evaluated qualitatively as follows: < 1%, almost certainly not; 1–5%, very unlikely; 5–25%, unlikely; 25– 75%, possibly; 75–95%, likely; 95–99%, very likely; and > 99%, almost certain. 4. Results Differences between the intervention and control group were unclear for baseline height, (Effect Size [ES] ± 90% confidence intervals=−0.36 ± 0.74; chance of difference being substantially positive/ trivial/substantially negative=10/25/65%), mass (ES=−0.07 ± 0.73; 26/35/38), or age (ES=−0.21 ± 0.74; 17/32/51%). There was a possible increase in LT2 for the intervention group for both hot (ES=0.27 ± 0.33; 65/33/1%) and temperate (ES=0.26 ± 0.26; 66/33/0%) conditions. However, these increases were unclear in comparison to the control group in the hot (ES=0.25 ± 0.19; 12/62/27%) and temperate (ES=0.10 ± 0.16; 29/65/6%) conditions (Fig. 2). 5. Discussion The current study aimed to determine the effect of a supplemental intermittent eight-day heat training protocol upon LT2. The primary finding was that heat training was insufficient to significantly improve LT2 in temperate or hot conditions comparable to a control group training only in temperate conditions. It is likely that the brief duration of the heat exposures was inadequate and/or the regularity of training was insufficient to optimise aerobic performance enhancements. To the author's knowledge, the current study is the first to examine the effect of an intermittent-day heat training protocol on performance in temperate conditions. Although consecutive-day heat training is widely researched (Tyler et al., 2016), evidence-based research is 18
Journal of Thermal Biology 65 (2017) 16–20
S. Gollan et al.
Fig. 2. Individual changes to the second lactate threshold for the (a) intervention training group in mild conditions, the (b) intervention training group in hot conditions, the (c) control training group in mild conditions, and (d) control training group in hot conditions. The straight line represents the line of equality. LT2 Second lactate threshold.
(≥4 sessions aerobic exercise per week), and, by virtue of them being volunteers, it was not possible to control exercise in the days between heat sessions. This could potentially influence the physiological response to heat training. Historical training load data could not be obtained from participants, therefore, we were unable to quantify the effect of heat training upon habitual sessions during the study period. Hydration status of the participants may influence adaptation potential (Neal et al., 2015), however, it was not controlled within this study (Table 1).
Four primary physiological signs of HA have been suggested by previous research; plasma volume expansion, reduced heart rate response to sub-maximal exercise, decreased core temperature response to exercise, and a greater sweat rate during exercise (Périard et al., 2015). Theoretically, the adaptations can lead to improved aerobic performance by minimising the physiological (i.e. cardiovascular and thermoregulatory) strain on the body (Taylor, 2014). The transfer of heat training to exercise in cooler conditions may potentially be less effective due to the reduced role of the aforementioned cardiovascular and thermoregulatory adaptations during performance in temperate conditions in comparison to hot conditions. Lastly, whilst net fluid loss is expected when exercising in hot conditions, allowing participants to drink water ad libitum may reduce thermal strain and restrict performance adaptions observed in comparison to permissive dehydration (Neal et al., 2015). The strength of the current study is the use of a comparable temperate training control group. Importantly, supplemental heat training as part of a habitual training routine was not detrimental to performance in temperate conditions. Athletes seeking to use intermittent day heat training as an aerobic performance booster should aim to train more frequently than twice a week, and extend each session duration beyond 30 min to ensure sufficient time above a target core temperature. Further research is required to investigate the resultant effect on performance by manipulating the duration between heat exposures. Some limitations are contained within the study. The participants were volunteers who reported regular training
6. Practical applications Longer duration training sessions ( > 30 min) in unison with shorter break periods (≤3 days) between heat exposures may be required to elicit a significant change in aerobic-based performance in trained athletes. 7. Conclusion The primary finding was that an intermittent eight-day heat training protocol was insufficient to significantly improve LT2 in temperate or hot conditions comparable to a control group training only in temperate conditions. Extended breaks of 72–96 h between short bouts of heat training appears too long to result in optimal aerobic-based performance benefits in trained athletes. 19
Journal of Thermal Biology 65 (2017) 16–20
S. Gollan et al.
Table 1 The change in LT2 for the control and intervention groups in temperate and hot conditions (mean ± SD); CI Confidence Intervals. Testing Conditions
Training group
Before (km h−1)
After (km h−1)
Change ± 90% CI (km h−1)
Change ± 90% CI (%)
Effect size ± 90% CI
Within group likelihood of change
Between group likelihood of change
Temperate
Intervention Control Intervention Control
13.35 ± 1.00 13.34 ± 1.46 13.26 ± 0.94 12.99 ± 1.22
13.63 ± 0.97 13.48 ± 1.11 13.54 ± 1.09 13.22 ± 1.31
0.28 ± 0.27 0.14 ± 0.24 0.29 ± 0.34 0.34 ± 0.26
2.1 ± 2.1 1.3 ± 3.1 2.2 ± 5.1 1.7 ± 3.1
0.26 ± 0.26 0.10 ± 0.16 0.27 ± 0.33 0.25 ± 0.19
Possible Likely trivial Possible Possible
Unclear
Hot
Unclear
on heat acclimation of women. Int. J. Biometeorol. 19 (1), 41–52. Garrett, A., Creasy, R., Rehrer, N., Patterson, M., Cotter, J., 2012. Effectiveness of shortterm heat acclimation for highly trained athletes. Eur. J. Appl. Physiol. 112 (5), 1827–1837. Gill, N., Sleivert, G., 2001. Effect of daily versus intermittent exposure on heat acclimation. Aviat. Sp. Environ. Med. 72 (4), 385–390. Hodge, D., Jones, D., Martinez, R., Buono, M.J., 2013. Time course of the attenuation of sympathetic nervous activity during active heat acclimation. Auton. Neurosci. 177 (2), 101–103. Hopkins, W.G., 2006. Spreadsheets for analysis of controlled trials, with adjustment for a subject characteristic. Sports Sci. 10, 46–50. Hopkins, W.G., Marshall, S.W., Batterham, A.M., Hanin, J., 2009. Progressive statistics for studies in sports medicine and exercise science. Med. Sci. Sports Exerc. 41, 3. Kelly, M., Gastin, P.B., Dwyer, D.B., Sostaric, S., Snow, R.J., 2016. Short duration heat acclimation in Australian football players. J. Sports Sci. 15 (1), 118–125. Lorenzo, S., Halliwill, J.R., Sawka, M.N., Minson, C.T., 2010. Heat acclimation improves exercise performance. J. Appl. Physiol. 109 (4), 1140–1147. Neal, R.A., Corbett, J., Massey, H.C., Tipton, M.J., 2015. Effect of short-term heat acclimation with permissive dehydration on thermoregulation and temperate exercise performance. Scand. J. Med. Sci. Sport.. Périard, J., Racinais, S., Sawka, M., 2015. Adaptations and mechanisms of human heat adaptation: applications for competitive athletes and sports. Scand. J. Med. Sci. Sports 25, 20–38. Racinais, S., Mohr, M., Buchheit, M., Voss, S., Gaoua, N., Grantham, J., Nybo, L., 2012. Individual responses to short-term heat acclimatisation as predictors of football performance in a hot, dry environment. Br. J. Sports Med. 0, 1–7. Racinais, S., Buchheit, M., Bilsborough, J., Bourdon, P.C., Cordy, J., Coutts, A.J., 2014. Physiological and performance responses to a training-camp in the heat in professional Australian football players. Int. J. Sports Physiol. Perf. 9 (4), 598–603. Rajaratnam, S., Arendt, J., 2001. Health in a 24-h society. Lancet 358, 999–1005. Sunderland, C., Morris, J.G., Nevill, M.E., 2008. A heat acclimation protocol for team sports. Br. J. Sports Med. 42 (5), 327–333. Taylor, N.A., 2014. Human heat adaptation. Compr. Physiol. 4 (1), 325–365. Tyler, C.J., Reeve, T., Hodges, G.J., Cheung, S.S., 2016. The effects of heat adaptation on physiology, perception and exercise performance in the heat: a meta-analysis. Sports Med., 1–26. Weinman, K., Slabochov, Z., Bernauer, M., Morimoto, T., Sargent, F., 1967. Reactions of men and women to repeated exposure to humid heat. J. Appl. Physiol. 22 (3), 553–558. Young, A., Sawka, M., Levine, L., Cadarette, B., Pandolf, K., 1985. Skeletal muscle metabolism during exercise is influenced by heat acclimation. J. Appl. Physiol. 59 (6), 1929–1935.
Acknowledgments The authors would like to thank the participants for volunteering their time and effort for the study. The authors report no conflict of interest and no financial assistance was obtained for the study. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References Barnett, A., Maughan, R., 1993. Response of unacclimatized males to repeated weekly bouts of exercise in the heat. Br. J. Sports Med. 27 (1), 39–44. Bassett, D.R., Jr, Howley, E.T., 2000. Limiting factors for maximum oxygen uptake and determinants of endurance performance. Med. Sci. Sports Exerc. 32 (1), 70–84. Bishop, D., Jenkins, D.G., Mackinnon, L.T., 1998. The relationship between plasma lactate parameters, Wpeak and 1-h cycling performance in women. Med. Sci. Sports Exerc. 30 (8), 1270–1275. Bonetti, D.L., Hopkins, W.G., 2009. Sea-level exercise performance following adaptation to hypoxia: a meta-analysis. Sports Med. 39 (2), 107–127. Buchheit, M., Voss, S.C., Nybo, L., Mohr, M., Racinais, S., 2011. Physiological and performance adaptations to an in-season soccer camp in the heat: associations with heart rate and heart rate variability. Scand. J. Med. Sci. Sports 21, e477–e485. Chalmers, S., Esterman, A., Eston, R., Bowering, K.J., Norton, K., 2014. Short-term heat acclimation training improves physical performance: a systematic review, and exploration of physiological adaptations and application for team sports. Sports Med. 44 (7), 971–988. Chalmers, S., Esterman, A., Eston, R., Norton, K., 2015. Standardisation of the Dmax method for calculating the second lactate threshold. Int. J. Sports Physiol. Perform. 10 (7), 921–926. Chalmers, S., Esterman, A., Eston, R., Norton, K., 2016. Brief heat training does not improve the lactate threshold in mild conditions. Int. J. Sports Physiol. Perform. 24, 1–24. Cheng, B., Kuipers, H., Snyder, A., Keizer, H., Jeukendrup, A., Hesselink, M., 1992. A new approach for the determination of ventilatory and lactate thresholds. Int. J. Sports Med. 13 (7), 518–522. Corbett, J., Neal, R.A., Lunt, H.C., Tipton, M.J., 2014. Adaptation to heat and exercise performance under cooler conditions: a new hot topic. Sports Med. 44 (10), 1323–1331. Fein, J.T., Haymes, E.M., Buskirk, E.R., 1975. Effects of daily and intermittent exposures
20
Contents lists available at ScienceDirect
Journal of Thermal Biology journal homepage: www.elsevier.com/locate/jtherbio
Supplemental intermittent-day heat training and the lactate threshold a,⁎
a,b
Stuart Gollan , Samuel Chalmers
a
, Stephen Alderton , Kevin Norton
MARK
a
a
Alliance for Research in Exercise, Nutrition, and Activity (ARENA), Sansom Institute for Health Research, University of South Australia, GPO Box 2471, Adelaide, SA 5001, Australia b Sport and Exercise Science, School of Science and Health, Western Sydney University, Australia
A R T I C L E I N F O
A BS T RAC T
Keywords: Performance Hot Acclimation Intermittent Running
Heat acclimation over consecutive days has been shown to improve aerobic-based performance. Recently, it has been suggested that heat training can improve performance in a temperate environment. However, due to the multifactorial training demands of athletes, consecutive-day heat training may not be suitable. The current study aimed to investigate the effect of brief (8×30 min) intermittent (every 3–4 days) supplemental heat training on the second lactate threshold point (LT2) in temperate and hot conditions. 21 participants undertook eight intermittent-day mixed-intensity treadmill exercise training sessions in hot (30 °C; 50% relative humidity [RH]) or temperate (18 °C; 30% RH) conditions. A pre- and post-incremental exercise test occurred in temperate (18 °C; 30% RH) and hot conditions (30 °C; 50% RH) to determine the change in LT2. The heat training protocol did not improve LT2 in temperate (Effect Size [ES] ± 90 confidence interval=0.10 ± 0.16) or hot (ES=0.26 ± 0.26) conditions. The primary finding was that although the intervention group had a change greater than the SWC, no statistically significant improvements were observed following an intermittent eight day supplemental heat training protocol comparable to a control group training only in temperate conditions. This is likely due to the brief length of each heat training session and/or the long duration between each heat exposure.
1. Introduction Repeated exercise in hot conditions elicits physiological and perceptual changes, termed heat acclimation/heat acclimatization (HA), that act as an ergogenic aid for aerobic performance in hot conditions. (Garrett et al., 2012; Lorenzo et al., 2010; Sunderland et al., 2008) Previous HA research has focused on physiological adaptations in thermally challenging environments, and subsequent changes of performance in similar conditions (Périard et al., 2016; Taylor, 2014). Less research has investigated performance changes in temperate conditions following HA training (Chalmers et al., 2014; Corbett et al., 2014). The potential to improve physical output by using an environmental stimulus different to the target environment has been demonstrated with altitude training and subsequent performance at sea-level (Bonetti and Hopkins, 2009). Improved shuttle running distance in temperate conditions (i.e. < 23 °C) has been reported following a seven (7%; Buchheit et al., 2011) and 14 (44%; Racinais et al., 2014) day training camp in a hot climate. Lorenzo et al. (2010) demonstrated that 10 consecutive days of heat training (100 min sessions per day) improved VO2max (5%), lactate threshold (6%), and time trial (5%) performance
⁎
in a mild environment. Interestingly, Neal et al. (2015) reported an enhanced lactate threshold in temperate conditions after just five days (90 min sessions per day) of heat training. The aforementioned studies highlight the benefit of a consecutive day heat training stimulus for improving performance, For athletes it can be impractical during many parts of the competitive season to undertake consecutive days of heat training due to the multifactorial demands (fitness, strength, skill, and tactical) required on a weekly basis. Hence, a pragmatic alternative may be employing an intermittent-day (i.e. every other day) HA protocol to supplement existing conditioning sessions, which may comparatively elicit less fatigue than consecutive day heat training whilst potentially stimulating an ergogenic effect. A separation of no more than 72 h between heat training sessions has been suggested to still retain the positive changes induced by HA (Chalmers et al., 2014), whereas seven days may be too long (Barnett and Maughan, 1993). Heat acclimation research involving heat training on intermittent days has been explored less than consecutive day programs. Whilst continuous heat training is desirable for effective adaptation, intermittent-day training may also promote adaptation, but to a lesser extent (Gill and Sleivert, 2001). Various intermittent training protocols have been employed, including; training on alternate
Corresponding author. E-mail address: [email protected] (S. Gollan).
http://dx.doi.org/10.1016/j.jtherbio.2017.01.011 Received 15 September 2016; Received in revised form 23 January 2017; Accepted 23 January 2017 Available online 26 January 2017 0306-4565/ © 2017 Elsevier Ltd. All rights reserved.
Journal of Thermal Biology 65 (2017) 16–20
S. Gollan et al.
took four testing sessions (pre- and post-testing) and eight treadmillbased training sessions in an environmentally controlled laboratory (Munters Pty Ltd., Albury, Australia) as outlined in Fig. 1. Each participant maintained habitual training and competition volume throughout the duration of the study. Participants completed all sessions at the same time of day ( ± 2 h) to minimize changes in performance caused by circadian variance (Rajaratnam and Arendt, 2001).
days (Gill and Sleivert, 2001; Weinman et al.,, 1967), every third day (Fein et al., 1975), and four times a week (Hodge et al., 2013). Sunderland et al. (2008) reported as little as four sessions of heat training (total of 150 min heat training) over 10 days was sufficient to improve the running performance of well-trained team-sport athletes in hot conditions. Kelly et al. (2016) described a brief in-season heat training program undertaken by professional Australian Football players (total of 135 min heat training), reporting that five sessions over nine days induced partial HA. This included a significant reduction in blood lactate concentration (Δ1.5 mmol/l) for the heat training group during sub-maximal exercise in hot conditions. Work produced at the lactate threshold is a strong predictor of endurance performance and sensitive to changes in aerobic-based performance (Bassett and Howley, 2000). The changes associated with HA appear to positively influence the lactate threshold (Chalmers et al., 2014; Corbett et al., 2014; Lorenzo et al., 2010). It is conceivable that adaptations from HA can impact the blood lactate response to exercise, and thus, influence the lactate threshold. Briefly, for example, an increase in plasma volume can lead to a diluted blood lactate concentration (Corbett et al., 2014), and a reduced reliance on carbohydrate as a primary fuel source during sub-maximal exercise can have significant implications for enhancing aerobic performance (Young et al., 1985) However, it remains unknown if an intermittentday HA protocol can translate to improved aerobic-based performance for athletes competing in a less thermally challenging environment. This is pertinent for sport scientists implementing heat training protocols because consecutive and intermittent-day protocols may promote different adaptation processes (Gill and Sleivert, 2001). Rather than provide a known dose of heat exposure that might improve performance (i.e. 1000 min) (Lorenzo et al., 2010), the current research aims to further expand knowledge surrounding brief and pragmatic supplemental heat training protocols for athletes. Therefore, the study investigated whether eight intermittent-day heat training sessions that supplemented habitual training was sufficient to improve running speed at the second lactate threshold (LT2) in both temperate and hot conditions for trained athletes.
2.3. Incremental exercise test (pre- and post-testing) Each participant was instructed about the procedures and risks of the study. Height (SECA stadiometer, Germany) and body mass (Soehnle scales [ ± 0.1 g] or FV-150KA1, A & D Co. LTD, Japan [ ± 0.05 g]) were measured. A laboratory-based test was chosen to ensure testing occurred in a controlled environment. Pre-testing occurred in a randomised order between hot (30 °C; 50% RH) and temperate (18 °C; 30% RH) conditions (Fig. 1). Two post-intervention incremental tests occurred in both temperate and hot conditions within five days of completing the training sessions (Fig. 1). The incremental exercise tests occurred on a calibrated motorized treadmill (TMX425CP, Trackmaster, Full Vision Inc., Newton, USA) set with no incline. The protocol of the incremental exercise test was in accordance with the guidelines of Chalmers et al. (2015), using four-minute incremental stages. Fingertip blood samples were collected during each incremental bout to calculate blood lactate concentration using a Lactate ProTM analyser (Arkray, Kyoto, Japan). Participants were not permitted to consume fluid during testing and were instructed to avoid strenuous exercise in the 24 h prior to each incremental exercise test. Running speed at LT2 was calculated using the paired standardised lactate threshold method (LTSDp) (Chalmers et al., 2015). The calculation of LT2 is modified from the method first described by Cheng et al. (1992) and later altered in the modified method (Bishop et al., 1998). 2.4. Training sessions A minimum of five days passed from pre-testing to the first training session to negate any cumulative effects and adaptations from the pretest in hot conditions (Fig. 1). Participants were randomly assigned to one of two groups: an intervention training group (30 °C; 50% RH) or a control training group (18 °C; 30% RH), and undertook eight training sessions within a 28 day period (Fig. 1). Each session involved 30 min of treadmill running at both high and moderate speeds (Fig. 1). The combined exercise time of the training sessions totalled 240 min. Participants from each training group completed the same relative intensity of exercise, since speeds were determined according to a target percentage of LT2 measured during pre-testing in the corresponding environment (Fig. 1). Participants were not made aware of their running speed at LT2 until completion of the study. Each session was separated by 72–96 h. Participants were able to consume water ad libitum and use a towel.
2. Methods 2.1. Participants Unacclimatized and healthy volunteers were recruited to participate in the study. Participants reported four or more aerobic-based sessions prior to the study beginning (no historical data pertaining to duration, distance or RPE could be obtained). Participants maintained a training diary (duration, distance) throughout the study and reported multiple aerobic-based training sessions (≥4 sessions per week). Five participants withdrew from the study due to training commitments, and injury or illness. Nine males and three females (n=12) completed the intervention (hot group) training (age 36 ± 11 y, height 1.75 ± 0.07 m, weight 71.9 ± 9.9 kg). Seven males and two females (n=9) completed the control (temperate group) training (age 39 ± 13 y, height 1.77 ± 0.07 m, weight 71.9 ± 9.9 kg). Participants wore athletic clothing and were instructed to consistently maintain a diet they considered typical before competitive exercise. Participants were considered unacclimatized because they were residents of the study location (Adelaide, South Australia) and confirmed that they had lived consecutively in a mild environment for the preceding ≥30 days. All participants provided written informed consent and the study was approved by the University of South Australia Human Ethics Committee.
3. Statistical analysis Changes in LT2 were analyzed using repeated measures analysis of variance (ANOVA) (SPSS). Independent samples t-tests compared the initial demographic data (height, mass, and age) of each group. Data were also analyzed using magnitude based inferences, as these indicate the magnitude of an effect, which may be considered more relevant to the practical detection of changes in LT2. Data were presented as mean ± standard deviation (SD), and also effect sizes (ES) with 90% confidence intervals. All data were log transformed before analysis to reduce bias arising from non-uniformity of error (Hopkins et al., 2009). LT2 data were reported as raw values for ease of interpretation. A modified statistical spreadsheet (Hopkins, 2006) was used to calculate ES between trials using pooled standard deviation. Chances of higher or lower differences were qualitatively evaluated as follows: < 1%,
2.2. Experimental design All sessions occurred in the winter/early spring months to minimize participants acquiring natural acclimatization (mean maximum temperature ~17 °C and relative humidity [RH] ~70%). Participants under17
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Fig. 1. Timeline representing the study design. LT2 Second lactate threshold. RH relative humidity.
limited that identifies the ergogenic potential of pragmatic intermittent-day training protocols aimed at supplementing habitual training. This is important since the adaptation process may differ between consecutive and intermittent day protocols (Gill and Sleivert, 2001). The current study adds to the body of research by suggesting that greater than 240 min of moderate-high intensity heat training over intermittent days may be required to substantially improve performance in temperate conditions for trained athletes. Despite no substantial improvements occurring to LT2, a small improvement beyond the typical error (1.5%; Chalmers et al., 2015) for the LTSDp method and SWC was observed for the intervention group in temperate and hot conditions. Similarly, Chalmers et al. (2016) observed that < 240 min of heat training did not significantly improve LT2 in mild conditions, but the athletes did demonstrate a small trend for improvement. Combined, these results support the suggestion that the duration of the heat exposures was inadequate and/or the regularity of training was insufficient to promote substantial adaptations within these athlete populations. The variable individual response to brief heat training observed in the current study (Fig. 2) has also been found in previous research (Racinais et al., 2012), indicating that some athletes may positively benefit from intermittent-day heat training (Sunderland et al., 2008) whereas others may not change (Kelly et al., 2016). While this suggests a possible genetic basis for response patterns, the overall extent of change is largely a function of environmental conditions, training protocol, test type, and hydration allowances during exercise. The break between heat exposures (3–4 days) during the current study is longer than commonly reported by other research (Gill and Sleivert, 2001; Sunderland et al., 2008; Kelly et al., 2016) and it is likely this may have influenced performance changes. Additionally, the brief nature of the training sessions (30 min) may have influenced adaptation potential by restricting time spent at an elevated core temperature (typically > 38.5 C) that is considered a critical threshold for the development of acclimation (Taylor, 2014). A degree of heat tolerance to exercise can develop in response to regular aerobic training in a temperate climate (Périard et al., 2015). The participants within the current study may have a limited adaptation potential due to their already-trained status, and the brief training program was insufficient to significantly improve performance.
almost certainly not; 1–5%, very unlikely; 5–25%, unlikely; 25–75%, possibly; 75–95%, likely; 95–99%, very likely; and > 99%, almost certain. If the chance of higher and lower differences was > 5%, the true difference was assessed as unclear. Probabilities to establish whether the true (unknown) differences were lower, similar, or higher than the smallest worthwhile change (as calculated by 0.2 multiplied by between subject standard deviation) were also calculated. Chances of higher or lower differences were evaluated qualitatively as follows: < 1%, almost certainly not; 1–5%, very unlikely; 5–25%, unlikely; 25– 75%, possibly; 75–95%, likely; 95–99%, very likely; and > 99%, almost certain. 4. Results Differences between the intervention and control group were unclear for baseline height, (Effect Size [ES] ± 90% confidence intervals=−0.36 ± 0.74; chance of difference being substantially positive/ trivial/substantially negative=10/25/65%), mass (ES=−0.07 ± 0.73; 26/35/38), or age (ES=−0.21 ± 0.74; 17/32/51%). There was a possible increase in LT2 for the intervention group for both hot (ES=0.27 ± 0.33; 65/33/1%) and temperate (ES=0.26 ± 0.26; 66/33/0%) conditions. However, these increases were unclear in comparison to the control group in the hot (ES=0.25 ± 0.19; 12/62/27%) and temperate (ES=0.10 ± 0.16; 29/65/6%) conditions (Fig. 2). 5. Discussion The current study aimed to determine the effect of a supplemental intermittent eight-day heat training protocol upon LT2. The primary finding was that heat training was insufficient to significantly improve LT2 in temperate or hot conditions comparable to a control group training only in temperate conditions. It is likely that the brief duration of the heat exposures was inadequate and/or the regularity of training was insufficient to optimise aerobic performance enhancements. To the author's knowledge, the current study is the first to examine the effect of an intermittent-day heat training protocol on performance in temperate conditions. Although consecutive-day heat training is widely researched (Tyler et al., 2016), evidence-based research is 18
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Fig. 2. Individual changes to the second lactate threshold for the (a) intervention training group in mild conditions, the (b) intervention training group in hot conditions, the (c) control training group in mild conditions, and (d) control training group in hot conditions. The straight line represents the line of equality. LT2 Second lactate threshold.
(≥4 sessions aerobic exercise per week), and, by virtue of them being volunteers, it was not possible to control exercise in the days between heat sessions. This could potentially influence the physiological response to heat training. Historical training load data could not be obtained from participants, therefore, we were unable to quantify the effect of heat training upon habitual sessions during the study period. Hydration status of the participants may influence adaptation potential (Neal et al., 2015), however, it was not controlled within this study (Table 1).
Four primary physiological signs of HA have been suggested by previous research; plasma volume expansion, reduced heart rate response to sub-maximal exercise, decreased core temperature response to exercise, and a greater sweat rate during exercise (Périard et al., 2015). Theoretically, the adaptations can lead to improved aerobic performance by minimising the physiological (i.e. cardiovascular and thermoregulatory) strain on the body (Taylor, 2014). The transfer of heat training to exercise in cooler conditions may potentially be less effective due to the reduced role of the aforementioned cardiovascular and thermoregulatory adaptations during performance in temperate conditions in comparison to hot conditions. Lastly, whilst net fluid loss is expected when exercising in hot conditions, allowing participants to drink water ad libitum may reduce thermal strain and restrict performance adaptions observed in comparison to permissive dehydration (Neal et al., 2015). The strength of the current study is the use of a comparable temperate training control group. Importantly, supplemental heat training as part of a habitual training routine was not detrimental to performance in temperate conditions. Athletes seeking to use intermittent day heat training as an aerobic performance booster should aim to train more frequently than twice a week, and extend each session duration beyond 30 min to ensure sufficient time above a target core temperature. Further research is required to investigate the resultant effect on performance by manipulating the duration between heat exposures. Some limitations are contained within the study. The participants were volunteers who reported regular training
6. Practical applications Longer duration training sessions ( > 30 min) in unison with shorter break periods (≤3 days) between heat exposures may be required to elicit a significant change in aerobic-based performance in trained athletes. 7. Conclusion The primary finding was that an intermittent eight-day heat training protocol was insufficient to significantly improve LT2 in temperate or hot conditions comparable to a control group training only in temperate conditions. Extended breaks of 72–96 h between short bouts of heat training appears too long to result in optimal aerobic-based performance benefits in trained athletes. 19
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Table 1 The change in LT2 for the control and intervention groups in temperate and hot conditions (mean ± SD); CI Confidence Intervals. Testing Conditions
Training group
Before (km h−1)
After (km h−1)
Change ± 90% CI (km h−1)
Change ± 90% CI (%)
Effect size ± 90% CI
Within group likelihood of change
Between group likelihood of change
Temperate
Intervention Control Intervention Control
13.35 ± 1.00 13.34 ± 1.46 13.26 ± 0.94 12.99 ± 1.22
13.63 ± 0.97 13.48 ± 1.11 13.54 ± 1.09 13.22 ± 1.31
0.28 ± 0.27 0.14 ± 0.24 0.29 ± 0.34 0.34 ± 0.26
2.1 ± 2.1 1.3 ± 3.1 2.2 ± 5.1 1.7 ± 3.1
0.26 ± 0.26 0.10 ± 0.16 0.27 ± 0.33 0.25 ± 0.19
Possible Likely trivial Possible Possible
Unclear
Hot
Unclear
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