Recent investigations examining the physiological demands imposed on international caliber female field hockey players have demonstrated the importance of elevated and sustained cardiovascular performance throughout training (33) and competition (29,32). Recent reviews focusing on the physiological adaptations to heat stress for maximizing elite athlete performance have highlighted the rapid development of hypervolemia and concomitant improvements in cardiovascular performance measures, typically observed within 7 exposures (19,34). This early onset of hypervolemia has demonstrated improvements in maximal cardiac output, stroke volume, and aerobic power (V̇o2max) in both hot and cool environments (28). Such physiological and performance adaptations may be beneficial for elite field hockey players before entering a competition period.
Although this evidence is compelling for coaches and practitioners, recent systematic reviews have highlighted the current paucity of inquiries focusing on heat stress techniques and their associated physiological and performance adaptions in elite female athletes (13), specifically in female team sport athletes (8). Moreover, a developing consensus for identifying new heat stress techniques tailored for elite athletes involving minimal exercise stimuli (7), while capable of simultaneously accommodating a team of athletes, warrants further exploration.
Recent evidence has demonstrated hot yoga to elicit minimal cardiovascular strain when performed in ambient environments ranging from 35 to 40° C and 20 to 40% relative humidity (5,26). Moreover, the typical size of a hot yoga studio can accommodate large numbers of participants and may be an ideal solution to exposing a team of athletes simultaneously to a heat stress. As such, the purpose of this investigation was to examine the effectiveness of hot yoga to act as an innovative heat stress technique capable of inducing hypervolemia and augmenting cardiovascular performance. A secondary focus examining a 6-day postintervention period, involving simulated competition within a national field hockey team was used to examine the utility of hot yoga to elicit a prolonged hypervolemic response before entering a competition period. In conducting this investigation, we hypothesized that 6 days of hot yoga would augment cardiovascular performance and elicit a hypervolemic response that would remain elevated over a 6-day competition period.
Experimental Approach to the Problem
This investigation involved a quasiexperimental design involving six 60-minute hot yoga sessions using permissive dehydration, over 6 consecutive days. A graded maximal exercise test was completed 24 hours before and 24 hours after intervention to examine cardiovascular performance adaptations. After completing 6 days of hot yoga, a descriptive correlation design over a 6-day national team camp, involving 2 intrasquad matches and a fitness test was used to examine the utility of hot yoga to elicit a prolonged plasma volume expansion during a simulated competition period (Figure 1).
When using the conventional limits of α (0.05) and β (0.20), an analysis for selecting an appropriate sample size was conducted using a predeveloped spreadsheet (25). When using the SE of 0.05 from a V̇o2max reliability study (40) involving similar duration between V̇o2 assessments, it was revealed 9 participants would provide enough power to detect the smallest meaningful change in performance when represented as an effect size (ES) of 0.20 (21). Two months before the investigation, participants trained within their national field hockey team in an outdoor environment and were exposed to ambient conditions ranging from 4 to 15° C. Environmental conditions throughout the investigation are represented as ambient temperature (° C), relative humidity (%), and wet-bulb globe temperature (WBGT).
Ten (n = 10) international caliber female field hockey players training as part of a national team acted as participants over a 2-week training block during preparation for the 2016 Rio Olympic Games. Mean age (± SD) for subjects was 25.5 yrs (3.0), additional characteristics are provided in Table 1. Only outfield players (non–goal keepers) (n = 9) were examined for changes in cardiovascular performance measures. Each athlete provided written informed consent to participate in the investigation and completed a health screening questionnaire (39) before the intervention. The current investigation was approved by and followed the recommendations of the Clinical Research Ethics Review Board at the University of British Columbia.
Cardiovascular Performance Assessment
A graded maximal exercise test was completed 24 hours before and 24 hours after intervention with the use of a computerized controlled treadmill (Woodway ELG, Woodway USA, Foster Court, WI, USA). Participants were informed to refrain from physical activity 24 hours before each exercise test and to standardize their hydration and nutrition practices before testing. Each exercise test was standardized to begin at the same time of day for both the preassessment and postassessment.
Participants were fitted with a mouth piece consisting of a 1-way breathing valve (Hans Rudolph, Kansas City, MO, USA) for the collection of expired gas concentrations and were analyzed using a calibrated metabolic cart (ParvoMedics TrueOne 2400). Each assessment involved a treadmill grade of 2.0% held throughout the test with a starting speed of 8.1 km·h−1 and involved increments of 0.8 km·h−1 every 3 minutes until volitional fatigue. Participants were verbally encouraged throughout each test.
Ventilatory thresholds (VT1 and VT2) were determined between each stage of the exercise test through examining alterations in minute ventilation (VE), fraction of expired oxygen (FEO2), fraction of expired carbon dioxide (FECO2), and the respiratory exchange ratio (RER) (37). Continuous heart rate (HR) was recorded using an HR monitor equipped to sample at 1,000 Hz and programed to record at 1-second intervals (Polar, Electro, Oy, Kempele, Finland). Run time to exhaustion was defined as the total time completed on test termination. Maximum oxygen pulse (ml·beat−1) was calculated using the absolute oxygen consumption (ml) divided by the HR recorded over the last 60 seconds of the graded exercise test. Maximum minute ventilation as expressed through body temperature, ambient pressure, saturated (VE-BTPS) (L·min−1) was calculated using the last 60 seconds of the graded exercise test. Change in the RER was calculated using 60 seconds of the last completed stage. V̇o2max was identified through an increase in workload that was followed by a plateau, decrease, or an increase in oxygen consumption of <150 ml·min−1 when averaging the last two 30-second sampling intervals of each stage (38).
Environmental conditions recorded within the laboratory during each assessment were as follows: TA (° C) = 23.2 ± 1.0, WBGT = 15.6 ± 0.8, and relative humidity (RH) % = 27.3 ± 9.4. All participants were familiar with completing maximal testing having routinely participated in similar protocols within the national field hockey team.
Hot Yoga Intervention
Six hot yoga classes consisting of 30 minutes of dynamic movements followed by 30 minutes of static stretching were performed over 6 consecutive days between 08:00 hour and 11:00 hour. Over the 6-day intervention period, hot yoga was the only form of exercise completed by each participant. Environmental conditions within the yoga studio were regulated by far-infrared heating ceiling panels and were recorded as follows: TA (° C) = 30.0 ± 1.8, WBGT = 24.4 ± 1.7, and RH% = 47.6 ± 8.8. Permissive dehydration, a technique previously demonstrated for enhancing the physiological response to repeated heat stress (18), was used during each hot yoga class and involved refraining from fluid consumption throughout each 60-minute class. Participants followed their daily nutrition and hydration regime as provided by the national team–registered dietician and were encouraged to standardize their intake before arriving for each hot yoga class in the effort to standardize pre-exercise hydration status. Consumption of fluid and sodium involved ad libitum throughout the intervention and national team camp period. Before and on completion of each hot yoga class, dry body mass (kg) was recorded for examining sweat loss. Anthropometric characteristics such as body surface area (BSA) and body surface to mass ratio (AD/M) were estimated using height and body mass (16).
Self-reported usage of ethyl estradiol-progestrin oral contraceptives (n = 10) and the typical length and current stage of their menstrual cycle was provided [follicular phase (n = 5) and luteal phase (n = 4)]. If participants were transitioning between menstrual phases (n = 1) during the intervention period, they were not included in the analysis when examining plasma volume percentage (PV%) differences between menstrual phases.
Postintervention and Competition Period (National Team Camp)
All participants took part in their national team camp 48 hours after completing the postintervention-graded exercise test. Environmental conditions throughout the camp were as follows: TA (° C) = 11.8 ± 0.50, WBGT = 9.8 ± 0.80, and RH% = 83 ± 4.2. The national team camp involved a Yo-Yo Intermittent Recovery Test (1) on day 9 (48 hours after intervention) and 2 intrasquad scrimmages, on day 10 and 11 (72 and 96 hours after intervention). Intrasquad matches involved four 10-minute periods with a 10-minute half-time break.
Cardiovascular Indices of Exercise Stress (Training Load)
Each participant was provided a Polar Team2 heart rate monitor (Polar, Electro, Oy), equipped to sample at 1,000 Hz, and programed to record at 1-second intervals 10 minutes before the start of each hot yoga class and for all field hockey–oriented activities throughout the investigation. Markers of cardiovascular stress examined throughout each hot yoga class consisted of percentage of maximum HR and an HR-derived training load. An HR-derived load was used to quantify the exercise stress experienced for all field hockey–oriented activities. Training loads were derived incorporating Banister's original training impulse method (2,3) that was customized with a recovery estimation model (31). Maximum HR values were determined from the preintervention-graded exercise test and were used in the equation for developing a training load. The calculation of a training load value was standardized to begin at the start of each yoga class or warm-up period preceding field hockey–oriented activities, until the completion of the hot yoga class or cool down after field hockey activities. Training loads were automatically calculated in real time using the Polar Team2 heart rate monitoring system and were exported into a Microsoft Excel spreadsheet for analysis.
Training loads were expressed as both arbitrary units (AUs) and as a percentage (% Max) of the most recent maximum weekly training load value experienced from on-field technical-, tactical-, and physiological-oriented hockey drills. This perspective was used to demonstrate the exercise stress elicited from hot yoga in relation to regular field hockey training and to examine the practicality of incorporating hot yoga into the daily training schedule without adding a significant training load.
Internal Core Temperature Assessment
Before each hot yoga class, participants consumed an U.S. Food and Drug Administration–approved ingestible thermometer pill (CorTemp; HQ, Inc., Palmetto, FL, USA), a device previously validated against rectal temperature the accepted gold standard (6). Each ingestible thermometer pill was calibrated before ingestion. Pills were immersed in a controlled water bath (±0.01° C) and allowed to stabilize for 15 minutes before developing a calibration slope and intercept using several data sets for each pill between the range of 35–45° C. To uphold an acceptable level of measurement reliability, each pill was consumed on an empty stomach a minimum of 60 minutes before each hot yoga class (15).
A schematic specifying the time points of each blood collection is provided in Figure 1. Each collection was taken between 07:00 hour and 10:00 hour throughout the 13-day investigation after lying supine for the recommended 20-minute period (27). A 4-ml blood collection taken from the antecubital vein using a butterfly venepuncture technique was used to examine alterations in hematocrit and hemoglobin for estimating change in PV% (14).
All data in text, figures, and tables are represented as mean ± SD and as a mean with a confidence limit (CL) of 90% (90% CL). A CL of 90% was selected because it has been suggested to be an appropriate default level as the probability of the true value residing below or above these limits are 5% each and interpreted as unlikely (24). All data were analyzed using a practical significance standpoint using magnitude-based inferences (4). Furthermore, a qualitative approach was adopted to provide meaningful significance around each performance measure examined (21). The qualitative likelihood of a change being either higher or lower, or harmful or beneficial was as follows: 1% almost certainly not, 1–5% very unlikely, 5–25% unlikely, 25–75% possible, 75–95% likely, 95–99% very likely, and >99% almost certainly (21). Probabilities were calculated to express a benefit-to-harm ratio as well as to identify whether the ES was beneficial (positive), trivial, or harmful (negative) as calculated using an Excel spreadsheet for practical significance and insight (22).
Within-trial standardized mean differences were used to reveal the magnitude of ES (9) using a predeveloped statistical spreadsheet (22).
The following thresholds for ES values were as follows: <0.2 trivial, >0.2 small, >0.6 moderate, >1.2 large, >2.0 very large, and >4.0 extremely large (21). Meaningful change as defined as the smallest worthwhile change in physiological measures was derived from the day-to-day variation over the 2 days before the first hot yoga class and are expressed as the coefficient of variation with a 90% CL (23). The day-to-day variations in core temperature were 0.57% (36.9–37.2° C), PV% = 2.3% (0.4–4.2%), and dry body mass 0.32% (65.6–65.8 kg). A Pearson correlation coefficient (r) analysis was used to examine the magnitude of correlation between the mean change in PV% and HR-derived training loads experienced over 6 days of hot yoga and from all field hockey activities during the postintervention period. The following principles were applied for identifying the strength of correlation: <0.1 trivial, 0.1–0.3 small, 0.3–0.5, 0.5–0.7 large, 0.7–0.9 very large, and 0.9–1.0 almost perfect (21). Throughout the complete analysis, if the 90% CL overlapped both a negative (harmful) and positive (beneficial) range, the ES was declared unclear; otherwise, the ES was declared the detected value (21).
Physiological Adaptations from 6 Days of Hot Yoga
The physiological responses after performing six 60-minute hot yoga classes are presented in Table 2. An increase in peak core temperature (° C) (37.3 ± 0.32 vs. 37.6 ± 0.48) (ES = 1.4, 90% CL [1.1–1.7]) was observed over 60 minutes. Variations in core temperature elicited minimal reductions in sweat loss as represented through mean change in dry body mass (kg) (65.8 ± 0.10 vs. 65.5 ± 0.10) (ES = −0.08, 90% CL [−0.09 to −0.07]). Minimal cardiovascular strain was observed as expressed through percentage of maximum HR (47 ± 4%), mean training load (AU) response over 60 minutes of hot yoga (24.1 ± 9.4), total training load over 6 days (AU) (145.0 ± 48.0), and the total 6-day training load expressed as a percentage of a maximum weekly training load (19.0 ± 8.0%).
A gradual reduction in mean PV% (−3.5%, 90% CL [−6.5 to −0.48]), residing below the smallest worthwhile change, was observed after 6 days of hot yoga (Figure 2).
A significant correlation was observed between mean change in PV% and the total training load (AU) experienced over 6 days of hot yoga (r = 0.48, 90% CL [−0.10 to 0.82]) (Figure 3). Significant inverse correlations were observed between mean change in PV% and body mass (r = −0.55, 90% CL [−0.80 to −0.14]), and PV% and BSA (r = −0.57 CI 90% [−0.8 to 0]). We failed to observe significant correlations between mean change in PV% and core temperature (r = 0.01), and PV% and sweat loss (kg) (r = 0.30, 90% CL [−0.30 to 0.70]).
Physiological Adaptations After Intervention—National Team Camp
A significant correlation between mean change in PV% and the training load experienced by day 10 (72 hours after intervention) was observed (r = 0.64, 90% CL [0.14 to 0.88]). This correlation became insignificant by day 13 (144 hours after intervention) (r = −0.11, 90% CL [−0.62 to 0.47]). A significant correlation was detected between mean change in PV% and body mass at day 13 (144 hours after intervention) (r = −0.55, 90% CL [−0.80 to −0.14]).
Physiological Adaptations Between Menstrual Phases
Before the first hot yoga class, participants who were tested while in the luteal phase (L) demonstrated an elevation in both resting core temperature (° C) ([L] 37.3 ± 0.6 vs. [F] 37.1 ± 0.16) (ES = 1.3, p < 0.04) and hematocrit ([L] 40.5% ± 1.7 vs. [F] 38.0% ± 1.40) (ES = 1.3, p < 0.05) when compared with participants in the follicular phase (F). When comparing change in core temperature between menstrual phases, participants in their follicular phase demonstrated an unclear elevation compared with participants in the luteal phase (ES = 0.71, 90% CL [−0.98 to 2.4]). Participants in each menstrual cycle phase demonstrated reductions in PV% (Table 3). An insignificant difference in PV% between menstrual phases ([L] −3.4% ± 7.3 vs. [F] −5.1 ± 4.2%) (ES = −0.35, 90% CL [−1.8 to 1.1]) was observed on day 8 (24 hours after intervention). Within-group PV% alterations after 6 days were unclear in participants in the luteal phase (−3.4%, ES = −1.2, 90% CL [−4.2 to 1.8]), whereas a significant reduction was observed in those tested in their follicular phase (−5.1%, ES = −2.3, 90% CL [−4.1 to −0.53]). Difference in PV% between menstrual phases at day 13 showed that participants in the follicular (F) phase had an unclear elevation compared with participants in their luteal phase (L) (ES = 0.33, 90% CL [−1.1 to 1.8]).
Cardiovascular Performance Adaptations
Change in cardiovascular performance measures are displayed in Table 4. V̇o2max when represented as an absolute (L·min−1) (3.05 ± 0.42 vs. 3.06 ± 0.36) (ES = 0.02, 90% CL [−0.16 to 0.20]) or relative measure (ml·kg−1·min−1) (46.2 ± 3.48 vs. 46.4 ± 3.11) (ES = 0.06, 90% CL [−0.16 to 0.28]) demonstrated trivial improvements. Trivial improvements in run time to exhaustion (min−1·s−1) (24.3 ± 3.26 vs. 24.5 ± 3.21) (ES = 0.11, 90% CL [−0.07 to 0.29]) and maximum HR (b·min−1) were observed (190 ± 12 vs. 191 ± 11) (ES = 0.09, 90% CL [−0.01 to 0.17]). Maximum oxygen pulse demonstrated a trivial reduction (ml·beat−1) (16.2 ± 1.9 vs. 15.9 ± 1.7) (ES = −0.15, 90% CL [−0.41 to 0.11]).
Small meaningful improvements in running speed (km·h−1) were observed at VT1 (8.7 ± 0.93 vs. 9.0 ± 1.0) (ES = 0.34, 90% CL [−0.08 to 0.76]) and VT2 (11.7 ± 0.92 vs. 12.2 ± 1.0) (ES = 0.53, 90% CL [−0.05 to 1.1]). In addition, a small meaningful adaptation in high-intensity substrate utilization (RER) during the last-completed stage was observed (1.10 ± 0.04 vs. 1.09 ± 0.02) (ES = −0.25, 90% CL [−0.62 to 0.12]).
The purpose of this investigation was to examine the efficacy of hot yoga as an alternative heat stress technique capable of inducing hypervolemia and augmenting cardiovascular performance in elite female field hockey players. The primary observations from this study were small meaningful improvements in running speed at VT1 and VT2, and positive adaptations in the RER during high-intensity exercise while in a hypovolemic state. In addition, a delayed hypervolemic response was observed during the simulated competition period and remained elevated for 6 days after intervention. The observed improvements in running speed at VT2 and positive alterations in high-intensity exercise substrate utilization are encouraging for field hockey coaches because recent evidence suggests most competition resides at or above this physiological performance marker (29). The observed submaximal performance improvements in the presence of hypovolemia parallel recent discoveries demonstrating altered physiological adaptations in women during short-term heat stress (30) and further suggest that meaningful change in cardiovascular performance in elite female athletes may be possible without the classical development of hypervolemia. Moreover, when further examining participants who demonstrated reductions in V̇o2max and run time to exhaustion (n = 2), small meaningful improvements in submaximal performance measures were observed. Self-reported delayed-onset muscle soreness, a consequence from performing certain yoga poses, elicited a premature test termination because of severe muscular discomfort. This singularity highlights a potential limitation within the study design regarding the timing of the postexercise assessment (35) and suggests that maximal performance measures may have been mitigated.
A primary component of this investigation was to examine a novel heat stress technique capable of accommodating multiple athletes simultaneously while eliciting minimal thermal load. A previous systematic review (8) examining heat stress techniques used within team sport revealed typical ambient temperatures ranging from 30 to 40° C consisting of high-intensity or intermittent exercise. These alternative techniques involve reduced exposure periods and have shown to produce similar physiological adaptations compared with traditional techniques. Although these techniques may be advantageous throughout the yearly training plan, the additional training load experienced is not always conducive for microcycles preceding competition and often fails to accommodate an entire team of athletes simultaneously unless using natural conditions. The findings from the current investigation demonstrate an alternative heat stress technique involving minimal thermal load and that allows for appropriate control over environmental conditions suitable to accommodate an entire team of athletes.
Although we observed a significant correlation between PV% change and training load over 6 days of hot yoga, the subsequent development of hypovolemia suggests hot yoga failed to elicit a robust exercise stimulus capable of preserving preintervention PV% levels. The significance of exercise for altering plasma volume, circulating plasma albumin, and fluid conservatory hormones has been well-reviewed (12). As such, the inclusion of exercise during heat stress techniques has demonstrated superior development in hypervolemia when compared with passive exposure (11), a consequence thought to be influenced by plasma albumin content and its subsequent oncotic pressure (36). Consistent with the renin angiotensin system's response to the cessation of exercise (10), short-term heat stress techniques involving permissive dehydration have demonstrated an upregulation of resting plasma aldosterone levels (18). Although fluid conservatory hormones were not examined in this study, we propose that the possible additive effects of recommencing exercise coupled with hypohydration may have been responsible for the significant rebound in PV% during the postintervention period. This can be postulated when examining 2 key observations. The first being the significant plasma volume expansion observed on day 10, after completing the postintervention-graded exercise test on day 8 and a maximal graded exercise test during the national team camp on day 9. Together, the accumulated training load from both tests was nominal (127 ± 20 AU) and was below the total training load experienced over 6 days of hot yoga (145 ± 48). As such, this evidence suggests that brief, moderate to high-intensity exercise may have been responsible for eliciting the rapid development of hypervolemia within 72 hours of completing hot yoga.
The second key observation is examining the 24-hour plasma volume expansion (2.3 ± 5.2%) on day 2 after completing a maximal-graded exercise test and using this expansion as the control for the expected hematological response. When examining the hypervolemic response on day 10 (5.0 ± 6.4%), the expansion observed was twice that of a single maximal exercise test. This expansion is supported because the total training load experienced was twofold of a single maximal exercise test having completed 2 maximal tests within 2 days. However, this expansion may be misleading because the PV% was calculated and compared with preintervention blood volume, failing to reflect the state of hypovolemia after 6 days of hot yoga. As such, when using the postintervention blood volume for calculating change in PV% on day 10, the observed hypervolemia becomes significantly greater (13.2 ± 5.9%). This expansion was 6 times greater than that of the control response and was elicited through a training load twice that of single maximal exercise test. These observations support our hypothesis that participants were primed for a plasma volume expansion after completing 6 days of hot yoga and required only a brief exercise stimulus for initiating a significant rebound. Nevertheless, we must acknowledge that the variance in day-to-day PV% throughout the investigation may have been influenced by sodium and fluid consumption because each component was not regulated nor calculated. Future investigations are encouraged to examine the PV response to performing hot yoga when supplemented into the weekly on-field training schedule whereby a regular exercise stimulus is experienced.
Although our sample size comparing menstrual phases was underpowered, we detected no significant differences in PV% after 6 days of hot yoga. However, within-group responses identified marked reductions in PV% only in participants currently in the follicular phase, an observation consistent with previous inquiry examining the response to mild exercise using identical heat stress when hypohydrated (17).
When examining the correlation between change in PV% and anthropometric characteristics, a significant inverse correlation between body mass and PV% was observed over 6 days of hot yoga and during the postintervention period. This correlation may have been a consequence of experiencing a compensable heat stress throughout the hot yoga and competition period. As such, thermoregulation may have been enhanced in larger individuals (20), potentially inhibiting a hypervolemic response. These observations highlight the influence of anthropometric characteristics in relation to physiological adaptations when performing mild heat stress techniques and should encourage practitioners to provide individual recommendations when exposing a uniform heat stress to a team of athletes.
This investigation provides coaches and practitioners an alternative and mild form of heat stress that involves minimal exercise stress while capable of comfortably accommodating a team of athletes simultaneously. The observed nominal cardiovascular strain allows practitioners to implement this heat stress technique during periodized rest weeks within the yearly training plan allowing for the maintenance of maximal cardiovascular performance and while concomitantly augmenting submaximal performance. Furthermore, a rapid hypervolemic response once recommencing training or competition may augment field hockey performance when entering competition periods lasting 6 days.
The authors acknowledge the support, dedication, and contributions from each participating athlete. They have no conflicts of interest to disclose. This investigation was funded through the Natural Sciences and Engineering Research Council of Canada. The results of the current study do not constitute endorsement of the products used by the authors or the National Strength and Conditioning Association. This work was supported by the Natural Sciences and Engineering Research Council of Canada.
1. Bangsbo J, Iaia FM, Krustrup P. The Yo-Yo intermittent recovery test: A useful tool for evaluation of physical performance in intermittent sports. Sports Med 38: 37–51, 2008.
2. Banister E. Modeling elite athletic performance. Physiol Test Elite Athletes: 403–424, 1991.
3. Banister E, Morton R, Fitz-Clarke J. Dose/response effects of exercise modeled from training: Physical and biochemical measures. Ann Physiol Anthropol 11: 345–356, 1992.
4. Batterham AM, Hopkins WG. Making meaningful inferences about magnitudes. Int J Sports Physiol Perform 1: 50–57, 2006.
5. Boyd CN, Lannan SM, Zuhl MN, Mora-Rodriguez R, Nelson RK. Objective and subjective measures of exercise intensity during thermo-neutral and hot yoga. Appl Physiol Nutr Metab 43: 397–402, 2017.
6. Casa DJ, Becker SM, Ganio MS, Brown CM, Yeargin SW, Roti MW, et al. Validity of devices that assess body temperature during outdoor exercise in the heat. J Athl Train 42: 333–342, 2007.
7. Casadio JR, Kilding AE, Cotter JD, Laursen PB. From lab to real World: Heat acclimation considerations for elite athletes. Sports Med 47: 1467–1476, 2017.
8. Chalmers S, Esterman A, Eston R, Bowering KJ, Norton K. Short-term heat acclimation training improves physical performance: A systematic review, and exploration of physiological adaptations and application for team sports. Sports Med 44: 971–988, 2014.
9. Cohen J. The t-test for means. In: Statistical power analysis for the behavioral sciences. Hillsdale, NJ: L. Erlbaum Associates, 1988. pp. 19–21.
10. Convertino V, Brock P, Keil L, Bernauer E, Greenleaf J. Exercise training-induced hypervolemia: Role of plasma albumin, renin, and vasopressin. J Appl Physiol 48: 665–669, 1980.
11. Convertino V, Greenleaf J, Bernauer E. Role of thermal and exercise factors in the mechanism of hypervolemia. J Appl Physiol 48: 657–664, 1980.
12. Convertino VA. Blood volume: Its adaptation to endurance training. Med Sci Sports Exerc 23: 1338–1348, 1991.
13. Daanen HAM, Racinais S, Periard JD. Heat acclimation decay and re-induction: A systematic review and meta-analysis. Sports Med 48: 409–430, 2017.
14. Dill D, Costill DL. Calculation of percentage changes in volumes of blood, plasma, and red cells in dehydration. J Appl Physiol 37: 247–248, 1974.
15. Domitrovich JW, Cuddy JS, Ruby BC. Core-temperature sensor ingestion timing and measurement variability. J Athl Train 45: 594–600, 2010.
16. Du Bois D, Du Bois EF. Clinical calorimetry: Tenth paper a formula to estimate the approximate surface area if height and weight be known. Arch Intern Med 17: 863–871, 1916.
17. Gaebelein CJ, Senay LC. Vascular volume dynamics during ergometer exercise at different menstrual phases. Eur J Appl Physiol Occup Physiol 50: 1–11, 1982.
18. Garrett AT, Goosens NG, Rehrer NJ, Patterson MJ, Harrison J, Sammut I, et al. Short-term heat acclimation is effective and may be enhanced rather than impaired by dehydration: Short-term heat acclimation. Am J Hum Biol 26: 311–320, 2014.
19. Guy JH, Deakin GB, Edwards AM, Miller CM, Pyne DB. Adaptation to hot environmental conditions: An exploration of the performance basis, procedures and future directions to optimise opportunities for elite athletes. Sports Med 45: 303–311, 2015.
20. Havenith G. Human surface to mass ratio and body core temperature in exercise heat stress—a concept revisited. J Therm Biol 26: 387–393, 2001.
21. Hopkins W, Marshall S, Batterham A, Hanin J. Progressive statistics for studies in sports medicine and exercise science. Med Sci Sports Exerc 41: 3, 2009.
22. Hopkins WG. A spreadsheet for deriving a confidence interval, mechanistic inference and clinical inference from a P value. Sportscience 11: 16–21, 2007.
23. Hopkins WG. Measures of reliability in sports medicine and science. Sports Med 30: 1–15, 2000.
24. Hopkins WG. Probabilities of clinical or practical significance. Sportscience 6: 16, 2002.
25. Hopkins WG. Sample sizes for magnitude-based inferences about clinical, practical or mechanistic significance: 2746. Med Sci Sports Exerc 38: S528–S529, 2006.
26. Larson-Meyer DE. A systematic review of the energy cost and metabolic intensity of yoga. Med Sci Sports Exerc 48: 1558–1569, 2016.
27. Lima-Oliveira G, Guidi GC, Salvagno GL, Danese E, Montagnana M, Lippi G. Patient posture for blood collection by venipuncture: Recall for standardization after 28 years. Rev Bras Hematol Hemoter 39: 127–132, 2017.
28. Lorenzo S, Halliwill JR, Sawka MN, Minson CT. Heat acclimation improves exercise performance. J Appl Physiol 109: 1140–1147, 2010.
29. McGuinness A, Malone S, Hughes B, Collins K. The physical activity and physiological profiles of elite international female field hockey players across the quarters of competitive match-play. J Strength Cond Res, 2018. doi: 10.1519/JSC.0000000000002483. Epub ahead of print.
30. Mee J, Gibson O, Doust J, Maxwell N. A comparison of males and females' temporal patterning to short-and long-term heat acclimation. Scand J Med Sci Sports 25: 250–258, 2015.
31. Nissilä J, Kinnunen H. Heart rate based training load and recovery time estimation. Polar Electro Oy 1: 1–4, 2008.
32. Perrotta AS, Held NJ, Warburton DER. Examination of internal training load parameters during the selection, preparation and competition phases of a mesocycle in elite field hockey players. Int J Perform Analaysis Sport 17: 813–821, 2017.
33. Perrotta AS, Taunton JE, Koehle MS, White MD, Warburton DER. Monitoring the prescribed and experienced heart rate derived training loads in elite field hockey players. J Strength Cond Res, 2018. doi: 10.1519/JSC.0000000000002474. Epub ahead of print.
34. Périard J, Racinais S, Sawka M. Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports. Scand J Med Sci Sports 25: 20–38, 2015.
35. Rose Chrismas BC, Taylor L, Siegler JC, Midgley AW. Muscle-damaging exercise 48 h prior to a maximal incremental exercise treadmill test reduces time to exhaustion: Is it time to reconsider our pretest procedures? Res Sports Med 25: 11–25, 2017.
36. Senay L, Mitchell D, Wyndham C. Acclimatization in a hot, humid environment: Body fluid adjustments. J Appl Physiol 40: 786–796, 1976.
37. Skinner JS, McLellan TH. The transition from aerobic to anaerobic metabolism. Res Q Exerc Sport 51: 234–248, 1980.
38. Taylor HL, Buskirk E, Henschel A. Maximal oxygen intake as an objective measure of cardio-respiratory performance. J Appl Physiol 8: 73–80, 1955.
39. Warburton DER, Jamnik VK, Bredin S, Gledhill N. The 2014 physical activity readiness questionnaire for everyone (Par-Q+) and electronic physical activity readiness medical examination (ePARmed-X+). Health Fitness J Can 7: 80–83, 2014.
40. Weltman A, Snead D, Stein P, Seip R, Schurrer R, Rutt R, et al. Reliability and validity of a continuous eincremental treadmill protocol for determination of lactate threshold, fixed blood lactate concentrations, and VO2max. Int J Sports Med 11: 26–32, 1990.