Subjects and screening

Subjects were recruited through a recruitment email and word of mouth to family and friends. Interested subjects that passed the screening questions were asked to attend a consent meeting. At this meeting, interested subjects learned about the study and had the opportunity to sign the consent form or decline involvement. Members of the research team facilitated the consent process. Each member of the research team had training in the protection of human subjects. They also signed a HIPAA form at this meeting and were given a copy of both the consent and the HIPAA for their records. All applicable institutional and governmental regulations concerning the ethical use of human volunteers were followed during this research.

All participants reported exercising at least five times per week with at least a six-week history of strength training three times per week. Participants were excluded for any of the following: known cardiac disease, uncontrolled hypertension, uncontrolled thyroid disease, uncontrolled diabetes, taking medications that could impair exercise performance (beta blockers), medical contraindications to exercise, an injury that prevented them from being able to complete movements in an exercise program, a doctor told them they cannot exercise or a VO2 below 35 mL/kg/min. Fifty-two healthy, physically fit males volunteered for the study. Data of seven subjects had to be removed as they started at least one exercise session in a dehydrated state. Therefore, 45 participants completed the trial (30.28 ± 5.4 yr, 1.77 ± 7.8 m, 83.46 ± 11.5 kg; 13.7 ± 4.8%BF; 49.8 ± 6.3 ml/kg/min V02) (Table1).

Table 1 Summary of participant characteristics Full size table

The study was approved by Compass Institutional Review Board (Mesa, Arizona) and written informed consent was obtained from each participant before enrollment.

Experimental design

The study was conducted in a cross-over, randomized design. The null hypothesis that cold water will not impact core temperature or performance measures was tested via a repeated measures analysis of variance and the criterion for significance for all tests was set at p < 0.05. Participants undertook two experimental trials that were administered in simple blocks, randomized, crossover order, followed by three performance tests: (1) 60% 1RM bench press to fatigue, (2) broad jump, and (3) time to exhaustion (TTE) on a stationary Keiser bike. As participant blinding to drink temperature is impossible, the subjects were informed that that the study outcome of interest was body temperature not performance. Participants, in randomized fashion, were given either cold (COLD) 4°C, or room temperature (RT) 22°C water provided in vacuum insulated hydration bottles with Thermos® technology during the exercise protocols and were given 12 ml/kg in equal aliquots to be consumed during the rest period between sets of exercises and to be completed before the end of the exercise session and before the first performance test. In the subsequent exercise session the participants were given the exact amount of water they consumed during the first trial. The trials were separated by a minimum of four days and no more than 21 days. Participants were asked to refrain from strenuous activity and abstain from alcohol and caffeine consumption 48 hours prior to both exercise sessions. Participants were then asked to consume 8 ml/kg body weight of water to ensure euhydration starting at 3 hours priors to training session and to be finished ~45 min before arriving to facility. Each trial commenced at the same time each day to control for the effect of circadian rhythm on body temperature. The day of the exercise session, the participants were asked to ingest a biodegradable temperature sensor pill (CoreTemp capsule, Mini Mitter Co. Inc., Bend, Oregon, USA), with a small meal, 6–8 hours prior to the exercise session to allow adequate time for motility into the small intestine and to minimize the effects of swallowing cold liquids on temperature readings. Core temperature was monitored using a VitalSense telemetric physiological monitoring system (Mini Mitter Co. Inc., Bend, Oregon, USA). To control for the effect of diet and hydration on exercise performance the participants were asked to arrive at the training facility 1.5 hours prior to their scheduled exercise sessions to receive a standardized meal of 1.0 g carbohydrate/kg body weight and 0.4 g protein per kg lean body mass in the form of a shake to be finished within Â½hour prior to commencing the exercise session. Upon arrival to the training facility, 1.5 before commencing exercise session, the participants were asked to provide a urine sample cup for urine specific gravity analysis (USG) using Roche USG 10 urine strips. If a participant was dehydrated they were instructed to continue the 8 ml/kg body fluid protocol and re-test 45 minutes later to confirm they were hydrated. Core temperature was taken at baseline and every 15 minutes with the VitalSense telemetric physiologic monitoring system (Mini Mitter Co. Inc., Bend, Oregon, USA). Body weight and USG were taken prior to the exercise session and immediately after performing the TTE test.

During both trials, each participant was assigned an identification number which was placed on their own vacuum insulated individual Thermos® brand bottle. They were instructed to only drink from their own Thermos® brand bottle. During the cold trial, the drinks were cooled using a domestic refrigerator and maintained at 4°C. During the RT trial, drinks were maintained at 22°C. Temperature of the water was measured using a standard long glass mercury thermometer (Indigo® Instruments, Waterloo, ON, Canada). After the initial exercise session was completed, participants were given a five minute rest before commencing the performance tests. The performance tests were administered by the same person/investigator to avoid potential bias when providing encouragement. The performance tests included: flat bench to fatigue at 60% of one rep max (RM) to determine muscular endurance[11], broad jump to determine force and power production[12] and time to exhaustion (TTE) on stationary bicycle to determine cardiovascular endurance. For the broad jump test the subjects were asked to jump as far as they could horizontally on a flat surface 2 times. Both jumps were averaged. The endurance test (TTE) was administered using a modified McArdle (1973) bike protocol. The protocol was based on the use of the Keiser stationary bike. The watts are based on the gear and the participants had to hold 80 rpms at each gear. The ramping was adjusted to fit the gearing designed of the Keiser stationary bike. We used it as a sub max test based on maintaining 80 rpms. If the participants could not keep above 80 rpm then the participant was instructed to stop and gear, time and Core temperature were recorded.

Preliminary measurements

Subjects completed the baseline testing at least four days prior to their first testing day. After the completion of the baseline testing, subjects were briefed on the study design and the drinking and exercise protocol. They were also able to familiarize themselves with the performance tests that they were to perform at the end of their exercise sessions. On their first trip to the facility, the participants’ weight, height, and 7-site skin fold thickness were measured. Skin fold thickness measurements were taken at seven sites (triceps, subscapula, chest, mid-axillary, abdominal, iliac create, front thigh) using calipers (Lange Skin fold Caliper, Beta Technology, Santa Cruz, CA). Percent body fat was determined using the Siri equation and body density was calculated with the Jackson-Pollock equation. After anthropometrics were taken participants proceeded to the flat bench press to determine the bench press 1RM performance test. Subjects were asked to bench press 60% of their 1RM as many times as they could. During the test subjects had a spotter behind them to take the weight once the subject fatigued. The participants were also fitted and assigned a stationary bike for the time to exhaustion performance test. Lastly, estimated peak oxygen consumption was assessed to determine fitness levels using a treadmill (Woodway, Waukesha, WI) via an 8–12 minute ramping protocol during which the American College of Sports Medicine graded walking equation was applied (American College of Sports Medicine, 2010). During the submaximal protocol, heart rate and ventilation were measured using the iMett system (Woodway, Waukesha, WI). Ventilation was measured with a flow meter and mask (Hans Rudolph) from which a ventilatory threshold was determined. Adjusted ACSM max norms to 95%, as a submax test was administered. A VO2 of ≥35 ml/kg/min was considered moderately fit and approved to participate in the study.

Exercise protocol

Subjects partook in an exercise session that combined both strength and aerobic exercise. The exercise session was supervised and led by a Performance Specialist at Athletes’ Performance. The subject to trainer ratio did not exceed 10:1. The 60 minute exercise session consisted of a 5 minute warm-up with dynamic stretching, 5 minutes of medicine ball exercises, 35 minutes of full body strength training and 15 minutes conditioning. The strength sessions exercises consisted of four exercise blocks which included a strength/power exercise followed by a corrective exercise/stretch to facilitate active rest between sets. There was a prescribed 2–3 minute rest between strength blocks 1 and 2, and 1–2 minutes rest between strength blocks 2 and 3, and 3 and 4. The strength session consisted of: 1st block (2x6):Dumbbell squat to press, power/velocity emphasis, loading was approximately 55% of 1RM, ½ kneeling quad hip flexor stretch for 30s-1m rest in between sets. Second block (3x10): Dumbbell flat bench press (strength/hypertrophy emphasis), loading was approximately 75% of 1RM with bent over T’s (3x6) during 30s-1m rest in between sets; Double leg, dumbbell Romanian deadlift (strength/hypertrophy emphasis), loading was approximately 75% of 1RM with bent knee hamstring stretch (3x6) during 30s-1m rest in between sets. Third block (2x8): 1 arm rotational row (power/velocity emphasis, loading was approximately 55% of 1RM with a push up position hold with alternating arm lifts during 30s-1m rest period. Fourth block (2x8): Lunge to curl to press (strength/hypertrophy emphasis) loading was approximately 80% of 1RM, with a front plank/pillar hold (1 minute) during rest in between sets; Eccentric only slide board leg curls. The 15 minute conditioning/cardiovascular exercise that followed the strength training was designed as high intensity interval training. Intensity was determined based on each participant’s individual heart rate zones which were prescribed off their sub max treadmill VO2 test results as 65-85% of ventillatory threshold (VT), 100-110% of VT, and 110% VT-Peak HR. The cardio session started with a 3 minute warm up (65%-85% VT) with goal of heart rate being in this zone by the end of the 3 minutes. Two minutes was spent at 100-110% VT, 1 minute at 100%VT-Peak, 1 minute at 65-85% VT, 2 minutes 100-110% VT, 1 minute 100% VT-Peak, 1 minute 65-85% VT, 2 minutes 100-110% VT, 1 minute 100% VT-Peak and 1 minute 65-85% VT.

During the exercise session, subjects were asked to drink their assigned beverage during rest periods between exercise sets. Consumption of water during the exercise session was ad libitum and the participants were instructed to completely finish the water by the end of the exercise session. If water was left-over it was recorded (only four participants did not consume all water). Core temperatures were measured every 15 minutes during the session via the VitalSense telemetric physiologic monitoring system (Mini Mitter Co. Inc., Bend, Oregon, USA) the researcher held near the subject’s body.

Environmental conditions

Wet and dry bulb temperatures were taken during each trial. Wet bulb temp averaged 14.9°C and 15°C (p=0.6273) for both RT and COLD trials respectively and dry bulb temp averaged 24°C and 24.2°C (p=0.1179).

Statistics

A statistical analysis was performed by the authors. Data were ensemble averaged across all 45 participants and standard deviations were calculated. The study design was a randomized cross-over study. Paired t-tests were used to compare performance between conditions and to compare the absolute change in body temperature from the pre-exercise session to the post-exercise session. A repeated measures analysis of variance was used to test for a significant effect of group, time and the interaction between the two during the hour of exercise. Tukeys post-hoc tests were used to determine significant differences between time points. Criterion for statistical significance was set at p<0.05.