The Effect of a Resistance Training Session on Physiological and Thermoregulatory Measures of Sub-maximal Running Performance in the Heat in Heat-Acclimatized Men

Ten healthy men (mean ± standard deviation; age 27.4 ± 4.1 years; height 1.78 ± 0.06 m; body mass 76.8 ± 9.9 kg; maximal oxygen uptake [VO_2max] 48.2 ± 7.0 mL kg⁻¹ min⁻¹) volunteered for this study. According to an a priori calculation, ten participants were sufficient to generate a power of 0.8 at an alpha level of 0.05 for the dependent measures based on previous studies [2, 11, 17]. Each participant had lived in a tropical climate of North Queensland, Australia, for at least 3 years and were undertaking regular (2–3 week⁻¹) running sessions outside, but had not performed lower body RT for the past 6 months. Biological variations were controlled for by refraining from high-intensity activity for at least 48 h prior to any testing session; avoiding caffeine and food intake for at least 2 h prior to any testing session; wearing the same shoes for every testing session; conducting each training and testing session at the same time of day within participants; and refraining from recovery-related activities, such as supplementation, medication, massage and cryotherapy, during the course of the study. Each participant provided written informed consent prior to taking part in any testing procedures, and did not report acute or chronic illness, disease and injury or medication that would contraindicate any training and testing procedures. The Institutional Human Research Ethics Committee approved all protocols, which were in line with the Declaration of Helsinki.

This study was conducted as a repeated measures design across 3 weeks (Fig. 1). The first week consisted of a familiarization session, followed by a VO_2max test 48 h thereafter. The familiarization session ensured each participant was acquainted with the procedures and equipment as well as to undertake a six-repetition maximum (6RM) test. During the second week, three RE tests, with at least 48 h of recovery in-between each testing sessions, were conducted to ensure familiarity with the RE protocol. The third RE test during the second week was used to report on baseline measures TBase. During the third week, each participant undertook a RT session. Subsequent RE tests were performed 24 (T₂₄) and 48 (T₄₈) hours following the RT session, which were then compared to TBase. Indirect muscle damage markers were also collected prior to (TBase), immediately post (T1) and 24 (T24) and 48 (T48) hours following the RT bout.

The 6RM protocol was conducted using exercises in the order of squats on a Smith Machine (MPL 706, Maxim Fitness, Australia), horizontal leg press (NS4000, Nautilus, Canada), leg extension (NS4000, Nautilus, Canada) and leg curls (NS4000, Nautilus, Canada) in the same session. Prior to the 6RM assessment, each participant undertook a standardized warm-up by completing ten repetitions of leg swings in the frontal and sagittal planes for each leg, followed by ten repetitions of squat exercises on the Smith Machine at approximately 50% of body mass. The 6RM protocol was conducted using previously described methods [20]. In summary, each participant completed 8–10 repetitions with a load approximately at 10RM based on perception of effort during the warm-up. Following 5 min of passive rest, the load was increased by 20% to attempt a 6RM. The load was adjusted by 5–10% heavier, or lighter, depending on the participant’s perceived load. A 5-min passive recovery period was provided between each attempt. The squats and horizontal leg press commenced with the knees fully extended at the start position and flexed to 45° at the end of the eccentric phase. Each participant was also requested to complete each repetition in approximately 2 s with 1 s for concentric and eccentric phases, respectively, to standardize contraction speed. In addition, the leg press exercise was performed unilaterally, commencing with the right leg. A qualified strength and conditioning coach ensured proper technique and correct loading was applied during each testing session. The highest load attempt in the 6RM test was considered for the calculations of training load of the RT protocol described below.

The VO_2max test was conducted in a custom-built climate-controlled chamber with the temperature and humidity set at 30 °C and 67.5%, respectively. These temperature and humidity settings were selected to replicate the average tropical environmental constraints of Far North Queensland, Australia. Prior to the VO_2max test, a progressive warm-up was conducted. The warm-up activities included dynamic stretches of the lower extremity; jogging at 8, 10 and 12 km h⁻¹ for 1 min, respectively; 2 min of walking at 5 km h⁻¹ on a treadmill (TM 601, Trackmaster, USA); and 1 min of passive recovery. The VO_2max test was conducted using a continuous incremental protocol [21], commencing at 9 km h⁻¹ and was increased by 1.5 km h⁻¹ every minute until volitional exhaustion was reached using verbal encouragement. The participants were deemed to have been reached VO_2max if there was no increase in VO₂ (mL kg min⁻¹), despite an increase in treadmill speed, or if the following criteria was achieved: respiratory exchange ratio > 1.1, maximum heart rate (HR) within ten beats of the age-appropriate reference value and Borg’s rating of perceived exertion of > 18 [22]. During the VO_2max test, an indirect calorimetry system (Quark CPET, Cosmed, Italy) was used to collect expired air and calculate the second ventilatory threshold (VT₂). The VT₂ was quantified from the inflection point of ventilation (VE) with respect to carbon dioxide production (VCO₂) on a scatter diagram from the VO_2max test [23]. The running intensity at VT₂ was then utilized to establish running speeds during the RE tests.

Similar to the VO_2max test, the RE test was conducted in a climate-controlled chamber with identical temperature and humidity settings. Prior to the RE test, a urine sample was collected to determine urine specific gravity using a calibrated urinary refractometer (Atago hand refractometer, model UNC-NE; Atago, Japan). Whole body sweating rate was also measured prior to, and following, each RE test to report changes as a result of the RE test. Due to technical difficulties with data collection, body mass change was reported for nine participants. Following an identical warm-up to the VO_2max test, the RE test was completed at 70% of VT₂ for 10 min [24]. During the RE protocol, respiratory measures were collected using an indirect calorimetry system (Quark CPET, Cosmed, Italy) to report oxygen consumption (VO₂; mL kg⁻¹ min⁻¹), carbon dioxide (VCO₂; mL kg^{− 1} min⁻¹), respiratory exchange ratio (RER), breath frequency (Bf; breath·min⁻¹) and ventilatory equivalents for oxygen (VE/VO₂) and carbon dioxide (VE/VCO₂). These respiratory measures were reported as the average of the last 3 min of the RE test. Potential existence of a VO₂ slow component was also assessed for the baseline by comparing VO₂ between the 7th and 10th minute of the RE test. The primary RE parameter was based on the gross caloric unit cost (CU_C) of running, using a previously reported method [25]. Initially, the caloric equivalent of VO₂ was determined, and the CU_C was then calculated as caloric unit cost (Kcal kg⁻¹ min⁻¹) = VO₂·caloric equivalent·s⁻¹·BM⁻¹·K, where VO₂ was in litres per minute, caloric equivalent was in kilocalories per litre, speed (s) was in metres per minute, body mass (BM) in kgs and K in 1000 m km⁻¹. Core temperature (T_C) was obtained by having participants ingest a telemetry pill (CorTemp; HQInc, Palmetto, USA) 8 h prior to each test. Heart rate (HR; RS800CX, Kempele, Finland), 6–20 Borg’s rating of perceived exertion (RPE), thermal discomfort (D_T) and thermal sensation (S_T) were also recorded on the 9th minute. The D_T was measured using a 1–5 visual analogue scale with 1 and 5 denoting ‘comfortable’ and ‘extremely uncomfortable’, respectively. The S_T was also a visual analogue scale, ranging from 1 to 13, with 1 and 13 denoting ‘unbearably cold’ and ‘unbearably hot’, respectively. All measures obtained from the RE test were reported from baseline to 48 h post-exercise. However, T_C will be reported for 24 h post-exercise due to participants voiding the sensor prior to the 48 h collection point.

The resistance exercises undertaken during the RT session were equivalent, and in the same order as the 6RM session. For each exercise, three sets of six repetitions were performed at 95% of 6RM to ensure that participants were able to complete each set without failure. Two minutes of passive recovery was provided in-between each set and exercise. During the RT session, participants rated the level of difficulty of each set from 1 to 10, with 1 and 10 denoting ‘very easy’ and ‘very difficult’, respectively [2]. Whilst no participants rated their level of difficulty below 8 following the second set, if participants rated their difficulty below 9 during the second set, the load was increased by 5% to ensure adequate stress. As a result, no participants rated their difficulty below 9 following the final set of each exercise.

The indirect muscle damage markers were countermovement jump (CMJ), creatine kinase (CK) and delayed onset of muscle soreness (DOMS). For the CMJ, participants undertook three jumps that were measured using a vertical jump apparatus (Yard Stick, Swift Performance, Australia). Participants were given at least 30 s of passive rest in-between each attempt, with the best score reported. Excellent test-retest reliability (intra-class correlation coefficient of 0.92) has previously been reported with an identical CMJ protocol for a similar group of moderately endurance-trained men [2, 20]. For CK, a 30 μL blood sample was collected via finger prick following 10 min of supine rest in a thermo-neutral condition of 22–23 °C. The blood sample was pipetted to a test strip and assessed for CK using a colorimetric assay method (Reflotron, Boehringer Mannheim, Germany). An in-house intra-assay coefficient of variation for CK within our laboratory was 7.2%. The DOMS was obtained using a 1–10 visual analogue scale with 1 denoted as ‘no soreness’ and 10 as ‘very, very sore’ [20]. To standardize the context of DOMS assessment, participants were requested to perform one repetition of a body weight squat and report the number of their perceived DOMS in the scale.

The measure of central tendency and dispersion for all data are reported as means ± standard deviation using Statistical Package of Social Sciences (SPSS, version 25; IBM Corp., Armonk, USA) software. The Shapiro-Wilk test was used to examine normality of the data distribution, and only CK and DOMS measures were departed from the norm. Thus, a one-way repeated measures analysis of variance (ANOVA) was employed for the majority of measures, with Bonferroni’s pairwise comparisons to determine the location of differences between each time point (i.e. TBase, T1 (only the indirect muscle damage markers), T24 and T48). A Friedman test was used to compare DOMS and CK across time points, with a Man-Whitney U test when a main time effect was identified. A paired sample t test was conducted for T_C, given that measures were only collected at TBase and T24 and for VO₂ slow component between the 7th and 10th minute of the RE test. Effect size (ES, Cohen’s d) was also calculated to report on the magnitude of differences between each time point, with 0.2, 0.5 and 0.8 classified as small, moderate and large, respectively [26]. The alpha level was set at 0.05 for all analyses.