Subjects

In total, 30 healthy male volunteers who were nonathletes participated in this study [mean ± standard deviation (SD): age, 20.9 ± 0.4 years; height, 170.2 ± 5.3 cm; and weight, 62.8 ± 4.2 kg]. All subjects participated in sports at a recreational level and had not been involved in any regular resistance or flexibility training. In addition, the subjects were instructed to not start a new resistance or flexibility training during the training intervention period. Subjects with a history of neuromuscular disease or a musculoskeletal injury of the upper extremities were excluded. Written informed consent was obtained from all subjects. The study was approved by the Ethics Committee of the Niigata University of Health and Welfare, Niigata, Japan (17678), and it followed the CONSORT recommendations. The study was registered with the University Hospital Medical Information Network Clinical Trials Registry (UMIN000036167; March 11, 2019).

Study protocol

This study followed a randomized, controlled design (Fig 1). After baseline measurements, the subjects were randomly allocated to the combined low-intensity resistance training with heat stress group (heat group; N = 15) or control group (N = 15) using a computerized random number function in Microsoft Excel (Microsoft Corp., Washington, WA, USA). In the heat group, low-intensity resistance training was performed after 20-min heat stress. In the control group, low-intensity resistance training was performed without heat stress. For each subject, 1RM for elbow extension was measured 1 week before the training session, and the resistance training load was set to 30% 1RM. The 1RM measurements were repeated every 2 weeks during the intervention, and the resistance training load was readjusted based on the actual 1RM value.

Fig. 1 Flow chart of the study protocol Full size image

Resistance training program

The triceps brachii of the dominant arm was investigated in this study because previous studies have reported that the triceps brachii shows high responsiveness to resistance training aimed at an increase in muscle volume [19]. The resistance training comprised lying triceps extension with a dumbbell. The subjects laid in the supine position with the shoulder and elbow both in 90° flexion, and they were instructed to extend the elbow concentrically for 2 s, then eccentrically for 2 s, and finally isometrically for 1 s at an angle of 90°. In both groups, the training load was 30% 1RM and the resistance training comprised 3 sets with 8 repetitions and 60-s intervals. The resistance training was performed 3 days per week for 6 weeks (18 sessions).

Muscle strength measurement

Each subject was instructed to perform a warm-up of 5 repetitions with a 3.5-kg dumbbell and 2 repetitions with a 5.0-kg dumbbell [20, 21]. After the warm-up, 1RM measurements were performed and the initial load was selected by each subject. The load was increased until the subject could not lift the weight anymore through a range of motion (elbow flexion from 90° to full extension) with the proper form. To avoid the effect of fatigue on 1RM, the rest period between two measurements was > 90 s. The 1RM measurements were performed at preintervention week 1 and postintervention weeks 2, 4, and 6.

Measurements of muscle thickness

The thickness of the triceps brachii muscle was measured using B-mode ultrasonography (Aplio 500; Toshiba Medical Systems, Tochigi, Japan) with a 5–14-MHz linear probe. The measurement point was halfway on a line from the acromial process of the scapula to the lateral epicondyle of the humerus [20, 21]. The subjects were instructed to lie in the prone position on a desk with their arms placed at their sides and the wrist pronated. Measurements were taken from the inner edge of the fascia to the humerus. Muscle thickness was measured before and after resistance training intervention. Measurements were performed > 48 h after the last resistance training session to avoid errors due to acute edema. All measurements were performed by the same experienced investigator.

Heat stress application

A hot pack was placed on the dominant upper arm for the application of heat stress. The subjects lied in the prone position, and the hot pack was applied to cover the muscle belly of the triceps brachii. Before application, the hot pack was heated to 75 °C in an hydrocollator and wrapped in a towel [18].

In a pilot study, we measured muscle temperature in 13 healthy males (mean ± SD: age, 21.2 ± 0.8 years; height, 171.7 ± 5.4 cm; and weight, 62.8 ± 4.3 kg) with a surface-type deep body thermometer (Core temp CTM-210; Telmo, Tokyo, Japan). The results showed that muscle temperature increased from 34.2 °C ± 1.0 °C (mean ± SD) before intervention to 36.9 °C ± 0.5 °C (mean ± SD) after low-intensity resistance training with heat stress using the same protocol as used in the heat group.

Statistical analysis

SPSS (version 24.0; IBM Corp., Armonk, NY, USA) was used for statistical analysis. Potential differences between the heat and control groups for 1RM and muscle thickness before measurements were assessed with an unpaired t-test. For all variables, a split-plot analysis of variance (ANOVA) using two factors [group (heat vs. control group) and test time (before vs. after measurements)] was used to analyze the interaction and main effect. When a significant interaction was observed, the Bonferroni multiple comparison test was used to determine the differences in 1RM among the pre intervention measurements as well as postintervention weeks 2, 4, and 6 measurements. Paired t-test was used to determine the differences in muscle thickness between the before and after measurements in the heat and control groups. The differences were considered significant at an alpha level of 0.05. Descriptive data are shown as mean ± SD.