Following ethical approval from the Burapha University Ethics committee (approval number: 18/2561) and in accordance with the Declaration of Helsinki, 72 university athletes, both male and female (18–30 years) were recruited.

Burapha university athletes in the Bangsaen Campus, Chonburi, Thailand were recruited in this study. Placards containing the study details were posted in the main areas of the Burapha University, such as the Faculty of Sport Science, the student dormitories, cafeteria, and library. Persons interested in participating in the study contacted a research assistant by phone and Line application. After making an appointment, they were screened through health questionnaires used to examine their general information, medical illness, history of exercise participation and supplementation intake, and mental health, in addition to a physical examination, which measured body mass, height, body mass index (BMI), blood pressure (BP), and heart rate (HR).

Prior to experiment commencement, each participant signed a written informed consent after completing study details. All participants were healthy of body and mind, were nonregular smokers or drinkers, free from medication and supplement intake, and of normal BMI. The participants were randomly recruited using a random number table into three groups (n=22/group): control group, RT group, and HIIT group as shown in Fig. 1.

The intervention design in the present study followed our previous protocol as referenced in Padkao and Prasertsri (2019), which was published in the Journal of Exercise Physiology online. (Padkao and Prasertsri, 2019). The control group was requested to maintain regular physical activity and dietary intake behaviors for 8 weeks. The RT group was instructed to perform a resistance exercise training program of the upper body (lateral pulldowns biceps curls, triceps extensions, seated dumbbell press, and lateral dumbbell raises) and lower body (leg press, lying leg curls, leg extensions, hip abductions, and adductions). In brief, the initial exercise was begun at 30% of 1RM in each muscle group, 15 repetitions/set/muscle group, then repeated at 70% of 1RM in each muscle group, 8 repetitions/set/muscle group, and finally performed at 30% of 1RM. Therefore, the total exercise session consisted of a couple of sets/muscle groups at low intensity (30% of 1RM) and a single set/muscle group at greater intensity (70% of 1RM). All subjects performed a 3-min warm-up and 3-min cool-down in the form of brisk marching. Exercise duration was 30 min/day, 3 days a week over 8 weeks. The HIIT group performed an exercise training program on a bicycle ergometer (Monark 828E ergomedic, Monark Exercise AB, Vansbro, Sweden) for 30 min/day, 3 days a week, for 8 weeks. Subjects began with a 3-min warm-up with free workload cycling. Then, a load was added to render 30% of maximal oxygen consumption (VO_2max) for 1 min (low intensity). After that, it was increased to an intensity of 70% of VO_2max (high intensity). Exercise intensity was interchanged between 30% and 70% VO_2max every min for 24 min. During exercise, subjects were required to maintain a speed of 50–60 rpm/min. At exercise completion, they performed a 3-min cool-down with free workload. Exercise duration in this group was the same as in the RT group. Assessments of parameters at pre- and posttraining were performed within 3 days prior to and subsequent to the training at the same duration of the day i.e., in the morning.

The 1RM is defined as the maximal weight that can be lifted once with the correct lifting technique. It is considered the gold standard for evaluating muscle strength in nonlaboratory situations (Seo et al., 2012). Participants in the RT group were evaluated for 1RM in each muscle group as recently described (Padkao and Prasertsri, 2019).

VO_2max was measured using a bicycle ergometer (Monark Ergomedic 828E, Monark Exercise AB, Sweden) according to the YMCA cycle test as previously defined (Padkao and Prasertsri, 2016).

Body composition and hemodynamics data were expressed as baseline characteristics for all participants. Body composition measurements were conducted in the standing position using a body composition analyzer (InBody 270, Ordamed Co., Ltd., Seoul, Korea) based on the principle of bioelectrical impedance analysis. Hemodynamic parameters were measured incorporating systolic BP (SBP), diastolic BP, mean arterial BP, and HR. HR pressure product (HRPP) was calculated by SBP multiplied by HR.

Plasma malondialdehyde (MDA) was determined by measuring thiobarbituric acid reactive substances as previously mentioned (Nakmareong et al., 2011). In brief, blood was collected in an ethylenediaminetetraacetic acid (EDTA) tube and centrifuged at 3,500 rpm, for 10 min, at 4°C. Plasma samples of 150 μL were combined with 10% trichloroacetic acid, 5 mM EDTA, 8% sodium dodecyl sulfate, and 0.5 μg/mL of butylated hydroxytoluene. This mixture was incubated for 10 min at room temperature with 500 μL of 0.6% 2-thiobarbituric acid added. The mixture was then boiled in a water bath for 30 min. After cooling to room temperature, the mixture was centrifuged at 10,000×g for 5 min. Supernatant absorbance was measured at 532 nm via a spectrophotometer. A standard curve was generated with appropriate concentrations of 1,1,3,3-tetraethoxypropane (0.3–10 μM).

Whole blood total glutathione (GSH) and Glutathione disulfide (GSSG) were measured in accordance with a previously described method (Sompamit et al., 2009). GSSG was determined using M2VP as a GSH scavenger. Briefly, a 100-μL whole blood sample was immediately reacted with 10 μL of 33 mM M2VP or distilled water and subsequently treated with 5% cold MPA to precipitate protein. Following centrifugation, the supernatant was used in the enzymatic coupling assay for GSH via a spectrophotometer (Ultrospec 6300 pro, Biochrom Ltd., Cambridge, UK). Lastly, redox ratio was calculated as GSH/GSSG.

Physical performance, flexibility, muscle strength, and endurance were determined in each participant. For the sit and reach test, a box was utilized for the assessment of trunk and leg flexibility. The highest value from three maneuvers was recorded. Each participant was asked to perform the maneuver in the sitting position with the legs straight and knees flat against the floor. Individuals were subsequently instructed to lean forward and reach out toward the box slowly as far as possible and hold for 2 sec. A handheld dynamometer was employed to measure leg muscle strength. The greatest value from two maneuvers was recorded. Each participant was requested to perform the maneuver in the standing position with the head and trunk straight along with slight hip and knee flexion. Subjects were instructed to pull the handheld dynamometer as much as possible and hold for 2 sec. They were allowed to rest for one minute and then repeat the maneuver. Muscle endurance performance to the point of fatigue was conducted utilizing the barbell bench press method. The YMCA test was administered according to procedures as previously described. A 36-kg (80 lb) barbell was used for men, with a 16-kg (35 lb) barbell used for women. The bar was either raised or lowered with each beat to achieve an overall lifting pace of 30 reps/min via metronome. Subjects were instructed to maintain a controlled pace throughout the test. The test was terminated when they could no longer lift the bar or were unable to maintain cadence (Kim et al., 2002). The maximum amount of barbell bench press repetitions was recorded. Briefly, participants lie back on a flat bench and grip the bar at shoulder width apart. From the starting position, subjects were instructed to breathe in and begin moving the weight down slowly until the bar touched the middle of the chest. They then pushed the bar upwards with the arms fully extended while breathing out. Subjects repeated the movement as many times as possible according to the pace of the metronome.

All data are presented as means±standard deviation. Normal distribution of data was applied according to the Shapiro–Wilk test. Differences among various groups were compared using one-way analysis of variance followed by Bonferroni post hoc test to determine the variances between groups. Paired t-test was applied to determine the differences before and after within groups. A P-value of <0.05 was considered statistically significant.

The mean age of participants was 21.8±1.0, 20.4±0.8, and 20.3±0.6 years in control, RT, and HIIT groups, respectively. The body weight in HIIT group was significant difference compared with RT group (P=0.015). This might partly be due to the difference in male to female ratio that was higher in HIIT group (Table 1). Amount of habitual exercise performed by the control, RT, and HIIT groups were 355.5±285.1 min/wk, 309.0±237.0 min/wk, and 277.2±218.4 min/wk, respectively. There were no statistically significant differences between groups. Moreover, there were no significant differences in baseline height, BMI, fat mass, lean mass, and hemodynamics parameters in all experimental groups (Table 1).

Oxidative stress is defined as an excessive of oxidants over antioxidants within a body system. The direct consequence of this condition is a shift in the redox state of biological compartments including DNA, mitochondria, lipid, and protein toward oxidizing and cause tissue damage.

A common approach to determine oxidative stress in biological systems involves assessment of the decrease or increase in a biomarker molecule that responds to oxidative stress. Evaluation of tissue antioxidant or lipid tissue damage, which MDA is a key by-product, is commonly attribute. To protect against oxidative damage, increases in antioxidant production and activity play a chief role in scavenging and reducing ROS. Thus, decline in the tissue antioxidant level is considered as to be oxidative stress. Besides, major tissue antioxidant enzymes including superoxide dismutase, catalase, and glutathione peroxidase (GPX) are existing and inducible in skeletal muscles. Among these antioxidant enzymes, GPX is indicated to increase in skeletal muscle fibers that are actively recruited during regular exercise training (Powers and Jackson, 2008).

Plasma MDA, a biomarker of lipid peroxidation is one criterion which has been identified as concerning oxidative stress status. At the outset period, plasma MDA exhibited no significant difference amid all experimental groups. Interestingly, after 8 weeks of RT, a significant reduction of plasma MDA compared with baseline (P=0.016) (Fig. 2) was observed.

GPX is an intracellular antioxidant enzyme. A greater level of blood glutathione indicates increased antioxidant levels. All GPXs are enzymes that catalyze the reduction of H₂O₂ to water by using reduced GSH as the electron donor. When GSH is the electron donor, it donates a pair of hydrogen ions, and GSH is oxidized to GSSG. High ROS production results in a decrease in the GSH/GSSG ratio, indicating lower levels of reduced GSH in favor of increased oxidized GSH. Surprisingly, the results of this study demonstrated that GSH significantly increased postexperimental period in all experimental groups (P<0.05) (Fig. 3A). After 8 weeks of both resistance and HIIT, GSH/GSSG redox ratios were markedly increased compared with baseline (P<0.05) (Fig. 3B). Thus, this data suggests that both types of exercise intervention improved antioxidant status in athletes. However, GSH and GSH/GSSG ratio were not significantly different between groups over time.

Physical performances were improved following RT and HIIT, with increased flexibility (P<0.05) (Fig. 4), leg strength (P<0.05) (Fig. 5) as well as an increase in weight lifting repetitions amid bench press testing (P<0.05) (Fig. 6). Though, this change was not observed in the control group.