Thirty-four healthy subjects (men n = 22 and women n = 12), volunteered to participate in the study. The process of recruiting volunteers for the study was conducted among Physical Education students during the academic semester. Participants were selected by distributing informational flyers and sharing announcements in classrooms. The materials contained details about the purpose of the study, eligibility criteria, procedures involved, and potential benefits of participating. Interested students were encouraged to sign up and participate voluntarily.

To be able to participate, participants were required not to present cardiometabolic conditions or medication usage that could interfere in the outcomes being assessed. Participants reported performing regular exercise for at least 150 min/week, predominantly running, cycling, fitness training, and ball sports (International Physical Activity Questionnaire-IPAQ) (14) according to the recommendations of the World Health Organization (15). After approval from the Research Ethics Committee (no. CAAE 41375120.6.0000.5542), participants read and signed an informed consent document. The study was carried out according to the Declaration of Helsinki.

The experimental period lasted 8 weeks: the first week was a familiarization period and pre-intervention tests (baseline); the second to seventh weeks were training intervention periods; and the eighth week was post-intervention tests. Assessments of muscle thickness (primary outcome) and aerobic fitness and general fitness (secondary outcomes) were performed in pre- and post-intervention moments.

Two testing days were adopted for both assessment moments, separated by at least 72 h. The first visit consisted of muscle thickness and aerobic fitness assessments. The volunteers were also weighed and measured for height. On the second visit, general fitness tests were performed. After baseline measurements, participants were randomly assigned, by simple draw, to one of the WB-HIIT groups: a group performing 30″ of all-out effort followed by 10″ of passive rest (G30 × 10; n = 17) or a group performing 40″ of all-out effort followed by 20″ of passive rest (G40 × 20; n = 17). Both groups displayed with 11 men and 6 women.

Participants were instructed to refrain from intense exercise and alcohol for 72 h before measurements. All assessments were performed by the same researchers. No restricted dietary control was adopted, but the subjects were instructed by a nutritionist not to change their dietary intake/usual nutrition during the entire study period. Both groups also received general guidance on healthy eating habits at the beginning of the study and were allowed to continue with their regular physical activity during the study period.

Ultrasonography was used to determine the muscle thickness of the vastus lateralis (MTVL), using an ultrasound-imaging unit (Mindray; DP10; Shenzhen, China), with a wave frequency of 7.5–10 megahertz (MHz). The ultrasound probe was applied perpendicularly to the skin for measurement. A water-soluble gel was used on the transducer to aid acoustic coupling and remove the need for excess contact pressure on the skin. Muscle thickness was defined as the distance between the interface of the muscle tissue and subcutaneous fat to the bones. Imaging was performed on the right side of the subjects' body. The subjects were asked to fast for 3 h before the tests, and muscle thickness (MT) assessments were performed at the same time of day at pre- and post-testing.

The assessments were performed at 50% distal between the lateral condyle of the femur and greater trochanter (16) with the subjects resting supine on an examination bed, with their knees fully extended and relaxed. The examined limb was secured to minimize undesired movements. The same researcher was responsible for carrying out both evaluations and was blinded to the intervention group of the participants during the experimental period. The values of the coefficient of variation and standard error of measurement were 3.18% and 0.76 mm, respectively (data from our laboratory).

Muscular endurance was evaluated by the sit-up, push-up, and Burpee's squat thrust tests. The sit-up test was conducted with participants initially positioned on a mat (supine position), with their feet fixed on the ground, heels together, and at 30–45 cm from the hip, with the fingers intertwined behind the head. The elbows were required to touch the knees at the anterior flexion of the spine and each repetition was counted as the subject returned to the initial position. Maximum repetitions performed correctly within 1 min were recorded (17).

For the push-up test, participants were instructed to initiate the test in a prone position (facing downward) with their hands placed on the floor, slightly wider than shoulder-width apart, and the feet (also placed on the floor) either together or shoulder-width apart, ensuring the body is straight from head to heels, forming a plank-like posture. In addition, the elbows were completely extended and the trunk away from the floor. In the descendent phase, the upper body needed to touch the floor, and the hands should be lifted for one second to ensure the body is completely flat on the floor. One repetition was counted when the body moved back to the starting position and the maximum number of repetitions completed was recorded (2). When the participant was unable to raise their elongated torso and lower body from the floor, the test was finished.

For the Burpee's squat thrust tests [adapted from Vandana et al. (18)], participants started in a standing position and were instructed to squat down and kick out their legs. The participants were then instructed to perform the reverse order of movements, to complete one full repetition (19). From the signal “go” the participant was asked to repeat this movement as rapidly as possible and the number of movements completed was recorded. When participants were unable to perform the movement properly, the test was finished. A 10 min rest interval was adopted between tests. All tests were performed in the same order during both pre- and post-intervention assessments. In addition, all researchers that carried out the assessments were blinded to the intervention group of the participants during the experimental period.

The maximum oxygen consumption (VO₂max) was measured using the Yo-Yo Endurance test. In summary, all participants were lined up along the starting line with one foot behind the line (cone A) and began running when instructed by the audio recording, when signaled to by the recorded audio beep (cone B), the volunteers turned and returned to the starting point. The participant continued to shuttle back and forth between the two lines 20 m apart, in time to the audio cues. At regular intervals, the time between audio signals was reduced and the running speed needed to be increased. The test was finished when the participants were not able to follow the beep two times in a row (20, 21). The distance (meters) covered in the test was then recorded and, in order to estimate the VO₂max, the following equation was adopted: VO₂max (ml.kg.min⁻¹) = (Distance covered in meters * 0.0084) + 36.4. The same researcher was responsible for carrying out both evaluations and was blinded to the intervention group that the participants were allocated to during the experimental period.

The G40 × 20 WB-HIIT training session involved 1 min of general warm-up (stationary running) followed by 9 exercises (40 s stimulus in all-out intensity of effort) divided into 3 blocks with 2 sets each. Passive rest intervals of 20 s between exercises, 40 s between sets, and 60 s between blocks were given (Table 1). The exercises adopted were, Block 1: Squat Jump, Curl up, and Skipping; Block 2: Wide arms push-up, “sumo” squat, and mountain climber; Block 3: Jumping jack, push-up, and spider plank.

The G30 × 10 WB-HIIT training session involved 1 min of general warm-up (stationary running), followed by 9 exercises (30 s stimulus in all out intensity) divided into 3 blocks with 3 sets each. Passive rest intervals of 10 s between exercises, 30 s between sets, and 60 s between blocks were allowed (Table 2). All exercises were the same as adopted in the G40 × 20 protocol. All sessions for both protocols were supervised by researchers not involved in data analysis.

The training routine lasted 6 weeks, since this time frame has been previously shown to induce relevant adaptations in both cardiorespiratory and morphological outcomes (22–24). Two weekly sessions were performed throughout the intervention period, since previous evidence suggest this training frequency as suitable for both beginner and intermediate individuals (11). Whenever a volunteer was absent, the training session was performed on another day in the same week. As a result, the adherence rate was 100% for both protocols.

The normality and homogeneity of variance were analyzed using the Shapiro-Wilk and Levene tests, respectively. The mean, standard deviation (SD), and 95% confidence interval (95% CI) were calculated for each dependent variable. To compare between-groups baseline characteristics, an unpaired student t-test was adopted. A repeated measures analysis of variance (ANOVA) was used to compare the effects of time (pre vs. post) and groups (G30 × 10 and G40 × 20), as well as the group vs. time interaction for the variables MTVL, Sit-up, Push-up, Burpee's squat thrust, and VO₂max. In case of significant F values, a Bonferroni post-hoc test was used for paired comparisons. The assumptions of sphericity were assessed using the Mauchly test. When violated, the Greenhouse-Geisser correction factor was applied. If any participant presented a pre-post change above 3 SD (outlier), his/her data was removed from the analysis. The effect size between groups (and the respective 95% CI) was calculated using Cohen's d and interpreted qualitatively as follows: trivial (<0.2), small (0.2–0.5), moderate (0.6–1.2), large (1.2–2.0), and very large (>2.0). A priori sample size calculation was performed considering fat free mass as the outcome measure, with a power of 0.80 and a target effect size of 0.38 (7), which required a minimum of 16 participants in each group to be included. The significance value adopted was p ≤ 0.05. All analyses were conducted in SPSS version 21 (IBM Corp, Armonk, NY).

For MTVL, a significant main effect of time (F_1,16 = 65.645; p = 0.001), but no effect of group (F_1,16 = 0.638; p = 0.436) or group × time interaction (F_1,16 = 2.750; p = 0.117) was observed (Table 4). No significant difference was noted for the absolute increase from baseline between groups (mean difference = 1.01 ± 0.58 mm, 95% CI = −0.18 to 2.20 mm). For the between-group ES comparison, a trivial effect was observed (d = −0.13). Twelve (70.5%) and sixteen (94.1%) participants from the G30 × 10 and G40 × 20 (respectively) responded above the typical error of measurement.

For the sit-up test, a significant main effect of time (F_1,16 = 23.000; p = 0.001), but no effect of group (F_1,16 = 2.323; p = 0.147) or group × time interaction (F_1,16 = 0.617; p = 0.444) was observed (Table 4). No significant difference was noted for the absolute increase from baseline between groups (mean difference = 1.58 ± 1.88 repetitions, 95% CI = −2.24 to 5.42 repetitions). For the between-group ES comparison, a trivial effect was observed (d = −0.09).

A significant main effect of time (F_1,16 = 10.798; p = 0.005), but no effect of group (F_1,16 = 3.105; p = 0.097) or group × time interaction (F_1,16 = 0.053; p = 0.821) was observed for the push-up test (Table 4). No significant difference was noted for the absolute increase from baseline between groups (mean difference = 0.7647 ± 3.336 repetitions, 95% CI = −6.03 to 7.56 repetitions). For the between-group ES comparison, a trivial effect was observed (d = −0.02).

Similar results were noted for the burpee's squat thrust, where a significant main effect of time (F_1,16 = 22.676; p = 0.001), but no effect of group (F_1,16 = 0.001; p = 0.980) or group × time interaction (F_1,16 = 0.093; p = 0.764) was observed (Table 4). After detecting outliers (individual responses above 3 SD), one subject from each group was removed, and no significant difference was noted for the absolute increase from baseline between groups (mean difference = 2.813 ± 3.314 repetitions, 95% CI = −6.031 to 7.560 repetitions). For the between-group ES comparison, a trivial effect was observed (d = −0.03).

A significant main effect of time (F_1,15 = 57.758; p = 0.001), but no effect of group (F_1,15 = 0.155; p = 0.709) or group × time interaction (F_1,15 = 0.287; p = 0.600) was observed for VO₂max (Table 4). After detecting outliers (individual responses above 3 SD), one subject from the G40 × 20 group was removed, and no significant difference was noted for the absolute increase from baseline between groups (mean difference = 0.06 ± 0.78 ml/kg/min, 95% CI = −1.56 to 1.69 ml.kg.min⁻¹). For the between-group ES comparison, a trivial effect was observed (d = −0.03).