Eight weeks equivalent-volume RT and HIIT performed within same session or alternated weekly: comparative effects on physiological adaptations in young adult males

Abstract

Background

This study aims to investigate the effects of 8 weeks equivalent-volume resistance training (RT) and high-intensity interval training (HIIT) performed within same session or alternated weekly on body composition, cardiorespiratory function, and muscular fitness in young adult males.

Methods

Twenty-three young adult males were randomly assigned to either the A group (RT and HIIT performed within the same session, n = 11) or the B group (RT and HIIT were alternated weekly, n = 12). Body composition, cardiorespiratory function, and muscular fitness were assessed at baseline (T1), the fifth week (T2), and the ninth week (T3). Baseline differences and training effects were analyzed using independent sample t-tests and repeated measures ANOVA, respectively.

Results

No significant group differences were found at baseline (P > 0.05). Post-intervention, body fat percentage, skeletal muscle mass, and thickness significantly improved over time (P < 0.05), but no significant group or interaction effects. Maximum power output significantly increased at T2 and T3 compared to T1 (P < 0.05), while VO₂max, METs, and Maximum Minute Ventilation significantly increased only at T3 compared to T1 (P < 0.05), with no significant group or interaction effects. Both groups showed significant improvements in maximal strength during the intervention (P < 0.05). Endurance repetitions, particularly in the squat exercise, showed significant improvement initially (P < 0.05), followed by a downward trend, and no significant group or interaction effects were found.

Conclusions

Concurrent training, whether performed within the same session or alternated weekly, elicited comparable improvements in body composition, cardiorespiratory, and muscular outcomes in young adult males. Both arrangements promoted beneficial adaptations, indicating flexibility in concurrent training design.

Trial registration

The study was retrospectively registered on 24 June 2025 at www.chictr.org.cn with identification number ChiCTR2500104848.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13102-025-01464-0.

Background

Few sports rely solely on strength or endurance, except for pure weightlifting or long-distance running on flat terrain. In fact, almost all sports require some combination of strength and endurance [4, 61]. Therefore, whether in competitive sports or general fitness, individuals often need to enhance both cardiorespiratory function and muscular fitness to achieve optimal performance or promote health [55]. Aerobic training (AT) (such as high-intensity interval training (HIIT)) generally helps improve the oxidation capacity of glucose and fat, increases mitochondrial quantity and function, enhances oxidative enzyme activity, and improves the contractile efficiency of respiratory and cardiac muscles, thereby significantly boosting cardiorespiratory function [22, 75]. Meanwhile, resistance training (RT), as the primary means of enhancing muscular strength, effectively promotes protein synthesis, induces muscle hypertrophy, and increases muscle cross-sectional area, thus improving muscular fitness [30, 54]. Against this background, concurrent training (CT)—defined as the simultaneous engagement in both AT and RT within the same session or training cycle—has become the preferred exercise strategy for many athletes and the general population [6]. Accumulating evidence suggests that CT is effective not only in enhancing cardiorespiratory and muscular strength but also in improving body composition. For instance, Martins et al. found that a six-week CT effectively improved cardiorespiratory function and muscle strength in young adult man [44]. Similarly, a meta-analysis revealed comparable results [43]. Furthermore, research shows that through the scientific integration of AT and RT, it not only helps enhance skeletal muscle mass (SMM) but also effectively reduces body fat percentage (BF%) in healthy adults [2, 10].

However, RT and AT often result in different, and even opposite, physiological adaptations, a phenomenon known as the "interference effect" in CT [52]. Hickson [31] was the first to report the interference effect in CT. His study found that pure RT led to continuous improvements in leg strength, while the CT initially showed similar strength gains. However, from weeks 9 to 10, the strength gains plateaued and even declined in CT [31]. This phenomenon suggests that when RT is combined with AT, the long-term effects on strength development may be attenuated. As mentioned earlier, AT enhances aerobic metabolism, improving the oxidative capacity of muscle fibers. In contrast, RT promotes muscle hypertrophy by increasing protein synthesis, which may lead to a relative insufficiency in capillary density within the muscle fibers per unit volume [49]. Therefore, during CT, these two types of training may induce distinct molecular biological adaptations, resulting in skeletal muscles being unable to simultaneously adapt to the metabolic and morphological changes required by both training types, thereby causing an incompatibility [64]. This incompatibility particularly negatively impacts the improvement of maximal strength [73] and strength endurance [19], while the interference with aerobic capacity is relatively weak [15].

It is worth noting that in order to minimize the impact of the interference effect, a series of studies have explored the potential factors influencing the interference effect in CT, including the order of the two types of training [43], the intensity of RT, whether to failure [70], the intensity of AT [65], training volume [58], and the mode of exercise [56]. Among these, the order of training is one of the main factors influencing the interference effect. Some studies have proposed that the acute interference of AT on subsequent RT may be the cause of the decline in muscle strength [15, 58]. Similarly, a systematic review by Eddens indicated that, compared to AT performed first, performing RT first is more beneficial for the physiological adaptation of lower limb dynamic muscle strength [43]. However, it is noteworthy that the effects of different training arrangements in CT have not been sufficiently addressed. Existing CT arrangements often place AT and RT within the same session [5, 65], which may present limitations in terms of practicality and flexibility [55]. Therefore, the present study aims to investigate the equivalent-volume RT and HIIT performed in same session vs alternated weekly on body composition, cardiorespiratory function, and muscular fitness in young adult males. Unlike traditional CT arrangements, the weekly alternating CT used in this study essentially represents a form of block periodization [59, 66], in which each week functions as a dedicated block emphasizing either RT or HIIT stimuli. We hypothesize that in this experimental design, weekly alternating CT (block periodization approach) would produce comparable physiological adaptations to same-session CT. This arrangement may provide greater flexibility for this population when choosing exercise times, thereby improving the operability and sustainability of the training.

Materials and methods

Participants

In this study, youth males aged 18 to 28 years were recruited through posters and social media from several prestigious universities in Hai Dian District, Beijing, with the recruitment process illustrated in Fig. 1. Inclusion criteria were: (1) male participants aged between 18 and 28 years; (2) healthy individuals confirmed through a health questionnaire, with no history of chronic diseases such as hypertension, heart disease, or osteoporosis, and not classified as obese; (3) no recent use of any form of vitamin supplements or sports nutrition supplements; (4) no history of significant injuries or recent surgeries; (5) no history of genetic disorders or any diseases affecting physical performance; (6) no severe allergies, particularly exercise-related allergic reactions such as exercise-induced asthma; (7) voluntary participation and willingness to comply with the study requirements. Exclusion criteria included: (1) a history of severe sports injuries or fractures within the past six months; (2) current cardiovascular, neurological, or endocrine disorders; (3) current use of any medications that may affect metabolism or physical performance, including steroids, hormones, or vitamin supplements; (4) any major surgical procedures or significant illnesses in the past three months. All participants provided written informed consent prior to participation in the study. This study was approved by the Ethics Committee of the Capital University of Physical Education and Sports in March 13, 2024, under the approval number 2024A207.

Fig. 1.

Participant recruitment flowchart

A priori power analysis was conducted based on the study by Faul et al., with sample size calculation performed using G*Power (3.1.9.7, University of Düsseldorf, Germany) for statistical power analysis [23]. Sample size estimation was based on detecting a time * group interaction for VO₂max (primary outcome) with an expected medium effect size (f = 0.30), α = 0.05, and power = 0.80, resulting in a minimum 10 participants per group [42]. To account for potential attrition and enhance statistical power, the sample size was increased to 12 participants per group, resulting in the recruitment of 24 participants. During the experiment, one participant withdrew for reasons unrelated to the study, and thus, 23 participants completed the study and were included in the data analysis. The basic characteristics of the participants are presented in Table 1.

Table 1.

Basic characteristics of participants

Variable	A (n = 11)	B (n = 12)	P
Year	22.73 ± 2.83	24.08 ± 2.39	0.227
Height	174.45 ± 6.12	178.83 ± 5.24	0.079
Weight (kg)	72.08 ± 8.19	76.83 ± 9.17	0.206
BF (%)	18.55 ± 3.75	18.38 ± 5.26	0.930
SMM (kg)	33.26 ± 4.49	35.48 ± 3.95	0.221
BB Muscle Thickness (cm)	1.83 ± 0.37	1.83 ± 0.21	0.972
RF Muscle Thickness (cm)	2.00 ± 0.22	1.92 ± 0.30	0.495
Wmax (W)	290.55 ± 51.57	293.75 ± 47.94	0.879
VO₂max (mL/kg/m)	47.16 ± 7.24	46.65 ± 2.65	0.828
METs	13.46 ± 2.07	13.33 ± 0.78	0.848
VEmax(L/m)	116.47 ± 20.68	115.70 ± 17.34	0.924
EQO2	34.09 ± 4.06	31.91 ± 3.26	0.170
Deep Squat 1RM (kg)	113.18 ± 34.52	112.5 ± 26.84	0.958
Bench Press 1RM (kg)	64.55 ± 23.07	63.33 ± 17.10	0.887
Rowing 1RM (kg)	68.27 ± 15.60	67.58 ± 10.08	0.900
Repetitions for Deep Squat	11.82 ± 3.25	13.92 ± 4.85	0.241
Repetitions for Bench Press	11.73 ± 4.08	13.25 ± 3.86	0.368
Repetitions for Rowing	12.45 ± 4.46	13.00 ± 4.55	0.775
Strength Endurance for Deep Squat (kg*reps)	1300.91 ± 462.52	1598.75 ± 713.54	0.253
Strength Endurance for Bench Press (kg*reps)	790.00 ± 435.20	868.33 ± 365.07	0.644
Strength Endurance for Rowing (kg*reps)	851.73 ± 385.32	884.58 ± 329.58	0.828

BF% Body Fat Percentage, SMM Skeletal Muscle Mass, BB Biceps Brachii, RF Rectus Femoris, Wmax Maximum Power Output, VO₂max Maximal Oxygen Uptake, METs Metabolic Qquivalents, VEmax Maximum Minute Ventilation, EQO₂ Oxygen Ventilation Equivalent

Experimental design

After signing the informed consent form and completing the personal health status questionnaire, all participants were randomly assigned to one of two groups using a computer-generated random number table (Y C): the same session group (A) (n = 11) and the different-week group (B) (n = 12). Given that previous studies have suggested that prioritizing RT in CT may help minimize the interference effect from AT, the present study adopted an RT-priority CT protocol [43]. Each group performed three CT sessions per week. The specific arrangements were as follows: in the A group, each training session involved RT first, followed by AT with a 10-min interval; while in the B group, each training session involved a doubled volume of either pure RT or AT, with RT was performed in the first week and AT in the second week, alternating between the two. Prior to the intervention, the research team conducted comprehensive measurements of participants' body composition (including weight, SMM, and BF%), muscle thickness (biceps brachii (BB) and rectus femoris (RF)), cardiorespiratory function (including maximum power output (Wmax), maximal oxygen uptake (VO₂max), metabolic equivalents (METs), maximum minute ventilation (VEmax), and oxygen ventilation equivalent (EQO₂)), and muscular fitness (maximal strength and strength endurance). In the 5th week of the intervention, participants' cardiorespiratory function and muscular fitness were re-evaluated to maintain the intended intensity of RT (70% of one-repetition maximum (1RM)) and AT (85% of VO₂max intensity) based on their progress. Finally, in the 9th week after the post-intervention, the aforementioned indicators were reassessed. Notably, cardiorespiratory function, maximal strength and strength endurance were evaluated on separate days to minimize fatigue-related interference between tests. Participants maintained a regular diet during the intervention period and were instructed to refrain from engaging in any additional physical activity.

Exercise protocol

In the CT protocol for the A group, RT consists of three exercises: squats, bench presses, and bent-over rows. The training intensity is set at 70% 1RM, with two sets of each exercise, performing 8–10 repetitions per set, and a 2-min rest interval between sets [3]. The exercise order follows: squat, bench press, and bent-over row. After completing RT, participants rest for 10 min, followed by HIIT. HIIT is performed on a treadmill, where participants run for 1 min at 85% of VO₂max intensity, followed by a 1-min recovery period at 35%. This alternating pattern is repeated for 5 cycles, lasting approximately 10 min. The RT protocol was primarily developed based on the exercise prescription guidelines from the ACSM [3]. The HIIT protocol was designed with reference to the study by Coe et al., which demonstrated the effectiveness of this approach, and the number of cycles was 5 [13]. The entire training session lasts about 40 min.

In the B group, the training content is alternated weekly, with participants completing three sessions per week of a single training type, either RT or HIIT (with RT in the first week). During RT weeks, the number of sets is increased from 2 to 4, while other parameters (including intensity, exercise type, repetitions, and rest intervals) remain consistent with the A group. In the AT weeks, only HIIT is implemented, with the number of sets adjusted to 10, while other training parameters remain unchanged. The entire training session lasts about 40 min. All participants completed over 90% of scheduled sessions through attendance logs, with an overall adherence rate of 95.8%; one participant withdrew during the study. Prescribed intensity zones (70% 1RM for RT and 85% VO₂max for HIIT) were maintained through real-time heart-rate monitoring using the Polar H10 device (Polar Electro, Finland) throughout the intervention, ensuring both adherence and safety.

Body composition measurement

On the day of measurement, participants underwent body composition measurement in the morning after an overnight fast, water restriction (at least 8 h), and sufficient rest. Prior to the measurement, participants were instructed to remove watches, jewelry, and all metal items to avoid interference with the results. To standardize dietary and activity conditions, participants were asked to maintain their regular dietary habits the day before the test and were allowed to engage in routine light activity. Height was measured using an ultrasonic height measurement device (DHM-200, Dingheng Technology Co., Ltd., China). Body composition was assessed using a bioelectrical impedance analyzer (InBody720, InBody Co., Ltd., South Korea), which measured weight, BF%, and SMM [29]. After calibrating the equipment according to the manufacturer's instructions, participants' basic information (such as height, age, and gender) was input into the device. Participants were instructed to stand barefoot and stable on the platform, holding the handles with their arms naturally extended at about a 15° angle from the body, ensuring that their palms, fingers, and soles of the feet made full contact with the electrodes, while participants were required to remain still during the test to ensure the accuracy of the data.

Muscle thickness

Muscle thickness was assessed using a portable B‑mode ultrasound system (LOGIQ Book XP, GE Healthcare, USA) at the BB and RF. Participants were placed supine on a treatment table in the standard anatomical position, with palms facing upward and both upper and lower limbs fully extended and relaxed. The BB measurement site was located at the two-thirds of the distance between the elbow fold and the tip of the acromion [32], while the RF site was defined as the midpoint between the anterior superior iliac spine and the superior border of the patella [25]. At each site, an adequate quantity of water‑based ultrasound coupling gel was applied to ensure optimal acoustic contact, and the transducer was positioned perpendicular to the skin without exerting excessive pressure to avoid tissue deformation. Transducer placement was marked to guarantee consistency across testing sessions. Three ultrasound images were captured at each site, and muscle thickness was determined from these images. All scans were performed by a single, experienced investigator to minimize inter‑operator variability. Test–retest reliability was evaluated using a two‑way mixed‑effects model: for the BB, the average measurement intraclass correlation coefficient (ICC) was 0.996 (95% CI: 0.992–0.998) and the single‑measurement ICC was 0.987 (95% CI: 0.977–0.993); for the RF, the average measurement ICC was 0.977 (95% CI: 0.960–0.988) and the single‑measurement ICC was 0.935 (95% CI: 0.888–0.965), indicating excellent measurement stability and consistency.

Cardiorespiratory function

The present study employed a treadmill to conduct a cardiorespiratory exercise test to assess the participants' cardiorespiratory function, including Wmax, VO₂max, METs, VEmax, and EQO₂. During the test, participants wore a bidirectional respiratory mask covering the nose and mouth, with gas exchange data recorded in real time by a gas analyzer (PowerCube Ergo, Ganshorn Medizin Electronic GmbH, Germany) [72]. Specifically, after a thorough warm-up, the test began with an initial speed of 2.74 km/h and a 10° incline. Every 3 min, the speed was increased by approximately 1.3 km/h, and the incline was raised by 2° until the participant reached exhaustion. The criteria for determining VO₂max include: (1) despite an increase in exercise load, oxygen uptake plateaus and no longer rises; (2) the respiratory exchange ratio exceeds 1.1; (3) the HR reaches within ± 10 beats/min or ± 5% of the age-predicted maximum heart rate (220 − age); and (4) the participant experiences subjective difficulty in maintaining the current exercise intensity [12, 57].

Muscular fitness measurement

The 1RM tests for the squat, bench press, and bent-over row were conducted following a standardized procedure. Initially, participants completed two sets of free-weight warm-up exercises, performing 8–10 repetitions per set to sufficiently activate the muscle groups and familiarize themselves with the movement patterns. Subsequently, participants performed a series of single repetitions, gradually increasing the weight to ultimately determine their 1RM. The weight increments were dynamically adjusted based on the participant's performance in the previous attempt and the execution of the movement, with smaller increments made as the maximum strength output approached, until they could no longer complete the movement or the researcher determined failure according to technical standards [26, 35]. Each participant was allowed up to three attempts, passive rest periods of 3 to 5 min were allotted between attempts. The day after completing the 1RM test, participants performed a strength endurance assessment at 70% of their 1RM by completing one set to failure, with the number of repetitions recorded. Throughout the test, researchers provided consistent verbal encouragement and continuously monitored the technique to ensure proper movement execution [11].

Statistical analysis

All data were recorded in electronic spreadsheets (Microsoft Excel, Redmond, WA, USA) and analyzed using SPSS statistical software (Version 29.0, Chicago, IL, USA). Data are presented as mean ± standard deviation (SD). Independent sample t-tests were used to assess baseline differences between the two groups. Additionally, to further explore the effects of different arrangements during CT on the dependent variables, repeated-measures analysis of variance (ANOVA) was employed. The experimental design followed a 2 (same session vs. different-week) * 2 (time: T1 and T2, involving body composition and muscle thickness) or 3 (time: T1, T2, and T3, involving cardiorespiratory function and muscular fitness) factorial design. Prior to analysis, data were tested for normality using the Shapiro–Wilk test, and if data violated the sphericity assumption, Greenhouse–Geisser correction was applied to adjust the degrees of freedom. In the case of significant group, time, or interaction effects, Bonferroni pairwise comparisons were used for further analysis. Effect size was assessed using partial eta squared (η²), with the following criteria: ∼0.01 for small effect, ∼0.06 for medium effect, and ≥ 0.14 for large effect [14]. R statistical software (Version 4.2.3, R Foundation for Statistical Computing, Vienna, Austria) was used to calculate the 95% confidence interval (CI) for η² using an approximate standard error method. Equivalence between groups was assessed using the two one-sided tests (TOST) procedure [38, 45]. Equivalence margins were defined based on the smallest worthwhile change (SWC), calculated as 0.2 × the baseline SD. For each outcome, the 90% CI of the between-group change-score difference was compared with the equivalence bounds [–SWC, + SWC] to determine whether equivalence was established. Statistical significance was set at P ≤ 0.05.

Results

At baseline, no significant statistical differences (P > 0.05) were observed between the two groups in terms of body composition, muscle thickness, cardiorespiratory function, and muscular fitness (Table 1), indicating good comparability between the groups.

In terms of body composition and muscle thickness, the results of repeated-measures ANOVA showed that weight did not reach a significant level for the main effects of time, group, or time * group interaction (P > 0.05). However, for BF%, SMM, and muscle thickness, a significant main effect of time was found (P < 0.05), although no significant differences were observed for the main effect of group or the time * group interaction (P > 0.05) (Table 2). Bonferroni post-hoc comparisons indicated that both groups exhibited a significant reduction in BF% (P < 0.05) and a significant increase in SMM, BB and RF thickness (P < 0.05) after the intervention. Equivalence testing indicated that for body weight, the between-group mean difference at T1 was –4.75, with a SWC of ± 1.7 and a 90% CI of [–12.27, 2.78]. The TOST single-sided p-values were 0.79 (lower bound) and 0.04 (upper bound), indicating that equivalence was not achieved. At T3, the between-group mean difference was –4.26, with the same SWC (± 1.7) and a 90% CI of [–11.56, 3.03]. The TOST single-sided p-values were 0.76 (lower bound) and 0.05 (upper bound), again indicating that equivalence was not achieved. Comparable Equivalence results were observed for BF%, SMM, and muscle thickness.

Table 2.

Repeated measures ANOVA of intervention effects

Variable	Condition	T1	T2	T3	Time	Group	Time*Group
Weight (kg)	A	72.08 ± 8.19	-	72.13 ± 8.31	F = 0.44, p = 0.513, Partial η2 = 0.02, 95%CI: 0.00–0.14	F = 1.60, p = 0.220, Partial η2 = 0.07, 95%CI: 0.00–0.28	F = 0.68, p = 0.420, Partial η2 = 0.03, 95%CI: 0.00–0.18
	B	76.83 ± 9.17	-	76.39 ± 8.50
BF (%)	A	18.55 ± 3.75	-	17.71 ± 3.86	F = 6.652, p = 0.017, Partial η2 = 0.24, 95%CI: 0.00–0.56	F = 0.01, p = 0.920, Partial η2 = 0.00, 95%CI: 0.00–0.02	F = 0.01, p = 0.946, Partial η2 = 0.00, 95%CI: 0.00–0.01
	B	18.38 ± 5.26	-	17.49 ± 5.27
SMM (kg)	A	33.26 ± 4.49	-	33.73 ± 4.51	F = 4.933, p = 0.037, Partial η2 = 0.19, 95%CI: 0.00–0.49	F = 1.49, p = 0.236, Partial η2 = 0.07, 95%CI: 0.00–0.27	F = 0.49, p = 0.492, Partial η2 = 0.02, 95%CI: 0.00–0.15
	B	35.48 ± 3.95	-	35.73 ± 3.71
BB Muscle Thickness(cm)	A	1.83 ± 0.37	-	2.34 ± 0.28	F = 33.79, p < 0.001, Partial η2 = 0.63, 95%CI: 0.37–0.89	F = 1.32, p = 0.264, Partial η2 = 0.06, 95%CI: 0.00–0.27	F = 3.01, p = 0.098, Partial η2 = 0.13, 95%CI: 0.00–0.41
	B	1.83 ± 0.21	-	2.10 ± 0.31
RF Muscle Thickness(cm)	A	2.00 ± 0.22	-	2.15 ± 0.30	F = 16.29, p < 0.001, Partial η2 = 0.45, 95%CI: 0.13–0.77	F = 0.02, p = 0.878, Partial η2 = 0.00, 95%CI: 0.00–0.03	F = 2.31, p = 0.144, Partial η2 = 0.10, 95%CI: 0.00–0.36
	B	1.92 ± 0.30	-	2.26 ± 0.29
Wmax (W)	A	290.55 ± 51.57	325.82 ± 61.60	343.09 ± 73.38	F = 5.72, p = 0.006, Partial η2 = 0.21, 95%CI: 0.00–0.43	F = 0.00, p = 0.994, Partial η2 = 0.00, 95%CI: 0.00–0.00	F = 0.06, p = 0.938, Partial η2 = 0.00, 95%CI: 0.00–0.04
	B	293.75 ± 47.94	329.00 ± 74.11	337.17 ± 63.23
VO₂max (mL/kg/m)	A	47.16 ± 7.24	48.71 ± 5.01	49.91 ± 4.00	F = 4.336, p = 0.019, Partial η2 = 0.17, 95%CI: 0.00–0.38	F = 0.02, p = 0.892, Partial η2 = 0.00, 95%CI: 0.00–0.03	F = 0.03, p = 0.973, Partial η2 = 0.00, 95%CI: 0.00–0.02
	B	46.65 ± 2.65	48.63 ± 5.16	49.76 ± 5.56
METs	A	13.46 ± 2.07	13.92 ± 1.44	14.25 ± 1.14	F = 4.339, p = 0.019, Partial η2 = 0.17, 95%CI: 0.00–0.38	F = 0.02, p = 0.905, Partial η2 = 0.00, 95%CI: 0.00–0.02	F = 0.02, p = 0.977, Partial η2 = 0.00, 95%CI: 0.00–0.02
	B	13.33 ± 0.78	13.91 ± 1.47	14.21 ± 1.59
VEmax(L/m)	A	116.47 ± 20.68	122.39 ± 11.42	127.21 ± 12.85	F = 5.184, p = 0.010, Partial η2 = 0.20, 95%CI: 0.00–0.41	F = 0.01, p = 0.919, Partial η2 = 0.00, 95%CI: 0.00–0.02	F = 0.04, p = 0.961, Partial η2 = 0.00, 95%CI: 0.00–0.03
	B	115.70 ± 17.34	120.94 ± 20.55	127.71 ± 13.09
EQO2	A	34.09 ± 4.06	35.18 ± 4.87	34.82 ± 3.63	F = 0.323, p = 0.726, Partial η2 = 0.02, 95%CI: 0.00–0.09	F = 3.89, p = 0.06, Partial η2 = 0.16, 95%CI: 0.00–0.44	F = 0.15, p = 0.865, Partial η2 = 0.01, 95%CI: 0.00–0.06
	B	31.92 ± 3.26	32.08 ± 4.19	32.50 ± 3.85
Deep Squat 1RM (kg)	A	113.18 ± 34.52	133.86 ± 36.49	143.82 ± 32.75	F = 107.62, p < 0.001, Partial η2 = 0.84, 95%CI: 0.75–0.93	F = 0.00, p = 0.984, Partial η2 = 0.00, 95%CI: 0.00–0.00	F = 0.08, p = 0.924, Partial η2 = 0.00, 95%CI: 0.00–0.04
	B	112.5 ± 26.84	134.58 ± 27.84	143.00 ± 25.06
Bench Press 1RM (kg)	A	64.55 ± 23.07	71.36 ± 22.59	74.95 ± 21.14	F = 39.06, p < 0.001, Partial η2 = 0.65, 95%CI: 0.48–0.82	F = 0.05, p = 0.821, Partial η2 = 0.00, 95%CI: 0.00–0.05	F = 0.10, p = 0.901, Partial η2 = 0.01, 95%CI: 0.00–0.05
	B	63.33 ± 17.10	69.17 ± 16.11	72.94 ± 13.68
Rowing 1RM (kg)	A	68.27 ± 15.60	77.23 ± 15.30	84.77 ± 11.17	F = 41.11 p < 0.001, Partial η2 = 0.66, 95%CI: 0.50–0.83	F = 0.03, p = 0.873, Partial η2 = 0.00, 95%CI: 0.00–0.03	F = 0.24, p = 0.790, Partial η2 = 0.01, 95%CI: 0.00–0.07
	B	67.58 ± 10.08	79.13 ± 13.28	85.90 ± 10.01
Repetitions for Deep Squat	A	11.82 ± 3.25	17.45 ± 9.48	16.09 ± 4.85	F = 7.48, p = 0.004, Partial η2 = 0.26, 95%CI: 0.00–0.52	F = 0.52, p = 0.479, Partial η2 = 0.02, 95%CI: 0.00–0.15	F = 0.17, p = 0.789, Partial η2 = 0.01, 95%CI: 0.00–0.07
	B	13.92 ± 4.85	18.83 ± 7.09	16.58 ± 2.87
Repetitions for Bench Press	A	11.73 ± 4.08	12.55 ± 3.47	13.55 ± 3.88	F = 1.37, p = 0.266, Partial η2 = 0.06, 95%CI: 0.00–0.20	F = 0.72, p = 0.405, Partial η2 = 0.03, 95%CI: 0.00–0.18	F = 0.21, p = 0.815, Partial η2 = 0.01, 95%CI: 0.00–0.07
	B	13.25 ± 3.86	13.25 ± 3.36	14.08 ± 1.51
Repetitions for Rowing	A	12.45 ± 4.46	14.82 ± 3.71	13.91 ± 3.51	F = 2.68, p = 0.080, Partial η2 = 0.11, 95%CI: 0.00–0.29	F = 0.11, p = 0.743, Partial η2 = 0.01, 95%CI: 0.00–0.07	F = 0.11, p = 0.899, Partial η2 = 0.01, 95%CI: 0.00–0.05
	B	13.00 ± 4.55	14.75 ± 3.55	14.67 ± 3.60
Strength Endurance for Deep Squat (kg*reps)	A	1300.91 ± 462.52	2135.23 ± 711.52	2300.55 ± 872.33	F = 1.93, p = 0.159, Partial η2 = 0.10, 95%CI: 0.00–0.28	F = 1.23, p = 0.282, Partial η2 = 0.06, 95%CI: 0.00–0.28	F = 0.35, p = 0.71, Partial η2 = 0.02, 95%CI: 0.00–0.11
	B	1598.75 ± 713.54	2570.83 ± 1193.78	2402.58 ± 656.28
Strength Endurance for Bench Press (kg*reps)	A	790.00 ± 435.20	872.73 ± 318.37	985.00 ± 321.65	F = 2.27, p = 0.118, Partial η2 = 0.11, 95%CI: 0.00–0.31	F = 1.44, p = 0.247 Partial η2 = 0.07, 95%CI: 0.00–0.31	F = 0.13, p = 0.878, Partial η2 = 0.01, 95%CI: 0.00–0.06
	B	868.33 ± 365.07	923.54 ± 316.04	1033.06 ± 244.39
Strength Endurance for Rowing (kg*reps)	A	851.73 ± 385.32	1124.55 ± 289.14	1184.00 ± 348.26	F = 2.73, p = 0.079, Partial η2 = 0.13, 95%CI: 0.00–0.34	F = 0.06, p = 0.809, Partial η2 = 0.00, 95%CI: 0.00–0.06	F = 0.10, p = 0.907, Partial η2 = 0.01, 95%CI: 0.00–0.05
	B	884.58 ± 329.58	1144.58 ± 264.48	1261.14 ± 359.84

BF% Body Fat Percentage, SMM Skeletal Muscle Mass, BB Biceps Brachii, RF Rectus Femoris, Wmax Maximum Power Output, VO₂max Maximal Oxygen Uptake, METs Metabolic Qquivalents, VEmax Maximum Minute Ventilation, EQO₂ oxygen ventilation equivalent, T1 Pre-intervention, T2 After 5 Weeks of Intervention, T3 After 9 Weeks of Intervention, Bold values indicate statistically significant differences (p < 0.05).

In terms of cardiorespiratory function, Wmax, VO₂max, METs, and VEmax showed significant main effects for time (P < 0.05), but no significant main effect for group or interaction effect (P > 0.05) (Table 2). Bonferroni post-hoc comparisons further revealed that both groups had significantly higher Wmax at T2 and T3 compared to T1 (P < 0.05) (Fig. 2A), while VO₂max, METs, and VE max showed significant increases only at T3 compared to T1 (P < 0.05) (Fig. 2B-D). In contrast, EQO₂ did not show statistically significant main effects for time, group, or time * group interaction (P > 0.05) (Fig. 2E). Significant equivalence was not observed for Wmax, VO₂max, METs, VEmax, and EQO₂.

Fig. 2.

Multiple comparison results of different CT arrangements on cardiorespiratory function. Panels A–E: W max (A), VO_2max (B), METs (C), VEmax (D), and EQO₂ (E) across three time points (T1, T2, T3); Bars: Mean ± SD for groups A (blue) and B (green); Dots: Individual participant values; Line graphs: Individual trajectories connecting pre- and post-intervention values (*: significant difference between two time points (P < 0.05))

In terms of muscular fitness, repeated-measures ANOVA showed significant main effects of time for maximal strength in squat, bench press, and bent-over row (P < 0.05), with no significant effects for group or time * group interaction (P > 0.05) (Table 2). Further analysis revealed that compared to T1, both groups had significantly higher maximal strength in the three exercises at T2 and T3 (P < 0.05), with deep squat and rowing in T3 significantly higher than T2 (P < 0.05) (Fig. 3A-C). Regarding repetitions, only squat exhibited a significant main effect of time (P < 0.05), with significant increases at T2 compared to T1 (P < 0.05). Although T3 was still higher than T1, the rate of increase was slower than at T2, and a slight decrease was observed at T3, indicating a decline in the later phase of the training intervention. For bench press and bent-over row repetitions, no significant effects for time, group, or their interaction (P > 0.05) were observed (Fig. 3D-F).

Fig. 3.

Multiple comparison results of different CT arrangements on muscular fitness. Panels A–C Maximal strength (1RM) outcomes for deep squat (A), bench press (B), and rowing (C) across three time points (T1, T2, T3); Panels D–F Strength endurance measured as repetitions performed at 70% 1RM for deep squat (D), bench press (E), and rowing (F) across T1–T3; Panels G–I Total RT volume (kg × reps) for deep squat (G), bench press (H), and rowing (I) across T1–T3; Bars: Mean ± SD for groups A (green) and B (orange); Dots: Individual participant values; Line graphs: Individual trajectories connecting pre- and post-intervention values (* indicates a significant difference between two time points (P < 0.05). ^a indicates a significant difference compared with T1 (P < 0.05). ^b indicates a significant difference compared with T2 (P < 0.05)

As repetitions was assessed at 70% of each updated 1RM, the absolute load increased across time points, which can decrease repetitions independent of endurance capacity, we employed a composite index—repetitions * maximal strength—and additionally included maximal strength as a covariate to better control for potential confounding. The results showed that all three strength endurance outcomes showed some improvements, but these were not statistically significant (P > 0.05) (Fig. 3G-I), and improvements in squat strength endurance appeared to plateau during the later phase of the intervention (Fig. 3G). Significant equivalence was not observed for the above-mentioned outcomes.

Discussion

The primary objective of this study was to investigate the effects of 8 weeks equivalent-volume RT and HIIT performed within the same session or alternated weekly on body composition, muscle thickness, cardiorespiratory function, and muscular fitness in young adult males. It is noteworthy that this study is the first to arrange CT across separate weeks, which can be considered as a training block approach. The results indicated that both within-session CT and training arranged across weeks produced similar positive effects on body composition, cardiorespiratory function, and muscular fitness. Moreover, in terms of muscular fitness, although both CT significantly improved maximal strength, their effects on strength endurance were relatively weaker. Specifically, maximal strength in squat, bench press, and bent-over row showed a significant upward trend with increased training duration. In contrast, the improvements in strength endurance were relatively limited (Figure E-I). It is important to note that although squat repetitions significantly increased during the early stages of the intervention, its rate of increase began to plateau by week 9 and even showed a declining trend. The "downward trend" from T2 to T3 is a critical observation from Fig. 3D and G, suggesting a potential plateau or interference effect in the strength-endurance dimension of this training intervention. Subsequently, an equivalence analysis was performed. It is important to note that, compared with conventional significance testing, equivalence testing generally requires a larger sample size [38]. In our study, the CIs were relatively wide, and as a result, the equivalence analysis did not reveal significant equivalence between groups.

Body composition

Previous studies have demonstrated that CT has a significant impact on improving body composition. Dupuit et al. conducted a 12-week CT with three sessions per week, which resulted in no significant changes in body weight and BMI in overweight or obese postmenopausal women, but significantly reduced BF% and visceral fat mass, while effectively increasing lower limb SMM [20]. Similarly, a randomized controlled trial targeting young males found that the sequence of CT may be a key factor influencing body composition improvements, with the "RT first" sequence showing more significant reductions in fat mass and BF% compared to "AT first" [40]. However, Moghadam et al.'s study indicated that regardless of the order of AT and RT, an 8-week CT could significantly increase SMM and reduce BF%. Notably, in the above-mentioned study, AT and RT were arranged on different days, with at least a 48-h interval between them [48]. Consistent with previous research, our findings also indicate that 8 weeks of CT effectively improves body composition (both groups significantly reduced BF%, increased SMM, and muscle thickness). Additionally, Müller et al.'s study further confirmed this result [51]. Although no significant changes in body weight were observed, this might be due to the offsetting effects of BF% reduction and SMM increase. In summary, both previous and current findings suggest that 8 weeks of CT is sufficient to improve body composition effectively, whereas the arrangements between the two training modalities in CT do not seem to be the decisive factor for improving body composition, provided the total exercise volume is comparable.

Cardiorespiratory function

Previous studies have consistently demonstrated that, regardless of intra-session CT sequence, performing RT prior to AT is more effective in enhancing lower-limb dynamic strength, whereas alterations in training order exert no significant influence on other interference-related outcomes, such as BF% or VO₂max [21]. However, limited evidence is currently available regarding the impact of the arrangements between RT and AT in CT on improvements in cardiorespiratory function. In the present study, comparable improvements in cardiorespiratory function were observed across CT protocols. Specifically, an increase in Wmax was detected as early as week 5, which likely reflects neuromuscular adaptations, such as enhanced maximal strength and strength endurance of the lower limbs [17]. These early adaptations may be attributed to improved neuromuscular efficiency, defined as enhanced coordination between neural activation and muscle recruitment [9]. Notably, significant changes in cardiorespiratory parameters—such as VO₂max—emerged predominantly during the later phase of the intervention (week 9), suggesting that unlike the early-phase neuromuscular responses to RT, improvements in aerobic capacity require a longer cumulative training stimulus [36]. Therefore, early increases in Wmax may reflect neural and motor coordination adaptations, whereas later VO₂max gains align with progressive mitochondrial and oxidative improvements. These adaptation timelines are consistent with previous observations [18, 36]. Moreover, no significant changes were detected in the EQO2 (VE/VO₂) throughout the training period. Given the significant elevation in VEmax, presumably due to increased cardiorespiratory function, a concurrent rise in VO₂ was anticipated, likely resulting from increased SMM and thickness [63]. Therefore, the observed improvement in VO₂max may be attributed to a dual enhancement in cardiorespiratory capacity and skeletal muscle oxygen utilization efficiency. This adaptation contributes to improved exercise economy [1], enabling individuals to sustain higher aerobic workloads and achieve greater METs outputs. In support of these findings, a substantial body of literature has demonstrated the broad applicability of CT in improving cardiorespiratory function across diverse populations. For instance, studies involving older adults have shown that CT significantly improves VO₂max, peak power output, and exercise economy [51]. Similarly, Trowell et al. reported that in endurance-trained runners without prior RT experience, a 10-week CT program—consisting of AT combined with RT performed 48 h apart—resulted in significant improvements in VO₂max, enhanced 2-km running performance, and reductions in total body fat [71]. Collectively, the present findings align with existing evidence, suggesting that both CT arrangements result in similar effects on cardiorespiratory function. Notably, although external volume and intensity were matched across arrangements, internal load indicators such as training impulse or time-in-zone were not quantified; therefore, differences in cardiovascular stimulus cannot be excluded.

Muscular fitness

Numerous studies have demonstrated that within CT programs, the interference effect is most pronounced in the development of maximal strength, followed by strength endurance [19, 74]. Two main hypotheses have been proposed to explain this phenomenon: the acute interference hypothesis and the chronic adaptation hypothesis. A key concept frequently cited in this context is “residual fatigue,” referring to the prolonged negative effects of prior training (e.g., AT), which may impair subsequent performance, particularly diminishing long-term adaptations to strength training [55]. Consequently, many studies suggest that when AT precedes RT within the same session, the residual fatigue may reduce the quality and total volume of RT, thereby impeding gains in maximal strength [5, 15, 56, 58, 60]. These residual effects largely reflect the influence of exercise sequence within a training session.

Notably, our study revealed that regardless of the arrangements between RT and AT, CT produced comparable improvements in both maximal strength and muscle hypertrophy. This may be attributed to our consistent practice of prioritizing RT within each session, which likely minimized any potential interference from subsequent AT [53]. Supporting this, previous research has shown that well-designed CT protocols do not necessarily impair maximal strength or muscle hypertrophy. For example, although not conducted on a weekly alternating schedule, Sale et al. reported that RT and AT did not interfere with each other, whether performed on the same day or on alternate days [62]. Similarly, Leveritt et al. found no significant differences in maximal strength when RT was separated from AT by either 8 or 32 h [39]. More recently, Hendrickse et al. demonstrated that in both young and older populations, maximal strength and muscle hypertrophy were not further improved by extending the interval between RT and cycling training, provided that RT was performed first—even when both modalities were scheduled within the same session, on the same day, or on separate days [27]. Although this study adopted a RT–first sequence to minimize the potential interference effects of AT on resistance adaptations, our findings still suggest the presence of interference to some extent.

Regarding repetitions for deep squat, significant improvements were detected during the initial five weeks of training (Fig. 3D), which is consistent with expected adaptations. Compared to upper-body musculature, both HIIT and RT jointly stimulated lower-body muscles, leading to more evident adaptations. However, as the intervention progressed, a downward trend in squat repetitions was observed at week 9 (Fig. 3D) which was also reflected in the total volume. Compared with T1, the average squat strength-endurance (kg * reps) across both groups increased by approximately 62.3% at T2, whereas the subsequent increase from T2 to T3 tended to plateau (−0.06%). These results suggest that although maximal strength may not have been significantly compromised by CT, a degree of "adaptive trade-off" may have occurred in strength endurance. The decline may result from cumulative neuromuscular fatigue and altered training priorities. After 5 weeks, the primary neuromuscular adaptations are realized. The continued high-intensity stimulus from both RT and lower-body-dominant HIIT may lead to a stagnation or overtraining effect specifically for repetitive performance (endurance) while maximal strength (a neural or structural adaptation) continues to improve [8, 16, 43, 69]. This aligns well with the concept of the "interference effect" being most pronounced for strength performance.

The plateau in squat strength endurance observed during the later stages of the intervention might be attributable to three factors: the overlap of muscle groups targeted by both training modalities [15], arrangements between RT and AT, and the intensity of RT. In the early phase of the study, significant improvements in both maximal strength and repetitions for deep squat were recorded, likely due to the synergistic effect of HIIT and RT, both of which predominantly activated lower-body musculature. However, despite the initial concurrent gains in strength and endurance, the cumulative training load imposed on the same muscle groups by both modalities gradually elicited an interference effect that constrained performance adaptations. Previous evidence suggests that such interference predominantly occurs in muscle groups that are substantially activated by both concurrent exercise components [15]. This interpretation is further supported by findings from Doncaster et al., who demonstrated that upper-body RT-induced muscle damage from bench press significantly increased perceived exertion during subsequent arm cranking, resulting in diminished upper-limb endurance capacity [19].

With arrangements between RT and AT (block-periodized (weekly) vs. same-session), the scheduling of "detraining" weeks may have played a role in attenuating strength endurance development (B group) [50]. In the current study, “detraining weeks” refer to planned periods when RT sessions were temporarily replaced by AT, resulting in an interruption of RT stimuli [47]. For instance, the interruption of RT during the second week, which was replaced with AT only, disrupted the continuity of RT stimuli, potentially necessitating neuromuscular re-adaptation in subsequent weeks. However, if this were the main factor, differences between the two groups would likely have emerged. Notably, previous studies suggest that detraining periods shorter than four weeks generally do not lead to a decline in adaptations to training performance [34, 68]. Therefore, we consider that a single week of detraining is unlikely to be the primary cause.

Furthermore, the RT intensity adopted in this study (70% 1RM) was within the conventional range for promoting local strength endurance and hypertrophy, inducing primarily peripheral muscular adaptations [67]. However, the hypertrophic effects induced by RT in lower-limb muscles may have conflicted with the capillary proliferation elicited by AT [28], exacerbating the incompatibility of adaptive responses in the concurrently trained muscle groups. In addition, from a molecular mechanism perspective, the AT activates 5' AMP-activated protein kinase (AMPK), which can inhibit mammalian target of rapamycin complex 1 (mTORC1) signaling and blunt hypertrophic responses [7, 24]. Therefore, the chronic, cumulative activation of AMPK from weeks of HIIT may have progressively inhibited the mTOR pathway, ultimately manifesting as a plateau or decline in the ability to sustain force output (strength endurance) by week 9. This molecular cross-talk may underlie the attenuated strength endurance observed in CT.

Finally, several limitations of this study should be acknowledged. First, Given the exclusive inclusion of young males, extrapolation to females or older adults should be made with caution. Comparable CT interventions in other age or sex groups have also demonstrated favourable body composition and functional outcomes [33, 37, 41]. Future research could examine whether sex-specific physiological or hormonal differences modify CT responses. Secondly, InBody is widely used for body composition measurement due to its convenience and relatively good accuracy, and this study minimized measurement bias by following standardized protocols and controlling participants’ diet and fluid intake prior to testing. However, hydration status was not instrumentally verified, which may have influenced measurement precision. Accordingly, small between-arrangement differences may have been undetectable using Bioelectrical Impedance analysis (BIA) alone [46]. Future studies could include urine specific gravity screening (USG ≤ 1.020) to ensure consistent hydration, and integrate Dual-Energy X-ray Absorptiometry measurements at pre- and post-intervention or triangulate BIA with ultrasound-based muscle thickness changes to validate sensitivity for detecting small differences. In addition, the observed patterns and practical considerations in the field suggest a degree of consistency between the two CT arrangements. However, no significant between-group equivalence was detected, and future studies including a RT-only control group with larger sample sizes are needed to further verify these findings.

Conclusion

Concurrent training, whether performed within the same session or alternated weekly, elicited comparable improvements in body composition, cardiorespiratory, and muscular outcomes in young adult males. Both arrangements promoted beneficial adaptations, indicating flexibility in concurrent training design.

Abbreviations

AT

Aerobic training

HIIT

High-intensity interval training

RT

Resistance training

CT

Concurrent training

SMM

Skeletal muscle mass

BF%

Body fat percentage

BB

Biceps brachii

RF

Rectus femoris

Wmax

Maximum power output

VO₂max

Maximal oxygen uptake

VEmax

Maximum minute ventilation

EQO₂

Oxygen ventilation equivalent

1RM

One-repetition maximum

HR

Heart rate

ICC

Intraclass correlation coefficient

SD

Standard deviation

ANOVA

Repeated-measures analysis of variance

η²

Partial eta squared

METs

Metabolic Qquivalents

AMPK

5' AMP-activated protein kinase

mTORC1

Mammalian target of rapamycin complex 1

BIA

Bioelectrical Impedance analysis

Authors’ contributions

Conceptualization: Y.C., S.B. J.M., R.Y., Q.L Data curation: Y.C. Formal analysis: Y.C. Funding acquisition: S.B. Investigation: Y.C., S.B. J.M., R.Y., Q.L Methodology: Y.C., S.B. J.M., R.Y. Project administration: S.B. Supervision: S.B. Writing – original draft: Y.C., S.B. J.M., R.Y., Q.L Writing – review & editing: Y.C., S.B. J.M., R.Y., Q.L All authors read and approved the final manuscript.

Funding

This work was supported by the Emerging Interdisciplinary Platform for Medicine and Engineering in Sports (EIPMES).

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

This study was conducted in accordance with the CONSORT guidelines. All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. The study was approved by the Ethics Committee of the Capital University of Physical Education and Sports in March 13, 2024, (No. 2024A207). This study was conducted on human subjects and subjects gave informed consent.

Competing interests

The authors declare no competing interests.