The effect of concurrent high-intensity interval training and resistance training on the lower body maximal strength and explosive power: A updated systematic review and meta-analysis

Abstract

BACKGROUND:

Endurance training can have a negative impact on strength training and may lead to reduced strength gains, known as the interference effect. However, high-intensity interval training (HIIT) as an endurance training mode may reduce this interference effect.

OBJECTIVE:

This systematic review and meta-analysis aim to investigate the effects of concurrent HIIT and resistance training (RT) on lower body explosive strength and maximum strength.

METHODS:

Five electronic databases were searched. Subgroup analyses were performed to assess the effects of HIIT modality, training status, and training duration on strength development following concurrent HIIT and RT.

RESULTS:

Meta-analysis showed that compared to RT alone, concurrent HIIT and RT will not affect the development of countermovement jump (CMJ) (WMD $=-$ 0.17, 95%CI $=-$ 1.45 to $-$ 1.11) and half squat (WMD $=-$ 0.05, 95%CI $=-$ 2.42 to 2.32). Further, subgroup analysis revealed that HIIT-running workout was conducive to the development of both CMJ and half squat. Longer training duration was found to be more effective in developing CMJ, while shorter training duration was more suitable for developing half squat. Additionally, athletes showed greater improvement than non-athletes.

CONCLUSIONS:

Combining HIIT and RT can enhance CMJ and half squat. The results of intervention are moderated by training variables and training status.

Keywords

Concurrent training HIIT strength training incompatibility

1. Introduction

Concurrent training refers to a training method in which strength and endurance tasks are arranged during the same training period [1]. In 2020, the World Health Organization recommended that individuals of all ages should work on developing their aerobic capacity while also enhancing their muscle strength [2]. Research has shown that compared to resistance training (RT), concurrent aerobic and strength training is a more effective intervention for improving physical activity levels [3, 4] such as impacting on body composition. In the field of competitive sports, due to the special nature of the events, athletes must improve their aerobic capacity while developing their strength to achieve better athletic performance. For example, in team sports, athletes need to have both direct physical confrontation ability and the ability to repeatedly run to increase their chances of scoring [5, 6] or in long-distance running events, athletes first need to have sufficient aerobic capacity to complete the race, and secondly, they need sufficient lower limb strength to help them improve their running speed, enhance the utilization of muscle elastic potential energy, improve running economy, and achieve better sports results. [7]. Countermovement jump and half squat are important indicators that affect running performance [8, 9]. At the same time, half squat is a commonly used indicator of lower limb strength [10, 11, 12], while jumping is a commonly used indicator of lower limb explosive strength [10, 11, 12]. However, in concurrent training, endurance training and strength training will lead to different biological adaptations. In certain qualities, such as strength and power, concurrent training may lead to negative adaptation. This phenomenon is called the interference effect [1].

Numerous studies have shown that gender, training level, endurance training modality, and the interval between endurance training and strength training are important influencing factors for interference effects [5, 6, 13, 14, 15, 16, 17]. For example, Wilson’s study suggests that using cycling as an endurance training mode reduces interference effects while using running as an endurance training mode increases interference effects [18]. However, a recent meta-analysis suggests that running may be a better endurance training mode with less interference effects [19]. This may be due to the inconsistency in the results of the combined analysis of endurance training models (high-intensity interval training and moderate-intensity continuous training) in this study. As is well known, the stimulation and adaptation brought by these two endurance training modes are different. When compared to moderate-intensity continuous training (MICT), high-intensity interval training (HIIT) is a time-efficient training method that effectively enhances both aerobic [20] and anaerobic capacity [21], as well as an individual’s short-term recovery ability [22]. In a comparative study conducted by Silva et al. 2012, concurrent HIIT and RT was compared with concurrent MICT and RT. Forty-four active women were randomly assigned to the moderate-intensity continuous running group ( $n=$ 10), the moderate-intensity continuous cycling group ( $n=$ 11), the high-intensity interval running group ( $n=$ 11), and the strength training group ( $n=$ 12). All subjects underwent intervention training twice a week for a total of 11 weeks. Although there was no significant difference between the groups, the high-intensity interval running group demonstrated advantages as the pre-post percentage change of 1RM leg press in this group was 46.8%, whilst the improvement in the moderate-intensity continuous cycling and moderate-intensity continuous running groups were 39.1% and 41.1% respectively [23]. Therefore, concurrent HIIT and RT can be considered a superior method of concurrent training for reducing the interference effect caused by combining strength and endurance training.

A Meta-analysis was carried out by Petré et al., (2021) as an attempt to compare the impact of concurrent training on maximum dynamic strength among individuals with different training levels. The study found that individuals with moderate trained or those who have not undergone systematic training were not affected by the interference effect on maximum dynamic strength after concurrent training. Conversely, individuals with systematic training experience were more affected by the interference effect [14]. According to the principle of diminishing returns, training adaptation does not continue to increase exponentially with increasing training volume. For trained individuals such as athletes, higher stimuli are required to improve athletic performance [24]. Therefore, how to scientifically set training variables has become extremely important [25].

Sabag et al. (2018)’s meta analysis included 13 studies exploring the impact of concurrent training on muscle strength and muscle hypertrophy. It was found that the negative effects of concurrent HIIT and RT do not seem to exist in upper limb strength and muscle hypertrophy, so the maximum strength and explosive force of the lower limbs may be the main targets affected [26]. At the same time, the study also found that the HIIT mode (high-intensity interval running or high-intensity interval cycling) is an important factor affecting training effectiveness [26]. The training duration seems to be a variable that has been overlooked by previous researchers. Early studies have shown that concurrent training after 8 weeks will lead to a decrease in training effectiveness, which may be due to the accumulation of fatigue [27]. Previous meta-analyses have not provided a clear report on the impact of individual training levels, training duration, and HIIT modes on the improvement of strength quality after concurrent HIIT and RT training [26, 28]. This article aims to comprehensively search for published randomized controlled trials related to concurrent HIIT and RT, and utilize meta-analysis to analyze the effect of concurrent training on lower body strength and lower body explosive strength, to evaluate the effects of concurrent HIIT and RT on different training levels of individuals, determine the optimal training duration and HIIT mode, and provide scientific evidence and guidance for the training of athletes and the general population.

2. Methods

2.1 Searching strategy

This study reports a previously registered systematic review and meta-analysis (PROSPERO: CRD42023396 886).

The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement [29, 30] served as the guidelines for this meta-analysis and systematic review. A search was put forward on e-databases including Scopus, SPORTDiscus, PubMed, Web of Science, and Embase from the earliest record and February 2023, inclusive. The search strategy was (((((resistance training[Title/Abstract]) OR (strength training[Title/Abstract])) OR (weightlifting training [Title/Abstract])) OR (concurrent training [Title/ Abstract])) OR (simultaneous training [Title/Abstract])) AND ((((high intensity interval training [Title/Abstract]) OR (sprint interval training[Title/Abstract])) OR (HIIT [Title/Abstract])) OR (SIT[Title/Abstract])). Two independent evaluators assessed the studies based on their titles and abstracts to ensure they met the inclusion criteria for the meta-analysis [30]. Any discrepancies were resolved by a third evaluator. The evaluators were blinded to the author, institution, and journal. Abstracts that met the inclusion criteria were subjected to full-text review, while those that lacked sufficient information were excluded. Moreover, the authors of potentially eligible studies were contacted to solicit clarification on any incomplete or missing data.

2.2 Eligibility criteria

The criteria for inclusion were defined based on the PICOS (Population, Intervention, Control, Outcomes, and Study) criteria [31].

(P): Population: healthy individuals aged from 18 to 40. (I): Intervention: concurrent HIIT and RT.

HIIT was defined as involving frequent activity bursts lasting up to 5 minutes, where intensities were either greater than 90% maximum oxygen uptake (VO2max), 100% lactate threshold, and 80% maximum heart rate, or based on best effort, interval running, or interval cycling [26]. RT was defined as skeletal muscle exercises that resist external resistance, such as bench presses and leg presses.

(C): Comparator: only RT, and actions, intensity, and volume must be the same as the experimental group. (O): Outcomes: half squat and countermovement jump.

2.3 Data extraction

Two reviewers independently assessed full-text publications using a consistent and predetermined method. The discussion was intended to resolve any discrepancies until a consensus was reached. In the case of any disagreements, a third reviewer was consulted for resolution. The extraction of data was conducted by one reviewer. Participants’ pertinent traits such as training level, sex, age, body weight, and height, as well as study characteristics such as measurement techniques for strength or explosive strength, were extracted. Simultaneously extracted the training frequency, period, intensity, and method of resistance training or HIIT training in concurrent training.

Table 1
Risk of bias assessment

Author	Random sequence generation	Allocation concealment	Blinding of participants and personnel	Blinding of outcome assessment	Incomplete outcome data	Selective reporting	Other bias
Robineau et al., 2017	$+$	?	$-$	$-$	$+$	$+$	$+$
Spiliopoulou et al., 2021	$+$	?	$-$	$-$	$+$	$+$	$+$
Tsitkanou et al.,2017	$-$	?	$-$	$-$	$+$	$+$	$+$
Cantrell et al.,2014	$+$	?	$-$	$-$	$+$	$+$	$+$
Balabinis et al.,2003	$+$	?	$-$	$-$	$+$	$+$	$+$
Robineau et al.,2016	$+$	?	$-$	$-$	$+$	$+$	$+$
Ross et al.,2009	$+$	?	$-$	$-$	$+$	$+$	$+$
Kotzamanidis et al.,2006	$+$	$+$	$-$	$-$	$+$	$+$	$+$
Laird et al., 2016	$+$	?	$-$	$-$	$+$	$+$	$+$
Hickson et al., 1980	$+$	$+$	$+$	$+$	$+$	$+$	$+$
Panissa et al., 2018	$+$	$+$	$+$	$+$	$+$	$+$	$+$

( $+$ ) $=$ low risk of bias; ( $-$ ) $=$ high risk of bias; (?) $=$ unclear risk of bias.

2.4 Quality analysis

The Cochrane tool includes seven items, namely random sequence generation, allocation consideration, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, selective reporting, and other biases [30]. If the study clearly meets the requirements of the entry, fill in “ $+$ ”. If the study clearly indicates that it does not meet the requirements of the entry, fill in “ $-$ ”. If the study does not mention the requirements of the entry, fill in “?”. Two assessors independently rated the studies. Conflicts between ratings were addressed through discussion, and a third assessor was consulted if no consensus was reached. Table 1 shows that the included articles have a moderate to high methodological quality. Furthermore, the Egger test was designed to assess publication bias [30, 32]. Figures 4 and 5 indicated that the two outcome indicators showed no publication bias. Sensitivity analysis was also conducted to identify whether a particular study accounted for the heterogeneity [33].

2.5 Statistical analysis

Weighted mean difference (WMD) was selected as the effect size indicator because the included studies measure the maximum strength or explosive strength of the lower limbs using different methods [34]. Results are presented as WMD or confidence interval (CI). Software R and Stata 16.0 were used for all analyses [35]. The within-group alteration in lower body maximum strength and explosive strength was determined by calculating the distinction between before and after the intervention.

The I² statistic, which quantifies inconsistency, was adopted to analyze between-study heterogeneity variability. The criteria for heterogeneity were defined at 0 $\leqslant$ I² < 25% (low), 25% $\leqslant$ I² < 50% (moderate) and I² 75% (high) [30]. Random-effect model of the meta-analysis was used to analyze data when I² 50%. A fixed-effect model was applied to pool data when heterogeneity was low. Publication bias was applied with the Egger test, P $<$ 0.1 delegate studies have publication bias [30, 32]. The primary analysis examined the results of concurrent HIIT and RT compared to RT alone on lower body strength and explosive strength. Sub-analyses were conducted on training status (athlete/non-athlete), modality of HIIT (running or cycling), and duration (less than 8 weeks or more than 8 weeks) of concurrent training to explore the factors influencing capacity development.

It is worth noting that although previous studies have divided the training status into three levels (untrained, moderately trained, and trained) [36], it may be due to the high intensity of endurance training, which poses certain risks for untrained individuals to participate. Therefore, no study has been conducted to explore the impact of concurrent training on untrained individuals.

Therefore, the individuals were divided into athletes and non-athletes based on their training status.

Table 2
Participant characteristics of included studies

Study	Group	Number of participants	Sex (M%)	Age (years)	Height (cm)	Weight (kg)	Training status (athlete/non-athlete)
Robineau et al., 2017a	HIIT $+$ RT	$n=$ 9	100.0%	25 $\pm$ 3.7	180 $\pm$ 7.7	86.2 $\pm$ 10.5	Athlete
	HIIT $+$ RT	$n=$ 10	100.0%	26.4 $\pm$ 3.0	179 $\pm$ 8.0	89.3 $\pm$ 10.3	Athlete
	RT	$n=$ 11	100.0%	27.5 $\pm$ 2.5	177 $\pm$ 5.6	89.4 $\pm$ 14.2	Athlete
Spiliopoulou et al., 2021	HIIT $+$ RT	$n=$ 10	0.0%	21.8 $\pm$ 2.8	169 $\pm$ 5.3	59.0 $\pm$ 8.3	Non-athlete
	RT	$n=$ 10	0.0%	21.8 $\pm$ 2.8	169 $\pm$ 5.3	59.0 $\pm$ 8.3	Non-athlete
Tsitkanou et al., 2017	HIIT $+$ RT	$n=$ 11	100.0%	21.8 $\pm$ 0.6	177 $\pm$ 1.5	74.2 $\pm$ 2.1	Non-athlete
	RT	$n=$ 11	100.0%	21.8 $\pm$ 0.6	177 $\pm$ 1.5	74.2 $\pm$ 2.1	Non-athlete
Cantrell et al., 2014	HIIT $+$ RT	$n=$ 7	100.0%	26.6 $\pm$ 6.6	176 $\pm$ 6.5	80.9 $\pm$ 11.2	Non-athlete
	RT	$n=$ 7	100.0%	24.7 $\pm$ 5.9	175 $\pm$ 9.1	78.1 $\pm$ 9.7	Non-athlete
Balabinis et al., 2003	HIIT $+$ RT	$n=$ 7	100.0%	22.6 $\pm$ 0.8	188 $\pm$ 0.5	86.1 $\pm$ 69	Athlete
	RT	$n=$ 7	100.0%	22.2 $\pm$ 0.4	188 $\pm$ 0.9	85.4 $\pm$ 54	Athlete
Robineau et al., 2016a	HIIT $+$ RT	$n=$ 15	100.0%	24.3 $\pm$ 3.8	172 $\pm$ 4.7	85.7 $\pm$ 11.5	Athlete
	HIIT $+$ RT	$n=$ 11	100.0%	28.0 $\pm$ 4.5	181 $\pm$ 9.3	90.4 $\pm$ 9.1	Athlete
	HIIT $+$ RT	$n=$ 12	100.0%	24.8 $\pm$ 3.9	177 $\pm$ 6.8	83.5 $\pm$ 14.9	Athlete
	RT	$n=$ 10	100.0%	25.2 $\pm$ 4.4	181 $\pm$ 7.4	90.8 $\pm$ 14.5	Athlete
Ross et al., 2009	HIIT $+$ RT	$n=$ 10	100.0%	19.8 $\pm$ 1.2	183 $\pm$ 5.0	93.3 $\pm$ 11.6	Athlete
	RT	$n=$ 9	100.0%	19.9 $\pm$ 1.4	182 $\pm$ 6.2	93.5 $\pm$ 12.6	Athlete
Kotzamanidis et al., 2006	HIIT $+$ RT	$n=$ 12	100.0%	18.0 $\pm$ 1.1	178 $\pm$ 35	73.5 $\pm$ 1.2	Athlete
	RT	$n=$ 11	100.0%	18.1 $\pm$ 1.1	175 $\pm$ 25	72.5 $\pm$ 2.2	Athlete
Laird et al., 2016	HIIT $+$ RT	$n=$ 14	0.0%	20.2 $\pm$ 1.5	171 $\pm$ 5.0	63.3 $\pm$ 9.9	Non-athlete
	RT	$n=$ 14	0.0%	20.4 $\pm$ 1.9	169 $\pm$ 2.2	62.6 $\pm$ 8.2	Non-athlete
Hickson et al., 1980	HIIT $+$ RT	$n=$ 7	71.4%	26.0 $\pm$ 8.0	$-$	82.2 $\pm$ 7.3	Non-athlete
	RT	$n=$ 8	87.5%	22.0 $\pm$ 3.5	$-$	75.8 $\pm$ 3.4	Non-athlete
Panissa et al., 2018	HIIT $+$ RT	$n=$ 11	100.0%	24.5 $\pm$ 3.7	179 $\pm$ 7	74.6 $\pm$ 6.8	Non-athlete
	RT	$n=$ 8	100.0%	28.7 $\pm$ 3.4	177 $\pm$ 8	77.5 $\pm$ 12.9	Non-athlete

Data are reported as mean $\pm$ SD.

Figure 1.

PRISMA flow diagram of literature screening process.

3. Results

3.1 Description of studies

A total of 2985 potential studies were found in the database (Fig. 1). Eleven studies were incorporated into the meta-analysis and systematic review because they satisfied the eligibility requirements. In all, there were 242 participants, consisting of 219 males and 23 females, with ages from 18 to 31 years. Of the eleven studies, five involved athlete populations, while the remaining studies involved non-athlete populations (Table 2).

Figure 2.

Effect of concurrent HIIT and RT versus RT alone on CMJ.

Figure 3.

Effect of concurrent HIIT and RT versus RT alone on half squat.

Table 3

Training program of included studies

Study	Group	HIIT prescription	RT prescription	Interval	Frequency (days/week)	Duration (weeks)	Outcomes
Robineau et al., 2017	HIIT $+$ RT	Sprint interval training, 4sets of 30s*8, 30s passive recovery	Actions: half squat, deadlift, leg extension, bench press, bench row Intensity: 70%–90%1RM, 2sets of 10rep	24 h	2	8	CMJ, HS
	HIIT $+$ RT	Short interval training, 4sets $<$ 60s*8, 4 minutes passive recovery	Actions: half squat, deadlift, leg extension, bench press, bench row Intensity: 70%–90%1RM, 2sets of 10rep	24 h	2	8	CMJ, HS
	RT	$-$	Same as above	$-$	$-$	$-$	$-$
Spiliopoulou et al., 2021	HIIT $+$ RT	Cycling with maximal aerobic explosive strength, 10 $\times$ 1 min	Actions: half squat, countermovement jump, drop jump Intensity: 40–100%1RM, 6–8 $\times$ 2–3rep	10 min	3	6	CMJ, HS
	RT	$-$	Same as above	$-$	$-$	$-$	$-$
Tsitkanou et al., 2017	HIIT $+$ RT	Cycling with maximal aerobic explosive strength, 10 $\times$ 60s	Actions: leg press, half squat, intensity: 6RM, 4 $\times$ 6rep	10 min	2	8	HS
	RT	$-$	Same as above	$-$	$-$	$-$	$-$
Kotzamanidis et al., 2006	HIIT $+$ RT	Interval running 30m with the best effort, 4–6 rep $\times$ 3sets	Actions: half squat, step up, leg curl, Intensity: 8RM-3RM, 60min	0	2	9	HS, CMJ
	RT	$-$	Same as above	$-$	$-$	$-$	$-$
Cantrell et al., 2014	HIIT $+$ RT	4–6 sets of modified 20 s Wingate protocol	Actions: back squat, bench press, leg extension, leg curl, pull-down, shoulder press Intensity: 85%1RM, 4-6rep $\times$ 3sets	24 h	2	12	HS
	RT	$-$	Same as above	$-$	$-$	$-$	$-$
Balabinis et al., 2003	HIIT $+$ RT	Interval running with 85% VO2max, 100 m $\times$ 8, 200 m $\times$ 8	Actions: half squat, leg press, bench press, lateral pull down Intensity: 75%–85%1RM, 4–6rep $\times$ 1–2sets	7 h	4	7	HS, CMJ
	RT	$-$	Same as above	$-$	$-$	$-$	$-$
Robineau et al., 2016a	HIIT $+$ RT	Interval running with 100% VO2max, 12 $\times$ 15 s, 3sets, 0 hour interval between endurance training and strength training	Actions: half squat, leg press, bench press, bench row, plyometrics Intensity: 70%–90% 1RM, 3–10 rep $\times$ 3–4sets	0	2	7	HS, CMJ
Robineau et al., 2016b	HIIT $+$ RT	Interval running with 100% VO2max, 12 $\times$ 15 s, 3sets, 6 hour interval between endurance training and strength training	Actions: half squat, leg press, bench press, bench row, plyometrics Intensity: 70%–90% 1RM, 3–10 rep $\times$ 3–4sets	6 h	2	7	HS, CMJ

Table 3, continued
Study	Group	HIIT prescription	RT prescription	Interval	Frequency (days/week)	Duration (weeks)	Outcomes
Robineau et al., 2016c	HIIT $+$ RT	Interval running with 100% VO2max, 12 $\times$ 15 s, 3sets, 24 hour interval between endurance training and strength training	Actions: half squat, leg press, bench press, bench row, plyometrics Intensity: 70%–90% 1RM, 3–10 rep $\times$ 3–4sets	24 h	2	7	HS, CMJ
	RT		Same as above
Ross et al., 2009	HIIT $+$ RT	Interval running with the best effort, 40 m–60 m $\times$ 12rep, 3sets	Actions: bench press, incline bench press, inverted fly, push press, seated shoulder press, dumbbell shrugs, dumbbell front raise, triceps push-down, triceps dumbbell, Squat, dumbbell lunges, leg expression, leg curl, standing calf raise, lateral pull down, seated row, dumbbell hammer curl, dumbbell biceps curl, core circuit Intensity: 4–10rep $\times$ 2–4sets	0	2	7	HS
	RT	$-$	Same as above	$-$	$-$	$-$	$-$
Laird et al., 2016	HIIT $+$ RT	20s sprint interval running*8	Actions: back squat, bent over row, bench press, sit-ups, squat jump, deadlift, standing press, back extension Intensity: 75%1RM, 3–10rep*3–5sets	4	3	11	HS
	RT	$-$	Same as above	$-$	$-$	$-$	$-$
Hickson et al., 1980	HIIT $+$ RT	5 min cycling at vVO2 max*6	Actions: leg presses, calf raises, parallel squats, knee flexions, knee extensions Intensity: 80%1RM, 3–5sets, 5–20rep	0	3	10	HS
	RT	$-$	Same as above	$-$	$-$	$-$	$-$
Panissa et al., 2018	HIIT $+$ RT	1min interval running at 100%maximal aerobic speed until complete 5 kilometers	Actions: bench press, half-squat, triceps extension, leg extension, seated row, leg curl and arm curl Intensity: 8–12RM, 8–12rep, 3sets	0	2	12	HS
	RT	$-$	Same as above	$-$	$-$	$-$	$-$

HIIT $=$ high intensity interval training; RT $=$ resistance training; rep $=$ repetitions; HS $=$ half squat; CMJ $=$ countermovement jump; LT $=$ lactate threshold; MVC $=$ maximal voluntary contraction; MIC $=$ maximal isometric contraction; vVO2max $=$ speed at VO2 max.

3.2 Intervention characteristics

Table 3 lists the parameters of the intervention, together with the HIIT and RT modalities, frequency, intensity, volume, and duration. In seven out of the eleven investigations, interval running was conducted as HIIT [5, 6, 14, 15, 37, 38, 39, 40], and the remaining four studies utilized cycling HIIT [13, 16, 17].

Figure 4.

Egger test of included studies with the effect of concurrent training on CMJ.

Figure 5.

Egger test of included studies with the effect of concurrent training on half squat.

Concerning training duration, four studies performed concurrent training which was less than 8 weeks [5, 14, 15, 16, 37] and seven studies performed concurrent training which was more than 8 weeks [6, 13, 17, 38, 39, 40].

There were five studies that tested the CMJ [5, 14, 16, 37, 38, 39, 40] and eleven studies tested the half squat [5, 13, 15, 16, 17, 37, 38, 39].

4. Primary analysis: The lower body maximum strength and explosive strength

Figures 2 and 3 depict the impact of concurrent HIIT and RT compared to RT alone on half squat and CMJ. There were no changes found between combined HIIT and RT by comparison to RT alone on CMJ (WMD $=-$ 0.17, 95%CI: $-$ 1.45 to 1.11, $p=$ 0.79) and half squat (WMD $=-$ 0.05, 95%CI: $-$ 2.42 to 2.32, $p=$ 0.97). There was little study-to-study heterogeneity being observed on CMJ (I² = 6%, $P=$ 0.38) and half squat (I² = 9%, $P=$ 0.35). All included studies have no publication bias (Figs 4 and 5).

4.1 Sub-analysis: CMJ and half squat

A sub-analysis was conducted to determine the impact of training status when performing concurrent HIIT and RT compared to RT alone on half squat (Table 4). Ten studies supplied adequate information for the computation of mean difference and 95% confidence intervals (95%CI). Five studies reported the change in half squat of athletes who experienced concurrent HIIT and RT versus RT alone, while six studies reported the change in half squat of non-athletes. There was a trend showing that non-athletes had a negative effect on the half squat (WMD $=-$ 3.38, 95%CI $=-$ 6.70 to $-$ 0.05, $p=$ 0.05). However, athletes who experienced concurrent HIIT and RT had a positive effect on the half squat (WMD $=$ 3.41, 95%CI $=$ 0.03 to 6.80, $p=$ 0.05). Low heterogeneity was observed among related studies on half squat for both athletes and non-athletes (I² = 0%, $p=$ 0.65 and I² = 0%, $p=$ 0.66, respectively), and there was a high subgroup difference between athletes and non-athletes on half squat (I² $=$ 87.3%, $p=$ 0.005).

A sub-analysis was performed to assess the effect of the modality of HIIT when performed with RT compared to RT alone on lower body strength and explosive strength (Table 3). In comparison to RT alone, concurrent HIIT-cycling and RT exhibited a tendency towards a negative effect on half squat (WMD $=-$ 3.63, 95%CI $=-$ 7.96 to 0.69, $p=$ 0.1). Compared to RT alone, concurrent HIIT-running increased half squat (WMD $=$ 1.50, 95%CI $=-$ 1.34 to 4.33, $p=$ 0.30).

Low (non-significant) heterogeneity was observed among HIIT-running studies for half squat (I² = 0%, $p=$ 0.1). Low heterogeneity can be observed among HIIT-cycling studies for half squat (I2 $=$ 0%, $p=$ 0.3), and there was a high subgroup difference between HIIT-cycling and HIIT-running on half squat (I2 $=$ 90.6%, $p=$ 0.001).

Table 4
Several subgroup analysis outcomes

Outcome	Subgroup	Including studies ( $n$ )	Heterogeneity		Meta-analysis
			I²	$P$	WMD	95%CI	$P$
HS	Athlete	5	0%	0.65	3.41	0.03, 6.80	0.05
	Non-athlete	6	0%	0.66	$-$ 3.38	$-$ 6.70, $-$ 0.05	0.05
	Subgroup difference	$-$	87.3%	0.005	$-$	$-$	$-$
HS	HIIT-cycling	4	0%	0.1	$-$ 3.63	$-$ 7.96, 0.69	0.10
	HIIT-running	7	0%	0.3	1.50	$-$ 1.34, 4.33	0.30
	Subgroup difference	$-$	73.5%	0.05	$-$	$-$	$-$
CMJ	Less than 8 weeks	3	58%	0.09	$-$ 0.34	$-$ 2.14, 1.46	0.71
	More than 8 weeks	2	0%	0.39	1.53	$-$ 0.81, 3.87	0.20
	Subgroup difference	$-$	35.3%	0.21	$-$	$-$	$-$
HS	Less than 8 weeks	4	0%	0.86	3.78	0.35, 7.20	0.03
	More than 8 weeks	7	0%	1	$-$ 3.56	$-$ 6.84, $-$ 0.27	0.03
	Subgroup difference	$-$	89.1%	0.002	$-$	$-$	$-$

Figure 6.

Effect of concurrent HIIT and RT of different training duration on CMJ and half squat.

A sub-analysis was conducted to assess the potential influence of the duration of concurrent HIIT and RT by comparison to RT alone on half squat and CMJ (Table 4 and Fig. 5). Five studies performed combined training for less than 8 weeks, while the remaining six studies performed combined training for more than 8 weeks (including 8 weeks). A negative tendency was observed for RT alone compared to concurrent HIIT and RT, lasting less than 8 weeks, on CMJ (WMD $=-$ 0.34, 95%CI $=-$ 2.14 to 1.46, $p=$ 0.71), but not on half squat (WMD $=$ 3.78, 95%CI $=$ 0.35 to 7.20, $p=$ 0.03). Concurrent HIIT and RT lasting 8 weeks or more showed a positive effect on CMJ by comparison to RT alone (WMD $=$ 1.53, 95%CI $=-$ 0.81 to 3.87, $p=$ 0.20), but not on half squat (WMD $=-$ 3.56, 95%CI $=-$ 6.84 to $-$ 0.27, $p=$ 0.03). Moderate heterogeneity was observed among studies whose duration was less than 8 weeks for CMJ (I² = 58%, $p=$ 0.09), while low heterogeneity was observed among studies whose duration was less than 8 weeks for half squat (I² = 0%, $p=$ 0.86). Low heterogeneity was observed among studies whose duration was more than 8 weeks (including 8 weeks) for both CMJ (I² = 0%, $p=$ 0.39) and half squat (I² = 0%, $p=$ 1). There is a high subgroup difference observed for half squat (I² = 89.1%, $p=$ 0.002).

5. Discussion

To the authors’ knowledge, this is the first meta-analysis that compares the effects of combined HIIT and RT to RT alone on the outcomes of CMJ and half squat. Additionally, this is also the first sub-analysis that explores the effect of different training statuses and durations for concurrent HIIT and RT on half squat and CMJ. This research shows that compared to strength training, concurrent high-intensity interval training and strength training will not affect the development of CMJ and half squat.

Our sub-analysis revealed that this interference effect may have a greater impact on non-athletic individuals, and can be improved by modifying HIIT modes or developing targeted training plans. This study found that HIIT modes, training status, and duration are the main factors that lead to heterogeneity in the three subgroup analyses. Due to limitations in the number of studies, we were unable to conduct a comprehensive subgroup analysis of CMJ. In summary, these included studies were of moderate methodological quality, and the results are relatively stable.

The results of this study differ from those of Sabag et al. (2018), who found that concurrent high-intensity interval training (HIIT) and strength training had a significant impact on lower limb strength (SMD $=-$ 0.248, 95% CI $=-$ 0.495 to $-$ 0.001, $p=$ 0.049). The reason for this difference may be that Sabag et al. (2018) included a wider range of indicators, such as leg press and leg extension exercises. However, according to the principle of specificity, squatting may be more effective in improving sports performance as it tests overall strength. During squatting, individuals not only need lower limb strength but also core stability, which is closely related to training and competition. For example, weightlifters require lower limb strength and joint stability during clean and jerk exercises [41], while leg press movements are less effective in developing these aspects.

Most of the previous meta-analyses did not analyze traditional endurance training (moderate intensity endurance training) and high-intensity interval training separately [18, 36, 42], which may have caused potential heterogeneity. Previous meta-analyses have found that concurrent high-intensity interval training and strength training do not appear to affect muscle hypertrophy. This may suggest that molecular-level conflicts may exist in the sustained form of moderate-intensity endurance training, rather than in the intermittent form of high-intensity endurance training [43, 44, 45, 46, 47]. Concurrent high-intensity interval training and strength training are less affected by interference effects. Compared to traditional endurance training, HIIT strengthens both central and peripheral adaptations [48], which require higher glycolysis for energy supply. High-intensity training further enhances muscle activation and neuromuscular control [49]. Fyfe et al. (2016) conducted a three-arm experiment and divided adult men engaged in recreational activities into three groups: concurrent high-intensity interval training and strength training ( $n=$ 8), concurrent moderate-intensity continuous endurance training and strength training ( $n=$ 7), and strength training only ( $n=$ 8). After eight weeks of training three times a week, although the leg press 1RM of the two concurrent training groups increased less than that of the strength training group (111 $\pm$ 56.24 kg), it should be noted that the concurrent high-intensity interval training and strength training group (84 $\pm$ 58.1 kg) had a greater increase than the concurrent moderate-intensity continuous endurance training and strength training group (75 $\pm$ 64.37 kg) [50]. The principle of “all” or “none” muscle fiber recruitment suggests that individuals must perform high-explosive strength exercises in a short time frame, such as during a full sprint or cycling [51]. In order to quickly mobilize large motor units to complete high-intensity actions within a short duration, the body may inhibit small motor units. As strength increases, the body’s fast-twitch muscle fibers become hypertrophied, which can promote explosive strength and maximum strength in the lower limbs.

Individual strength performance and hormone levels are also important factors that affect concurrent training [1]. Petré et al. (2021) conducted a meta-analysis to investigate the impact of concurrent aerobic and strength training on individuals’ lower limb strength with different training levels and found that interference effects occur in individuals with systematic training experience. Interestingly, this study found through subgroup analysis that, compared to non-athletic individuals, the interference effects on lower limb explosive strength and maximal strength of athletes after concurrent HIIT and RT are smaller, which is inconsistent with the conclusion of Petré et al. When non-athletes perform high-intensity interval training, the accumulation of metabolic by-products (such as lactic acid), hydrogen ions, and inorganic phosphates in the muscles can inhibit contractility, thus affecting the quality of subsequent strength training. Athletes have stronger metabolism levels than non-athletes, so it is reasonable to think that athletes can recover faster without affecting the effectiveness of strength training. The accumulation of fatigue caused by endurance training can hinder the effectiveness of individual strength training, and excessive training time is likely to result in overtraining [52].

There are significant differences in understanding the optimal endurance training mode. Wilson et al., (2012) posited that cycling as an aerobic training mode produces a smaller interference effect compared to running, possibly attributable to the longer duration of eccentric muscle contractions during running [18]. This pattern arises due to the need for skeletal muscles to alternate constantly between concentric and eccentric contractions during running. Metabolic byproducts and fatigue accumulation will hinder the training effect of concurrent HIIT and RT. However, a recent study suggests that endurance training modes do not seem to affect the effectiveness of concurrent training. The cognitive difference may be due to the diversity of factors that cause interference effects, and multiple factors collectively cause interference effects, making it difficult to separate individual influencing factors. The interpretation is inconsistent with the above research, which may be because the aerobic exercise mode included in this study is all HIIT. This study found that after concurrent high-intensity interval cycling training and strength training, the adverse effect of interference on half squatting increased ( $-$ 3.63, 95%CI $=-$ 7.96 to 0.69). Therefore, compared to intermittent cycling, choosing intermittent running as HIIT mode can better reduce the interference effects of concurrent training. This may be because intermittent running and RT lead to similar improvements in exercise performance, reflected in the increase in explosive strength, lower limb strength, and anaerobic capacity [53]. Another interpretation is that whereas cycling doesn’t require you to bear any weight, running does, so running will lead to greater physiological stimulation [54]. However, related studies have shown that intermittent cycling does not affect the hypertrophy of lower limb muscles [17, 55]. It is worth mentioning that Tsitkanou et al. (2017) found that the percentage of IIx type muscle fibers of quadriceps femoris in the experimental group decreased by 7% after 8 weeks of RT, while the percentage of type IIx muscle fibers in the control group decreased by 7.6% after concurrent training with intermittent cycling as endurance training. This indicates that the interference effect has little impact on muscle fibers, but for high-level athletes, even subtle differences should be noted.

Training periodization has always been an essential variable in endurance and strength training [56, 57, 58]. For RT periodization of less than 8 weeks, muscle adaptation mainly manifests as the improvement in the neural control of muscle contraction and the reduction of the recruitment proportion of type IIx muscle fibers [58]. However, for training periods of more than 8 weeks, skeletal muscle adaptation mainly focuses on enhancing muscle-tendon stiffness [57]. The work of skeletal muscles is closely related to their energy supply. Skeletal muscle glycogen is an important form of energy storage, and related studies have shown that glycogen is difficult to fully recover within 24 hours after high-intensity training [59]. Although the literature included in this study mostly used training intervals greater than 24 hours, there is still reason to believe that the energy depletion caused by HIIT may not be fully recovered at the beginning of RT for individuals without systematic training experience. In addition, Doma et al., (2015) found that high-frequency concurrent training could lead to a decrease in maximum strength, as well as muscle soreness and fatigue. Therefore, the improvement of muscle strength may not be linear, and fatigue and energy depletion are the main factors that affect skeletal muscle adaptation [60]. Prolonged, high-frequency, high-intensity endurance training will lead to chronic consumption of skeletal muscle glycogen [61], and similarly, high-intensity sprint training and resistance training can also cause glycogen depletion [62]. This study found that over an 8-week training session would impair the improvement of half squat 1RM after concurrent HIIT and RT, but the impact on the enhancement of explosive force in the lower limbs is small. This result is consistent with the results of Hickson et al. (1980), where the maximum strength is affected by concurrent training, and after 8 weeks, the maximum strength begins to decrease. [27]. This indicates that adaptation to concurrent training for lower limb strength may be selective. [27].

5.1 Limitations

Before drawing conclusions, it is important to consider the limitations of this study. Firstly, this study only focused on two outcomes, and the results cannot be derived from other indicators of lower limb explosive strength (e.g. squat jump or drop jump) or maximum lower limb strength (e.g. leg press). Secondly, there is a lack of research on concurrent HIIT and RT, and the analysis includes studies on different genders, training frequencies, and rest intervals between endurance and strength training which may lead to biased results. Finally, according to the overtraining hypothesis, the shorter the rest interval between endurance and strength training, the more likely it is to cause overtraining, which will impair strength quality improvement [48]. Related studies have shown that an interval of more than 24 hours between endurance and strength training can help eliminate the fatigue caused by endurance training [63]. Most of the literature included in this study used training intervals of more than 24 hours [5, 6, 14, 16, 17, 37, 38, 39], which may also cause some result bias. Coaches and practitioners should be aware of these limitations when using the findings to inform their training programs. Further research is needed to address these limitations and provide more robust evidence on the effects of concurrent HIIT and RT.

6. Conclusion

In conclusion, compared with strength training, concurrent high-intensity interval training and strength training have no negative impact on the improvement of lower body explosive strength and maximum strength. Athletes are less likely to be affected by interference, while non-athletes are more susceptible to interference effects, which can be reduced by developing targeted training plans. In terms of training content, using interval running as the HIIT modality can further reduce the interference effect. The ideal training regimen for enhancing lower body maximum strength is one that lasts no more than eight weeks, while concurrent HIIT and RT lasting more than 8 weeks are more suitable for improving CMJ.

Author contributions

DESIGN OF THE STUDY: Y.H.C. and J.M.

SEARCH STRATEGY, LITERATURE SEARCH AND DATA EXTRACTION: Y.H.C., L.M.H. and X.M.F.

RISK OF BIAS SCORING AND STATISTICAL ANALYSIS: X.M.F.

PREPARATION OF THE MANUSCRIPT: Y.H.C., and J.M.

All authors have read and approved the final manuscript.

Ethical considerations

Our data were extracted from published papers that had received ethical approval, so previous researchers had helped us obtain qualified ethical approval and we did not need to apply for ethical approval again.

Funding

The systematic review with meta-analysis did not receive any funding.

Footnotes

Acknowledgments

The authors have no acknowledgments.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

Fyfe

Bishop

Stepto

. Interference between concurrent resistance and endurance exercise: mol ecular bases and the role of individual training variables. Sports medicine (Auckland, NZ). 44; 743-762. doi: 10.1007/s40279-014-0162-1.

Bull

Al-Ansari

Biddle

et al. World Health Organization 2020 guidelines on physical activity and sed entary behaviour. British Journal of Sports Medicine. 54: 1451-1462. doi: 10.1136/bjsports-2020-102955.

Villareal

Aguirre

Gurney

et al. Aerobic or Resistance Exercise, or Both, in Dieting Obese Older Adults. N Engl J Med. 2017; 376: 1943-1955. doi: 10.1056/NEJMoa1616338.

Pugh

Faulkner

Turner

et al. Satellite cell response to concurrent resistance exercise and high-int ensity interval training in sedentary, overweight/obese, middle-aged i ndividuals. European Journal of Applied Physiology. 118: 225-238. doi: 10.1007/s00421-017-3721-y.

Robineau

Babault

Piscione

et al. Specific Training Effects of Concurrent Aerobic and Strength Exercises Depend on Recovery Duration. Journal of Strength and Conditioning Research. 30: 672-683. doi: 10.1519/JSC.0000000000000798.

Robineau

Lacome

Piscione

et al. Concurrent Training in Rugby Sevens: Effects of High-Intensity Interva l Exercises. International Journal of Sports Physiology and Performance. 12: 336-344. doi: 10.1123/ijspp.2015-0370.

Blagrove

Howe

Cushion

et al. Effects of Strength Training on Postpubertal Adolescent Distance Runne rs. Medicine and Science in Sports and Exercise. 50: 1224-1232. doi: 10.1249/MSS.0000000000001543.

Kjær

Herskind

Kristiansen

et al. Applicability of the Load-Velocity Relationship to Predict 1-Repetitio n Maximum in the Half-Squat in High-Level Sprinters. International Journal of Sports Physiology and Performance. doi: 10.1123/ijspp.2022-0334. 1-8.

Loturco

Fernandes

Bishop

et al. Variations in Physical and Competitive Performance of Highly Trained S printers Across an Annual Training Season. Journal of Strength and Conditioning Research. 37: 1104-1110. doi: 10.1519/JSC.0000000000004380.

10.

Gaamouri

Hammami

Cherni

et al. The effects of upper and lower limb elastic band training on the chang e of direction, jump, power, strength and repeated sprint ability perf ormance in adolescent female handball players. Frontiers in Sports and Active Living. 5: 1021757. doi: 10.3389/fspor.2023.1021757.

11.

Schneiker

Fyfe

Teo

SYM

et al. Comparative Effects of Contrast Training and Progressive Resistance Tr aining on Strength and Power-Related Measures in Subelite Australian R ules Football Players. Journal of Strength and Conditioning Research. doi: 10.1519/JSC.0000000000004423; 10.1519/JSC.0000000000004423; 10.1519/JSC.0000000000004423.

12.

Loturco

Pereira

Bishop

et al. Effects of a resistance training intervention on the strength-deficit of elite young soccer players. Biology of Sport; 39: 615-619. doi: 10.5114/biolsport.2022.106157.

13.

Cantrell

Schilling

Paquette

et al. Maximal strength, power, and aerobic endurance adaptations to concurre nt strength and sprint interval training. European Journal of Applied Physiology. 114: 763-771. doi: 10.1007/s00421-013-2811-8.

14.

Petré

Löfving

Psilander

. The Effect of Two Different Concurrent Training Programs on Strength a nd Power Gains in Highly-Trained Individuals. Journal of Sports Science & Medicine. 17: 167-173.

15.

Ross

Ratamess

Hoffman

et al. The effects of treadmill sprint training and resistance training on ma ximal running velocity and power. Journal of Strength and Conditioning Research. 23: 385-394. doi: 10.1519/JSC.0b013e3181964a7a.

16.

Spiliopoulou

Zaras

Methenitis

et al. Effect of Concurrent Power Training and High-Intensity Interval Cyclin g on Muscle Morphology and Performance. Journal of Strength and Conditioning Research. 35: 2464-2471. doi: 10.1519/JSC.0000000000003172.

17.

Tsitkanou

Spengos

Stasinaki

et al. Effects of high-intensity interval cycling performed after resistance training on muscle strength and hypertrophy. Scandinavian Journal of Medicine & Science in Sports. 27: 1317-1327. doi: 10.1111/sms.12751.

18.

Wilson

Marin

Rhea

et al. Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises. J Strength Cond Res. 2012; 26: 2293-2307. doi: 10.1519/JSC.0b013e31823a3e2d.

19.

Sabag

Najafi

Michael

et al. The compatibility of concurrent high intensity interval training and resistance training for muscular strength and hypertrophy: a systematic review and meta-analysis. J Sports Sci. 2018; 36: 2472-2483. doi: 10.1080/02640414.2018.1464636.

20.

Kumari

Singh

Varghese

. Effects of high-intensity interval training on aerobic capacity and sports-specific skills in basketball players. J Bodyw Mov Ther. 2023; 34: 46-52. doi: 10.1016/j.jbmt.2023.04.032.

21.

Shi

Tong

Nie

et al. Repeated-sprint training in hypoxia boosts up team-sport-specific repe ated-sprint ability: 2-week vs 5-week training regimen. European Journal of Applied Physiology. doi: 10.1007/s00421-023-05252-x.

22.

Bishop

Girard

Mendez-Villanueva

. Repeated-sprint ability – part II: recommendations for training. Sports Medicine (Auckland, NZ). 41: 741-756. doi: 10.2165/11590560-000000000-00000.

23.

Silva

Cadore

Kothe

et al. Concurrent training with different aerobic exercises. Int J Sports Med. 2012; 33: 627-634. doi: 10.1055/s-0031-1299698.

24.

Cardozo

de Souza Destro

. Pyramidal resistance training: A brief review of acute responses and l ong-term adaptations. Journal of Bodywork and Movement Therapies. 35: 21-27. doi: 10.1016/j.jbmt.2023.04.070.

25.

Brenner

. American Academy of Pediatrics Council on Sports M, Fitness. Overuse injuries, overtraining, and burnout in child and adolescent at hletes. Pediatrics. 119: 1242-1245. doi: 10.1542/peds.2007-0887.

26.

Sabag

Najafi

Michael

et al. The compatibility of concurrent high intensity interval training and r esistance training for muscular strength and hypertrophy: a systematic review and meta-analysis. Journal of Sports Sciences. 36: 2472-2483. doi: 10.1080/02640414.2018.1464636.

27.

Hickson

. Interference of strength development by simultaneously training for st rength and endurance. European Journal of Applied Physiology and Occupational Physiology. 45: 255-263. doi: 10.1007/BF00421333.

28.

Kang

Yin

et al. Effects of Concurrent Strength and HIIT-Based Endurance Training on Ph ysical Fitness in Trained Team Sports Players: A Systematic Review and Meta-Analysis. International Journal of Environmental Research and Public Health. 19: 14800. doi: 10.3390/ijerph192214800.

29.

[Anonymous]. Reprint – Preferred Reporting Items for Systematic Reviews and Meta-Analyses: The PRISMA Statement. Physical Therapy. 2009; 89: 873-880.

30.

Muka

Glisic

Milic

et al. A 24-step guide on how to design, conduct, and successfully publish a systematic review and meta-analysis in medical research. European Journal of Epidemiology. 35: 49-60. doi: 10.1007/s10654-019-00576-5.

31.

Liberati

Altman

Tetzlaff

et al. The PRISMA statement for reporting systematic reviews and meta-analyse s of studies that evaluate healthcare interventions: explanation and e laboration. BMJ (Clinical Research ed). 339: b2700. doi: 10.1136/bmj.b2700.

32.

Hunter

Saratzis

Sutton

et al. In meta-analyses of proportion studies, funnel plots were found to be an inaccurate method of assessing publication bias. Journal of Clinical Epidemiology. 67: 897-903. doi: 10.1016/j.jclinepi.2014.03.003.

33.

DerSimonian

Laird

. Meta-analysis in clinical trials. Control Clin Trials. 1986; 7: 177-188. doi: 10.1016/0197-2456(86)90046-2.

34.

Lau

Ioannidis

Schmid

. Quantitative synthesis in systematic reviews. Ann Intern Med. 1997; 127: 820-826. doi: 10.7326/0003-4819-127-9-199711010-00008.

35.

Cheung

Vijayakumar

. A Guide to Conducting a Meta-Analysis. Neuropsychol Rev. 2016; 26: 121-128. doi: 10.1007/s11065-016-9319-z.

36.

Petré

Hemmingsson

Rosdahl

et al. Development of Maximal Dynamic Strength During Concurrent Resistance a nd Endurance Training in Untrained, Moderately Trained, and Trained In dividuals: A Systematic Review and Meta-analysis. Sports Medicine (Auckland, NZ). 51: 991-1010. doi: 10.1007/s40279-021-01426-9.

37.

Balabinis

Psarakis

Moukas

et al. Early phase changes by concurrent endurance and strength training. Journal of Strength and Conditioning Research. 17: 393-401. doi: 10.1519/1533-4287(2003)017<0393:epcbce>2.0.co;2.

38.

Hermassi

Haddad

Bouhafs

et al. Comparison of a Combined Strength and Handball-Specific Training vs. I solated Strength Training in Handball Players Studying Physical Educat ion. Sportverletzung Sportschaden: Organ der Gesellschaft fur Orthopadisch – Traumatologische Sportmedizin. 33: 149-159. doi: 10.1055/a-0919-7267.

39.

Kotzamanidis

Chatzopoulos

Michailidis

et al. The effect of a combined high-intensity strength and speed training pr ogram on the running and jumping ability of soccer players. Journal of Strength and Conditioning Research. 19: 369-375. doi: 10.1519/R-14944.1.

40.

Makhlouf

Castagna

Manzi

et al. Effect of Sequencing Strength and Endurance Training in Young Male Soc cer Players. Journal of Strength and Conditioning Research. 30: 841-850. doi: 10.1519/JSC.0000000000001164.

41.

Reilly

. Some observations on weight-training [proceedings]. British Journal of Sports Medicine. 12: 45-47. doi: 10.1136/bjsm.12.1.45.

42.

Murlasits

Kneffel

Thalib

. The physiological effects of concurrent strength and endurance trainin g sequence: A systematic review and meta-analysis. Journal of Sports Sciences. 36: 1212-1219. doi: 10.1080/02640414.2017.1364405.

43.

Ben-Zeev

Okun

. High-Intensity Functional Training: Molecular Mechanisms and Benefits. Neuromolecular Medicine. 23: 335-338. doi: 10.1007/s12017-020-08638-8.

44.

Glass

. Skeletal muscle hypertrophy and atrophy signaling pathways. Int J Biochem Cell Biol. 37: 1974-1984. doi: 10.1016/j.biocel.2005.04.018.

45.

Inoki

Zhu

Guan

K-L

. TSC2 mediates cellular energy response to control cell growth and surv ival. Cell. 115: 577-590. doi: 10.1016/s0092-8674(03)00929-2.

46.

Nader

. Concurrent strength and endurance training: from molecules to man. Medicine and Science in Sports and Exercise. 38: 1965-1970. doi: 10.1249/01.mss.0000233795.39282.33.

47.

Solsona

Pavlin

Bernardi

et al. Molecular Regulation of Skeletal Muscle Growth and Organelle Biosynthe sis: Practical Recommendations for Exercise Training. International Journal of Molecular Sciences. 22: 2741. doi: 10.3390/ijms22052741.

48.

Vechin

Conceição

Telles

et al. Interference Phenomenon with Concurrent Strength and High-Intensity In terval Training-Based Aerobic Training: An Updated Model. Sports Medicine (Auckland, NZ). 51: 599-605. doi: 10.1007/s40279-020-01421-6.

49.

Buchheit

Laursen

. High-intensity interval training, solutions to the programming puzzle. Part II: anaerobic energy, neuromuscular load and practical applicati ons. Sports medicine (Auckland, NZ); 43: 927-954. doi: 10.1007/s40279-013-0066-5.

50.

Fyfe

Bartlett

Hanson

et al. Endurance Training Intensity Does Not Mediate Interference to Maximal Lower-Body Strength Gain during Short-Term Concurrent Training. Frontiers in Physiology. 7: 487. doi: 10.3389/fphys.2016.00487.

51.

Morton

Sonne

Farias Zuniga

et al. Muscle fibre activation is unaffected by load and repetition duration when resistance exercise is performed to task failure. The Journal of Physiology. 597: 4601-4613. doi: 10.1113/JP278056.

52.

Cheng

Jude

Lanner

. Intramuscular mechanisms of overtraining. Redox Biology. 35: 101480. doi: 10.1016/j.redox.2020.101480.

53.

Slater

Sygo

Jorgensen

. SPRINTING…Dietary Approaches to Optimize Training Adaptation and Performance. International Journal of Sport Nutrition and Exercise Metabolism. 29: 85-94. doi: 10.1123/ijsnem.2018-0273.

54.

Doma

Deakin

Schumann

et al. Training Considerations for Optimising Endurance Development: An Alternate Concurrent Training Perspective. Sports Med. 2019; 49: 669-682. doi: 10.1007/s40279-019-01072-2.

55.

Pugh

Faulkner

Jackson

et al. Acute molecular responses to concurrent resistance and high-intensity interval exercise in untrained skeletal muscle. Physiological Reports. 3: e12364. doi: 10.14814/phy2.12364.

56.

Billat

. Interval training for performance: a scientific and empirical practice. Special recommendations for middle- and long-distance running. Part I: aerobic interval training. Sports medicine (Auckland, NZ); 31: 13-31. doi: 10.2165/00007256-200131010-00002.

57.

de Souza

Tricoli

Roschel

et al. Molecular adaptations to concurrent training. International Journal of Sports Medicine. 34: 207-213. doi: 10.1055/s-0032-1312627.

58.

Staron

Karapondo

Kraemer

et al. Skeletal muscle adaptations during early phase of heavy-resistance tra ining in men and women. Journal of Applied Physiology (Bethesda, Md: 1985). 76: 1247-1255. doi: 10.1152/jappl.1994.76.3.1247.

59.

Namma-Motonaga

Kondo

Osawa

et al. Effect of Different Carbohydrate Intakes within 24 Hours after Glycoge n Depletion on Muscle Glycogen Recovery in Japanese Endurance Athletes. Nutrients. 14: 1320. doi: 10.3390/nu14071320.

60.

Gacesa

JZP

Klasnja

Grujic

. Changes in strength, endurance, and fatigue during a resistance-traini ng program for the triceps brachii muscle. Journal of Athletic Training. 48: 804-809. doi: 10.4085/1062-6050-48.4.16.

61.

Costill

Bowers

Branam

et al. Muscle glycogen utilization during prolonged exercise on successive da ys. Journal of Applied Physiology. 31: 834-838. doi: 10.1152/jappl.1971.31.6.834.

62.

Robergs

Pearson

Costill

et al. Muscle glycogenolysis during differing intensities of weight-resistanc e exercise. Journal of Applied Physiology (Bethesda, Md: 1985). 70: 1700-1706. doi: 10.1152/jappl.1991.70.4.1700.

63.

Bentley

Zhou

Davie

. The effect of endurance exercise on muscle force generating capacity o f the lower limbs. Journal of Science and Medicine in Sport. 1: 179-188. doi: 10.1016/s1440-2440(98)80013-3.

The effect of concurrent high-intensity interval training and resistance training on the lower body maximal strength and explosive power: A updated systematic review and meta-analysis

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Searching strategy

2.2 Eligibility criteria

2.3 Data extraction

Table 1 Risk of bias assessment

2.5 Statistical analysis

Table 2 Participant characteristics of included studies

3.1 Description of studies

4.1 Sub-analysis: CMJ and half squat

Table 4 Several subgroup analysis outcomes

5.1 Limitations

6. Conclusion

Author contributions

Ethical considerations

Funding

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Risk of bias assessment

Table 2
Participant characteristics of included studies

Table 4
Several subgroup analysis outcomes