Effects of Concurrent Strength and HIIT-Based Endurance Training on Physical Fitness in Trained Team Sports Players: A Systematic Review and Meta-Analysis

round: Concurrent strength and HIIT-based endurance training (CT) has merit in time-saving in team sports. However, the effect of CT on physical tness remained equivocal. This meta-analysis aimed to determine whether CT would produce an interference effect on the development of physical tness when compared to strength training (ST) or HIIT-based endurance training (HET) alone in trained team sports players. Methods: A total of 2478 studies from three databases were screened. 52 full texts were reviewed. Seven studies were nally included and then subgroups were used for quantitative analysis. Results: Compared to ST alone, CT had a signi cant effect on the development of maximal lower-body strength in trained team sports players (MD 4.20 kg, 95% CI 0.71–7.68,p= 0.02, I 2 = 20%), but there was no signi cant difference between the groups on training adaptation in lower-body power (SMD 0.08, 95% CI 0.23–0.39,p= 0.62,I 2 = 26%).

analysis. Results: Compared to ST alone, CT had a signi cant effect on the development of maximal lower-body strength in trained team sports players (MD 4.20 kg, 95% CI 0.71–7.68,p= 0.02, I 2 = 20%), but there was no signi cant difference between the groups on training adaptation in lower-body power (SMD 0.08, 95% CI 0.23–0.39,p= 0.62,I 2 = 26%). Furthermore, a sub-group analysis based on the internal organization order of CT revealed that there was no statistically signi cant subgroup effect between CT and ST alone in all parameters. Conclusions: Well-designed CT regimens did not interfere with the development of physical tness of trained team sports players. Keywords:concurrent training; strength training; endurance training; physical tness; interference 1. Introduction The most common team sports, such as soccer, rugby, American football, basketball, and handball are based on strength and endurance. Players need to develop multiple aspects of physical tness to achieve the desired athletic performance in these sports. This necessitates players developing a well-rounded tness to meet the physical demands of match-play. Within these criteria of physical tness, maximal lower-body strength, power, and aerobic capacity are the most important determinants of athletic performance [1]. In a 90-min soccer match, for instance, the total distance a player runs ranges from 10–12 km at an average intensity of up to the anaerobic threshold (80–90% of maximum heart rate) [2], Within this context, they may be engaged in approximately 150–250 actions of 15–20 m of high-intensity sprints and perform numerous explosive actions [3,4]. These actions strongly in uence the performance of players and the team, and can potentially change a match's outcome [5]. Thus, in addition to constantly improving techniques and tactics, the development of physical tness is currently being given greater consideration by strength and conditioning professionals. However, within current sporting contexts, the available training time may be extremely limited owing to congested competitive schedules [6]. It is dif cult to schedule strength training (ST) and endurance training (ET) on alternate days. Strength and conditioning professionals must improve the time-ef ciency of training on the premise and of guaranteeing the quality of training.

strength and conditioning professionals. However, within current sporting contexts, the available training time may be extremely limited owing to congested competitive schedules [6]. It is dif cult to schedule strength training (ST) and endurance training (ET) on alternate days. Strength and conditioning professionals must improve the time-ef ciency of training on the premise and of guaranteeing the quality of training. Consequently, most team Int. J. Environ. Res. Public Health2022,19, 14800.

Int. J. Environ. Res. Public Health2022,19, 14800 2 of 17 sports currently combined ST and ET on the same day to develop players' physical t- ness within routine training. Such a time-ef cient training regimen that simultaneously combines ST and ET within a training cycle is called concurrent training [6–13]. Moreover, repeated-sprint training (RST) and sprint interval training (SIT) based high-intensity inter- val training (HIIT) methods are increasingly applied to team sports ET protocol, in view of the HIIT is considered one of the most effective and time ef cient means for improving cardiorespiratory and metabolic function. Since Robert C. Hickson rst conducted concurrent training studies in 1980, an ex- tensive body of research has been produced. Some studies have shown that concurrent training can potentiate the individual effects produced by ST and ET more than ST or ET alone [9,14–17]. For example, it was documented well that ST contributes to enhancing endurance performance by improving leg stiffness [18–21], which is a notion introduced from physics to characterize properties of certain types of deformable bodies under an in uence of external forces [22–26]. However, other studies suggested that concurrent train- ing compromises speci c adaptive responses compared to ST or ET alone, more speci cally, concurrent training may attenuate gains in muscle hypertrophy [27,28], strength [29–31], and power [32–35], but has little to no effect on endurance outcomes, such as VO2max or VO2peak and Yo-Yo test performance [11,12,35,36]. This phenomenon was de ned as the “interference effect” of concurrent training [11,13,37,38]. However, to date, evidence for an interference effect has remained equivocal in humans. Previous studies have shown that variables such as intensity, volume, frequency, training status, ET protocol, organiza- tion order, duration of the recovery period, and nutrition supplements strongly in uence individual adaptation to concurrent training. Although some meta-analyses [11,39–44] address the effects of the above-mentioned factors in athletic performance outcomes and physiological changes, there is a lack of robust evidence-based guidelines for trained team sports players. Given the necessity for trained team sports players to simultaneously develop strength and endurance, as well as the popularity of HIIT-based

strongly in uence individual adaptation to concurrent training. Although some meta-analyses [11,39–44] address the effects of the above-mentioned factors in athletic performance outcomes and physiological changes, there is a lack of robust evidence-based guidelines for trained team sports players. Given the necessity for trained team sports players to simultaneously develop strength and endurance, as well as the popularity of HIIT-based endurance training in team sports. There is a need for a systematic review and meta-analysis to draw conclusions from the inconsistencies. The purpose of this study was to determine whether concurrent strength and HIIT-based endurance training (CT) would produce an interference effect on physical tness development compared to ST or HIIT-based endurance training (HET) alone in trained team sports players. The hypothesis was as follows: ST carried out rst in CT with an adequate interval time between ST and HET would not induce an interference effect on physical tness development. Our ndings will enhance understanding regarding the application of CT in team sports and assist strength and conditioning professionals to design better CT regimens. 2. Materials and Methods This systematic review and meta-analysis was performed in line with the recommenda- tions of the Preferred Reporting Items for Systematic Reviews and Meta-Analysis statement (PRISMA). 2.1. Literature Search Strategy A search from 1980, the year that seminal research relating to concurrent training was published, to and including June 2022 was carried out using the following electronic databases: Web of Science, PubMed, and ScienceDirect. The search strategy used the following Boolean search syntax: “(concurrent training or concurrent exercise or combined training or concurrent strength and endurance training) and (soccer or football or associa- tion football or American football or rugby or basketball or handball or hockey or softball or team sports).” The search strategy is presented in Table. The search was limited to peer-reviewed English language articles. Following this, a primary exclusion based on titles and abstracts of retrieved articles was conducted individually by two authors (J.K. and Z.Y.) to assess their eligibility for review and meta-analysis. A secondary exclusion

strategy is presented in Table. The search was limited to peer-reviewed English language articles. Following this, a primary exclusion based on titles and abstracts of retrieved articles was conducted individually by two authors (J.K. and Z.Y.) to assess their eligibility for review and meta-analysis. A secondary exclusion

Int. J. Environ. Res. Public Health2022,19, 14800 3 of 17 thereafter was also conducted individually by two authors (J.K. and X.Y.), based on a review of full-text articles. Any disagreements were solved by consensus with a third author (B.G.). Table 1.Web of Science search strategy performed on 30 June 2022. Concept Search Strategy Line No. Entry Concurrent training 1 Concurrent training 2 Concurrent exercise 3 Combined training 4 Concurrent strength and endurance training 5 1 or 2 or 3 or 4 Sports 6 Soccer 7 Football 8 Association football 9 American football 10 Rugby 11 Basketball 12 Handball 13 Hockey 14 Softball 15 Team sports 16 6 or 7 or 8 or 9 or 10 or 11 or 12 or 13 or 14 or 15 17 5 and 16 2.2. Eligibility Criteria According to the Cochrane Collaboration methods, the PICOS (Participants, Interven- tion, Comparison, Outcomes, Study design) approach was used to build inclusion criteria for this systematic review and meta-analysis: (1) cohorts of team sports players aged >13 with no restriction for gender; (2) studies needed to have incorporated a concurrent training regimen, containing at least one group that followed single-mode ST/ET; (3) means and standard deviations of one or more outcome measures had to be reported for all groups pre- and post-intervention; (4) outcome measures included one or more of the following, lower-body 1-repetition maximum(1RM) of half squat, countermovement jump (CMJ) or squat jump (SJ), VO2max/peak or Yo-Yo test. These indicators represented lower-body maximum muscle strength, maximum immediate muscle power, and aerobic capacity, respectively; (5) randomized controlled or matched trials with at least 5 weeks of follow-up duration; (6) the original study. 2.3. Data Extraction Two authors (X.Y. and Z.Y.) independently conducted data extraction from the in- cluded studies using an electronic data sheet. Data extracted comprised participant charac- teristics (including mean age, gender, and training status), study characteristics (including trial design, participant number, group set, intervention duration, training frequency, and exercise protocol), and outcome measures. Discrepancies about study conditions were discussed until a consensus was reached with a third author (B.G.). 2.4. Statistical Analyses Though included studies reported

using an electronic data sheet. Data extracted comprised participant charac- teristics (including mean age, gender, and training status), study characteristics (including trial design, participant number, group set, intervention duration, training frequency, and exercise protocol), and outcome measures. Discrepancies about study conditions were discussed until a consensus was reached with a third author (B.G.). 2.4. Statistical Analyses Though included studies reported multiple outcome measures, only those that were relevant to the meta-analysis were extracted. The differences in means in each group were calculated for each study using a comparison of mean change from pre- to post-intervention. The standard deviation (SD) of the mean change was also calculated and used to generate forest plots with study-speci c point estimates and respective 95% con dence intervals (CIs). SD of the difference of the means was computed using the following formulation according to Deeks and Higgins [45]:

Int. J. Environ. Res. Public Health2022,19, 14800 4 of 17 SD change= s (NT 1)SD 2 T +(NC 1)SD 2 C NT+NC 2 (1) where NT, NC,and SDT, SDCrepresent the sample size in the experiment group and control group as well as the standard deviation of their responses, respectively. According to Morris [46], the SDTand SDChere use the standard deviation of the pretest mean of each group. For each study, if the same measures were applied to the same outcome, pooled mean difference (MD) was calculated in Review Manager software (v.5.4. The Cochrane Collabo- ration, 2020). If not, between-group standardized mean differences (SMD) were calculated in Review Manager software using Hedges' adjusted g, which is corrected for sample size. Analysis of the pooled data was conducted with a random-effects model, where weighting was based on inverse variance. According to Cohen, the overall effect size was interpreted as trivial (value < 0.2), small (0.2 value < 0.5), moderate(0.5 value < 0.8), or large (value 0.8), respectively. Statistical signi cance was considered forp 0.05.SMDs were also used to create funnel plots so that all estimates could be placed into one funnel plot. 2.5. Quality Assessment The Physiotherapy Evidence Database (PEDro) scale was used to rate the risk of bias and methodological quality of trials of the included studies. This rating scheme is reported as valid and reliable [47]. It quanti ed internal study validity using scoring from 0 (high risk of bias) to 10 (low risk of bias). Owing to the nature of sports training, it is often dif cult to establish blinding within exercise interventions for both subjects and testing personnel. Thus, we modi ed the original version of the PEDro scale so that blinding of participants and investigators was not considered for quality assessment. The modi ed PEDro scale included an item indicating that the training load was controlled and reported, similar to the study of Ludyga et al. [48]. The scores of the seven included publications included studies ranged from 6 to 9, with an average score of 7.7, indicating moderate to high methodological quality (Table). Heterogeneity between studies

considered for quality assessment. The modi ed PEDro scale included an item indicating that the training load was controlled and reported, similar to the study of Ludyga et al. [48]. The scores of the seven included publications included studies ranged from 6 to 9, with an average score of 7.7, indicating moderate to high methodological quality (Table). Heterogeneity between studies was assessed using I 2 statistics for each outcome and interpreted as low, moderate, and high, corresponding to an I 2 statistic of 25%, 50%, and 75%, respectively, according to Higgins et al. [49]. Furthermore, publication bias was assessed using funnel plots produced by Review Manager software. The funnel plot showed that the effects were relatively symmetrically distributed around the overall pooled effect size (Figure). Sensitivity analyses were also conducted to identify whether a particular study accounted for the heterogeneity. Table 2.Methodological quality assessment for inclusion in the study. Study 1 2 3 4 5 6 7 8 9 10 Total Balabinis et al., 2003 [9] 1 0 0 1 1 0 1 1 1 1 7 Kotzamanidis et al., 2005 [50] 1 1 1 1 1 0 1 1 1 1 9 Ross et al., 2009 [51] 1 1 0 0 1 0 1 0 1 1 6 Makhlouf et al., 2016 [52] 1 1 0 1 1 0 1 1 1 1 8 Robineau et al., 2016 [53] 1 1 0 1 1 0 1 1 1 1 8 Robineau et al., 2017 [7] 1 1 0 1 1 0 1 1 1 1 8 Hermassi et al., 2019 [54] 1 1 0 1 1 0 1 1 1 1 8 The modi ed PEDro scale item. 1. Eligibility criteria speci ed. 2. Random allocation. 3. Concealed allocation. 4. Groups similar at baseline. 5. Training load controlled and reported. 6. Assessor blinding. 7. Less than 15% dropouts. 8. Intention-to-treat analysis. 9. Between-group statistical comparisons. 10. Point measures and variability data. Each satis ed item contributes 1 point to the total PEDro score (range 0–10 points).

4. Groups similar at baseline. 5. Training load controlled and reported. 6. Assessor blinding. 7. Less than 15% dropouts. 8. Intention-to-treat analysis. 9. Between-group statistical comparisons. 10. Point measures and variability data. Each satis ed item contributes 1 point to the total PEDro score (range 0–10 points).

Int. J. Environ. Res. Public Health2022,19, 14800 5 of 17Int. J. Environ. Res. Public Health 2022, 19, x FOR PEER REVIEW 5 of 17 The modified PEDro scale item. 1. Eligibility criteria specified. 2. Random allocation. 3. Concealed allocation. 4. Groups similar at baseline. 5. Training load controlled and reported. 6. Assessor blinding. 7. Less than 15% dropouts. 8. Intention-to-treat analysis. 9. Between-group statistical comparisons. 10. Point measures and variability data. Each satisfied item contributes 1 point to the total PEDro score (range 0–10 points). Figure 1. Funnel plots of standardized mean differences effect size versus standard error. 2.6. Study Characteristics Through systematic database searching, 1365, 921, and 671 relevant articles were initially identified in PubMed, Web of Science, and ScienceDirect, respectively. After duplicates were identified via Endnote software (v.9.3.3. Clarivate Analytics) and screening was performed based on the title and abstract, 52 articles remained. Through a manual search of the reference lists an additional nine studies were selected. After the initial full-text examination, 15 potential studies were further assessed. Further trial details of 15 studies were examined according to the eligibility criteria, and seven studies [7,9,50–54] were finally included in the systematic review and meta-analysis. Figure 2 shows the article selection process. All subjects in the seven studies were from six team sports: soccer, rugby, American football, basketball, and handball. Their training status was defined as “trained” and “athlete”, in accordance with Wilson et al. [11]. Five additional records [8,10,17,33,55] were not eligible for inclusion in the meta-analysis but were included in the qualitative analysis. In one study [51] of unreported data, the means and SDs of outcomes were estimated from figures using GetData Graph Digitizer (http://www.getdata-graph-digitizer.com/, accessed on 16 July 2022). In most studies the ET protocol was mainly repeated-sprint training (RST) and/or sprint interval training (SIT) HIIT models; the ST protocol included combined free weights, machine resistance, circuit resistance training, and plyometrics to maintain and develop maximum strength levels while further improving explosive strength. The intervention duration ranged from 5 to 12 weeks, and two to three times per week. More details can be found in Table

The study reviews the effects of concurrent training on physical fitness in team sports.