Description of Studies
The database search yielded 1464 potential studies for inclusion (Fig. 1). Twenty-seven studies met the inclusion criteria, and were included in the meta-analysis [3,4,5,6, 8, 10,11,12,13,14,15,16, 21,22,23,24,25, 35, 36, 38,39,40,41,42,43,44,45]. A total of 750 participants were included (523 men and 227 women), aged 20–38 years. Seven studies involved untrained individuals [11,12,13,14, 21,22,23], 10 studies involved moderately trained individuals [3, 5, 10, 15, 16, 38,39,40,41, 45], and 10 studies involved trained individuals [4, 6, 8, 24, 25, 35, 36, 42,43,44]. The corresponding authors of 16 studies were contacted [3,4,5,6, 14,15,16, 21, 22, 25, 36, 38,39,40,41,42] for clarification or missing information via e-mail, of whom five responded with additional information [5, 15, 16, 36, 39]. In two of the included studies, it was not clear in which form the variation (SD and SEM) was reported. Scientific reasoning by comparisons with equivalent studies were then applied to recalculate the variation in the outcomes, as the authors would not respond to requests for clarification [6, 24]. For more detailed information about the participant characteristics, see Table 1.
Table 1 Participant characteristics Intervention Characteristics
A summary of training design variables for lower-body strength and endurance training in each study, including recovery between sessions, the sequential order, the frequency and duration of endurance training per session, and the intervention length, is presented in Table 2. The frequency and duration of endurance training per week ranged from 2 to 6 sessions per week and from 12 to 260 min per week. In six of the included studies, information regarding the rest and work durations during intervals was missing. As the authors did not respond with additional information, the endurance duration per session was estimated through scientific reasoning based on the distance that was covered during the endurance training and comparisons with other similar studies [4, 6, 24, 25, 39, 44]. The length of the studies ranged from 6 to 21 weeks. The mean frequency of the endurance training sessions was 2.9 per week for untrained participants, 2.8 for moderately trained participants, and 2.6 for trained participants. The mean duration of the endurance training per session was 37 min for untrained participants, 31 min for moderately trained participants, and 29 min for trained participants. The frequency of strength training ranged from 2 to 5 sessions per week with a mean frequency of 2.9 sessions per week for untrained participants, 2.7 for moderately trained participants, and 2.5 for trained participants. For untrained participants, 4 studies performed endurance training first, and in 3 studies, the intra-session order was not specified. For moderately trained participants, 4 studies performed endurance training first, and 2 performed resistance training first; and in 6 studies, the intra-session order was not specified. For trained participants, 4 studies performed endurance training first, and 5 studies performed resistance training first; and in 3 studies, the intra-session order was not specified.
Table 2 Training design characteristics The endurance exercise type (interval/continuous/mixed) was 0/6/1 for untrained, 9/6/1 for moderately trained, and 11/2/2 for trained participants. Of the studies, 12 performed concurrent resistance and endurance training within the same session (< 20 min between sessions) [4, 5, 13, 14, 16, 21, 23, 25, 36, 40, 43, 45], 13 performed concurrent resistance and endurance trainings during different sessions (> 2 h between sessions) [3, 6, 10,11,12, 15, 24, 25, 35, 38, 39, 41, 44], two of the studies mixed performing concurrent resistance and endurance training during the same and different sessions during the training programme [8, 22], and one study did not report whether the trainings were performed in the same or different sessions [42]. With regard to the outcome variables, 15 of the studies measured the maximal dynamic strength with leg press exercise (and two of these also measured squat exercise) [5, 6, 10,11,12,13, 15, 16, 21, 23, 24, 39,40,41, 45], and 14 with squat exercise [3, 4, 8, 14, 22, 24, 25, 35, 36, 38, 40, 42,43,44].
Strength Improvement: Concurrent Training Compared with Resistance Training only
The strength improvement for the different interventions included in this meta-analysis is presented in Table 3.
Table 3 Effect of concurrent resistance and endurance training compared with resistance training only on maximal strength development Primary Analyses: Training Status
The effect of concurrent resistance and endurance training compared with that of resistance training only on the maximal strength for the three categories, untrained, moderately trained, and trained, is shown in Fig. 2. For untrained and moderately trained participants, there were no significant negative effects of concurrent resistance and endurance training as compared with resistance training alone (ES = 0.03, 95% CI – 0.29 to 0.35; p = 0.87 and ES = – 0.20, 95% CI – 0.42 to 0.02; p = 0.08). For trained individuals, there was a small significant negative effect favouring resistance training alone compared with concurrent resistance and endurance training (ES = – 0.35, 95% CI – 0.59 to – 0.11; p < 0.01). There were no indications of heterogeneity in terms of ES as follows: untrained, I2 = 0, p = 0.99; moderately trained, I2 = 0, p = 1.0; and trained, I2 = 1.5, p = 0.43. The relative weight contributions of the included studies were evenly distributed.
Subgroup Analyses: Same Session Compared with Different Sessions
A subgroup analysis was performed to compare the ES between studies that performed resistance and endurance trainings within the same session (< 20 min between sessions; Fig. 3) or during different sessions (> 2 h between sessions; Fig. 4). For untrained and moderately trained individuals, there was no significant difference in effect between conducting same session concurrent resistance and endurance training compared with conducting resistance training alone (ES = 0.01; 95% CI – 0.44 to 0.46; p = 0.98 and ES = – 0.23, 95% CI – 0.54 to 0.08, p = 0.14). However, for trained individuals, the results showed a moderately negative effect favouring resistance training alone compared with conducting resistance and endurance training within the same training session (ES = – 0.66, 95% CI – 1.08 to – 0.25, p < 0.01). Low and nonsignificant heterogeneities among the studies were observed for the untrained, moderately trained, and trained individuals (I2 = 0, p = 0.94; I2 = 0, p = 0.96; and I2 = 17.1, p = 0.31, respectively).
Training during different sessions (> 2 h between sessions; Fig. 4) showed no significant difference in effect between concurrent resistance and endurance training and resistance training alone for any of the training status categories (untrained, ES = 0.12, 95% CI – 0.41 to 0.65, p = 0.65; moderately trained, ES = – 0.16, 95% CI – 0.48 to 0.16, p = 0.32; and trained, ES = – 0.10, 95% CI – 0.43 to 0.23, p = 0.55). There were no indications of heterogeneity in terms of ES as follows: untrained, I2 = 0, p = 0.57; moderately trained, I2 = 0, p = 0.95; and trained, I2 = 0, p = 0.93.
The stratified funnel plots for training status showed no detectable differences among the groups (data not shown). We also performed a sensitivity analysis using Hedges’ g in our main analysis of training category (which may be less sensitive to small sample sizes than the standardised difference in means), but these findings did not materially differ from our main results (data not shown).