Concurrent training (CT) has become a contemporary topic for coaches, strength and conditioning professionals, and researchers because a large number of sports require both strength and aerobic capacities for maximizing performance (30,41,51). However, resistance and endurance trainings produce divergent metabolic and morphological adaptations with little overlap between them (12,41). Therefore, it seems necessary to find optimal combinations of both types of training regimes to obtain maximum simultaneous development of strength and endurance capacities.
Studies analyzing the neuromuscular adaptations and performance improvements associated with CT have reported inconsistent results. Although CT does not alter the ability to positively adapt to endurance training (6,51), most studies have indicated that CT regimens appear to inhibit strength, hypertrophy, and power development compared with resistance training alone (15,18,27). Nevertheless, some experiments have reported little or no negative effect on strength gains with the addition of aerobic training (1,35,48).
In addition to large influence of the interindividual variation in response to a training program (25,32), the effects of CT on strength gains may vary markedly due to a large number of design factors, including the mode, frequency, duration, type of exercises, volume, and intensity used during both resistance and aerobic training, different sequences and recovery times between resistance and aerobic training sessions, training history of participants, and dependent variables selected (13,29,51). The effect of most of these variables has already received considerable attention in previous studies and reviews (41,51). However, to the best of our knowledge, a question that remains ignored in the literature is the possibility of manipulating the load magnitude during resistance training. In addition, most of resistance exercises used in studies analyzing the effect of CT on physical performance (3,7,15,18,23,25,27) were open-chain, isolated, isotonic, or machine-based exercises (i.e., leg extension and flexion, seated hamstring curl, leg curl, leg press, isometric plantar flexion, calf rise). It appears that resistance training programs which preferably include open-chain exercises may not provide adequate movement pattern specificity for optimal performance improvements in closed-chain sporting movements such as running (2). Therefore, it has been indicated that future investigations should include traditional multijoint resistance exercises because that are believed to be superior for eliciting optimal neuromuscular adaptations and increasing the force capabilities of the leg musculature (2). Because (a) the training load seems to be the most important variable to consider when designing a resistance training program (11) and (b) the exercises selected in a resistance training program can influence the magnitude of neuromuscular adaptations (2), gains in strength and endurance variables during CT may be directly influenced by the load magnitude and exercise used during resistance training. Thus, the first aim of this study was to analyze the effect of 3 CT programs that only differed in the load magnitude used during the full squat (FS) training on performance in vertical jumping, sprint, leg strength, and endurance capacity.
In addition, interruptions in training sessions due to several factors are normal in any sport (26,38,40,42). For this reason, knowing the effects of a detraining (DT) period could be important for designing better training strategies. The DT adaptations after strength or endurance training alone have been widely studied in different populations (8,9,26,31,36). Unfortunately, the effect of training cessation after CT has received less scientific attention (4,45,46,49). Moreover, although abrupt cessation of intense physical training is associated with a decline of physical performance (38–40), DT-induced changes in performance after CT are linked with multiple factors (16,31,36,46,49,50) among which is included the relative intensity used during previous resistance program. Therefore, the second aim of this study was to analyze the effects of 4-week DT after CT programs differing in load magnitude used during resistance training on different strength and aerobic parameters.
Experimental Approach to the Problem
An experimental research design was used to compare the effects of 3 concurrent resistance and aerobic training programs only differing in load magnitude used during resistance training (40–55% 1 repetition maximum [1RM] vs. 55–70% 1RM vs. 70–85% 1RM) on physical performance and the subsequent DT adaptations. To address this, 36 physically active men were randomly assigned to the control group (CG) or resistance training group with low loads (LLG), moderate loads (MLG), or high loads (HLG). The players assigned to experimental groups performed resistance training combined with endurance, whereas players assigned to CG merely undertook daily life activities. All the experimental groups trained twice a week for 8 weeks using a CT regimen. All subjects were evaluated using a battery of tests performed in 2 sessions separated by a 48-hour rest interval. During the first testing session, the participants performed the 20-m running sprints and the 20-m shuttle run test. During the second testing session, subjects executed the countermovement vertical jump (CMJ) test and an isoinertial loading test in FS exercise. During the 2 week preceding this study, 4 preliminary familiarization sessions were undertaken to ensure a proper execution technique in both FS and CMJ exercises. To evaluate the DT effects, the strength and aerobic parameters were tested after 4 weeks of training cessation. Throughout this period, the participants were asked refrain from participating in regular exercise programs aimed at developing or maintaining strength and aerobic capacity.
Thirty-six physically active men volunteered to participate in this study. After an initial evaluation, the participants were matched according to their estimated 1RM (1RMest) in FS exercises and then randomly assigned to 4 groups depending on the loading magnitude used during resistance training, as follows: (a) a LLG (40–55% 1RM), a MLG (55–70% 1RM), a HLG (70–85% 1RM), and a CG. Because of injury or illness, 4 participant (1 from the HLG and 3 from the CG) were absent from the post-testing sessions. Thus, of the 36 initially enrolled participants, only 32 successfully completed the entire study. Player characteristics are displayed in Table 1. Participants in the CG were asked not to perform any type of resistance or aerobic training during the experimental period. All the participants provided written informed consent to the experimental procedures after the possible benefits and risks of participation were explained to them. The investigation was conducted in accordance with the Declaration of Helsinki and was approved by the University of Beira Interior Research Ethics Committee.
Neuromuscular performance was assessed before (pre), after the 8-week training period (post 1), and after the 4-week DT period (post 2) using a battery of tests performed in 2 sessions separated by a 48-hour rest interval. Testing sessions were performed at the same time of day for each participant under the same environmental conditions (∼20° C and ∼60% humidity). Body mass and height (Seca Instruments, Ltd., Hamburg, Germany) were measured before the warm-up protocol in the first testing session. Strong verbal encouragement was provided during all tests to motivate participants to give a maximal effort.
Each participant performed three 20-m sprints separated by a 3-minute rest. Photocell timing gates (Brower Timing System, Fairlee, VT, USA) were placed at 0, 10, and 20 m so that the times needed to cover 0–10 m (T10) and 0–20 m (T20) could be determined. A standing start with the lead-off foot placed 1 m behind the first timing gate was used. The average of the best 2 sprints was used for the analysis. Warm-up consisted of 5 minutes of running at a self-selected intensity, 5 minutes of joint mobilization exercises, followed by several sets of progressively faster 30-m running accelerations. Reliability for T20 as measured by the coefficient of variation (CV) was 3.8%, whereas the intraclass correlation coefficient (ICC) was 0.94.
Shuttle Run Test
The 20-m multistage shuttle run test was administered according to the original version described by Léger et al. (28). The initial running velocity was set at 8.5 km·h−1 and was gradually increased in 0.5 km·h−1 each minute (14). The test was terminated when a participant failed to reach the appropriate marker in the allotted time twice or could no longer maintain the pace. The number of laps completed was recorded. Estimated maximum oxygen consumption (V[Combining Dot Above]O2max, ml·kg−1·min−1) was calculated based on the maximal speed (MAS) reached before participants were unable to keep up with the audio recording, as follows: −27.4 + 6 MAS (28).
Vertical Jump Test
The jump height was determined using a contact mat connected to an electronic power timer, control box, and handset (Globus Ergojump, Italy). Each participant performed 3 maximal CMJs with their hands on their hips, separated by 1-minute rests. The highest value was recorded for the subsequent analysis. The ICC was 0.96, and the CV was 3.2%.
Isoinertial Squat Loading Test
A Smith machine (Multipower Fitness Line; Peroga, Murcia, Spain) was used for this test. A detailed description of the testing procedures used in this study was recently reported elsewhere (14). The initial load was set at 17 kg and progressively increased in 10 kg increments until the attained mean propulsive velocity (MPV) was ∼1.00 m·s−1 (range 0.95–1.05 m·s−1) (14). The participants performed 3 repetitions with each load, with 3-min recovery. A linear velocity transducer (T-Force System; Ergotech, Murcia, Spain) was used to register bar velocity. The 1RMest was calculated based on the MPV attained against the heaviest load lifted, as follows: (100 load)/(−5.961 MPV2) − (50.71 MPV) + 117 (44).
The descriptive characteristics of the training programs completed by each group are presented in Table 2. The resistance training session comprised FS, vertical jump, and sprint exercises. Approximately 2- to 3-min rest periods were allowed between each set and exercise. The participants were instructed to perform all exercises at maximal intended velocity to obtain the highest possible gains (43). The loads used by each participant in the FS exercise were assigned according to 1RMest obtained in the initial isoinertial squat loading test. Thus, the relative intensity of the FS exercise progressively increased from 40 to 55% 1RM, 55–70% 1RM, and 70–85% 1RM for LLG, MLG, and HLG, respectively. Because strength was expected to increase with training, an intermediate isoinertial squat loading test was performed after 4 weeks of training to perform the necessary load adjustments for each training group. Aerobic training was performed 20 minutes after the participants completed the resistance training. All the experimental groups completed the same aerobic training regimen, which consisted of 15–20 minutes performing the 20-m shuttle run exercise at 75% of the maximal individual speed reached during the 20-m multistage shuttle run test. As for strength training, participants were assessed in the 20-m shuttle run test after 4 weeks of training to perform the necessary adjustments for each training group. At least 2 trained researchers supervised each workout session and recorded the compliance and individual workout data during each training session. All participants were instructed to maintain their normal daily activities throughout the study. The participants did not undertake any additional strength or aerobic training activities during the testing, training, and DT periods.
The values of each variable are presented as mean ± SD. Homogeneity of variance across groups (LLG vs. MLG vs. HLG vs. CG) was verified using the Levene test, whereas the normality of distribution of the data was examined with the Shapiro–Wilk test. Four (group: LLG, MLG, HLG, and CG) × 3 (time: pre, post 1, and post 2) repeated measures analysis of variances (ANOVAs) were calculated for each variable. Sphericity was checked using Mauchly's test. Percentage of change for each variable was calculated ([post – pre/pre] × 100) and a 1-way ANOVA was conducted to examine between-group differences with Tukey post hoc comparisons (LLG vs. MLG vs. HLG vs. CG) to clarify the interaction. In addition to this null hypothesis testing, the data were assessed for clinical significance using an approach based on the magnitudes of change (20). The effect sizes (ES) were calculated using Cohen's d (9) to estimate the magnitude of the training effect on the selected neuromuscular variables within each group. The threshold values for assessing the magnitudes of the standardized effects were 0.20, 0.60, 1.20, and 2.00 for small, moderate, large, and very large magnitudes, respectively. Probabilities were also calculated to establish whether the true (unknown) differences were lower than, similar to, or higher than the smallest worthwhile difference or change (0.2 multiplied by the between-subject SD) (20). The quantitative chances of obtaining higher or lower differences were evaluated as follows: 1%, almost certainly not; 1–5%, very unlikely; 5–25%, unlikely; 25–75%, possible; 75–95%, likely; 95–99%, very likely; and 99%, almost certain. If the chances of having higher or lower values than the smallest worthwhile difference were both >5%, the true difference was assessed as unclear. Inferential statistics based on the interpretation of the magnitude of effects were calculated using a purpose-built spreadsheet for the analysis of controlled trials (19). The statistical analyses were performed using SPSS software version 18.0 (SPSS, Inc., Chicago, IL, USA). Statistical significance was established at the p ≤ 0.05 level.
Data for all variables analyzed were homogeneous and normally distributed (p > 0.05). There were no significant differences between groups at baseline for any analyzed variable. The mean values, percentage of change, and intragroup ES for all variables analyzed during pre, post 1, and post 2 are reported in Table 3 (LLG), Table 4 (MLG), and Table 5 (HLG), respectively.
All the experimental groups showed improvements (p ≤ 0.05–0.001) in all the variables assessed except the LLG in T10 and the HLG in T20 (Tables 3, 4 and 5). No changes took place in the CG. The magnitude of change for LLG was from small (T10, T20, 1RMest, and V[Combining Dot Above]O2max) to moderate (CMJ). For MLG, the standardized effects were small (T10, T20 and 1RMest) and moderate (CMJ and V[Combining Dot Above]O2max), whereas for HLG, the qualitative outcome relative to ES was small (T20 and 1RMest), moderate (T10 and V[Combining Dot Above]O2max), or large (CMJ), depending to the assessed variable.
After the training period, significant “time × group” interactions were observed for T10 (p < 0.001), CMJ (p < 0.01), 1RMest (p < 0.01), and V[Combining Dot Above]O2max (p < 0.001), whereas there was no “time × group” interaction in T20 (p = 0.349). The 1-way ANOVA indicated that all the experimental groups showed significantly greater percent changes from Pre to post 1 for 1RMest (p ≤ 0.05–0.01) and V[Combining Dot Above]O2max (p ≤ 0.05–0.05) compared with CG, whereas the HLG and MLG also showed greater percentage of change than CG in T10 (p < 0.001) and CMJ (p ≤ 0.05), respectively (Table 6; Figure 1).
The 4-week DT period produced an important detriment effect on all the variables analyzed for all the experimental groups. Most of these variables returned to initial values or lower after the rest period (Tables 3, 4 and 5). In fact, no differences were found between pre and post 2 in any studied variable for any experimental group. In addition, no significant differences were found between the 3 trained groups and the CG at post 2 for any variable.
To the best of our knowledge, this was the first study focused on analyzing the training and DT effects after CT programs differing in the relative intensity (%1RM) used during resistance training regime on strength and aerobic performance in physical active men. The main finding of this study was that the all 3 experimental groups showed significant and practical improvements in different performance variables including jump, running sprint, maximal strength, and V[Combining Dot Above]O2max. Thus, it appears that resistance training programs consisting in FS exercise with low (40–55% 1RM), moderate (55–70% 1RM), or high (70–85% 1RM) loads combined with the same low-intensity aerobic training (75% V[Combining Dot Above]O2max) could be equally effective for producing significant gains in strength and endurance capacities. In addition, the DT period resulted in significant performance decrements in all variables assessed for all experimental groups. These results could be of great interest for coaches and strength and conditioning professionals to optimize training programs in those sports modalities to require combinations of both components of strength and endurance training for maximize performance.
All 3 experimental groups showed significant (p ≤ 0.05–0.01) improvements in 1RMest after training period. However, changes reported in LLG (13.9%; ES: 0.57), MLG (9.9%; ES: 0.40), and HLG (11.4%; ES: 0.47) were lower than those reported in previous studies (∼20%) (15,23,35) and meta-analyses (ES: 1.30) (51) that assessed the effects of CT on strength development in untrained male participants. Although have been described that continuous aerobic training would be predicted to have minimal interference on strength gains using either high-load or moderate-load resistance training protocols (6,13), it is possible that the short rest period between resistance and aerobic training in this study (∼20 minutes) may have induced a greater degree of interference than previous studies (51). Thus, our results confirmed the need to separate resistance and aerobic training sessions to optimize strength gains (13). In addition, the use of only 1 resistance exercise (FS) has also been able to influence in the lower gains found in this study compared with other studies (7,15,23,35) in which several resistance exercises were used (e.g., knee flexion and extension, leg curl, leg press, calf raise). Comparison between experimental groups showed no significant differences in strength gains between LLG, MLG, and HLG. However, the analysis of practical inferences resulted in a possible better effect on 1RMest for LLG compared with MLG and HLG. These results are in agreement with previous studies indicating that resistance training programs that include training with moderate to high loads and repetitions at or near the point of muscle failure lead to lower strength gains compared with the use of a moderate number of repetitions for not training to repetition failure (6,13,22,37).
Only few studies (5,21,34) have analyzed the effect of CT on jump performance, while, to the best of our knowledge, no studies have examined the influence of CT on running sprints in adult individuals. In this study, all 3 combinations resulted in low-moderate improvements in CMJ (11.6–13.9%; ES: 0.61–1.27) and sprint times (1.0–3.5%; ES: 0.20–0.63). These improvements in CMJ were greater than previous studies conducted with untrained individuals (9.0–3.3%). Thus, although aerobic training (continuous and high-intensity run training) has been reported to cause deterioration in the capacity of the neuromuscular system to rapidly generate force (17), it appears that adding explosive exercises (jumps and accelerations) along with the FS exercise executing each repetition at maximal intended velocity could attenuate the interference on adaptations to short- and high-intensity efforts.
Regarding the load magnitude used during resistance training, this study showed no significant differences for pre-post changes in jump and sprint variables between experimental groups. However, there was a slight trend toward greater intragroup ES for HLG compared with LLG and MLG in T10 and CMJ. In addition, HLG showed a likely greater effect than LLG and MLG in T10, whereas practically worthwhile difference was possibly more beneficial in favor of MLG compared with LLG. For the rest of comparisons, the differences between LLG, MLG, and HLG were unclear. These results appear to be in contrast with a recent meta-analysis (47), which indicated that high-load resistance training alone resulted in lesser sprint ES (ES = 0.52) compared with lower loads (ES = 0.97). However, our results seem to indicate that, when resistance training is combined with continuous aerobic training, using moderate to high loads is more effective for improving jump and sprint performance than those with low loads.
The training period resulted in similar improvements in V[Combining Dot Above]O2max for all 3 experimental groups. These changes (∼12–15%) were comparable to those reported in previous studies (∼7–18%) performing CT or aerobic regimens alone (3,15,18,21,34). Therefore, although this study did not include a group that underwent aerobic training alone, our results appear to be in line with previous reports, suggesting that CT does not affect the development of V[Combining Dot Above]O2max in untrained or resistance-trained individuals. In addition, as a remarkable contribution of this study, our results suggest that load magnitude used during resistance training do not effect on changes in aerobic performance, as no significant differences were found in V[Combining Dot Above]O2max gains between LLG, MLG, and HLG.
The DT period resulted in a marked and similar reduction in physical performance for all 3 experimental groups, with a partial (CMJ and 1RMest) or complete (T10, T20 and V[Combining Dot Above]O2max) reversals of the adaptations obtained during an 8-week training period. This is in accordance with previous studies that have shown important V[Combining Dot Above]O2max declines (4–14%) with short-term training cessation in trained and untrained individuals (38). However, studies conducted with elementary school students using CT have shown both significant loss (45) and no changes (46) in this variable. In relation to sprint performance, several studies using a CT training period (36,45,46) have shown that the sprint time in 10, 20, and 30 m remained unchanged or only decreases slightly during the DT period. Discrepancies with our results could be due to differences in the age of the participants and the training program configurations (36,45,46).
According to several studies and review analyzing the effect of DT period after CT training or resistance training alone (26,38), the loss of maximal strength (4–7%) and CMJ height (5–9%) in this study were lower compared with aerobic performance. Because CMJ performance depends largely on the maximal strength of the leg extensors (10,52), it is possible that the lower reduction in CMJ performance was associated with the maintenance of 1RMest. In accordance with our results, other studies have shown no significant changes in CMJ performance after 4–6 weeks of cessation of resistance training (24,26,33). However, it appears that when resistance training is combined with aerobic training, both maximal strength and CMJ height trend to descend to a greater extent after DT period (45,46).
This study has some limitations need to be addressed. Obviously, one of the main limitations of this study is the low number of subjects in each group. Thus, some effects are associated with large confidence limits for the intragroup and between-group change differences. Therefore, we can not be sure whether differences within and between groups would have been clearer with a greater number of subjects in each experimental group. In addition, this study evaluated the efficacy of aerobic training and a specific resistance training regimen consisting in FS exercise alone. It is possible that the use of only 1 resistance exercise may have been a limitation for strength gains during CT. In addition, this type of resistance training has also been able to influence the degree of loss of physical performance during the DT period. However, because the main aim of the this was to analyze the training and DT effects of combined resistance training programs against 3 different external loads with the same aerobic training on strength and aerobic variables, we consider it appropriate not to include additional resistance exercises to avoid increasing the number of confounding factors such as number of exercises, rest time between exercises, type of exercises (e.g., multijoint vs. isolated, closed- vs. open-chain, isoinertial vs. isotonic), or fatigue accumulated. However, a comparison of the relative efficacy of different resistance training regimens combined with different aerobic training seems to be an interesting topic for future research. Finally, we should acknowledge that different participants, for instance, experienced ones could lead to other results and further investigation should also be developed in this regard.
In brief, the results of this study indicated that 8 weeks of resistance training programs with different loads combined with low-intensity aerobic training improved strength and aerobic capacities, regardless of training intensity used during resistance training. Despite the similar improvements, resistance trainings with loads higher than 55% of 1RMest are suggested to increase changes in explosive efforts, such as short runs (T10 m) and CMJ. In addition, 4 weeks of DT compromised previous gains, mainly in V[Combining Dot Above]O2max and sprint time variables.
The results seem to suggest that performing strength training with low-, moderate-, or high-external loads combined with low-intensity aerobic training regimen is beneficial for strength and aerobic development in healthy adult men. Furthermore, choosing higher loads during strength training can lead to increased gains in explosive efforts. Despite our data highlight that 8 weeks of training are sufficient to verify enhancements, it takes only 4 weeks without training to return to the initial values. This should be considered when designing CT in sports clubs to improve its efficiency. Thereupon, this experiment provides a new path to integrate both strength and aerobic regimens in the same session.
This project was supported by the National Funds through FCT—Portuguese Foundation for Science and Technology (UID/DTP/04045/2013)—and the European Fund for Regional Development (FEDER) allocated by European Union through the COMPETE 2020 Programme (POCI-01-0145-FEDER-006969)—competitiveness and internationalization (POCI). The authors disclose funding received for this work from any of the following organizations: National Institutes of Health (NIH); Welcome Trust; Howard Hughes Medical Institute (HHMI); and other(s).
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