Concurrent Training: Effects on Strength and Speed Abilities

Historically, strength-related variables have been the most affected by the so-called interference effect, notably those linked to strength and speed capacities (Schumann et al., 2022), which are highly relevant to sports performance (Suchomel et al., 2016). Consequently, optimizing CT is of critical importance for the development of these neuromuscular qualities in athletic populations. Although a CT program might be expected to negatively impact maximal strength and sprint speed, to date, some studies have reported no CT-related impairments in these neuromechanical measures (Huiberts et al., 2024; Sabag et al., 2018). In this section, we present a detailed discussion of studies examining the influence of CT approaches on strength and speed performance, as well as the specific contexts in which interference-related effects may be more pronounced. These data are summarized in Table 2. These findings are of particular interest for team sports characterized by congested schedules, in which strength and speed qualities must often be developed and maintained under limited recovery conditions (Petre et al., 2021; Schumann et al., 2022).

Different ST methods and CT configurations have been used. For example, Balabinis et al. (2003) compared a CT group performing ET in the morning and ST in the afternoon (7 hours apart) with an ST-only group of amateur basketball players. The ST protocol included four exercises (i.e., bench press [BP], half-squat [HS], leg press [LP], and pull-down [PD]) at intensities ranging from 40% to 95% one repetition maximum (1RM), while ET consisted of interval training from the 85% maximum heart rate (HR_max) to all-out sprints. Both groups improved the strength-power-related variables (1RM in the HS, BP, LP and PD exercises and countermovement jump [CMJ] height) similarly. Robineau et al. (2016) investigated three CT protocols differing in the rest interval between ST and ET (0, 6, and 24 h), along with an ST-only group in amateur rugby players. ET consisted of running intervals at 120% maximal aerobic speed (MAS), and ST involved four exercises at 70–90% 1RM, performed before ET. Those authors reported that when sessions were separated by 6 hours, no differences emerged between CT and ST-only groups in the 1RM BP, the bench row, the squat, or CMJ height. In a subsequent study, two CT protocols—one with ET performed as sprint interval training (SIT) (30-s sprints with 4 min of rest) and the other using short intervals (30-s intervals at 100% MAS with 30 s of recovery)—both with a 24-h separation from ST (four exercises at 70–90% 1RM) were compared with an ST-only control group of amateur rugby players (Robineau et al., 2017). No significant differences in strength-related variables were observed, except for concentric torque at low speeds (60°•s⁻¹ at isokinetic knee extension), where the SIT group showed smaller changes compared with the other groups. In summary, current evidence suggests that the interference phenomenon can be mitigated through careful manipulation of training variables. When ST and ET are executed within the same training session, it is recommended that ST precedes ET to minimize the detrimental effects of accumulated fatigue on neuromuscular performance. Conversely, when it is possible to separate the two modalities by at least six hours, session sequencing appears to have little to no impact on performance outcomes. Thus, the available evidence highlights the importance of strategic session planning to optimize adaptations in CT contexts. In real-world team-sport settings, such systematic planning must also account for accumulated training and match loads, which may further influence acute and chronic fatigue and recovery dynamics (Robineau et al., 2016).

On the other hand, Hennessy and Watson (1994) examined the effects of a CT protocol versus an ST protocol in amateur rugby and Gaelic football players. The ST program comprised three sessions per week, each including distinct lower- and upper-body exercises (e.g., arm curl, back squat, BP, lunge, and PD), whereas ET consisted of two continuous and two interval training sessions at 70–85% HR_max. The CT group (who trained five times per week) exhibited smaller improvements in lower-body strength and jump height compared to the ST group, although upper-body strength gains were similar between the two groups. In addition, 20-m sprint performance improved only in the ST group. Similarly, in the previously mentioned study by Robineau et al. (2016), the CT group that performed both training modalities consecutively without rest (i.e., CT0) demonstrated smaller improvements in HS, BP, and bench row strength than the other CT groups and the ST group. Petre et al. (2023) compared the effects of two distinct CT configurations—one with ET (i.e., HIIT) performed prior to ST and the other in the reverse order (with a 10-min rest interval between modalities)—with a group performing only ST in elite bandy players (i.e., an ice-based team sport played with skates, sticks, and a ball, on a large field). The ST protocol consisted of four sets of squats using a flywheel device, whereas the HIIT protocol involved short cycling intervals at 130% MAS. All groups improved maximum isometric force in the squat exercise equally; however, the group that performed HIIT prior to ST showed a significant reduction in CMJ performance. Additionally, the HIIT + ST group exhibited lower power output during ST sessions, suggesting that prior HIIT may have induced a transient neuromuscular fatigue state, likely exacerbated by residual metabolic stress. Belkadi et al. (2025) compared the effects of two CT structures in elite handball players: one including short shuttle sprints and the other comprising 30-s sprints with directional changes. In both interventions, ST consisted of complex-contrast training, which consisted of four exercises of varying intensities, ranging from heavy-load exercises (up to 85% 1RM) to body mass movements. Although both groups improved strength-related variables (e.g., squat-1RM and the rate of force development), the short-sprint group showed a significant decrease in 5-m sprint performance and in the peak sprint speed achieved during a repeated sprint ability (RSA) test. Thus, when both modalities are executed within the same training session (with less than one hour of rest) or under conditions of high ET volumes (e.g., five training days per week including more than two high-intensity sessions), the likelihood of delayed or compromised adaptations appears to increase, especially for ST-related variables such as 1RM, vertical jump height, and sprinting speed. It is important to emphasize that these effects may also be influenced by the excessive demands imposed during the initial training block (i.e., first training session) and by insufficient recovery time-frames, rather than being solely attributed to a conceptual interference phenomenon (Petre et al., 2023). This distinction is critical in elite team sports, where high training and match density can also intensify recovery-related constraints (Seipp et al. (2023)).

Concurrent Training: Effects on Endurance Performance

Although substantial evidence indicates that ST can improve endurance-related variables, such as running economy and anaerobic capacity (Rønnestad and Mujika, 2014), most of these findings pertain to individual sports, where there is minimal interference from additional factors such as technical-tactical training components (e.g., small-sided games [SSGs]) (Casamichana et al., 2014; Karahan, 2020). In addition, distinct ET variables (e.g., volume, intensity, and training strategy) play an important role in CT adaptations (Sabag et al., 2018). Accordingly, this section details the influence of CT on endurance performance outcomes (additional information is presented in Table 3). Of note, in team-sport environments, endurance-oriented stimuli are frequently imposed through both training sessions and matches, thus reinforcing the importance of integrating CT within real-world performance demands. For example, Balabinis et al. (2003) showed in amateur basketball players that the CT group achieved nearly double the improvement in maximal oxygen uptake (VO_2max) compared with the ET group. The CT group performed ET in the morning and, after a 7-h recovery period, completed the ST session, a scheduling strategy that effectively minimized negative interactions between the two modalities. Likewise, Hennessy and Watson (1994) found no meaningful differences between CT and ET in amateur rugby players, possibly due to the greater training volume accumulated by the CT group compared with the unimodal groups. In line with these findings, Sanchez-Sanchez et al. (2019) demonstrated that combining ET with prior ST—using multiple lower-body exercises on an inertial device—did not compromise training outcomes compared with ET performed alone via HIIT in regional-level basketball and soccer players. The CT group showed greater improvements in change of direction performance and the RSA, whereas the ET group achieved superior gains in a 20-m sprint test. Overall, when comparing endurance adaptations between CT and ET, well-designed ST appears capable of further enhancing ET-induced adaptations.

Belkadi et al. (2025) reported that the long-sprint group achieved greater gains in total time during an RSA test and improved time to exhaustion, indicating that the configuration of ET can induce specific adaptations within the context of CT. In the study by Robineau et al. (2017) in amateur rugby players, two CT protocols were evaluated: one involving short-interval ET at 100% MAS and the other combining SIT (30-s sprints) with ST completed before ET, using four exercises at 70–90% 1RM. The SIT approach proved to be more effective in enhancing peak VO_2max and RSA performance, but appeared to interfere more with strength development, particularly at low movement velocities (60°•s⁻¹ at isokinetic knee extension). Petre et al. (2018) investigated two distinct CT protocols in highly trained ice hockey players. The same ST was performed with 2–5 sets of heavy squats (above 80% 1RM). One group performed ET in the form of HIIT (20-s intervals at 150% MAS), while the other undertook continuous training at 70% VO_2max, both on a cycle ergometer. Improvements were similar across all variables except for VO_2max, which increased exclusively in the HIIT group. Arslan et al. (2025) examined amateur soccer players’ responses to two different CT protocols. Both groups completed the same ST protocol, incorporating heavy-load squat and BP exercises, complemented by CMJs and sprints. One group performed ET in the form of HIIT, consisting of 20-s running intervals covering 85% of the Yo-Yo test final distance, whereas the other group followed the same 20-s/10-s work-rest structure using calisthenic exercises (e.g., burpees and jumping jacks). Both groups exhibited comparable enhancements in VO_2max; however, only the HIIT group improved 20-m sprint performance. Similarly, Botonis et al. (2016) compared the effects of two HIIT-based CT protocols in elite water polo players, both using identical ST protocols, which included BP, LP, PD, shoulder press, and triceps press exercises at 85–90% 1RM. Following the first protocol (HIIT4), athletes swam 4-min bouts at 106% of the velocity corresponding to a 4 mmol•L^-1 blood lactate concentration (V₄), while in the second protocol (HIIT100), they performed 100-m efforts at the same intensity. The HIIT4 protocol elicited greater improvements in swimming velocities associated with various lactate thresholds. In general, the configuration of ET appears to directly and specifically influence the adaptations derived from CT. Therefore, it is recommended that the specific demands of the sport be carefully considered when selecting and structuring ET and ST variables within CT programs (Dolan et al., 2024).

Influence of Recovery Interval Duration between Sessions

Robineau et al. (2016) examined the effects of varying rest intervals between ST and ET (0, 6, and 24 h). Findings indicated that a 6-h recovery period was sufficient for strength-related outcomes to improve similarly to a 24-h interval; however, the greatest improvement in VO_2max occurred in the 24-h separation group. In a related study, Bern et al. (2021) compared elite female rugby players performing CT within the same training session versus those performing ST first (i.e., six exercises at 65–90% 1RM), followed by ET (i.e., SSGs combined with sprint intervals) seven hours later. Both groups improved equally in strength- and endurance-related variables. Current evidence suggests that separating ST and ET into different days or by at least six hours may optimize strength adaptations, whereas a 24-h interval appears more favorable for enhancing VO_2max. From an applied perspective, this is unsurprising, as positive adaptations in physical performance are generally enhanced when distinct training stimuli (e.g., ST and ET) are strategically separated, with longer intervals between sessions (i.e., ≥ 6 h and up to ~24 h) likely providing additional benefits (Robineau et al., 2016). In applied practice, complementary monitoring of external and internal loads (e.g., GPS-derived metrics, session-RPE, or heart rate-based measures) may help practitioners contextualize these recovery-related effects without altering the fundamental structure of CT programs (Casamichana et al., 2014; Seipp et al. (2023)). Nonetheless, given the limited number of available studies and the practical challenges of prescribing ST and ET sessions on separate days or with extended recovery intervals, as well as implementing adequate recovery strategies in team-sport environments (Coffey and Hawley, 2017; Petre et al., 2021; Schumann et al., 2022), definitive conclusions cannot yet be drawn.

Influence of Training Status

Another moderating factor discussed in the literature is athletes’ training status (Coffey and Hawley, 2017; Huiberts et al., 2024), particularly strength levels typically assessed via 1RM testing (Santos-Junior et al., 2021). While Coffey and Hawley (2017) hypothesized that the CT-related impairments might be more pronounced in individuals with greater training experience, other investigations have not supported this assumption, provided that sufficient recovery time is allowed between sessions (Schumann et al., 2022). Hence, the following section presents results categorized according to strength levels.

Highly trained athletes are defined as those with a 1RM in the squat exceeding 150% of body mass or more than 120% in the BP for men (Santos-Junior et al., 2021). Studies comparing the effects of ST alone versus CT (Robineau et al., 2016; 2017) found no negative influences on strength adaptations when at least six hours of rest were provided between sessions. Similarly, research examining the influence of the ET format (Botonis et al., 2016; Petre et al., 2018; Robineau et al., 2017) reported no detrimental effects on strength adaptations, regardless of whether both modalities were completed in the same training session (Petre et al., 2018) or on alternate days (Botonis et al., 2016; Robineau et al., 2017). Enright et al. (2015) noticed that the ET + ST group achieved superior results, although with a longer rest interval (120 min vs. 30–45 min) between sessions. Overall, these findings suggest that highly trained athletes can optimize CT adaptations by manipulating variables such as inter-session recovery duration.

Advanced athletes are classified as those with a 1RM in the squat between 120% and 150% of body mass in men and between 100% and 130% in women (Santos-Junior et al., 2021). Studies comparing different ET formats (Arslan et al., 2025; Belkadi et al., 2025) observed training-specific improvements: groups performing running or sprint drills with shorter rest intervals exhibited greater sprint-related gains, while those using longer rest intervals showed greater VO_2max enhancements. Hennessy and Watson (1994) reported reduced strength gains in the CT group compared with the ST group but similar endurance improvements (i.e., greater VO_2max values) between CT and ET. Mcgawley and Andersson (2013) found no significant differences when altering the order of the training modalities, and Bern et al. (2021) reported equivalent adaptations whether CT was performed within the same training session or separated by a 7-h interval.

Intermediate athletes are defined as those with a 1RM in the squat exercise ranging from 80% to 120% of body mass (Santos-Junior et al., 2021). Balabinis et al. (2003) reported that the CT group, performing ET followed by ST with a 7-h interval between sessions achieved the most balanced improvements across all assessed variables compared with ST and ET executed in isolation. These findings suggest that training status, as assessed via relative strength levels, does not play a major role in influencing adaptations to CT. Instead, the modulation of training outcomes (and, consequently, physical and technical performance) seems to depend primarily on the specific configuration of training variables, such as the rest interval between sessions, which should therefore be carefully considered in both research and applied practice.

Limitations

Considering the characteristics of the specific training sessions in team sports, all studies that analyzed the effects of strength or speed training sessions are, in essence, CT-based programs. Nevertheless, since the aerobic-based ET content is not always controlled or manipulated in terms of volume, intensity, and intermittent phases (Coffey and Hawley, 2017; Dolan et al., 2024; Wilson et al., 2012), it is not possible to precisely determine the superiority of one training program over the other. In addition, the available literature is characterized by a limited number of longitudinal interventions lasting longer than 12 weeks, which restricts inferences about the long-term (i.e., chronic) responses of different CT configurations. Moreover, the control of concurrent technical and tactical training sessions (e.g., small-sided games), which represent a substantial component of the global training load in team sports, is often insufficiently reported or standardized. For this reason, certain specific studies were not considered in this narrative review, which consequently limits the findings reported here.