Perspectives on Concurrent Strength and Endurance Training in Healthy Adult Females: A Systematic Review

A systematic electronic search for articles was performed in July 2021 using PubMed and Medline. Search terms were defined a priori and included: [“combined” OR “concurrent] AND [“strength” OR “resistance”] AND [“aerobic” OR “endurance”] AND [“training” OR “exercise”] AND [“women” OR “females”]. The lists of relevant articles and reviews obtained through the search were examined individually to identify any further studies that were subsequently added manually (see the Electronic Supplementary Material search strategy in Appendix 1). A follow-up systematic electronic search was performed in PubMed in December of 2022 to identify additional articles published later in 2021 and 2022. This review was not registered, and the review protocol was not published.

Two reviewers (RSM and JKI) independently used a two-phase screening strategy to identify relevant articles. First, the title and abstract were assessed against the predetermined inclusion and exclusion criteria described above. Studies that did not meet predetermined inclusion criteria or that met at least one of the exclusion criteria were excluded. Next, full-text articles were read and assessed against the predetermined inclusion and exclusion criteria. Conflicts were resolved by consensus with the whole group. No automation tools were used in this process. The flow chart for the literature search and selection of studies is presented in Fig. 1.

Inclusion criteria were as follows: (1) fully published peer-reviewed publications; (2) studies published in English; (3) participants were healthy normal weight or overweight (not obese, as classified in the publication and verified < 30 kg/m²) females of reproductive age (mean age between > 18 and < 50) or presented as a group (n > 5) in studies including both females and males and where female results were reported separately; (4) participants were randomly assigned to intervention groups, when warranted, and the study included measures of maximal strength and endurance performance; where only one group was included or training groups were defined by, e.g., hormonal contraceptive use, randomization was not required; (5) the duration of the intervention was a minimum of 8 weeks to ensure a meaningful training duration likely to induce training adaptations.

Strength training was defined as weighted exercises (free weights or machines) including maximal, hypertrophic, explosive (power), and/or muscle endurance type training. Strength training classification (e.g., hypertrophic, maximal, explosive, etc.) is based on the language used by authors in their peer-reviewed publications, whereas specific loading and repetition ranges are reported in the tables later in the text. It should be noted that strength training adaptations in untrained individuals are generally both neural (muscle activation) and muscular (hypertrophy) with both submaximal and maximal loads but that trained individuals require more specific training loads to achieve desired adaptations (see, e.g., [69, 70] for more information). Endurance training was defined as continuous running, cycling, cross-country skiing, and rowing including both steady-state and interval training (excluding marching, walking, dancing, water-based exercise, and step aerobics). Study-specific endurance training volume and intensity are reported in the tables later in the text. Studies without clearly defined/described endurance training (frequency/volume, mode); those including combinations of endurance training modes with marked differences in force-production strategies, such as running and cycling; those evaluating concurrent training for rehabilitation or in populations with diseases; those including nutritional supplements or tactical military training; and/or those examining only acute responses to concurrent strength and endurance exercise/loading were excluded.

Data extraction was completed by one reviewer (RSM) and was verified by one reviewer (JKI). Conflicts were resolved by consensus with the whole group. Studies were divided into groups by endurance training mode (running, cycling, or “other”) for further analysis, as endurance training mode appears to influence adaptations to strength training [10, 12]. The following information was compiled into tables that were subsequently edited and included in the “Results” section below: study hypothesis; participant information, including training status, sample size, and age; training program overview/example; main strength and endurance outcome(s); and study conclusion.

Although one function of the a priori inclusion criteria was to aid in the selection of higher quality studies, the quality and bias of included studies were further assessed using a modified version of the Downs and Black checklist [71], which was specifically modified for this review, similar to [72] (see the Electronic Supplementary Material Downs and Black modified checklist in Appendix 2). The quality assessment was completed by one reviewer (RSM) and verified independently by one reviewer (JKI). Conflicts were resolved by consensus with the whole group. This checklist is made up of 15 outcomes from five domains: (1) reporting, (2) external validity, (3) internal validity—bias, (4) internal validity—confounding (selection bias), and (5) power. The maximum attainable score was 16, and study quality was categorized as follows: “high” (14–16), “moderate” (10–13), “low” (6–9), or “very low” (0–5). The results of the Downs and Black assessment were used to assign a quality rating to each study.

Seven of the included studies examined concurrent strength and endurance running (Table 1). In this subset of studies, the duration of training interventions ranged from 8 to 16 weeks and included 2–3 strength training sessions per week. Participant training background ranged from recreationally active to competitive collegiate level athletes (tier 1 to tier 3 according to classification by McKay et al. [73]). Endurance training intensity ranged from lower intensity distance (higher volume/duration) training to higher intensity interval training as well as combinations of these training intensities. Strength training included primarily hypertrophic and heavy/maximal intensities (as described by study authors), including combinations of, e.g., maximal and explosive strength training. The quality of the studies was rated as moderate (10–13). In two studies, concurrent strength and endurance training were compared to strength training only, endurance training only, and a control group. In untrained females, ~ 8 weeks of concurrent strength and endurance running was more effective at increasing both upper and lower body strength than endurance running or no formal training (control group) and equally as effective as strength training alone. In addition, concurrent training was found to be more effective at increasing endurance capacity (measured as peak/maximal oxygen consumption [VO_2peak/VO_2max]) than strength training or no formal training (control group) and equally as effective as endurance training alone [74, 75].

In two studies, strength training was added to endurance running. In competitive runners, improvements in running economy were attributed to strength training, while maximal aerobic capacity (VO_2max) remained unchanged after 10 weeks of concurrent strength and endurance running versus running alone [76]. In recreational runners, no significant differences between concurrent strength and endurance running and endurance running only groups were reported in VO_2max, running economy, or body composition adaptations [77].

One study compared strength training versus body weight (muscle endurance) training, and a combination of maximal and explosive strength training performed concurrently with endurance running in recreational runners appeared to be superior to endurance running performed concurrently with body weight (muscle endurance) training for increasing maximal strength; however, improvements in peak running speed were similar between groups [78]. Another study compared adaptations between recreationally trained naturally menstruating females and recreationally trained females using hormonal contraceptives. This study indicated that 10 weeks of high-intensity, different-day concurrent strength and endurance running improved maximal strength of the leg extensors and countermovement jump height, whereas no differences between groups were observed in 3000-m running time improvements [43].

Four of the above studies [43, 74, 78, 79] included assessment of fast-force production using either countermovement jump, squat jump, or 5-jump (straight-leg) plyometric jump test peak force. Mixed maximal and explosive strength training performed concurrently with endurance running [43, 78] improved countermovement jump, while non-linear periodized strength training (with loads ranging from 3 repetitions maximum [RM] to 12 RM) performed concurrently with aerobic training increased squat jump [74]. Heavy resistance training appeared to be more effective at improving force production in a 5-jump plyometric test than volume-load–matched plyometric training, which could be relevant in terms of force production for running performance [79].

Four of the included studies examined concurrent strength and endurance training in which the endurance training mode was cycling (Table 2). In this subset of studies, the duration of the training intervention ranged from 9 to 24 weeks and included 1–2 strength training sessions per week. Participant training background ranged from untrained/sedentary to recreationally active (tier 0 to tier 1 according to classification by McKay et al. [73]). Endurance training intensity ranged from low-intensity distance training to high-intensity interval training including combinations and progressions of these training intensities. Strength training intensity ranged from muscle endurance to hypertrophic and heavy/maximal strength training in addition to combinations and progressions of these strength training modes. The quality of the studies was rated as moderate (10–11).

One study examined low-volume strength training performed concurrently with endurance cycling within a single group. Untrained participants improved isometric strength of the leg and arm extensors and maximal aerobic capacity as well as blood lipid profile [80]. Three studies examined longer-term periodized concurrent strength and endurance cycling interventions where training order (endurance before strength, strength before endurance, or alternate day training) was a central theme. All three training orders induce favorable changes in maximal strength of the lower extremities, endurance capacity, and body composition [81‐83]. Performing strength and endurance training on different days was, however, suggested to have additional advantages in terms of improving endurance performance and decreasing fat mass due to a potential for increased daily physical activity that may accrue via warm-ups and cool-downs as well as commuting to the gym (possibly walking or biking). Interestingly, these studies suggest that endurance (cycling) training before strength training may be more effective for improving submaximal endurance performance in females than in males although marked differences in maximal strength development of the lower extremities were not observed. [78]

Only one study examined fast-force production [80], reporting no change in the rate of force development analyzed from maximal isometric leg press.

Three of the included studies examined concurrent strength and endurance training in which the endurance training mode was either rowing or cross-country skiing (Table 3). In this subset of studies, intervention length ranged from 9 to 16 weeks and included 2–3 strength training sessions per week. Participant training background ranged from recreationally active to trained cross-country skiers (tier 1 to tier 3 according to classification by McKay et al. [73]). Endurance training in the studies where the endurance training mode was rowing was completed at a heart rate equivalent to the ventilation threshold 3–4 times per week [39, 84], whereas the cross-country skiing was performed primarily at lower intensities but also included higher intensity interval training [85]. Strength training in these studies included heavy/maximal strength training. The quality of the studies was rated as low to moderate (9–10).

Two studies combined rowing with whole body strength training with a focus on measuring the lower extremities [39] (and left ventricular morphology [84]). One study examined concurrent strength and endurance (ski ergometer) training in the upper body, where both testing and training were targeted at the upper body [85]. Understandably, the study participants were trained cross-country skiers that were accustomed to using their upper bodies as part of their sport. The investigation revealed improved upper body strength and significantly greater time to exhaustion. Overall, these three studies indicated that concurrent rowing or cross-country skiing and strength training appeared to increase 1-RM strength (bilateral leg press or double pole) and endurance performance/capacity, whereas only Bell et al. [39] suggested endurance training may inhibit strength development (measured as bilateral incline leg press) in previously trained females. Regrettably, fast-force production was not assessed in any of the included studies.

Only five of the 14 included studies incorporated jumping tests (e.g., squat jump, peak power, countermovement jump, and straight-leg 5-jump) to evaluate fast-force production. Overall, these studies indicate that strength training combined with endurance training improves measures of fast-force production. Of note is that all of the included studies indicating increases in fast-force production trained strength concurrently with running rather than cycling. Squat jump peak power increased by ~ 23% and ~ 29% in concurrent strength and running and strength only training, respectively, while it only increased by ~ 15% and ~ 17% in endurance training only and control groups [74]. An ~ 11% improvement in countermovement jump height was observed in a group performing a mixture of maximal and explosive strength training combined with running, where the relative increase in performance of the group that completed mixed maximal and explosive strength training combined with endurance running was greater than the control group that combined endurance running with body weight circuit training [78]. In contrast, improvements in peak force from a straight-leg 5-jump plyometric jump test (Nkg⁻¹) were larger in a group combining running with heavy resistance training than a group performing volume-matched plyometric and heavy resistance training (change score of 7.5 ± 14.8 vs. 1.1 ± 14.3) [79].

Two studies examined fast-force production from a different perspective. Myllyaho et al. [43] reported a greater increase in countermovement jump height in naturally menstruating females (11%) compared with hormonal contraceptive using females (4%), although the difference between groups was not statistically significant. Kyröläinen et al. [80] reported that isometric rate of force development was unchanged in a single group performing concurrent strength and endurance training, but the lack of a single-mode control group makes it impossible to determine whether this was a result of training strength and endurance concurrently or simply a function of the prescribed training not improving rate of force development.

A limited subset of studies reported, or took into consideration, menstrual status or hormonal contraceptive use in terms of timing of testing/measurements or during recruitment and subsequent data analyses. Nindl et al. [75] and Hendrickson et al. [74] reported that their investigation included “regularly menstruating” females, and the authors indicated that blood draws were always at the same (unreported) time (phase) of the menstrual cycle for analysis of serum hormones. Kyröläinen et al. [80] reported that none of the participants were using hormonal contraceptives, and Eklund et al. [83] reported that hormonal contraceptive users and non-users were included in the same groups. Kyröläinen et al. [80] and Eklund et al. [83] did not time testing according to menstrual cycle phase. Of the included studies, only Myllyaho et al. [43] considered both menstrual cycle status and hormonal contraceptive use during recruitment and subsequently compared a group of females using monophasic oral contraceptives and self-reported eumenorrheic/naturally menstruating females. Reported weaknesses in this study were the inclusion of several formulations of monophasic combined oral contraceptives in addition to a lack of hormonal verification throughout the study (including ovulation). Nevertheless, levels of progesterone and estrogen were analyzed from blood serum in an effort to confirm that performance testing was completed in the early follicular phase (days 1–5 of the menstrual cycle) [43].

The methodological quality of the selected studies is presented in Fig. 2. Overall study quality was low to moderate (9–12), with an average rating of 10.7, which is at the lower end of the “moderate” study quality classification. Several studies lacked power calculations and/or did not report compliance with the intervention or whether or not participants were randomized into intervention groups, and the majority of studies did not consider hormonal status. The majority of studies lacked external validity and had problems regarding internal validity in terms of selection bias.

Included studies reported training interventions ranging widely from 7 to 24 weeks in populations that ranged from untrained to trained according to the classification by McKay et al. [73]. Included studies generally incorporated linear training progression in terms of intensity and volume, but did not necessarily utilize a specific periodization as is common, for example, in athletes [88], where dividing training into consecutive phases with specific objectives based on training period/season is common [70]. While the studied combinations of strength and endurance training appeared to consistently result in improvements in maximal strength and other characteristics that contribute/or may contribute to endurance capacity/performance, further combinations and approaches to programming may need to be explored. Indeed, the combinations of strength and endurance training used in included studies may not have employed a training load or frequency high enough and/or training interventions that were long enough to induce significant changes in maximal strength, fast-force production, endurance performance/capacity, or an “interference effect.”

The endurance training modes utilized in the included studies were overwhelmingly running or cycling, although several other endurance training modes may be used by physically active and athletic females. Endurance training modes such as running and cycling employ different force-production strategies [89‐91] that should be taken into consideration when evaluating training adaptations. Running requires repetitive and relatively fast force production using the stretch–shortening cycle [92], where even maximal uphill running does not elicit maximal muscle activation of the lower extremities [93]. Cycling requires prolonged repeated force production [91]. In addition, the quadriceps femoris and gluteal muscles play a more significant role in cycling than in running, where the biceps femoris is more involved [94]. Rowing and cross-country skiing are important endurance training modes that employ the upper body, which is often overlooked in research on concurrent strength and endurance training and females, in general. Included studies primarily focused on training the lower extremities, where only three studies included upper body strength training and testing (as a part of whole body training or upper body training alone (Hoff et al. [85] in cross-country skiers as well as Haykowsky et al. [84] and Bell et al. [39] in rowers). Females generally have lower upper body strength than males [95, 96], and more research regarding concurrent strength and endurance training in the upper body is warranted in all populations that require efficient use of the upper body for performance. Endurance performance was primarily assessed using sport-specific VO_2max tests, although field tests to assess endurance performance were also utilized. It is important to remember that laboratory tests may not be fully representative of sport-specific or functional performance requirements; thus, caution should be used when interpreting results.

The studies in this review included “hypertrophic” or maximal and/or explosive/plyometric training (or a progression from “hypertrophic” towards maximum and explosive/plyometric training) ~ 2–3 times per week combined with running, cycling, rowing, or cross-country skiing up to 4 times per week. The strength training loads employed varied considerably, ranging from 2 to > 10 repetitions per 2–6 sets. Improvements in maximal strength were assessed primarily using bilateral 1-RM leg press or squat. Again, these types of laboratory tests for maximal strength may not be fully representative of sport-specific or functional performance requirements; thus, caution should be used when interpreting results.

Fast-force production, also known as the rate of force development or the ability of the neuromuscular system to generate force rapidly particularly early in rapid contractions, is essential for performance in fundamental movements needed for sports performance such as jumping, throwing, and sprinting [9]. Indeed, the fast-force production appears to be linked to sport-specific and functional daily tasks and is more sensitive for detecting changes in neuromuscular function than maximal strength testing [97], although fast-force production in females is an understudied topic [98]. We might expect that concurrent strength and endurance training increases fast-force production in females, because countermovement jump (height, peak power, and peak velocity) correlates well with measures of maximal strength like 1-RM squat and power clean [99], which generally appear to improve as a result of concurrent strength and endurance training. This is supported by Hendrickson et al. [74], who reported similar increases in squat jump peak power (watts) between females performing strength training and females performing concurrent strength and endurance training, and Barnes et al. [79], who reported that peak force from a straight-leg 5-jump plyometric jump test (Nkg⁻¹) was larger in females combining running with heavy resistance training than in females performing volume-matched plyometric and heavy resistance training. Regrettably, further analysis of the influence of concurrent strength and endurance training on fast-force production in females is limited by a lack of data.

Athletes/exercisers and coaches/personal trainers may worry about endurance training interfering with strength development and fast-force production [7, 8] and/or muscle hypertrophy [10] or muscle hypertrophy “interfering” with endurance performance; however, the present literature does not appear to substantiate these worries in females. Regrettably, the presence of an “interference effect” and/or potential mechanism behind this “interference effect” are difficult to evaluate in female participants as only six of the included studies employed study designs that might be used to assess “chronic interference” [39, 74‐77, 85]. Two studies can be used to examine the influence of concurrent strength and endurance training in comparison to strength and endurance training alone [74, 75], three studies can assess the differences or the possibility of “interference” of concurrent training on endurance performance [76, 77, 85], and only one study can assess the possibility of “interference” of concurrent training on strength development [39]. Importantly, the primary aim of these six studies was not to explore “interference,” and none of the included studies demonstrated clear disadvantages for strength or endurance adaptations when strength and endurance training were performed concurrently. Instead, strength training added to endurance training improved strength and running economy [76], although observed improvements in strength were not reflected in increased endurance performance more than endurance training alone [77]. In the context of cross-country skiing and rowing, upper body endurance training combined with maximal whole body strength training improved work economy/endurance performance more effectively than endurance or strength training alone [39, 85]. The only studies that compared strength only, endurance only, concurrent strength and endurance, and control groups were Nindl et al. [75] and Hendrickson et al. [74]. These studies demonstrated “specificity of training,” where strength training significantly improved maximal strength and endurance training significantly improved endurance capacity. More specifically, Hendrickson et al. [74] demonstrated that concurrent strength and endurance training improved strength performance at comparable levels to strength training alone, while also demonstrating that concurrent strength and endurance training improved endurance capacity at comparable levels to endurance training alone. Collectively, these results suggest an absence of an “interference” effect, as performing strength and endurance training concurrently did not appear to be less advantageous than strength or endurance training alone. Due to the limited data available, sweeping conclusions regarding the presence or absence of “interference” in apparently healthy female populations cannot be made. Moreover, the heterogeneity in training background and training combinations in addition to the relatively lower volume of overall training make it impossible to draw any conclusions about “interference” in apparently healthy females.

A unique approach to concurrent strength and endurance research was used by Eklund et al. [82, 83] and Schumann et al. [81], who examined whether or not acute “interference” due to the order of exercise, i.e., endurance before strength or strength before endurance, on the same day and in the same session versus performing strength and endurance on separate days might result in chronic “interference.” Eklund et al. [82] reported that endurance before strength, strength before endurance, and different-day concurrent training were all effective in improving measures of maximal strength and endurance capacity, but that different-day concurrent training induced a greater magnitude of improvement in endurance performance than same-day endurance followed by strength or strength followed by endurance. Likewise, training endurance before strength or strength before endurance on the same day yielded similar increases in muscle cross-sectional area [83]. Additionally, Schumann et al. [81] reported that endurance before strength, strength before endurance, and different-day strength and endurance training improves endurance capacity in both females and males, while females appear to have additional improvements in submaximal endurance capacity when endurance training is performed before strength training on the same day.

Total training volume and the ratio of strength training to endurance training influences training adaptations. In general, a higher volume of endurance training in combination with strength training is associated with “interference” in males [100]. The studies included in this review employed a relatively low volume and frequency of concurrent strength and endurance training. Likewise, the duration of training interventions may influence “interference” or a lack thereof. Shorter interventions, such as those utilized in the included studies, may not reveal “interference,” particularly in untrained populations [101] and especially if training frequency is low [102, 103]. Medium-length or so-called prolonged interventions (lasting, e.g., 9–12 or 13–24 weeks, respectively) are more likely to reveal “interference,” especially if training frequency/volume is high [8, 104]. While the included studies ranged in duration from 7 to 24 weeks and included 1–3 strength training sessions per week, most of the studies lasted between 9 and 16 weeks and all of the 24-week studies were from the same laboratory. Furthermore, as previously mentioned, the frequency and volume of strength and endurance training remained low to moderate in all of the included studies, thus limiting our ability to evaluate the potential for “interference” in situations where a higher volume and intensity of training are used.

Finally, training status also influences susceptibility to “interference.” Studies included in this review investigate concurrent strength and endurance training in a relatively heterogeneous population including participants that were untrained, recreationally trained, and trained [73]. When endurance training is added to strength training, it may lead to positive adaptations in strength in moderately trained and untrained individuals, but in trained individuals, the influence may even be negative [18]. On the other hand, untrained individuals may be more sensitive to physiological stress than trained individuals [8], where starting with low to moderate training frequency and volume would be recommended.

The current review identified only a limited number of studies investigating concurrent strength and endurance training studies in females. It is possible that some studies were overlooked during the review process due to searching only two databases. Training studies are understandably a challenging undertaking; many laboratories do not have adequate resources to complete long-term and/or well-controlled training studies. The quality of the included studies ranged from low to moderate, with the majority receiving a score indicating “moderate” quality. Several studies lacked power calculations and did not report compliance with the intervention or whether participants were randomized into intervention groups. The group size for the included studies was consistently < 20, which may have influenced the statistical power as well as the generalizability of results. Importantly, the a priori inclusion criteria employed in the present review excluded several studies (see Fig. 1). The lack of consideration and/or reporting of menstrual status and hormonal contraceptive use should be considered in future research.

It is worth noting that most of the included studies did not take into consideration or report menstrual status or hormonal contraceptive use, which may be among the limiting factors in understanding adaptations to concurrent strength and endurance training (as well as adaptations to training in general) in females. Indeed, exercise testing and research has often been completed with little or no consideration of hormonal profiles, while some research has, undoubtedly, been unfulfilled due to the potential “confounding factors” incurred by the hormonal fluctuations or use of exogenous hormones that are a significant part of the lives of most females of “reproductive age.” The most recent meta-analyses indicate that the effect of the menstrual cycle on performance is only trivial [105], while the influence of hormonal contraceptive use on training adaptations is relatively small [72, 106], Overall, however, studies examining the influence of the menstrual cycle on performance or influence of hormonal contraceptives on training adaptations are of low quality. Thus, unequivocal conclusions regarding the influence of hormonal profile on performance cannot be made. Importantly, the individual effects of hormonal profiles on training, while not necessarily statistically significant, may be meaningful for individual athletes [107].

A growing body of research, which has been primarily performed in females, indicates that low energy availability could be a factor in the observed plateaus and/or decreases in performance and health (including bone density) [108, 109]. We hypothesize that observed plateaus and decreases in performance that have been described as or identified as “interference” might be explained, in part, by hormonal dysfunction related to inadequate energy availability. Regrettably, the current body of concurrent strength and endurance training research does not control for, or monitor, e.g., menstrual status or energy availability during training interventions. Energy availability is considered a prerequisite for both high-quality training sessions and recovery [110‐112], where even short-term deficits in energy availability can result in decreased muscle protein synthesis [113, 114] and short- to medium-term deficits in energy availability can blunt training response and/or impair recovery, thus predisposing athletes to undesired overreaching or overtraining [67, 115‐117]. To advance female exercise physiology and sport science research, methodology including participant selection, experimental design, and exploration of hormonal profiles, and, for example, confirmation of hormonal status (menstrual status and hormonal contraceptive use) should be considered to further elucidate the diversity and complexities associated with female physiology [118, 119]. In practice, simply taking into consideration and recording of the menstrual cycle status/phase/hormonal contraceptive use may help to explain more about increases, decreases, and plateaus in training adaptations that could be related to hormonal status.

Finally, while not a focus of this paper, programming concurrent strength and endurance according to the phases of the menstrual cycle has been promoted in popular and social media, and it is worth mentioning that the present review found no evidence to support this approach. While some research has indicated that periodizing strength training according to menstrual cycle phase (e.g., higher volume training during the follicular phase than in the luteal phase) might be beneficial for increasing strength and muscle mass in the lower extremities [120‐123], these benefits are not reported in the upper extremities [124]. Furthermore, no training studies exist to date that demonstrate benefits from periodizing endurance training according to menstrual cycle phase, although cross-sectional studies indicate an increased dependency on fat oxidation during the luteal phase [125, 126]. Lastly, there are no training studies published to date that would indicate any physiological advantages from programming or planning concurrent strength and endurance training by menstrual cycle phase.