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Review

Concurrent Training and the Acute Interference Effect on Strength: Reviewing the Relevant Variables

Panissa, Valéria Leme Gonçalves1; Greco, Camila C. PhD2; Ribeiro, Natalia MSc3; Julio, Ursula F. PhD1; Tricoli, Valmor PhD3; Franchini, Emerson PhD1

Author Information
Strength and Conditioning Journal: June 2022 - Volume 44 - Issue 3 - p 46-57
doi: 10.1519/SSC.0000000000000668
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Abstract

INTRODUCTION

To attain good competitive results in some sports, it is necessary to concomitantly develop several physical capacities during a training period. Among the required capacities, maximal strength, power, and endurance are the most prominent (42). Strength and power exercises are used to improve skeletal muscle contractile capacity (12), whereas aerobic endurance exercises improve oxygen delivery to the muscle and its ability to extract oxygen from blood (26). Thus, athletes in various sports perform multiple types of training exercises aimed at aerobic fitness and strength development to optimize performance during competitions (4,5). In addition, individuals seeking health improvement also make use of concurrent training (CT) to reduce body fat and increase muscle mass (21).

In this discussion, CT is defined as the combination of aerobic and strength exercises during the same session or training period. This nomenclature is due to the antagonistic adaptations that these 2 types of exercises may produce in the organism (6,24,25,28). It has been demonstrated that this training strategy may impair strength-related performance (14,37,39) as well as muscle hypertrophy and strength development (6,24,25,28).

Some studies have proposed that an acute negative effect of aerobic exercise might be responsible for the long-term strength and hypertrophy impairments; thus, changes in strength performance and muscle mass are smaller after CT compared with strength training (ST) alone (11,13,18,36,38,46). This hypothesis suggests a reduction in strength production and/or volume performed in each CT session when the aerobic exercise precedes the strength exercise, which in turn would generate a decreased ST stimulus (compared with the ST alone) that could explain the long-term impairment in strength gains. In addition, several studies suggest that the acute negative effects may be dependent on some factors such as aerobic exercise intensity (15,52), duration of recovery interval between exercises (37,52), muscle groups involved in both exercises (15,47), aerobic exercise mode (39), and aerobic exercise volume (43). Moreover, some ergogenic aids might also play a role in attenuating the acute interference effects (2,3,47).

Therefore, this review aims to examine the acute effects of aerobic exercise on subsequent strength performance (strength endurance or maximal strength) and identify the factors which influence negative effects. This knowledge may contribute to the organization and prescription of CT sessions designed to attenuate the aerobic exercise acute interference effect and, consequently, result in more effective training programs for both health and performance purposes.

HISTORICAL ANALYSIS OF CONCURRENT TRAINING

The first study to demonstrate the interference phenomenon was published by Hickson in 1980 (25). This investigation used 3 experimental groups of active men in 10 weeks of ST (lower-body exercises at 80% of 1 repetition maximum—1RM), aerobic training (AT, continuous as fast as possible in 30 minutes and intermittent sessions at the work rate that approached the maximal oxygen uptake), or the combination of both training regimens (CT, same exercises at the same intensities as performed by strength and endurance groups). The ST group trained 5 times a week, whereas the AT (3 high-intensity interval sessions and 3 moderate-intensity continuous sessions in alternative days) and CT groups (usually at least 2 hours of rest or inactivity separating 2 types of training) trained 6 times. The author observed that the CT group experienced a deficit in strength development over the last 2 weeks of training compared with the ST but obtained similar gains in maximal oxygen uptake when compared with the AT group. Therefore, this study demonstrated that CT might compromise strength gains. Subsequent to this study (25), other related investigations resulted in mixed outcomes with some (6,13,22,46) reporting similar results as those of Hickson (25), and some (5,23,33,49) reporting different alternative observations. The discrepancy in the results may be because of different training protocol characteristics, such as intensity, duration, and the time interval between exercises, and subjects' training status.

Since the identification of the interference effect, several studies have attempted to identify the mechanisms behind such phenomenon (11,14). Two hypotheses have gained special attention: the chronic and the acute hypotheses. The chronic hypothesis is supported by the different training-induced changes promoted by the ST compared with the AT, resulting in a conflictive adaptive environment in the skeletal muscle. For example, in general, ST causes increased contractile capacity and muscle fiber size but decreases mitochondrial and capillary density and oxidative enzyme activity (12). By contrast, AT increases mitochondrial and capillary density as well as oxidative enzymes concentration and activity, maintains or reduces muscle fiber size, and decreases contractile capacity, whereas strength decreases or remains unchanged (26). The acute hypothesis attributes the impairment of strength gains because of a carryover detrimental effect when strength exercise is preceded by aerobic exercise. By this hypothesis, fatigue induced by previous aerobic exercise would lead to lesser total work performed during strength exercise (13,46), which in turn might interfere with strength development. Therefore, it seems important to understand the variables that may influence negative effects. Thus, the present review focuses on the acute hypothesis.

ACUTE HYPOTHESIS

Several studies use the term “residual fatigue” to refer to the carryover detrimental effects generated by previous exercise (i.e., aerobic exercise), which would cause impairment on subsequent exercise performance (e.g., strength exercise) (8,30,31,37,39,43,45,47). Thus, owing to residual fatigue from previous aerobic exercise, the individual would start ST without adequate rest or recovery, decreasing the capacity to develop force and/or to accomplish the same total training volume during the ST session (8,37,39,43,45,47).

Therefore, over time, the quality and load volume would be negatively affected in the concurrent condition compared with the strength workout performed without the influence of the aerobic exercise. Load volume has been considered an important ST variable that is positively related to strength and muscle hypertrophy gains (29,40,44,50,51). Thus, it is reasonable to suggest that it is necessary to preserve the load volume of strength exercise during CT sessions to avoid or mitigate the long-term interference effect.

One of the first studies to suggest the acute effect as detrimental to strength improvement during CT was conducted by Craig et al. (13). The investigators divided 36 men into 3 training groups: ST (lower-body and upper-body exercises at 75% of 1RM), AT (running, continuous at 75% of maximal heart rate), and CT (in which subjects underwent ST after AT). After 10 weeks, there were no strength gains in the lower limbs for the CT and the AT groups, whereas the ST group experienced a ∼6% increase in strength. These results revealed compromised strength gains in the concurrent group, and this impairment was attributed to a carryover effect (i.e., insufficient recovery) caused by the aerobic exercise (running) performed before strength exercises.

The relationship between total load volume and strength gains during CT was demonstrated in a study by Sale et al. (46). In this study, 16 physically active subjects were submitted to 10 weeks of CT. Participants were divided into 2 groups: the A2d group, which performed aerobic (cycling, intermittent, 6 bouts of 3 minutes at 90–100% of maximal oxygen uptake) and ST (unilateral leg press, 6 sets of 15–20 RM) in the same day, twice a week (exercise order was alternated in each session), and the B4d group, which performed aerobic and ST in different days, 4 times a week (AT and ST on different days). At the end of the study, the authors reported greater increases in maximum strength in the B4d group (25%) compared with the A2d group (13%), although changes in muscle hypertrophy and strength endurance did not differ between groups. The observed difference in maximal strength was attributed to the decreased ST volume performed in each session because the B4d group had a higher mean absolute (10%) and relative (2%) load volume than the A2d group. In addition, this study provided important data regarding the acute interference effect. When the A2d group performed the aerobic + strength exercises in order, there was a 4% decrease in absolute and relative total training volume compared with the opposite order. Unfortunately, the study did not include a ST-only group. Nevertheless, it can be concluded that the acute interference effect may be one important factor for constraining strength gains over time.

De Souza et al. (14) also observed interference effects when aerobic and strength exercises were performed in the same training session. The investigators randomly divided 37 physically active men into 4 groups which undertook an 8-week training period: ST (3 lower-body exercises, 3 to 5 sets of 6–12 RM), AT (running, intermittent, 20 to 15 bouts of 1 minute at 80–100% of the speed to elicit maximal oxygen uptake), CT (in which subjects underwent both ST and AT in the same training session, and the order was balanced and altered during the training period), and control. It was observed that increases in type IIa (17%) and I (18%) muscle fiber cross-sectional area occurred only in the ST group. Although the exercise order (aerobic and strength) was balanced and alternated during CT sessions, the residual fatigue during sessions in which the AT preceded the ST might be responsible for the absence of muscle fiber hypertrophy gains in the CT group. Similar to Craig et al. (13), this study also did not control the acute total load volume performed during each training session.

Two other studies investigated the acute interference effect on strength adaptations exploring exercise order (11,18). Cadore et al. (11) divided participants into 2 groups: one performed aerobic exercise (AT, cycling, 80–95% of maximal heart rate during first 6 weeks; 6 bouts of 4 minutes at maximal heart rate in the last 2 weeks) before strength exercise (ST, upper-body and lower-body exercises, 2–3 sets of 18–20 RM to 6–8 RM) and the other performed the reverse order (ST + AT). After a 12-week (3×/week) training period, the increase in force production was significantly higher in the ST + AT order (35%) than in the AT + ST order (22%).

Similarly, Eklund et al. (18) investigated 3 different combinations of strength and aerobic exercises during CT: aerobic exercise (AT, cycling, alternating moderate and high-intensity determined by heart rate zones corresponding to the aerobic and anaerobic thresholds) before strength exercise (ST, lower body exercises, 2–5 sets of 10–20 repetitions at 40–80% 1RM or 3–10 repetitions at 80–95% 1RM) in the same session (AT + ST), strength exercise before aerobic exercise in the same session (ST + AT), and aerobic and strength exercises in different days. The investigators observed similar gains in strength and muscle hypertrophy for all 3 groups after 24 weeks of training. However, neural adaptations (i.e., voluntary activation capacity and electromyographic activity) were compromised in the AT + ST group compared with the 2 other groups. The authors speculated that the periodized nature of ST that was composed by different intensities allowed the AT + ST group to improve strength and hypertrophy even with attenuated neural adaptations.

Finally, 2 recent meta-analyses (17,36) showed that strength followed by the aerobic exercise order was superior to provide maximal dynamic strength gains than the reverse order. Murlasits et al. (36) selected studies with a minimum of 8 weeks of training and observed a difference of 3.96 kg (1 maximal repetition, only in lower-body exercises) between orders, whereas Eddens et al. (17) selected studies with a minimum of 5 weeks and showed a difference of 6.91% (lower-body dynamic strength) between orders. In addition, Eddens et al. (17) also analyzed the role of the intrasession exercise order on maximal static strength and muscle hypertrophy (lower-body exercises), concluding that these variables were not affected by the strength/aerobic exercise order. Therefore, the ST + AT order seems to be superior only for the dynamic strength but not static strength development, indicating that the training specificity is important provided that the exercises during the training program are dynamic and not static. Thus, the acute interference effect from each training session may contribute to the long-term impairment in strength gains after a CT program.

Moreover, considering the evidence in the literature demonstrating that total training volume is an important variable for strength and muscle hypertrophy gains (29,40,44,50,51) and that acute volume performed is reduced in a CT session (8,30,31,37,39,43,45,47), it is quite plausible to expect lower strength and hypertrophy adaptations after a period of CT. However, although most studies have shown acute interference effects, some have demonstrated that in specific conditions; these effects can be attenuated or even suppressed (15,37,47). The divergences between the findings may be due to some training protocol components such as aerobic exercise intensity and interval recovery between ST and AT, which seem to influence the acute interference effect occurrence.

The main changeable variables of AT protocols are exercise intensity (15,52), mode (16,39), and volume (43); duration of recovery interval between exercises (7,8,37); and active muscles involved (15,47). In addition, ergogenic aids are also a factor that have been considered in possibly offsetting the deleterious interference effect on strength performance during a CT session (2,3,19,20,47). If the training program's priority is muscle strength development, all these factors should be considered to optimize CT organization and prescription.

Although the acute hypothesis has received considerable attention from researchers, it must be clear that it does not explain all the chronic adaptations promoted by the CT. This hypothesis is related just to the reduction in strength performance during a single training session when aerobic exercise precedes strength exercise and is not related to other effects that may contribute when the CT is performed in the reverse order or even on different days. All investigations related to the variables included in this review are summarized in Table.

Table - Studies involving the acute interference on concurrent training sessions (aerobic followed by strength exercise)
Authors Variables manipulated Aerobic protocol Strength protocol Main outcomes
Abernethy (1) Aerobic exercise intensity Cycle ergometer
Moderate-intensity continuous—150 min at 35% iV̇O2max or HIIE—5×5:5 min (work rates for each minute within a repetition corresponding to 40, 60, 80, 100, and 100% iV̇O2max)
Isokinetic unilateral leg extension (torques generated for 10 contractile speeds (0.52–5.20 rad•s−1) ↓ ∼4% in torque in both aerobic exercise intensities at different contractile speeds
De Souza et al. (15) Aerobic exercise intensity Muscle groups involved in the exercises Running on a treadmill—5 km moderate-intensity continuously at 90% of the AT velocity or HIIE—1 min at iV̇O2max: 1 min PR 1RM and MNR at 80% 1RM in leg press and bench press exercises ↓ 27% in MNR in leg press after HIIE
Salles Painelli et al. (47) Aerobic exercise intensity
Muscle groups involved in the exercises
Ergogenic aids: creatine supplementation
Running on a treadmill—5 km
moderate-intensity continuously at 90% of VT
HIIE—1 min at 100% iV̇O2max: 1 min of PR
1RM and MNR in 4 sets at 80% 1RM in leg press and bench press exercises ↓ ∼22% in MNR in leg press after HIIE
Creatine supplementation was effective to maintain the MNR in leg press after HIIE
Sporer and Wenger (52) Aerobic exercise intensity
Duration of interval recovery between exercises
Cycle ergometer
Moderate-intensity continuously for 36 min at ∼70% iV̇O2max or HIIE—6 × 3 min at 95–100% iV̇O2max interspersed by 3 min at 40% iV̇O2max
Interval recoveries of 4, 8, and 24 h between exercises
MNR in 4 sets at ∼75% 1RM in leg press and bench press exercises ↓ ∼25 and ∼9% in MNR after 4 and 8 h, respectively
Divljak (16) Aerobic exercise mode Running on a treadmill or cycle ergometer in moderate-intensity continuously—40 min, 80% of HRmax 3 repetitions measuring peak power followed by total volume with MNR in 3 sets at 80% 1RM in squat exercise ↓ 30% in total volume in the cycle ergometer condition
Panissa et al. (39) Aerobic exercise mode Running on a treadmill or cycle ergometer—15 × 1 min at 100% of MAP or MAV: 1 min PR MNR in 4 sets at 80% 1RM in half-squat exercise ↓ 35% and 47% of volume in the first set for a treadmill and cycle ergometer, respectively
↓ ∼36% of volume in the s set for a cycle ergometer
Bentley et al. (7) Duration of recovery interval between exercises Cycle ergometer—30 min at LT plus 4×1:1 min at 120% iV̇O2max, performed 6 or 24 h before strength exercise 6-s all-out cycling test; peak isokinetic leg extension at 60, 120, and 180°·s−1; and a maximal concentric squat jump ↓ 11% in maximal peak torque at 60°·s−1 after the 6-h interval
↓ 5% in maximum force of the vertical jump after the 6-h interval
Bentley et al. (8) Duration of recovery interval between exercises Cycle ergometer—30 min continuously at 80% V̇O2max plus 4×1:1 min at 120% iV̇O2max, performed 10 min or 6 h before strength exercise Isometric maximal voluntary contractions of the knee extensor muscle ↓ maximal voluntary contraction force after 10 min (12%) and after 6 h (6%)
Leveritt et al. (32) Duration of recovery interval between exercises Cycle ergometer—50 min at 70–110% of CP, performed 8 or 32 h before strength exercise Unilateral leg extension
Peak isometric torque (5-s maximal contractions) (0.52 rad.s−1)
Isokinetic peak torque (at 1.04, 2.08, 3.14, 4.19, 5.23, and 8.37 rad.s−1)
Isotonic strength (2 sets of 10 repetitions at 80% of maximal torque)
There was no interference
Panissa et al. (37) Duration of recovery interval between exercises Running on a treadmill—5 km of HIIE—1 min at 100% of MAV: 1 min of passive recovery performed 30 min, 1, 4, 8, and 24 h before strength exercise MNR in 4 sets at 80% 1RM in half-squat exercise ↓23 and 15% in MNR after 30 min and 1 h, respectively
Reed et al. (41) Muscle groups involved in the exercises Cycle ergometer—45 min at 75% of HRmax MNR in 6 sets at 80% 1RM in bench press and back-squat exercises ↓ ∼15% in MNR cumulative at set 3 for back squat
Ribeiro et al. (43) Aerobic exercise volume Running on a treadmill—3, 5 or 7 km (∼18, 30, and 42 min, respectively) in moderate-intensity continuously at 90% of the AT velocity 1RM and MNR in 4 sets at 80% 1RM in leg press exercise ↓ 12 and 22% of strength endurance after 5 and 7 km, respectively
Aoki et al. (3) Ergogenic aids: carbohydrate supplementation Running on a treadmill continuously—45 min at 70% iV̇O2peak 1RM followed by MNR in 2 sets of 70% 1RM in leg press exercise Carbohydrate supplementation was ineffective to avoid interference
Freitas et al. (19) Ergogenic aids: beta-alanine supplementation Running on a treadmill—5 km of HIIE—1 min at 100% of MAV: 1 min PR Total volume and MNR in 4 sets of 80% 1RM in leg press exercise Beta-alanine supplementation was ineffective to avoid interference
Freitas et al. (20) Ergogenic aids: capsaicin supplementation Running on a treadmill—5 km of HIIE—1 min at iV̇O2max: 1 min passive recovery Total volume and MNR in 4 sets at 70% 1RM in half-squat exercise Capsaicin supplementation ↑∼13% total volume
Leveritt and Abernethy (30) Ergogenic aids: 2 days of low carbohydrate diet Cycle ergometer—60 min at 75% of MAP plus 4 × 1 min at 100% of MAP: 1 min PR MNR in 3 sets at 80% 1RM in squat exercise
Unilateral leg extension isokinetic peak torque (0.52 rad.s−1) and knee extension (5 sets of 5 repetitions at 1.05, 2.09, 3.14, 4.19, and 5.24 rad.s-1)
There was a moderate negative effect on the first 2 sets of squats and a negligible effect on the final set
Rossi et al. (45) Ergogenic aids: caffeine supplementation Running on treadmill—5 km of HIIE—1 min at iV̇O2max: 1 min of PR Total volume and MNR in 4 sets at 80% 1RM in half-squat exercise Caffeine supplementation was ineffective to avoid interference
↓ = decrease; iV̇O2max = intensity associated with maximal oxygen uptake intensity; iV̇O2peak = intensity associated with peak oxygen uptake; min = minutes; 1RM = 1 repetition maximum; AT velocity = anaerobic threshold velocity; CP = critical power; HIIE = high-intensity intermittent exercise; HRmax = maximal heart rate; MAP = maximal aerobic power; MAV = maximal aerobic velocity; MNR = maximal number of repetitions; PR = passive recovery; VT = ventilatory threshold.

AEROBIC EXERCISE INTENSITY

Aerobic exercise intensity seems to be a key variable in CT prescription. The manipulation of intensity results in different demands over the neuromuscular, aerobic, and anaerobic energy systems components (10), which in turn may generate different effects on acute strength performance (15). All studies that investigated the effect of aerobic exercise intensity on strength performance have used high-intensity interval training, which was composed of short efforts (<1 minute) performed between 100% and 120% of intensity associated with maximal or peak oxygen uptake (iV̇O2max or iV̇O2peak) or long efforts (>1 minute) performed between maximal lactate steady state and iV̇O2max or iV̇O2peak (1,15,47,52).

Sporer and Wenger (52) tested 2 aerobic exercise protocols, 1 high-intensity intermittent (6 bouts of 3 minutes at 95–100% iV̇O2max interspersed by 3 minutes of active recovery at 40% iV̇O2max) and another consisting of moderate-intensity continuous exercise (36 minutes at 70% iV̇O2max), both performed 4 or 8 hours before the strength exercise. Similar decreases in the maximal number of repetitions in leg press exercise (4 sets at 75% 1RM to failure) were observed between protocols (∼9 and ∼25% after 4 and 8 hours of AT) independent of the exercise intensity.

Similarly, Abernethy (1) did not find an influence of exercise intensity when strength exercises (leg extension on a Cybex II isokinetic dynamometer) were performed after a session of either continuous moderate-intensity exercise (150 minutes at 35% iV̇O2max) or more intense intermittent aerobic exercise protocol (5 bouts of 5 minutes at 40, 60, 80, 100, and 100% iV̇O2max separated by 5 minutes of recovery). The reduction in maximal isokinetic peak torque (from 0.52 to 5.20 rad/s) was similar (∼4%) after both aerobic exercise protocols.

On the other hand, De Souza et al. (15) investigated the impact of 2 aerobic exercise protocols on strength-endurance (maximal number of repetitions at 80% 1RM) and maximal dynamic strength (1RM) performance. Subjects ran 5 km intermittently at high-intensity (1 minute:1 minute at iV̇O2max) and continuously at moderate intensity (90% of the anaerobic threshold). Only high-intensity interval exercise was observed to affect lower limb strength-endurance performance (27% decrease in maximal number of repetitions), whereas there was a nonsignificant trend in 1RM reduction.

Similarly, Salles Painelli et al. (47) investigated the effects of 5 km continuous (at 90% of the ventilatory threshold) or intermittent running (1 minute:1 minute at iV̇O2max) on maximum dynamic strength (1RM) and strength-endurance (4 sets of maximum repetitions at 80% 1RM) performance in the leg press exercise. Corroborating the results of the study by De Souza et al. (15), a 22% reduction was observed in total strength-endurance volume only after the high-intensity intermittent aerobic exercise protocol.

In summary, it seems that strength-endurance performance is more affected by aerobic exercise intensity (greater reduction after high-intensity interval exercises), whereas maximum dynamic strength seems not to be affected in isotonic exercises; however, it is affected in isokinetic exercise, when the same muscle groups are considered.

AEROBIC EXERCISE MODE

Another factor that should be considered when analyzing CT interference effects is the aerobic exercise mode. Usual aerobic exercises such as running and cycling show different movement characteristics, physiological demands, predominance of muscle action (concentric and eccentric), and even motor unit recruitment patterns (9,34,35). Therefore, the aerobic exercise mode may affect the subsequent strength performance during a CT session.

Panissa et al. (39) studied the acute effects of 2 aerobic exercise modes on strength-endurance performance. Ten physically active individuals were submitted to 3 experimental conditions: ST only, running aerobic exercise followed by ST, and cycling aerobic exercise followed by ST. The ST consisted of 4 sets of half-squat maximum repetitions at 80% 1RM and aerobic exercise performed intermittently (15 bouts of 1 minute:1 minute at 100% of maximal aerobic power). Both aerobic exercise modes caused attenuation on total volume performed in the first exercise set when compared with the first set of the ST-only condition (−35% and −47% for treadmill and cycle ergometer, respectively). However, for the second set, only the cycling exercise showed a significant reduction (∼36%) on strength performance compared with the second set of the ST-only condition.

Similar outcomes were shown in a study by Divljak (16). In this study, the total ST volume performed (3 sets at 80% 1RM to task failure in the squat exercise) was ∼30% lower after 40 minutes of cycling (80% of maximal heart rate) when compared with running at the same intensity. Thus, both studies (16,39) showed that the interference magnitude is higher after cycling exercise. However, the influence of other aerobic exercise modes (e.g., swimming, rowing, elliptic exercise, etc.) on subsequent strength performance needs to be investigated.

The present review does not intend to approach chronic effects, but we must mention that although acute decrements were observed in greater magnitude after aerobic cycling exercise, there is some evidence that in long term, running may be more impactful on strength and hypertrophy gains (22,53). Despite the divergent results between acute and chronic effects, it is possible to note that the aerobic exercise mode can alter the magnitude of negative interference on strength performance.

DURATION OF RECOVERY INTERVAL BETWEEN EXERCISES

Although strength performance may be impaired after an aerobic exercise bout, it seems that if there is a sufficient time interval for recovery between both exercises, the interference effect may be eliminated or at least minimized (7,8,37).

Bentley et al. (7) investigated 2 different rest interval durations (6 and 24 hours) between aerobic and strength sessions. The aerobic exercise was composed of 30 minutes cycling at the lactate threshold (plus 4 bouts of 1 minute:1 minute at 120% iV̇O2max). Reductions in knee extension peak torque (11%) and maximal force in the concentric phase of the vertical jump (5%) were observed only after the 6-hour interval, suggesting that a 24-hour recovery period is sufficient to eliminate the acute interference effect. In a later study, but using a similar aerobic exercise protocol, Bentley et al. (8) found statistically significant reduced performance after 6-hour intervals in maximal voluntary contraction force.

Leveritt et al. (32) also explored the effect of different interval durations (8 and 32 hours) on maximal strength (1RM) and strength-endurance performance (5 sets of consecutive maximal repetitions in 6 contractile speeds 1.04, 2.08, 3.14, 4.19, 5.23, and 8.37 rad·s−1 of leg extension). No reductions were observed in strength after any condition preceded by 50 minutes of aerobic exercise at 70–100% of the critical power (i.e., an asymptote of the power × duration curve). However, because the subjects did not execute the strength exercises immediately after the aerobic bout, it is difficult to precisely determine the role of the recovery interval in the attenuation of the interference effect.

In a more elaborate experimental design, Sporer and Wenger (52) evaluated strength-endurance performance (4 sets of maximal repetitions at 75% 1RM) 4, 8, and 24 hours after an aerobic exercise performed with 2 different protocols: 36 minutes of continuous running at 70% iV̇O2max or intermittent running consisting of 6 bouts of 3 minutes (at 95–100% iV̇O2max: 3 minutes of recovery). Decreases in leg press total exercise volume were observed after 4 and 8 hours of recovery, with greater effect after 4 hours (25%) compared with 8 hours (9%). On the other hand, a 24-hour interval did not result in a significant interference effect. These outcomes occurred independently of the aerobic exercise protocol.

Finally, Panissa et al. (37) investigated a high-intensity intermittent aerobic exercise (5 km, 1 minute:1 minute at 100% of maximal aerobic velocity) on strength-endurance performance (4 sets of maximal repetitions at 80% 1RM) performed after 30 minutes, 1, 4, 8, and 24 hours recovery intervals. Half-squat exercise total volume decreased only after 30 minutes and 1 hour (23 and 15%, respectively), but it was not affected after the 4, 8, and 24 hours recovery interval.

Thus, it seems that the interference effect may be dependent on the recovery interval duration between strength and aerobic exercises. According to the evidence found in the literature, strength performance (i.e., maximal strength and strength endurance) is not significantly affected after a recovery interval between 4 and 8 hours, with no performance changes after 24 hours. It should be noted that the studies of Bentley et al. (7,8) conducted strength tests on the same day, so that results must be interpreted with caution because they may overestimate the interference effect.

MUSCLE GROUPS INVOLVED IN THE EXERCISES

Regardless of the CT protocol applied, there is a consensus that the interference effect will only occur in muscle groups that are active in both aerobic and strength exercises (15,41,47,52). For example, De Souza et al. (15) and Salles Painelli et al. (47) used similar aerobic exercise protocols, running 5 km intermittently at high-intensity (1 minute:1 minute at iV̇O2max) and continuously at moderate intensity (90% of the anaerobic threshold). The effects on strength-endurance performance were then examined on lower-body and upper-body exercises (maximal number of repetitions at 80% 1RM in the leg and bench press). No interference effect was found on upper-body strength exercise; however, lower-body strength endurance was negatively affected after the high-intensity intermittent running.

Another study that supports these findings was conducted by Reed et al. (41). In this investigation, 9 resistance-trained individuals were submitted to 4 experimental conditions: a cycle ergometer bout followed by bench press exercise, bench press exercise only, cycle ergometer bout followed by back-squat exercise, and back-squat exercise only. The cycle ergometer aerobic bout was performed for 45 minutes at 75% of the maximal heart rate, and the strength exercises comprised 6 sets of a maximum number of repetitions at 80% 1RM. Only the back-squat exercise showed a significant reduction in repetitions number until set 3 (∼15%) after the aerobic exercise bout. Thus, confirming that the interference only occurs in muscle groups that are primarily recruited in both exercises (i.e., lower limbs for aerobic and strength bouts).

Therefore, we conclude that the inclusion of aerobic exercise does not affect subsequent strength performance when aerobic and strength exercises are performed by different muscle groups.

AEROBIC EXERCISE VOLUME

It has been demonstrated that there is an antagonistic relationship between muscle strength performance and running exercise duration, with greater strength reduction after more prolonged distance or time (34). The impairment in strength occurs because of a reduction in the voluntary drive to the active muscles (i.e., central fatigue) and in muscle force production capacity (i.e., peripheral fatigue) (35).

We found only 1 study investigating the effect of running exercise volume on subsequent strength performance (43). In this study, 21 physically active men were submitted to 3, 5, and 7 km (duration of ∼18, 30, and 42 minutes, respectively) of continuous running (90% of the anaerobic threshold velocity), followed by a 1RM or a strength-endurance test (4 sets of maximal repetitions at 80% 1RM) in the leg press exercise. Significant reductions in strength-endurance performance were observed after 5 km (12%) and 7 km conditions (22%) when compared with strength-only condition. In addition, the strength-endurance performance in the 7 km condition was lower than that in the 3 km (14%) and 5 km conditions (12%), whereas no effect on maximal strength was observed. Thus, this study demonstrated that performing 3 km running at 90% of the anaerobic threshold did not impair subsequent strength performance; however, when subjects ran at the same intensity but cover 5 and 7 km, the impairment on strength endurance was observed, mainly after 7 km.

Therefore, the reduction in strength performance depends on aerobic exercise volume at least in running activity. It is important to note that the leg press exercise did not use the same muscles as running, and it is a limitation in this topic.

ERGOGENIC AIDS

To eliminate or at least minimize the acute interference effects, the utilization of ergogenic aids has been considered by some investigations (2,3,19,20,30,45).

It is well known that muscle glycogen is an important fuel for both aerobic and strength exercises (3,30). Therefore, one possible explanation for reduced performance in subsequent strength exercises would be the decrease in liver and muscle glycogen stores (30). To test whether the acute interference might be associated with lower energy stores, Leveritt and Abernethy (30) investigated the effect of decreased carbohydrate reserve on maximal strength and strength-endurance performance. After an aerobic exercise session (60 minutes of cycling at 75% of maximal aerobic power, followed by 4 1 minute bouts at 100% of maximal aerobic power) combined with 2 days of low carbohydrate diet (1.2 ± 0.5 g/kg/d), they observed that strength-endurance (3 × 80% 1RM squat exercise total volume) performance was negatively affected when compared with a control condition (only strength exercise).

Similar to the work conducted by Leveritt and Abernethy (30), Aoki et al. (3) examined whether carbohydrate intake could attenuate acute interference effects. In this study, one group consumed 500 mL of a 6% maltodextrin solution before and during the aerobic exercise bout (45 minutes at 70% of iVO2peak), whereas the other group received a placebo. No differences were found between groups in strength performance (2 sets in leg press at 70% of 1RM with repetitions until failure), although the maximal number of repetitions was lower in both situations compared with the control condition. It seems that a marked reduction in glycogen stores may cause negative interference in total volume performed as showed by Leveritt and Abernethy (30); however, the findings of Aoki et al. (3) suggest that the aerobic exercise probably did not reduce glycogen stores low enough to cause interference, making supplementation unnecessary.

Conversely, Salles Painelli et al. (47) found the positive effects of creatine supplementation (20 g·d−1 for 7 days followed by 5 g·d−1 throughout the study) on maximum dynamic strength (1RM) and strength-endurance (4 sets of maximal repetitions at 80% 1RM) performance after 5 km running bouts (continuously at 90% of ventilatory threshold or intermittently 1 minute:1 minute at iV̇O2max). It is important to note that the supplementation protocol used in the study conducted by Salles Painelli et al. (47) has been previously demonstrated to be effective in increasing the creatine stores in muscles. Creatine supplementation may increase muscle-free creatine which favors a more rapid replenishment of phosphate creatine and improves recovery from intense exercise. The authors observed a 22% total volume reduction in leg press exercise after the intermittent aerobic protocol only for the placebo group. The creatine-supplemented individuals showed no signs of any acute interference effects. Thus, it seems that increased muscle creatine stores before a CT session may prevent the decrease in strength-endurance performance after aerobic exercise.

Another type of ergogenic aid used in CT studies is caffeine (45). Caffeine can delay fatigue because of its action in the central nervous system (27). Rossi et al. (45) investigated the effects of caffeine supplementation on strength-endurance (4 sets of maximal repetitions at 80% 1RM) performance in the half-squat exercise after running 5 km intermittently (1 minute:1 minute at iV̇O2max). These findings did not demonstrate any significant effect on strength-endurance performance (total volume) compared with the control condition.

Beta-alanine supplementation has been tested as well, relative to its potential effects on CT (19). Beta-alanine may increase the intramuscular carnosine that is a buffer hydrogen ion and attenuate reductions in pH after intense exercise (48). Freitas et al. (19) used 6.4 g/d of beta-alanine during 28 days; before and after this period, participants performed 5 km high-intensity intermittent run (1 minute:1 minute at maximal aerobic velocity) followed by ST (4 sets of 80% of 1RM in leg press). The strength loss after the aerobic exercise was not prevented or attenuated after 28 days of beta-alanine supplementation.

Finally, Freitas et al. (20) compared the maximal number of repetitions (4 sets at 70% 1RM) in the half-squat exercise preceded by 5 km high-intensity intermittent run (1 minute:1 minute at iV̇O2max) between a placebo and a capsaicin analog (12 mg) supplement. The capsaicin analog is a bioactive substance found in various species of pepper. Capsaicin and its analogs agonize the transient receptor potential vanilloid-1 (TRVP-1) in the muscle, increasing the calcium release, resulting in improved actin–myosin filament interaction and greater force production. Training volume (maximum number of repetitions × load) was higher (2077.6 ± 465.2 versus 1838.9 ± 624.1 kg) in the capsaicin analog condition.

In light of the aforementioned findings, carbohydrate, caffeine, and beta-alanine supplementation are not able to eliminate or attenuate the effects of acute interference, but creatine and capsaicin analog supplementation may be useful tools to mitigate the decrement in strength performance after a bout of aerobic exercise, at least in the protocols investigated by these authors.

CONCLUSIONS

Based on studies included in this review, we can conclude that regarding aerobic exercise intensity, high-intensity interval exercises resulted in more pronounced negative effects on strength-endurance performance but not in maximal strength compared with moderate-intensity exercise. In respect to the aerobic exercise mode, cycling resulted in more negative effects on strength-endurance performance than running. Concerning aerobic exercise volume, although we found only 1 study investigating this variable, it indicated that low volume (3 km, ∼18 minutes) did not diminish strength-endurance performance, whereas higher volumes (5 and 7 km, ∼30 and ∼42 minutes, respectively) generated impairments.

Some evidence pointed out that strength-endurance performance is recovered after 4-hour to 8-hour recovery interval between activities. In addition, reduction in strength-endurance performance is located only in muscle groups involved in both exercises. Finally, although caffeine, carbohydrate, and beta-alanine were not able to revert the deleterious effect on strength-endurance performance, creatine and capsaicin analog supplementation were able to do it. The conclusions of this study may also be seen in the infographic (Figure).

F1
Figure

It is important to note that conclusions made in this review are limited because of the lack of a larger number of studies with a systematic changing of variables. Because the concurrent aerobic and ST sessions are composed of many variables, it is important that to attenuate or avoid acute impairments on strength, professionals and coaches take into consideration exercise protocol characteristics before applying the conclusions described in the present review. It is also necessary to note that avoiding the acute impairments on strength will not necessarily nullify possible long-term effects in strength and hypertrophy development because the acute hypothesis is only a part of other possible factors that may culminate in interference effects.

Acknowledgments

Valéria LG Panissa thanks Nicolas Clark for his help with the infographic elaboration.

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Keywords:

concurrent exercise; maximum strength; strength endurance

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