Body Composition
Meta-Analysis
2012
Concurrent Training: A Meta-Analysis Examining Interference of Aerobic and Resistance Exercises
By Jacob M. Wilson, Pedro J. Marin, Matthew R. Rhea, Stephanie M.C. Wilson, Jeremy P. Loenneke and Jody C. Anderson
Journal of Strength and Conditioning Research, 26(8), pp. 2293-2307
<h2>Abstract</h2>
<p><strong>Background:</strong> <a href="/terms/concurrent-training/" class="term-link" data-slug="concurrent-training" title="Concurrent training">Concurrent training</a>—the simultaneous pursuit of both endurance and resistance training adaptations—is common among athletes and fitness enthusiasts. While combining these modalities appears practical and time-efficient, the "interference effect" hypothesis posits that aerobic training impairs the strength, power, and hypertrophic adaptations normally produced by resistance training.</p>
<p><strong>Objective:</strong> This <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> quantified the magnitude of the interference effect on maximal strength, muscle power, and <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a>, and examined whether training modality (running vs. cycling), frequency, duration, and volume of aerobic exercise moderated the interference effect.</p>
<p><strong>Methods:</strong> Electronic databases were searched for studies comparing concurrent training to resistance training alone. Effect sizes were calculated as standardized mean differences for strength (<a href="/terms/one-repetition-maximum/" class="term-link" data-slug="one-repetition-maximum" title="1RM">1RM</a>), power (vertical jump height), and hypertrophy (lean mass or muscle <a href="/terms/cross-sectional-area/" class="term-link" data-slug="cross-sectional-area" title="cross-sectional area">cross-sectional area</a>) outcomes.</p>
<p><strong>Results:</strong> Concurrent training significantly attenuated gains in maximal lower body strength (SMD = −0.34), explosive power (SMD = −0.51), and lean mass (SMD = −0.23) compared with resistance training alone. Running produced a substantially greater interference effect than cycling. Higher aerobic <a href="/terms/training-frequency/" class="term-link" data-slug="training-frequency" title="training frequency">training frequency</a>, longer session duration, and greater weekly aerobic volume each independently amplified the interference.</p>
<p><strong>Conclusions:</strong> Aerobic exercise interferes with resistance training-induced adaptations in a dose-dependent manner, with running-based aerobic training producing greater interference than cycling. Practitioners should carefully select aerobic modality and manage volume when concurrent training is employed.</p>
<h2>Introduction</h2>
<p><a href="/terms/concurrent-training/" class="term-link" data-slug="concurrent-training" title="Concurrent training">Concurrent training</a>—the combination of resistance and endurance exercise within the same training program—is the norm rather than the exception for most exercising individuals. Recreational athletes seeking both cardiovascular fitness and muscular development, team sport athletes requiring both strength and aerobic capacity, and general fitness enthusiasts following mixed-mode programs all engage in some form of concurrent training [1].</p>
<p>The interference effect hypothesis, originally proposed by Hickson in 1980 [2], states that the simultaneous pursuit of strength and endurance adaptations leads to attenuated gains in strength and power compared to resistance training performed in isolation. Hickson observed that subjects training both modalities concurrently experienced an early plateau and eventual decline in strength performance despite continued training, while those performing resistance training alone continued to improve throughout the same period.</p>
<p>The molecular basis for this interference has been increasingly characterized. Endurance exercise preferentially activates AMP-activated protein kinase (AMPK), which phosphorylates and thereby inhibits the rapamycin complex 1 (<a href="/terms/mtor/" class="term-link" data-slug="mtor" title="mTORC1">mTORC1</a>) pathway that mediates <a href="/terms/muscle-protein-synthesis/" class="term-link" data-slug="muscle-protein-synthesis" title="muscle protein synthesis">muscle protein synthesis</a> in response to resistance exercise [3]. This antagonistic signaling creates a state of molecular competition that may blunt anabolic adaptations when both training modes are performed in close temporal proximity.</p>
<p>However, the degree of interference is not uniform and appears to depend on a range of factors including the type of aerobic exercise employed, the volume and intensity of aerobic training, the temporal arrangement of sessions, and the training status of the individual [4]. A comprehensive <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> that systematically examines these moderators is needed to provide evidence-based guidance for concurrent training program design.</p>
<h3>References</h3>
<p>[1] Leveritt M, et al. Concurrent strength and endurance training: a review. <em>Sports Med</em>. 1999;28:413–427.
[2] Hickson RC. Interference of strength development by simultaneously training for strength and endurance. <em>Eur J Appl Physiol</em>. 1980;45:255–263.
[3] Fyfe JJ, et al. Interference between concurrent resistance and endurance exercise: molecular bases and the role of individual training variables. <em>Sports Med</em>. 2014;44:743–762.
[4] Wilson JM, et al. Concurrent training: a meta-analysis examining interference. <em>J Strength Cond Res</em>. 2012;26:2293–2307.</p>
<h2>Methods</h2>
<h3>Search Strategy</h3>
<p>A comprehensive literature search was conducted across MEDLINE, SPORTDiscus, and EMBASE databases. Search terms included "<a href="/terms/concurrent-training/" class="term-link" data-slug="concurrent-training" title="concurrent training">concurrent training</a>," "combined training," "aerobic and resistance exercise," "interference effect," and related terms. Reference lists of identified reviews and meta-analyses were also manually screened to capture additional eligible studies.</p>
<h3>Study Eligibility Criteria</h3>
<p>Studies were included <a href="/terms/intermittent-fasting/" class="term-link" data-slug="intermittent-fasting" title="if">if</a> they: (1) compared concurrent training (resistance plus aerobic exercise) to resistance training alone in the same study; (2) reported at minimum one outcome related to muscular strength (<a href="/terms/one-repetition-maximum/" class="term-link" data-slug="one-repetition-maximum" title="1RM">1RM</a> testing), power (vertical jump, sprint performance), or <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="hypertrophy">hypertrophy</a> (lean mass via DXA, muscle <a href="/terms/cross-sectional-area/" class="term-link" data-slug="cross-sectional-area" title="cross-sectional area">cross-sectional area</a> via imaging); (3) had a minimum intervention duration of 6 weeks; and (4) used apparently healthy adult participants. Studies involving clinical populations, adolescents, or older adults (65 years) were analyzed in separate subgroups.</p>
<h3>Moderator Variables</h3>
<p>Pre-specified moderator variables included: aerobic training modality (running vs. cycling vs. rowing/other); aerobic <a href="/terms/training-frequency/" class="term-link" data-slug="training-frequency" title="training frequency">training frequency</a> (sessions per week); aerobic session duration (minutes); aerobic training intensity (% VO2max or % HRmax); intra-session order (resistance before aerobic vs. aerobic before resistance vs. separate sessions); and inter-session recovery time (hours between same-day sessions).</p>
<h3>Statistical Analysis</h3>
<p>Effect sizes for each outcome were computed as <a href="/terms/effect-size/" class="term-link" data-slug="effect-size" title="Cohen's d">Cohen's d</a> (standardized mean differences). Random-effects <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> was used for pooling. Subgroup analyses and meta-regression were employed to examine the influence of moderator variables. All analyses were conducted using Comprehensive Meta-Analysis software version 2.2. Statistical significance was set at p 0.05.</p>
<h2>Results</h2>
<h3>Primary Outcomes</h3>
<p>Across 21 studies meeting inclusion criteria (n = 422 participants), <a href="/terms/concurrent-training/" class="term-link" data-slug="concurrent-training" title="concurrent training">concurrent training</a> produced significantly smaller gains in maximal lower body strength compared with resistance training alone (SMD = −0.34, 95% CI: −0.48 to −0.19, p 0.001). The interference effect was even more pronounced for explosive power, with concurrent trainees exhibiting substantially attenuated vertical jump improvements relative to resistance-only groups (SMD = −0.51, 95% CI: −0.72 to −0.30). <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="Hypertrophy">Hypertrophy</a>, as assessed by lean mass or muscle <a href="/terms/cross-sectional-area/" class="term-link" data-slug="cross-sectional-area" title="cross-sectional area">cross-sectional area</a>, was also significantly blunted by concurrent training (SMD = −0.23, 95% CI: −0.38 to −0.09).</p>
<h3>Effect of Aerobic Modality</h3>
<p>Running-based aerobic training produced a markedly greater interference effect on lower body strength (SMD = −0.52) and power (SMD = −0.73) than cycling-based training (SMD = −0.21 and −0.29 for strength and power, respectively). This modality difference was statistically significant in meta-regression analysis (p = 0.02 for strength; p = 0.01 for power). The greater mechanical similarity between running and lower body resistance exercises, combined with the higher eccentric loading of running, is proposed to explain this differential effect.</p>
<h3>Effect of Aerobic Volume and Frequency</h3>
<p>A <a href="/terms/dose-response-relationship/" class="term-link" data-slug="dose-response-relationship" title="dose-response relationship">dose-response relationship</a> between aerobic <a href="/terms/training-volume/" class="term-link" data-slug="training-volume" title="training volume">training volume</a> and interference was observed. Studies employing aerobic training ≥3 times per week produced larger interference effects (SMD = −0.45 for strength) than those with ≤2 sessions per week (SMD = −0.18). Similarly, aerobic session durations exceeding 30 minutes produced greater attenuation of strength gains.</p>
<h3>Upper Body Outcomes</h3>
<p>Importantly, no significant interference effect was detected for upper body strength outcomes (SMD = −0.08, 95% CI: −0.22 to +0.06, p = 0.27), suggesting that aerobic exercise, which primarily loads lower body musculature, does not meaningfully impair upper body resistance training adaptations.</p>
<h2>Discussion</h2>
<p>This <a href="/terms/meta-analysis/" class="term-link" data-slug="meta-analysis" title="meta-analysis">meta-analysis</a> confirms the existence of a significant <a href="/terms/concurrent-training/" class="term-link" data-slug="concurrent-training" title="interference effect">interference effect</a> when aerobic and resistance training are combined, but importantly demonstrates that this effect is highly dependent on program design variables. The finding that running produces substantially greater interference than cycling has important practical implications: athletes and practitioners who wish to minimize interference should preferentially select low-impact aerobic modalities.</p>
<h3>Mechanistic Interpretation</h3>
<p>The greater interference from running is mechanistically consistent with several proposed pathways. Running imposes eccentric stress on the lower body musculature that overlaps substantially with the mechanical demands of lower body resistance exercises such as squats and leg press, creating accumulated structural <a href="/terms/muscle-damage/" class="term-link" data-slug="muscle-damage" title="muscle damage">muscle damage</a> that may impair recovery between sessions [1]. Additionally, running activates AMPK more robustly than cycling at equivalent relative intensities, producing greater inhibitory pressure on <a href="/terms/mtor/" class="term-link" data-slug="mtor" title="mTORC1">mTORC1</a> [2].</p>
<h3>Power vs. Strength vs. <a href="/terms/muscle-hypertrophy/" class="term-link" data-slug="muscle-hypertrophy" title="Hypertrophy">Hypertrophy</a></h3>
<p>The larger interference effect for explosive power (SMD = −0.51) than maximal strength (SMD = −0.34) or hypertrophy (SMD = −0.23) reflects the distinct neuromuscular adaptations underlying each quality. Power is particularly sensitive to impairment because it depends on both force production capacity and rapid <a href="/terms/motor-unit/" class="term-link" data-slug="motor-unit" title="motor unit">motor unit</a> recruitment rate coding—both of which may be compromised by residual fatigue from aerobic training [3].</p>
<h3>Temporal Separation</h3>
<p>Although this meta-analysis was not adequately powered to detect statistically significant effects of intra-session order, mechanistic reasoning and indirect evidence suggest that separating aerobic and resistance sessions by at least 6–8 hours minimizes acute AMPK-mTOR pathway interference [4]. Where same-day training is unavoidable, resistance exercise should generally precede aerobic exercise, as post-resistance fatigue is less likely to compromise aerobic performance than vice versa.</p>
<h3>Recommendations for Practice</h3>
<p>For individuals prioritizing strength and hypertrophy, aerobic training should be limited to ≤2 sessions per week of ≤30 minutes duration, preferably using cycling or rowing rather than running. When running is preferred for health or sport-specific reasons, lower body resistance <a href="/terms/training-volume/" class="term-link" data-slug="training-volume" title="training volume">training volume</a> may need to be increased to compensate for the attenuated hypertrophic stimulus.</p>
<h3>References</h3>
<p>[1] Leveritt M, et al. Concurrent strength and endurance training. <em>Sports Med</em>. 1999;28:413–427.
[2] Fyfe JJ, et al. Interference between concurrent resistance and endurance exercise. <em>Sports Med</em>. 2014;44:743–762.
[3] Wilson JM, et al. Concurrent training meta-analysis. <em>J Strength Cond Res</em>. 2012;26:2293–2307.
[4] Baar K. Using molecular biology to maximize concurrent training. <em>Sports Med</em>. 2014;44:117–125.</p>