The interference effect is the observation that combining endurance and resistance training in the same program can blunt the adaptations to one or both modalities. First described by Robert Hickson in 1980, the finding has shaped how athletes and coaches program concurrent training for decades. But the magnitude of the interference — and how to minimize it — depends heavily on context.
The Molecular Basis
At the cellular level, endurance and resistance training activate competing signaling pathways. Resistance training stimulates mTORC1 (mechanistic target of rapamycin complex 1), the master switch for muscle protein synthesis. Endurance training activates AMPK (adenosine monophosphate-activated protein kinase), a sensor of cellular energy status that promotes mitochondrial biogenesis and fat oxidation.
The problem is that AMPK directly inhibits mTORC1. When AMPK is activated by a long run or cycling session, it suppresses the anabolic signaling that would otherwise drive muscle growth. This molecular tug-of-war is the mechanistic foundation of the interference effect.
However, the magnitude of this inhibition is dose-dependent. A 20-minute jog does not activate AMPK to the same degree as a 90-minute tempo run. And the duration of AMPK elevation is finite — typically 1 to 3 hours post-exercise, depending on the intensity and duration of the cardio session.
What the Research Shows
Hickson's original 1980 study had subjects performing both heavy strength training and intense endurance training (40 minutes of running or cycling at high intensity) on the same day, 6 days per week. The concurrent group gained strength at the same rate as the strength-only group for the first 7 weeks, then began to lose strength while the strength-only group continued to progress.
More recent and better-controlled studies have painted a more nuanced picture. A 2012 meta-analysis by Wilson and colleagues found that the interference effect is primarily a concern for lower-body power and hypertrophy. Upper-body gains were largely unaffected by concurrent cardio training. The type of cardio also mattered: running (which involves eccentric muscle damage to the legs) produced greater interference than cycling (which is concentric-dominant and produces less damage).
A 2019 study published in the British Journal of Sports Medicine found that when cardio volume was kept moderate (less than 3 sessions per week, less than 30 minutes per session), there was no measurable interference with strength or hypertrophy gains. The interference effect appeared primarily when cardio volume was high — more than 3 hours per week of moderate-to-high intensity endurance work.
Practical Strategies for Minimizing Interference
1. Separate Sessions by Time
If you must perform both cardio and weights on the same day, separate them by at least 6 hours. This allows the AMPK elevation from cardio to subside before mTORC1 is needed for the anabolic response to lifting. Morning cardio followed by evening weights is a common and effective split.
2. Prioritize the Modality That Matters More
If muscle growth is your primary goal, perform weight training first when you are freshest. Save cardio for after, or better yet, for a separate day. Training in a pre-fatigued state reduces the mechanical tension you can generate during your lifts, compromising the primary stimulus for hypertrophy.
3. Choose Low-Impact Cardio Modalities
Cycling, swimming, rowing, and elliptical training produce less eccentric muscle damage than running. Less damage means less inflammation, faster recovery, and less interference with the adaptive response to resistance training. If you enjoy running, keep the volume modest and avoid high-mileage weeks during hypertrophy-focused training blocks.
4. Keep Cardio Volume and Intensity Moderate
Two to three sessions per week of 20 to 30 minutes at a moderate intensity (heart rate zone 2, roughly 60 to 70 percent of max heart rate) is sufficient for cardiovascular health without meaningfully impairing strength or hypertrophy gains. High-intensity interval training (HIIT) can be effective in small doses but generates more systemic fatigue than steady-state work.
5. Fuel Appropriately
Much of the interference effect may be mediated by energy availability. AMPK is a sensor of low cellular energy. If you are eating in a caloric surplus or at maintenance, the AMPK signal from moderate cardio is dampened. If you are dieting in a deficit and adding cardio on top, the combination of energy restriction and AMPK activation creates a strongly anti-anabolic environment. During fat-loss phases, keep cardio modest and caloric deficits moderate.
Who Needs to Worry About This?
For the average gym-goer who lifts 3 to 4 times per week and does 2 to 3 moderate cardio sessions, the interference effect is likely negligible. The people most at risk are hybrid athletes training for both a marathon and a powerlifting meet, or those who add excessive HIIT on top of already high-volume resistance programs.
For most people, the health benefits of regular cardio — improved cardiovascular function, insulin sensitivity, mood, and longevity — far outweigh the theoretical cost of a slightly blunted hypertrophy response. The key is keeping both modalities at sensible volumes rather than maximizing both simultaneously.