Concurrent Training
The interference effect between strength and endurance training
Plain English
Concurrent training is the practice of combining resistance training and endurance training in the same program. It is what most people who lift and do cardio are doing without naming it. The challenge is that the two adaptations activate partially conflicting cellular pathways, and when volume or proximity of sessions is not managed, endurance training can blunt strength and hypertrophy gains. This conflict is called the interference effect.
The Mechanism
Resistance training primarily activates the mTOR pathway, which drives muscle protein synthesis and hypertrophy. Endurance training primarily activates the AMPK pathway, which promotes mitochondrial biogenesis, fat oxidation, and cardiovascular adaptation. These two pathways have an antagonistic relationship: AMPK activation inhibits mTOR signaling, which is why high-volume endurance work in close proximity to resistance training can suppress hypertrophic adaptation.
The interference effect was first described by Robert Hickson in 1980 in a study showing that adding concurrent endurance work reduced strength gains by roughly 50% compared to strength training alone over 10 weeks. Subsequent research has clarified that the interference effect is dose-dependent and modality-specific: high-volume running produces significantly more interference than cycling, likely because of the eccentric muscle damage from running adding fatigue on top of the molecular conflict.
Critically, the interference effect is not binary. Low to moderate volumes of Zone 2 cardio (150 to 180 minutes per week) can be combined with resistance training without meaningful hypertrophy loss, particularly when sessions are separated by at least 6 hours or placed on different days. The interference effect becomes practically significant at high endurance volumes, during intense cardio (Zone 4 to 5), or when cardio and lifting are performed in close succession.
Why It Matters
Cardio and lifting can coexist; the interference effect is a dosing problem, not a binary conflict.
If your primary goal is muscle growth or maximal strength, program structure matters: high-volume running immediately before resistance training, or very high weekly running volume alongside lifting, will reduce hypertrophy outcomes. But the interference effect is manageable. Most people chasing body composition and health can combine moderate cardio with resistance training effectively by separating sessions, prioritizing lifting, and keeping cardio in Zone 2.
Common Misconception
A common overcorrection is to eliminate all cardio from a hypertrophy program for fear of the interference effect. This is unnecessary at moderate cardio volumes. The more common and damaging error is the opposite: performing high-intensity cardio immediately before heavy lifting, which both depletes glycogen and adds central fatigue before the resistance session begins.
How to Improve It
3 Things to Remember
The interference effect (AMPK inhibiting mTOR) is dose-dependent: low to moderate Zone 2 cardio does not meaningfully blunt hypertrophy.
High-intensity running immediately before lifting produces the worst interference; separating sessions by 6+ hours or using different days eliminates most of the conflict.
Cycling produces less interference than running, making it the better concurrent training choice when hypertrophy is the priority.
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