When cardio and lifting compete
Concurrent training means combining endurance work and resistance training in the same program. The interference effect is the observation that, under some conditions, heavy endurance training can blunt the strength, power, or hypertrophy gains that the resistance work would otherwise produce. The effect is most often reported for power and high-velocity strength qualities, while pure muscle size appears more robust.
Importantly, the interference is not universal; many people improve both qualities together, especially at moderate training loads.
Dose, modality, and timing decide the outcome
Several variables shape whether interference shows up. Higher endurance volume and frequency raise the risk, as does running compared with lower-impact cycling, partly through residual fatigue and muscle damage. Separating the two sessions in time and avoiding placing exhausting cardio immediately before key lifts can reduce the conflict. Proposed molecular explanations involve competing energy-sensing pathways, though these are only part of the story.
This is general educational information rather than a prescription. The right balance of cardio and lifting depends on individual goals, recovery, and health status, and is best planned with a qualified coach.