Interference effect

The interference effect is the reduction in strength gains that can occur when endurance exercise and lifting are performed back-to-back, compared with lifting alone, even as endurance performance improves.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 229.} Muscle growth can suffer too: a meta-analysis found that the effect size for muscle gains fell by almost 50% when aerobic training was added to lifting.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 169.}

Two proposed mechanisms explain this concurrent training effect.^{3Rønnestad and Mujika, Optimizing strength training for running and cycling endurance performance, p. 5.},^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 164.} Endurance exercise activates AMPK, which may inhibit mTOR signaling and suppress the myofibrillar protein synthesis triggered by lifting.^{3Rønnestad and Mujika, Optimizing strength training for running and cycling endurance performance, p. 5.} The AMPK response rises with endurance intensity.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 169.} Aerobic work can also leave residual fatigue and depleted fuel, reducing the quality and amount of work in the next lifting session.^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 164.},^{5Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 172.}

What remains uncertain

Schoenfeld describes mixed evidence for the cell-level explanation: some studies show impeded anabolic signaling and others show no harm.^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 164.} These measurements cover hours, while interference would develop over weeks or months.^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 164.} Several studies also found similar strength gains regardless of whether aerobic or resistance work came first in the same session.^{5Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 172.}

In practice

• Space the sessions: interference grows as they get closer, and same-day training impedes gains more than alternate days.^{6Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 245.} A long recovery gap, or possibly separate days, appears to minimize it.^{5Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 172.}
• Keep aerobic frequency low when muscle gain is the priority.^{7Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 171.} In one study, 0, 1 or 3 aerobic days alongside lifting produced limb girth gains of 4.3%, 2.8% and 1%, respectively.^{7Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 171.}
• Higher-intensity endurance work appears to interfere more than lower-intensity work.^{2Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 169.}
• Heavier strength loads are less compatible with endurance training.^{6Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 245.}