The fear that running will undermine progress in muscle size and strength is a common concern among people who lift weights. This worry stems from the principle of specificity, suggesting the body cannot efficiently adapt to two opposing training stimuli simultaneously. “Gains” refer to muscle hypertrophy (increased muscle fiber size) and maximal strength (the ability to produce force). While combining endurance training, such as running, with strength training can create a biological conflict, the degree to which this occurs depends highly on how the training is structured. The potential for running to “ruin” gains is not absolute, but rather a nuanced issue of volume, intensity, and timing.
The Physiological Interference Effect
The core reason for the potential conflict lies in the body’s cellular signaling pathways, which respond differently to endurance and strength exercise. Strength training primarily activates the mammalian target of rapamycin (mTOR) pathway, promoting muscle protein synthesis, hypertrophy, and anabolism. This pathway is responsible for the growth and repair of muscle tissue following a lifting session.
In contrast, prolonged running activates the 5′ adenosine monophosphate-activated protein kinase (AMPK) pathway, which acts as the cell’s energy sensor. When cellular energy stores are depleted during long-duration cardio, AMPK is activated to promote energy utilization and mitochondrial biogenesis—adaptations necessary for endurance. The activation of AMPK has been shown to transiently suppress or inhibit the mTOR pathway, essentially putting a brake on muscle growth signaling.
This competition between signaling mechanisms is the molecular basis of the “interference effect.” The body receives mixed messages about whether to conserve energy for endurance or build muscle mass. Consequently, the simultaneous activation of both pathways can reduce the optimal anabolic response that a strength-only program would produce. The interference effect is most pronounced when strength and endurance training create excessive fatigue and energy depletion.
Running Volume and Intensity
The extent of the interference effect is determined by the specific characteristics of the workout, not the act of running itself. High-volume, long-duration running, often classified as Low-Intensity Steady State (LISS) cardio, causes the most significant interference. This exercise depletes muscle glycogen stores over a prolonged period, which is a major trigger for the catabolic AMPK pathway.
Long, continuous running also creates a high degree of cumulative fatigue and muscle damage, particularly in the lower body. This residual fatigue directly impacts the quality of subsequent strength training sessions. High-volume, moderate, and frequent endurance training is the combination most likely to diminish the strength-building stimulus and negatively affect strength adaptations.
However, incorporating short bouts of High-Intensity Interval Training (HIIT) or moderate-volume aerobic exercise minimizes the negative effects on muscle gains. HIIT sessions, due to their shorter duration, avoid the chronic glycogen depletion and accumulated fatigue associated with long runs. Utilizing low-impact modalities, such as cycling or rowing, further reduces the mechanical stress and muscle damage from repetitive impact, making them safer options for preserving lower-body strength gains.
Strategic Scheduling for Concurrent Training
Minimizing the physiological interference effect requires a deliberate approach to scheduling and recovery. The most effective strategy is to separate strength training and running sessions by a minimum of six to eight hours. This separation allows the acute signaling cascades from the first workout, such as AMPK activation from running, to return toward baseline before the second workout begins. If both sessions must occur on the same day, performing the strength session first is recommended to ensure the highest quality of lifting and a maximal anabolic stimulus.
For those prioritizing muscle and strength development, strength training should be performed when the body is freshest and fully fueled. If sessions cannot be adequately separated, it is beneficial to train different muscle groups. For example, an upper-body lifting session followed by a lower-body focused run can reduce localized fatigue.
Recovery is also a significant factor in managing concurrent training. Adequate caloric intake, particularly carbohydrates and high-quality protein, is necessary to quickly replenish glycogen stores and support muscle protein synthesis. Ensuring proper nutrition and sufficient sleep acts as a buffer against the catabolic signals induced by endurance exercise, allowing the body to sustain both types of adaptation over time.