Combining resistance training for muscle growth with endurance training like running is known as concurrent training. While highly effective for overall health and physical fitness, this approach presents a physiological conflict. The fundamental challenge is finding the precise balance where one type of training does not undermine the adaptations of the other. The body receives mixed signals from these two divergent stimuli, creating an “interference effect” that can diminish muscle and strength gains.
The Biological Interference: Why Running and Lifting Compete
The conflict between building muscle and improving endurance occurs at a molecular level within muscle cells. Resistance exercise triggers a signaling pathway that promotes muscle growth, primarily through the activation of a complex called mTOR. This pathway is the body’s primary mechanism for increasing muscle protein synthesis, leading to hypertrophy.
Running, especially long-duration or high-intensity running, triggers signals aimed at optimizing endurance and energy efficiency. This process activates a central energy sensor known as AMP-activated protein kinase (AMPK). When a person runs for an extended period, the resulting energy stress activates AMPK, which acts as a molecular “brake” on energy-consuming processes like muscle growth.
AMPK can inhibit the mTOR pathway, telling the muscle cell to prioritize survival and energy conservation over building new tissue. When the body is under significant metabolic stress from running, endurance adaptations take precedence, blunting the anabolic response stimulated by resistance training. This interference effect is most pronounced when both types of training are performed with high frequency and intensity.
Practical Thresholds: When Running Volume Becomes Detrimental
The point at which running volume becomes detrimental is individualized, but research points to clear thresholds in duration, frequency, and intensity. The negative impact on muscle size and strength gains is directly proportional to the total volume of endurance work performed.
Studies indicate that incorporating endurance training three or more times per week can begin to hinder improvements in muscle fitness compared to resistance training alone. Maintaining muscle mass becomes difficult when running frequency is high. For those serious about hypertrophy, limiting running to two sessions per week has a much lesser impact on strength gains.
The duration of individual running sessions is another factor. Sessions longer than 45 to 60 minutes are often cited as a tipping point for significant interference. Longer durations deplete muscle glycogen stores and sustain the activation of AMPK, extending the catabolic signaling period. Running long distances, such as a half-marathon or marathon, causes sustained muscle damage that compromises recovery.
The type of running also dictates the degree of interference. Low-intensity steady-state (LISS) running contributes to overall volume and can deplete energy reserves if performed for too long. High-intensity interval training (HIIT) is often more favorable for muscle retention. HIIT uses short, powerful bursts similar to resistance training and creates less sustained metabolic stress than long-distance running.
Optimizing Concurrent Training: Strategies to Preserve Muscle Mass
Minimizing the interference effect requires strategic planning around the timing and composition of your workouts, alongside proper nutritional support. Separating your running and lifting sessions is the most effective way to allow the molecular signaling pathways to reset. Aim to space the two types of exercise by at least six hours, giving the AMPK signal time to return to its baseline level before introducing the anabolic stimulus of lifting.
Prioritizing your resistance training session before your run can also be beneficial. This ensures the muscle-building stimulus is delivered when you are fresh and energy stores are full, maximizing the mTOR response before the AMPK signal from running is introduced.
Nutrition plays a key role in mitigating the catabolic effects of high running volume. A consistently high intake of protein, distributed evenly throughout the day, is necessary to maximize muscle protein synthesis and recovery. Maintaining adequate carbohydrate availability is critical to fuel runs and resistance sessions, preventing the body from breaking down muscle tissue for energy.
Finally, be selective about the kind of running you do. Replace long, steady-state runs with short-duration, high-intensity efforts like sprints or hill repeats. This approach minimizes the total volume and metabolic stress that triggers the interference effect while still improving cardiovascular fitness. This high-intensity work also promotes the recruitment of fast-twitch muscle fibers, which are the fibers most responsible for muscle size and power.