Bodybuilding is a discipline focused on maximizing muscle mass while simultaneously minimizing body fat to achieve a highly defined physique. While resistance training is the foundation of muscle development, running and other forms of cardio are viewed with caution due to their potential to interfere with hypertrophy. The relationship is not one of complete avoidance but one of careful, calculated integration to achieve specific body composition goals.
The Physiological Conflict Between Running and Muscle Growth
The primary concern bodybuilders have with running, especially high-volume endurance running, is the interference effect. This effect describes how combining endurance training and resistance training can diminish the gains from resistance work. At the cellular level, resistance exercise triggers the mTOR pathway, which is the mechanism responsible for muscle protein synthesis and growth.
Endurance activities activate a competing cellular pathway called the AMPK pathway. AMPK promotes the breakdown of fuel sources for immediate energy, which is a catabolic process. When activated, AMPK can inhibit the mTOR pathway, essentially slowing down the signal for muscle growth.
Performing significant amounts of running can create a cellular environment that counteracts the muscle-building signals generated by weight lifting. The body struggles to adapt optimally for both muscle endurance and muscle size simultaneously. The conflict is particularly pronounced with prolonged, moderate-intensity running, which keeps the AMPK pathway highly active.
Strategic Use of Running in Bodybuilding Phases
Despite the physiological conflict, bodybuilders do use running strategically. The two main phases are “bulking” and “cutting,” each with distinct goals that dictate the role of cardio. In the bulking phase, the goal is to gain as much muscle and strength as possible by eating in a calorie surplus.
During bulking, running is minimized to conserve calories and recovery resources needed for muscle hypertrophy. If cardio is used at all, it is typically low-intensity to maintain cardiovascular health without impeding muscle gain. The cutting phase requires a calorie deficit to shed body fat while preserving the hard-earned muscle mass.
Running becomes a valuable tool in the cutting phase to create the necessary calorie deficit for fat loss. To mitigate the interference effect during this time, bodybuilders often separate their running sessions from their weight training sessions by several hours, ideally six or more, or place them on different days. This separation allows the mTOR pathway to be fully activated after resistance training before the AMPK pathway from running is stimulated.
Choosing the Right Cardio Modality for Muscle Preservation
The type of running or cardio chosen is just as important as the timing when the goal is muscle preservation. Long-distance running is avoided because its volume and duration maximize the systemic fatigue and cellular interference effect. Bodybuilders prefer modalities that efficiently burn calories with the lowest possible impact on muscle recovery.
Low-Intensity Steady State (LISS) cardio involves maintaining a consistent, manageable pace, such as a brisk walk or a slow jog. LISS is typically performed at about 60-70% of maximum heart rate for 30 to 60 minutes. This lower intensity is less taxing on the central nervous system, reduces the activation of the AMPK pathway, and is less likely to break down muscle tissue for fuel.
High-Intensity Interval Training (HIIT) involves short bursts of near-maximal effort followed by brief rest periods. HIIT sessions are shorter, and are highly efficient at burning calories. While HIIT can be highly effective for fat loss, its high-impact nature and significant fatigue can pose a greater risk of injury or overtraining if performed too frequently or too close to heavy leg workouts.