Bulking is a deliberate nutritional phase aimed at maximizing muscle growth through resistance training and a controlled calorie surplus. The primary goal is to provide the body with more energy and raw materials than it expends, creating an environment where muscle tissue can be built efficiently. Running, a form of exercise known for increasing energy expenditure, creates tension with this goal. While running offers health benefits, its inclusion during a bulk must be carefully managed to prevent it from undermining the central objective of gaining muscle. This balance requires understanding how each activity influences energy balance and molecular signaling.
The Metabolic Conflict of Energy Expenditure
Bulking fundamentally depends on maintaining a consistent positive energy balance, meaning daily calorie intake must exceed Total Daily Energy Expenditure (TDEE). For optimal muscle gain with minimal fat accumulation, a modest daily surplus of approximately 250 to 500 calories above maintenance is recommended. This specific surplus ensures the body has enough energy to fuel intense resistance training and synthesize new muscle protein without leading to excessive storage of fat.
Running significantly increases TDEE by burning a substantial number of calories. A moderate-intensity run can easily burn several hundred calories, which immediately erodes the carefully calculated surplus needed for muscle growth. If a person burns 400 calories during a run, they must consume an additional 400 calories just to return to their maintenance level, plus the standard bulking surplus.
Failing to compensate for the calories burned during a run effectively shifts the body from an anabolic state toward a maintenance or even a catabolic state. When the body is forced to rapidly mobilize energy stores because the calorie intake is insufficient, it may begin to break down tissues, including muscle, for fuel. This metabolic shift directly counteracts the purpose of the bulking phase, making meticulous nutritional tracking necessary.
Understanding the Interference Effect
Beyond the simple calorie equation, running introduces a physiological conflict at the cellular level known as the “interference effect” or concurrent training effect. Resistance training and endurance training send competing signals to the muscle cells, which can dampen the muscle-building response. This conflict is rooted in the activation of two different molecular pathways.
Resistance training, the stimulus for muscle growth, activates the mammalian target of rapamycin (mTOR) pathway, a molecular signal that drives protein synthesis and ultimately leads to muscle hypertrophy. In contrast, endurance activities like running activate the adenosine monophosphate-activated protein kinase (AMPK) pathway. The AMPK pathway is primarily responsible for sensing low energy levels and promoting energy conservation and fat oxidation.
When activated simultaneously or in close succession, the AMPK pathway can inhibit the mTOR pathway, effectively putting a brake on muscle protein synthesis. This cross-talk means that even if a person consumes enough calories, high-volume or high-intensity running can still reduce the magnitude of muscle gain achieved from the resistance training. The intensity of the endurance exercise is a key factor.
Strategic Cardio for Muscle Preservation
Incorporating running while bulking is possible, but it requires strategic planning to minimize both the metabolic and molecular conflicts. The type of running performed is the first variable to control, with Low-Intensity Steady State (LISS) cardio being the preferred method. LISS, such as a light jog or brisk walk, maintains a lower heart rate, burns fewer calories per minute, and creates less molecular signaling interference compared to High-Intensity Interval Training (HIIT).
The timing of the running session relative to the weightlifting session is also a significant factor in mitigating the interference effect. It is advisable to separate the two types of workouts by a minimum of six to eight hours, or ideally, to perform them on entirely separate days. Performing cardio immediately after a resistance training session is the least optimal approach, as the endurance signals would immediately compete with the active muscle-building signals.
Running during a bulk should focus on maintaining cardiovascular fitness rather than improving endurance performance. Limiting running to two or three short sessions per week, with each session lasting no more than 20 to 30 minutes, is a common recommendation. This frequency and duration are sufficient to gain the health benefits of cardio, such as improved recovery and blood flow, without compromising the primary goal of muscle hypertrophy.