The decision to include aerobic activity, or cardio, during a muscle-building phase is often debated. Bulking is a dedicated training period where an individual intentionally consumes a calorie surplus to maximize muscle hypertrophy alongside resistance training. This approach conflicts with cardio’s primary function: burning calories and increasing energy expenditure. The central question is whether the benefits of maintaining cardiovascular fitness outweigh the risk of burning the surplus calories required for muscle growth.
Systemic Benefits of Aerobic Activity During Bulking
Maintaining aerobic fitness during bulking offers significant advantages beyond calorie expenditure. Regular low-to-moderate intensity activity strengthens the heart and improves circulation, helping manage blood pressure and cholesterol, which supports general well-being even with a higher-calorie diet.
Aerobic exercise also enhances muscular recovery after intense weightlifting. Low-intensity activity increases blood flow to trained muscles, accelerating the clearance of metabolic waste products like lactate. This improved circulation ensures that nutrients are efficiently delivered to the muscle tissue for repair and growth.
Consistent aerobic conditioning can improve nutrient partitioning. By maintaining insulin sensitivity, the body directs surplus calories toward muscle cells for growth rather than storing them as fat. Conditioning also increases work capacity, allowing an individual to sustain a higher volume of training or maintain better performance during heavy lifting.
Minimizing Metabolic Interference with Hypertrophy
The primary concern regarding cardio during bulking is the “interference effect,” where simultaneous endurance and strength training can blunt muscle growth. This effect stems from competing molecular signaling pathways. Resistance training triggers the mTOR (Mechanistic Target of Rapamycin) pathway, the regulator for protein synthesis and muscle hypertrophy.
Prolonged or high-intensity endurance exercise activates the AMPK (AMP-Activated Protein Kinase) pathway, responsible for energy regulation. AMPK activation can inhibit the mTOR pathway, sending a mixed signal to the muscle and compromising the anabolic environment necessary for maximal growth.
To avoid this antagonism, the intensity and duration of aerobic work must be carefully controlled. High-volume or high-intensity cardio, especially that causing significant energy depletion, is most likely to activate AMPK and interfere with mTOR. Activities pushing the heart rate into the 65% to 80% range for extended periods are most likely to cause this metabolic clash. Therefore, minimizing duration and keeping intensity low is the most effective strategy to preserve muscle-building signals.
Strategic Implementation: Type, Duration, and Timing
Strategic implementation involves selecting the right type, managing frequency and duration, and carefully timing sessions relative to resistance training. The two main types of cardio are Low-Intensity Steady State (LISS) and High-Intensity Interval Training (HIIT). LISS, maintaining a consistent pace at a moderate effort level, is generally the preferred choice during bulking.
LISS activities, such as brisk walking or light cycling for 20 to 30 minutes, are less metabolically taxing and cause minimal AMPK activation, avoiding interference with the mTOR pathway. HIIT involves short bursts of near-maximal effort and is highly effective at burning calories and activating AMPK. This makes HIIT a less ideal choice for bulking due to its high potential for signaling interference and calorie expenditure.
A practical guideline for LISS is two to three sessions per week, keeping each session under 40 minutes to minimize energy burn. Timing is the most important factor for preventing the interference effect. Ideally, aerobic activity should be performed on separate days from weight training to allow molecular signaling to commit to one adaptation. If same-day training is necessary, separate cardio and resistance sessions by a minimum of six hours. This allows the mTOR signal from weight training to peak without acute inhibition by AMPK activation.