Bulking is a training phase focused on maximizing muscle growth by consuming more calories than the body expends, creating a caloric surplus. Cardiovascular exercise (cardio) elevates the heart rate for an extended period. Historically, cardio was seen as a direct inhibitor of muscle gains. However, combining cardio with a bulking phase is possible and beneficial, provided the strategy is carefully managed to prevent physiological competition.
Understanding the Physiological Conflict
The primary concern regarding concurrent training stems from the “interference effect,” where endurance work blunts the adaptations stimulated by resistance training. This conflict occurs at a molecular level, involving two competing signaling pathways. Resistance training activates the mammalian target of rapamycin (mTOR) pathway, the primary driver of muscle protein synthesis and hypertrophy.
Endurance exercise, especially when prolonged or intense, activates the adenosine monophosphate-activated protein kinase (AMPK) pathway. When AMPK is highly active, it suppresses the mTOR pathway, signaling the body that energy should not be used for building new muscle tissue. This molecular crosstalk compromises the anabolic signal sent by weightlifting. Furthermore, a simple energy conflict exists because cardio burns a significant number of calories. To remain in the caloric surplus necessary for bulking, the calories expended during cardiovascular sessions must be added back into the daily food intake, which can be challenging to manage consistently.
Strategic Integration of Cardio Types
The type of cardiovascular exercise performed is a major determinant in minimizing the interference effect. Low-Intensity Steady State (LISS) cardio is generally the preferred choice during a bulking phase.
Low-Intensity Steady State (LISS)
LISS activities include brisk walking, light cycling, or using an elliptical machine at a sustainable, moderate pace, typically keeping the heart rate between 60% and 70% of its maximum. LISS is less systemically fatiguing than other forms of cardio, which allows for better recovery between demanding weight training sessions. Because the intensity is low, the body relies more on fat for fuel, which helps to preserve the glycogen stores needed for lifting performance and minimizes the activation of the competing AMPK pathway. Incorporating two to three LISS sessions per week, with a duration of about 20 to 30 minutes, is a common recommendation to gain health benefits without hindering muscle growth.
High-Intensity Interval Training (HIIT)
High-Intensity Interval Training (HIIT) is characterized by short bursts of all-out effort followed by brief rest periods. While HIIT is very time-efficient, its high-intensity nature creates a much greater recovery cost and dramatically increases the caloric expenditure. The significant physiological stress from intense HIIT can heavily activate the AMPK pathway and interfere more substantially with muscle adaptations. If an individual chooses to include HIIT, it should be limited to a maximum of one session per week and kept short, around 10 to 15 minutes of work intervals. For those prioritizing muscle mass gain, the high fatigue and potential for interference from multiple HIIT sessions outweigh the benefits.
Managing Energy Balance and Session Timing
Successful integration of cardio requires meticulous management of the energy equation to ensure the caloric surplus remains intact. The calories burned during each cardio session must be accurately estimated and promptly consumed through food to maintain the energy excess necessary for muscle hypertrophy. This nutritional adjustment is non-negotiable; otherwise, the additional activity will reduce the surplus and stall muscle growth.
Session Timing
The timing of the cardio session relative to resistance training is also a major factor in minimizing the interference effect. It is highly beneficial to physically separate the training modalities by a minimum of four to six hours. This separation allows the acute signaling effects of the first workout, particularly the AMPK activity induced by cardio, to subside before the second session begins. If both must be performed in the same session, resistance training should be completed first to maximize the muscle-building signal. Proper fueling around these sessions is equally important, particularly consuming carbohydrates before and after the cardio to replenish muscle glycogen stores needed for high-quality weightlifting performance and recovery.