Cardiovascular exercise, commonly referred to as cardio, is any activity that raises your heart rate and keeps it elevated for a sustained period (e.g., running, swimming, or cycling). The primary goal of this training is to improve heart health and muscular endurance. The question of whether cardio also contributes to muscle growth, or hypertrophy, is common and nuanced, depending on the specific type of cardio performed and the individual’s training status.
Understanding the Difference Between Hypertrophy and Endurance Adaptations
The body utilizes two distinct molecular pathways to regulate adaptations to different forms of exercise. Resistance training, the primary driver of muscle growth, activates the mammalian target of rapamycin (mTOR) pathway. The mTOR pathway senses mechanical tension and metabolic stress, promoting the synthesis of new muscle proteins and leading to increased muscle size and strength. This process primarily targets the fast-twitch, Type II muscle fibers responsible for powerful, explosive movements.
Conversely, traditional, long-duration endurance cardio activates the adenosine monophosphate-activated protein kinase (AMPK) pathway. AMPK is the cell’s energy sensor, activating when energy stores, particularly glycogen, are depleted during sustained activity. Its activation promotes mitochondrial biogenesis—the creation of new powerhouses within muscle cells—enhancing their ability to use oxygen and resist fatigue. This adaptation is characteristic of slow-twitch, Type I muscle fibers.
These two pathways often compete, an effect sometimes referred to as antagonistic crosstalk. When activated, AMPK can inhibit the mTOR pathway, telling the muscle cell to prioritize energy conservation and efficiency over growth. Consequently, performing long-duration, low-intensity cardio, which strongly activates AMPK, can blunt the muscle-building signals generated by resistance training, limiting hypertrophy.
When Cardio Can Stimulate Muscle Growth
While long, slow cardio is not a significant stimulus for hypertrophy, certain forms can promote muscle growth. This occurs when the activity is structured to mimic the high mechanical tension and metabolic stress of resistance training, recruiting fast-twitch muscle fibers. The structure of the exercise, rather than the duration, becomes the defining factor for muscle gain.
High-Intensity Interval Training (HIIT) is the most prominent example, involving short bursts of near-maximal effort followed by brief rest periods. These intense intervals demand the rapid recruitment of Type II muscle fibers and create significant metabolic stress, a powerful signal for the mTOR pathway. Studies suggest that HIIT protocols can activate a high percentage of Type II muscle fibers, creating conditions for protein synthesis and measurable muscle development.
Cardio modalities that incorporate resistance can bridge the gap between endurance and strength training. Activities like weighted sled pushes, heavy resistance cycling sprints, or uphill sprints place a high mechanical load on the leg muscles. This heavy load forces the muscle to contract powerfully, similar to lifting weights, generating sufficient tension to stimulate growth signals. Maintaining high intensity and short duration provides a dual benefit of cardiovascular fitness alongside a growth stimulus for fast-twitch muscle fibers.
Optimizing Concurrent Training for Maximum Gains
Individuals aiming to maximize both muscle hypertrophy and cardiovascular fitness must use strategic planning when integrating both types of training—known as concurrent training—to manage the interference effect. The primary goal is to prevent the AMPK-driven endurance adaptations from suppressing the mTOR-driven muscle growth signals. Simply combining the workouts without thought can lead to suboptimal results in both strength and endurance.
Timing the sessions is the most effective way to minimize competition between signaling pathways. Research suggests separating resistance training and endurance training sessions by at least six hours is beneficial, with more than eight hours being optimal. This separation allows the initial molecular signaling cascade from the first workout to largely subside before the second workout introduces a competing signal.
Nutritional support is important when engaging in concurrent training. Adequate calorie intake is necessary to fuel both intense activities and support muscle tissue repair and growth. Consuming sufficient protein is particularly important, as it provides the amino acid building blocks required to maximize the mTOR response following resistance exercise. Prioritizing the resistance training session when energy levels are highest, such as performing it earlier in the day, ensures the highest quality of work for the hypertrophy goal.