The desire to maximize both cardiovascular fitness and muscle size simultaneously often leads to a scheduling challenge known as concurrent training. Achieving both goals requires a strategic approach, as poorly timed cardio can interfere with the physical adaptations needed for muscle growth. The primary conflict lies in how the body prioritizes the molecular signals sent by endurance exercise versus those triggered by resistance training. Strategic timing is the foundation for effectively combining both types of exercise to prevent one from hindering the other.
Understanding the Interference Effect
The interference effect describes the physiological conflict that occurs when the body attempts to adapt to two opposing training stimuli close together. Resistance training activates a signaling pathway that stimulates muscle growth, while endurance exercise triggers a separate pathway prioritizing energy efficiency. Doing both types of workouts too closely can cause the body to send mixed signals, blunting the gains from resistance training.
The core of this conflict lies in two competing cellular energy sensors: the mechanistic target of rapamycin (mTOR) and adenosine monophosphate-activated protein kinase (AMPK). Resistance training strongly activates the mTOR pathway, which initiates muscle protein synthesis and hypertrophy. Endurance training, particularly high-volume or long-duration activity, significantly increases the activation of AMPK, which functions as a cell’s energy monitor.
When energy stores are depleted during prolonged cardio, the resulting increase in adenosine monophosphate (AMP) triggers AMPK activation. This activated AMPK then acts to inhibit the mTOR pathway, telling the cell to conserve energy and stop the process of building muscle. This biochemical crosstalk creates a temporary molecular environment that works against the muscle-building signal generated by lifting weights. Therefore, strategic timing allows the anabolic mTOR signal to fully propagate before the catabolic AMPK signal can interfere.
Optimal Separation: Time Gaps Between Workouts
The most effective strategy to prevent the interference effect is to separate your cardio and resistance training (RT) sessions by a sufficient amount of time. Performing cardio immediately before a weightlifting session is not recommended because it depletes glycogen stores and causes fatigue, which compromises the quality and intensity of the resistance workout. A lower-quality weightlifting session provides a weaker muscle-building stimulus.
If you must perform both types of exercise on the same day, current evidence suggests aiming for at least a six-hour separation between the sessions. This gap allows post-exercise fatigue to dissipate and gives the molecular signaling pathways time to progress toward their intended adaptation. Research indicates that waiting four or eight hours after intense cardio may still not be enough time for full recovery, potentially impacting the ability to lift heavy weights during the RT session.
For optimal recovery and to ensure peak strength, a 24-hour separation between a major resistance workout and an intense cardio session is often considered the ideal. This 24-hour window minimizes lingering fatigue or molecular interference, allowing the anabolic signaling from the RT session to proceed unhindered. If you combine the two types of training into one session, prioritize resistance training first to ensure energy and focus are directed toward the muscle-building stimulus.
Minimizing Interference Through Cardio Selection
When separating workouts by many hours is not practical, the type and duration of cardio you choose become the primary tools for minimizing interference. Not all cardio is equally detrimental to muscle gain; the degree of interference depends on the total volume and intensity of the endurance activity. High-Intensity Interval Training (HIIT) and Low-Intensity Steady State (LISS) cardio affect the muscle-building process differently.
Research suggests that shorter bouts of HIIT may cause less interference with hypertrophy than prolonged, moderate-intensity, or LISS cardio. This is partly because HIIT, with its short, powerful bursts, is metabolically closer to resistance training and less likely to trigger the high-volume AMPK activation that blunts mTOR signaling. Low-impact HIIT, such as cycle sprints, is often recommended as it reduces muscle damage and recovery cost associated with high-impact activities like running sprints.
Conversely, long-duration, moderate-intensity cardio, especially sessions exceeding 30 to 40 minutes, can be more problematic for muscle gain when performed too close to resistance training. The sustained nature of this exercise mode can lead to significant energy depletion, strongly activating the AMPK pathway and potentially interfering with anabolic signals. If you must perform LISS cardio on the same day as weightlifting, keep the sessions short, ideally under 30 minutes, and use low-impact modalities like walking or cycling at a Zone 2 intensity, which primarily uses fat for fuel and helps preserve muscle glycogen stores.
Structuring Your Weekly Concurrent Schedule
Translating these principles into a weekly schedule requires intentional planning to maximize recovery and signaling pathways. A practical approach is to use dedicated “cardio days” or “rest days” for your most taxing endurance work, ensuring a full 24-hour gap from the previous or next major weightlifting session. For example, if you lift heavy on Monday morning, schedule intense cardio for Tuesday morning or evening.
Another effective strategy involves separating cardio and resistance training by body part splits. You can perform cardio that primarily uses the upper body, like rowing, on a day you train lower body, and vice versa. This minimizes localized fatigue in the muscles being targeted for hypertrophy. Scheduling a light, low-impact LISS session after a resistance workout is acceptable, provided the duration is short, as the RT session has already sent the primary anabolic signal.
If your schedule forces you to train both on the same day, aim to schedule resistance training in the morning and cardio in the late afternoon or evening, achieving the six-hour minimum separation. This allows you to perform the most demanding exercise, weightlifting, while you are fresh, leading to a higher quality stimulus for muscle growth. By prioritizing the quality of your weightlifting sessions and strategically managing the timing and type of your cardio, you can successfully pursue both strength and endurance adaptations.