Spacing out cardio and resistance training workouts is recommended because of how these activities affect your body on a molecular, hormonal, and recovery level.
1. Conflicting Molecular Pathways
Cardio and resistance training activate different molecular pathways that can interfere with each other when performed back-to-back.
Cardio (Endurance Training): Activates the AMPK pathway, which enhances mitochondrial biogenesis and aerobic capacity. AMPK is often referred to as a “catabolic” pathway — it promotes energy breakdown.
Resistance Training: Activates the mTOR pathway, which promotes muscle protein synthesis and growth. This is an “anabolic” pathway, supporting tissue building and repair.
When these pathways are activated simultaneously, AMPK can inhibit mTOR, potentially reducing the effectiveness of muscle-building processes.
2. Hormonal Interference
Cardio increases cortisol, a catabolic hormone associated with energy metabolism and stress. Excess cortisol can interfere with muscle recovery and protein synthesis.
Resistance Training stimulates anabolic hormones like testosterone and growth hormone, which are crucial for muscle repair and growth.
Performing cardio immediately before or after resistance training can elevate cortisol levels, potentially blunting the anabolic effects of lifting.
3. Energy Systems and Fatigue
Cardio primarily uses the oxidative (aerobic) energy system. Resistance Training primarily uses the phosphagen and glycolytic (anaerobic) energy systems.
If you perform cardio before resistance training, you may deplete glycogen stores and reduce the intensity of your lifting session. Conversely, doing resistance training first can impair your endurance. Performing both in close succession can overtax your muscles, delay recovery, and increase the risk of overtraining.
Space Them Out by at Least 3 Hours
When both types of workouts are done on the same day, waiting at least 3 hours allows your body to partially recover and reset the molecular and hormonal environment. This reduces the interference between AMPK and mTOR pathways.
Spacing cardio and lifting to be on separate days is preferred.
If you must combine them, prioritize your primary goal:
- If your goal is endurance, do cardio first.
- If your goal is strength or muscle growth, do resistance training first.
Alternatively, low-intensity cardio (walking or cycling) can be used as active recovery after resistance training without significant interference.
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