Education
Cardio & the interference effect
Fat-burning zone and fasted cardio myths, mTOR vs AMPK, Pontzer’s constrained expenditure, and Henselmans integration rules.
1. Demolishing cardio myths for fat loss
Most people start cardio to lose fat using outdated magazine rules that metabolic research has invalidated.
The "fat burning zone" illusion
Myth: train strictly at ~60–70% HRmax because a higher fraction of fat is burned acutely. Reality: lower intensity does raise fat oxidation during the bout, but body composition follows 24-hour energy balance—not session substrate. Burn glycogen hard and fat oxidation rises later. The zone adds no practical advantage for subcutaneous fat loss.
Fasted cardio misconception
Myth: morning fasted work burns more fat with low insulin and depleted glycogen. Reality: meta-analyses show identical fat loss for fasted vs fed cardio. Extra morning fat oxidation is compensated over the next 24 hours. Hard fasted cardio cuts power output and can increase muscle protein breakdown.
2. The interference effect
Chasing maximal hypertrophy and high aerobic fitness together triggers concurrent training interference.
Antagonistic signaling
- Resistance training: mTOR drives protein synthesis and hypertrophy.
- Prolonged cardio: AMPK drives mitochondrial adaptations and endurance but suppresses mTOR.
Mechanical damage
High-impact cardio (road running) imposes eccentric micro-trauma. Recovery goes to repair instead of hypertrophy supercompensation—strength and effective volume fall.
3. Constrained energy expenditure
Coaches often assume cardio calories stack additively on resting metabolism. Herman Pontzer’s work supports a constrained model instead.
Large cardio increases trigger energy saving elsewhere—NEAT drops (less fidgeting, more sitting). Metabolic adaptation offsets much of the gross burn. Cardio is a weak primary deficit tool versus nutrition control.
4. Henselmans rules for integrating cardio
Cardio is not required for leanness; add it when further food cuts are psychologically or physically untenable. If you use it, apply three rules to limit interference:
- Low-impact, concentric-biased modes: cycling or incline walking—no running-style eccentric impact; preserve recovery for squats.
- Temporal separation: cardio on non-lifting days; same day—≥6 hours apart, or split into very short pre/post lifting blocks.
- Non-target muscles: leg hypertrophy goal—avoid long cycling; use rowing or upper-body ergometry to spare the lower-body pool.