Episode Details

Should endurance athletes complete high-intensity intervals while fresh, or place them later in a workout after accumulating fatigue? Paul Warloski and Dr. Paul Laursen explain why the answer depends on the type of fatigue involved. General systemic or neuromuscular fatigue may be a reason to postpone intensity, while lower muscle-glycogen availability can amplify signals associated with mitochondrial development and fatigue resistance. The discussion covers AMPK, fat oxidation, fast-twitch muscle fibers, running mechanics, heat, race specificity, athlete type, training experience, and why experienced athletes may benefit from occasionally performing intervals later in a session—even when their power output is lower.

Key takeaways

• Not all fatigue is the same. Suppressed HRV, neuromuscular fatigue, and feeling systemically depleted are different from deliberately training with reduced glycogen availability.
• Do not force intensity when genuinely under-recovered. When the central nervous system appears flat or muscle tone and coordination are impaired, recovery or easy aerobic work may be more appropriate.
• Lower glycogen can increase the adaptive signal. The episode explains that training with reduced glycogen may amplify AMPK signaling, encouraging mitochondrial development.
• Lower power does not necessarily mean a poor session. Intervals performed later in a workout may produce less power while still creating a strong physiological stimulus.
• The aerobic system supports repeatable power. Training fast-twitch fibers under depleted conditions may help those fibers become more fatigue-resistant while retaining their strength.
• Running requires greater caution than cycling. Fatigue can alter stride length, ground-contact time, stiffness, and movement patterns, potentially increasing injury risk.
• Experienced athletes are better candidates for fatigue-state intervals. Beginners should generally complete a manageable warm-up and perform their intervals with good form and quality.
• Athlete profile matters. “Twitchy” athletes may require more recovery than diesel-type athletes, both between intervals and between demanding training days.
• Heat adds another stressor. High temperatures may increase glycogen use and reduce the ability to sustain interval quality, so heat-based sessions require careful control.
• Use fatigue-state intervals selectively. They are a tool for building durability, not a requirement for every high-intensity workout.

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