When to Perform High-Intensity Intervals

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. TRY ATHLETICA.AI | Adaptive Endurance Training Plans Powered by Sports Science Athlete Profiling Primer Dr. Paul Laursen Paul Warloski - Simple Endurance Coaching