A typical over-under session: 3 × 10 min as (2 min at 105% FTP / 2 min at 95% FTP, repeated). The physiological stimulus is the challenge of clearing lactate while still producing significant power. Over-unders build the lactate shuttle — the body's ability to move lactate from producing muscles to consuming ones — which translates directly to race ability on hilly terrain and in group rides with surges. More race-specific than pure threshold intervals. Demanding session; one per week maximum during build phases.
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