WHY THIS HAPPENS
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Cardiac drift on long rides — HR rises for the same power as fatigue accumulates
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Dehydration — blood volume drops, HR climbs to maintain cardiac output
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Under-recovery from previous sessions, illness, or poor sleep
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Heat stress — warm days can add 10-15 bpm at the same power
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Caffeine, alcohol, or stress the night before
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Zone miscalibration — your zones may be wrong if FTP hasn't been tested recently
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Overtraining — elevated resting HR for 3+ consecutive days is a red flag
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Under-fuelling — low glycogen drives HR up at lower intensities