PHYS-22 · Principle
Why are my legs wrecked after a trail race but fine after five hours on the bike?
Running, above all downhill, lengthens working muscle under load, which damages it; pedalling is mostly shortening work. Strain, not effort, decides.
How the body works, agreed across the reviews below; a study entry shows what one experiment measured.
Practical Application
Why are my legs wrecked after a trail race but fine after five hours on the bike? Running, above all downhill, lengthens working muscle under load, which damages it; pedalling is mostly shortening work. Strain, not effort, decides.
Mechanism
When a contracting muscle is forcibly lengthened, the weakest sarcomeres are pulled beyond overlap and disrupted, and the strain spreads: damage tracks mechanical strain, not metabolic cost (REF-704). Done eccentrically, an exercise costs about a quarter of the energy; damage spreads over 2-3 days (REF-1347); the whole response is "incompletely understood" (REF-1343).
Common misreading
"Hard means damaging": a maximal but smooth effort on the trainer can cost more energy and less tissue than a long descent.