Why are my legs more wrecked after a long run than a long ride?

Millet and Lepers start from the "task dependency" of muscle fatigue: where strength is lost, from the motor cortex to the contractile elements, depends on the type of contraction, the muscles and the duration and intensity of the exercise. They review the then-recent studies of exercise lasting 30 minutes to several hours.

In the abstract central fatigue contributes to muscle fatigue in long-distance running, shown by twitch interpolation, the ratio of EMG during a maximal voluntary contraction to the M-wave, and voluntary against electrically evoked force.

Some central activation deficit was also seen in the knee extensors after cycling, but central fatigue after activities inducing low muscle damage was attenuated compared with running.

Spinal mechanisms — inhibition from group III and IV muscle afferents, or less facilitation from muscle spindles — may contribute to the reduced neural drive, though supraspinal fatigue cannot be ruled out.

After a 30-km run, the individuals with the greatest loss of knee-extensor strength had a significant activation deficit; but central fatigue alone cannot explain the whole loss, and impaired neuromuscular propagation, failure of excitation-contraction coupling and a reduced intrinsic capacity for force may be involved too.

Our reading: this is the review behind the base's one line on running versus cycling neuromuscular fatigue (REF-537); its abstract ties the larger central fatigue of running to running's greater muscle damage, without numbers.

Note one difference in detail: REF-537 (Millet, Vleck and Bentley 2009, full text) reports a 13% strength loss after 2 h of cycling "without a central fatigue contribution", while this abstract reports "some central activation deficit" after cycling — different studies, the same direction (less central fatigue on the bike).