its because strength and endurance adaptations can interfere with each other. For best results doing same day cycling/strength, do cycling in the morning and strength in the evening.
As for the science behind it, this paper: Using Molecular Biology to Maximize Concurrent Training - PMC was mentioned by Coach Chad on a podcast a year or two ago. It was also mentioned this weekend on the Empirical Cycling feed:
There is a paper just referenced on Emperical Cycling’s weekend instagram. The biochemical signaling from intense endurance exercise subsides after 3 hrs. The signaling after strength exercise lasts 18 hrs. At 66 I do an hr early in the morning, the weights at 1 pm. Take the next day completely off. If you ride after lifting too soon you block the biochemical signaling and dont benefit in terms of strength or hypertrophy
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