Nothing in the muscle explained why ibuprofen changed it
Biopsies from the 31 people in that trial found growth signalling, ribosome production, satellite cells, capillary supply and fibre size no different between groups.
What the study found
Every candidate mechanism for ibuprofen's muscle effect came back null
Biopsies from the 31 people in the ibuprofen versus low-dose aspirin training trial were read for the machinery of growth. The growth-signalling pathway, ribosome production, satellite cell number, capillary supply and muscle fibre size all came back no different between the groups.
The caveat · Same 31 people, not a new trial
This is a second analysis of the same 31 participants, not an independent replication, so it inherits that trial's size and design unchanged. A null result across the markers measured narrows the candidates rather than showing that no mechanism exists. Muscle fibre size specifically was unaffected, which sits awkwardly next to the whole-muscle scan difference the earlier report described, and the authors do not reconcile the two.