Researchers used deep phosphoproteomic analysis of human muscle biopsies to compare molecular signaling after endurance versus resistance exercise. They discovered a signaling cascade — MKK3b → p38 → MK2 → mTORC1 — that is activated specifically and persistently after resistance exercise, not endurance exercise. Activation of MKK3b strongly correlated with increased muscle protein synthesis (R=0.87) in both male and female participants. Genetic activation of MKK3b alone in mice was sufficient to trigger the full cascade, boost protein synthesis, and increase muscle fiber size. This identifies core molecular machinery responsible for resistance exercise's unique muscle-building effects, with implications for treating muscle wasting and designing targeted therapies.
