TL;DR
The body has a brake on muscle growth called myostatin, and drugs that ease off that brake have crossed two lines this fall. One is now an approved medicine, for a rare muscle-wasting disease in children. Another, given alongside the weight-loss drug semaglutide, cut the muscle people lost roughly in half while they lost just as much weight. That matters because the new weight-loss drugs take muscle along with fat, and muscle is what keeps older people standing, walking and living on their own. If the next trials show that kept muscle also means kept strength, losing weight and losing strength could stop being a package deal, and the same drugs could become a treatment for the slow muscle loss of old age.
Below the line, for paid subscribers: The full story: where it stands, how we got here and what's dated next; our Odds board on 5 milestones, starting with "Scholar Rock reports first healthy-volunteer data for SRK-439 by 2026"; our labeled speculation on what it adds up to; and every source.