TL;DR
Muscle weakness in old age has long been treated as wear and tear. This fall, research pinned it on specific, fixable parts: the junction where nerves trigger muscle stops passing signals reliably, muscle's power plants lose a fat that holds them together, and a gene called DEAF1 rises with age and jams the cell's cleanup crew, a brake that exercise turns down. Each failure has been partly reversed in animals. None has been reversed in people, and no drug against DEAF1 exists. But a named broken part is something a drug can be aimed at, and the first year-long, placebo-controlled test of a mitochondrial drug against general aging decline is funded. If any of this carries over to people, frailty, the falls and lost independence that come with weak muscle, could become a treatable condition instead of a fate.
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 6 milestones, starting with "Regeneron starts its trevogrumab trial in older patients by 2027"; our labeled speculation on what it adds up to; and every source.