Muscle & Aging

The decline, decomposed: three clocks run at different speeds — muscle power (speed × force) declines earliest and fastest; strength follows, measurably ahead of mass — the visible 1–2% yearly loss arrives last, meaning function erodes while the mirror still looks fine. The wiring explains it: motor neurons die at roughly one percent per year after 60, fast-twitch (type II) fibers lose their innervation first — reinnervation leaves the characteristic fiber grouping of aged muscle. And the response dulls: anabolic resistance means older muscle needs roughly double the protein per meal and genuinely heavy loading to hear the same signal (the Healthy Aging page's threshold story, told here from the muscle side).

The readout and the stakes: grip strength has emerged as one of the most powerful objective biomarkers of aging — large cohorts find it predicts all-cause mortality and future disability better than most blood panels; gait speed is similarly respected, to the point clinicians call it the sixth vital sign. The functional stakes are the frailty cascade: strength fades → balance thins → falls arrive → fractures follow → immobility compounds everything (the Bone page's fall story, driven from the muscle end). The cascade is why muscle is aging's load-bearing wall — not cosmetic.

The reversibility — the proof that reframes everything: the training trials in the very old are among the most encouraging in exercise science: frail ninety-year-olds doubling leg strength in months of progressive resistance work; muscle biopsies confirming new fiber growth at ages once written off. The prescription is specific: load heavy enough to matter — light activity is health, but aged muscle needs genuine resistance to hear the signal — with the risen protein threshold met at each meal, leucine included. Twice weekly, progressive, forever: the pattern that turns compound disuse into compound capability.

The reframe that closes the page: what's popularly called 'aging' in muscle is a blend — the wiring loss is biology, but a large share of the functional decline is compound disuse, decades of the load never arriving. Disuse responds to loading at every age tested. The screening story exists too (grip, gait, and mass make the clinical criteria), and the annual-visit conversation about grip strength is one worth requesting — an objective readout of the one system whose decline is negotiable.

• Three clocks: power fades first, then strength, then size — the mirror lies last.
• Motor neurons die ~1%/yr after 60; fast-twitch fibers fade first.
• Grip strength predicts mortality better than most blood panels.
• The frailty cascade runs strength → balance → falls → fractures → immobility.
• Frail 90-year-olds doubled leg strength in progressive-training trials.
• Aged muscle needs heavy load — light activity alone doesn't reach the threshold.