Muscle mass is one of the strongest predictors of healthy aging. Low muscle mass — clinically called sarcopenia — is associated with falls, fractures, longer hospital stays, metabolic dysfunction, and higher mortality. SMI is the field standard for quantifying it in a way that's comparable across body sizes.

The Formula (Janssen, 2000)

SM (kg) = 0.407 × weight(kg) + 0.267 × height(cm) − 0.049 × age + 2.55 × sex − 2.898 SMI = SM ÷ height²(m²) This equation was derived against MRI-measured skeletal muscle in 388 adults and validated in another 343. It explains about 85% of the variance in muscle mass compared to imaging — accurate enough for screening, not for clinical diagnosis.

Mass Is Only Half the Picture

Full sarcopenia diagnosis (EWGSOP2) requires both low muscle mass and low function — grip strength below 27 kg (men) or 16 kg (women), or gait speed under 0.8 m/s. Strength can drop faster than mass with aging, so a low SMI warrants a functional check too.

What Builds Muscle

Progressive resistance training: 2-3 sessions/week, challenging loads, full ROM. Adequate protein: 1.6-2.2 g/kg bodyweight, distributed across meals. Recovery: sleep and training frequency that allows full adaptation. Consistency: beats intensity. Two years of "good enough" beats six weeks of perfect. Creatine monohydrate (3-5 g/day) — one of the best-studied ergogenic aids for older adults.