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Coach Sheet

Glossary entry — body-composition

Lean Body Mass LBM

Total body weight minus fat mass. Includes muscle, organs, bone, water, connective tissue. The metabolically active portion of body weight. 

LBM = total_weight × (1 − body_fat_fraction)

What LBM means

Lean body mass is everything in your body that isn’t fat: muscle, organs, bone, blood, water, connective tissue. Almost all of metabolic activity (BMR) comes from LBM, which is why formulas like Katch-McArdle key BMR off LBM rather than total weight.

A 70 kg person at 15% body fat has LBM = 70 × 0.85 = 59.5 kg.

Why LBM matters

For coaches:

  • BMR calibration: Katch-McArdle (370 + 21.6 × LBM_kg) is more accurate than Mifflin-St Jeor for clients above ~25% body fat
  • FFMI: built directly off LBM, used for tracking lean mass changes during cutting
  • Cut targeting: maintaining LBM during a cut is the goal; weight loss should be fat loss, not muscle loss

How to estimate LBM

LBM = total weight × (1 − body fat fraction). The accuracy depends entirely on the body fat estimate.

Body fat measurement methods, ordered by accuracy:

  1. DEXA scan: ±2% accuracy
  2. Bod Pod, hydrostatic weighing: ±3%
  3. Navy method (circumference): ±3%
  4. Skinfolds with calipers (trained measurer): ±3-4%
  5. BIA scale: ±5-10% (frequently worse)

A 5% error in body fat percentage produces a 3-5 kg swing in LBM estimate. For coaching purposes, re-measure body fat quarterly using a consistent method, not weekly with different methods.

Common confusion

“My weight stayed the same but LBM went up. Am I gaining muscle?” Possibly, if the body fat measurement is reliable. If you measured body fat with two different methods (Navy this month, DEXA last month), the change might be measurement noise, not real composition shift.

“LBM dropped 2 kg during my cut, I’m losing muscle.” Possibly, but a 2 kg LBM drop is at the edge of normal measurement noise for most non-DEXA methods. Track the trend over 8-12 weeks before concluding muscle loss. A 3-4 kg LBM drop sustained across 12 weeks of cutting is more worrying.

In Coach Sheet

The Profile tab takes Weight and Body Fat % once. Quick Stats auto-calculates LBM for use in FFMI and as an alternate BMR input via Katch-McArdle. The Measurements tab tracks LBM trend over time. A flat LBM during weight loss confirms muscle preservation.

Sources

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