Appendicular Skeletal Muscle Mass Index (ASMI)
The appendicular skeletal muscle mass index divides the muscle in the arms and legs by height squared, giving a single number used to define low muscle mass in sarcopenia - and it can be low in people who are overweight.
Plain English
Patient-Friendly Explanation
The ASMI is a way of expressing how much muscle a person carries in their arms and legs relative to their height. It comes from a body composition scan such as DEXA or a good bioimpedance machine: the scanner reports the muscle in the four limbs, and the clinician divides that by height squared. A low value is one of the criteria for sarcopenia, the loss of muscle that comes with age and illness. The important point for a weight clinic is that a person can have a low ASMI while still being overweight or obese, because excess fat hides the loss of muscle. Measuring it before and after weight loss shows whether the weight lost was fat or muscle.
Medical
Clinical Definition
The ASMI is calculated as appendicular skeletal muscle mass, the sum of lean mass in both arms and both legs, divided by height in metres squared. It is derived from dual-energy X-ray absorptiometry, from multifrequency bioelectrical impedance validated against DEXA, or from magnetic resonance imaging and computed tomography. The Asia Working Group for Sarcopenia 2019 defines low muscle mass as an ASMI below 7.0 kg/m2 in men and below 5.4 kg/m2 in women by DEXA, with corresponding bioimpedance thresholds of below 7.0 and below 5.7 kg/m2; the European consensus uses below 7.0 and below 5.5 kg/m2. Sarcopenia requires low muscle mass together with low muscle strength, typically grip strength below 28 kg in men and below 18 kg in women, or impaired physical performance such as gait speed below 1.0 m/s or a chair stand time of 12 seconds or more, so the ASMI alone is not a diagnosis. Height-squared adjustment systematically under-detects low muscle in people with obesity, which is why weight- or BMI-adjusted alternatives such as appendicular lean mass divided by BMI are also used. During intentional weight loss, a fall in ASMI indicates that lean tissue is being lost, which argues for adequate protein intake and resistance training.
Key Insight
Why It Matters
It is the measurement that separates good weight loss from bad weight loss: a programme should be judged on fat lost while muscle is preserved, and the ASMI is how that is checked rather than assumed.
Sources
Reviewed against standard medical references.
Medically reviewed by
Family Physician & Registered Acupuncturist, Accord Medical Clinic. MBBS (NUS), Graduate Diploma in Family Medicine (NUS), Graduate Diploma in Acupuncture (Singapore College of TCM).
Last reviewed: September 2026
This page is general health education, not medical advice. Whether any treatment is appropriate for you is a decision made in consultation after assessment.
