SMI stands for Skeletal Muscle Index, a measure that compares your muscle mass to your body size. It’s the go-to metric for determining whether someone has a healthy amount of muscle or is at risk for sarcopenia, the medical term for significant muscle loss. The most widely used formula takes the combined muscle mass of your arms and legs (called appendicular skeletal muscle mass) and divides it by your height squared, producing a number in kg/m². This height adjustment is what makes SMI useful: it lets clinicians compare muscle levels fairly between a person who is 5’2″ and someone who is 6’1″.
How SMI Is Calculated
The core formula is straightforward. First, the muscle mass in all four limbs is added together. This combined number, abbreviated ASM, captures the skeletal muscle that matters most for mobility, balance, and daily function. Then ASM is divided by height squared:
SMI = ASM (kg) ÷ height² (m²)
Major international guidelines from groups in Europe, Asia, and North America all use this same basic calculation. There is an alternative version that divides ASM by body weight instead of height, expressing the result as a percentage. This weight-based version can be helpful for people with obesity, since a height-only adjustment might overestimate their muscle adequacy. In practice, though, the height-squared version is the one you’ll see most often in clinical reports and research.
What the Numbers Mean
SMI values differ significantly between men and women, and they shift with age. A large Brazilian study of over 18,000 adults found that men aged 18 to 29 had an average SMI of about 8.3 kg/m², while women in the same age range averaged around 7.0 kg/m². By age 80 and older, men’s average dropped to 7.3 kg/m² and women’s to 6.8 kg/m². In women, muscle mass typically peaks between the ages of 40 and 49, then declines at roughly 5.7% per decade starting in the sixties. Men follow a similar pattern, with muscle peaking in middle age and declining steadily from the seventh decade onward.
Those averages matter because the diagnostic cutoffs for low muscle mass are set relative to healthy young adults. The European Working Group on Sarcopenia in Older People (EWGSOP2) defines low muscle mass as an SMI below 6.0 kg/m² for men and below 4.3 kg/m² for women when measured by DXA. The Asian Working Group for Sarcopenia uses slightly different thresholds: below 7.0 kg/m² for men and below 5.4 kg/m² for women by DXA, or below 7.0 and 5.7 respectively when measured by bioimpedance. The Asian cutoffs are higher partly because the reference populations and body composition norms differ across ethnic groups.
How SMI Is Measured
Three technologies are commonly used to determine the muscle mass that feeds into your SMI calculation, and each has trade-offs.
- DXA (Dual-energy X-ray Absorptiometry) is considered the clinical standard. You lie on a table while a low-dose X-ray scanner differentiates bone, fat, and lean tissue throughout your body. It’s precise, but it requires specialized equipment typically found in hospitals or imaging centers.
- BIA (Bioelectrical Impedance Analysis) is far more accessible. You stand barefoot on a scale-like platform and grip handles while a weak electrical current passes through your body. The device estimates muscle mass based on how easily the current flows through different tissues. BIA correlates well with DXA overall, but it tends to overestimate trunk muscle mass and its readings can shift depending on hydration status.
- CT scans are used mainly in clinical settings where a scan already exists, such as during cancer treatment. Researchers analyze a cross-sectional image taken at the level of the third lumbar vertebra (L3), measuring the total area of muscles including the psoas, erector spinae, and abdominal wall muscles. That area is then divided by height squared to produce an SMI value. This approach is highly precise but involves radiation and specialized software, so it isn’t used for routine screening.
If you’ve had a body composition analysis at a gym or wellness clinic, the device was almost certainly BIA. The number it produces for SMI is useful for tracking your own trends over time, even if its absolute accuracy is slightly lower than DXA.
Why SMI Matters for Sarcopenia
SMI exists primarily to identify sarcopenia, a condition where muscle loss becomes severe enough to affect strength, mobility, and health outcomes. Sarcopenia isn’t just an issue for the elderly. Prolonged bed rest, chronic illness, poor nutrition, and sedentary lifestyles can all accelerate muscle loss at any age. An SMI value below the established cutoffs is one of the key criteria for a sarcopenia diagnosis, though clinicians also assess grip strength and walking speed before making a formal determination.
The practical consequences of low SMI go beyond feeling weak. People with sarcopenia have a higher risk of falls, fractures, longer hospital stays, and reduced ability to live independently. Identifying low SMI early creates a window to intervene with resistance exercise and adequate protein intake before those downstream problems develop.
SMI in Cancer and Chronic Illness
Outside of age-related muscle loss, SMI has become an important marker in cancer care. Patients with low muscle mass tend to experience higher rates of treatment complications and greater toxicity from chemotherapy. Low SMI has been linked to increased overall mortality in breast cancer survivors in several studies, though the relationship is more nuanced than it first appears. In younger patients, some research has found that higher SMI may actually be associated with greater chemotherapy toxicity, possibly because more muscle tissue increases the exchange of drugs within capillaries. This complexity is why oncologists increasingly look at SMI as part of a broader nutritional and physical assessment rather than interpreting it in isolation.
SMI vs. Fat-Free Mass Index
If you’ve encountered body composition testing, you may have also seen FFMI, or Fat-Free Mass Index. The two metrics sound similar but measure different things. FFMI takes your entire fat-free mass, which includes skeletal muscle plus organs, bone, and water, and divides it by height squared. SMI (or its equivalent, ASMI) focuses exclusively on skeletal muscle in the arms and legs.
This distinction has real clinical significance. Research on cancer patients found that FFMI was better at identifying severe malnutrition, while ASMI was a stronger predictor of survival. The logic makes sense: fat-free mass reflects your overall nutritional reserves, but limb muscle mass specifically reflects the kind of functional tissue that resists wasting the longest. When limb muscle drops measurably, it signals a deeper level of physical depletion. For general fitness tracking, either metric gives you useful information. In a clinical context, your care team will choose the one that best fits the question they’re trying to answer.
How to Improve Your SMI
Resistance training is the single most effective way to build and maintain skeletal muscle mass. Even in adults over 70, progressive strength training consistently increases muscle size and improves SMI. Two to three sessions per week targeting major muscle groups is the general recommendation supported by sarcopenia guidelines worldwide.
Protein intake plays a supporting role. Muscle tissue can’t grow without adequate amino acids, and older adults in particular often fall short of the 1.0 to 1.2 grams of protein per kilogram of body weight per day that’s recommended to preserve muscle. Spreading protein intake across meals rather than concentrating it at dinner appears to be more effective for muscle protein synthesis.
If you’ve received a low SMI result from a body composition test, it’s worth getting a follow-up measurement after several months of consistent training and dietary adjustment. Tracking SMI over time gives you a concrete, objective way to see whether your efforts are translating into actual muscle gains.