SMI on an InBody scan stands for Skeletal Muscle Index. It’s a measure of how much muscle you carry relative to your height, calculated by dividing your appendicular skeletal muscle mass (the muscle in your arms and legs) by your height squared. The result is expressed in kg/m², and it gives you a standardized way to assess whether your muscle mass is adequate, low, or above average for your frame.
How SMI Is Calculated
The InBody machine uses bioelectrical impedance analysis (BIA) to estimate how much lean muscle tissue sits on your limbs. It sends small electrical currents through your body and measures how quickly they travel, since muscle and fat conduct electricity differently. The machine totals the estimated muscle mass in both arms and both legs to get your appendicular skeletal muscle mass, or ASM. Then it divides that number by your height in meters, squared.
The formula looks like this: SMI = ASM ÷ height². So if your arms and legs contain a combined 22 kg of skeletal muscle and you’re 1.70 m tall, your SMI would be 22 ÷ 2.89 = 7.6 kg/m². Dividing by height squared adjusts for the fact that taller people naturally carry more muscle. Without that correction, a 6’2″ person would almost always outscore a 5’4″ person regardless of actual muscularity.
What Counts as Normal or Low
The most widely used cutoffs come from research on large populations and are endorsed by international sarcopenia working groups. For men, an SMI below 7.0 kg/m² is generally considered low muscle mass. For women, the threshold is 5.4 kg/m². Falling below these numbers doesn’t mean you’re sick, but it does flag that your muscle mass is in a range associated with reduced physical function and higher health risks.
These cutoffs were originally established using DXA (dual-energy X-ray absorptiometry), which is considered the gold standard for body composition measurement. InBody devices correlate well with DXA for body composition metrics, with correlation values around 0.96 for body fat percentage and 0.84 for fat-free mass. However, InBody tends to slightly overestimate fat-free mass by about 2.7 kg on average, which means your SMI on an InBody scan may read a bit higher than it would on a DXA scan. Keep this in mind if your number is close to a cutoff.
Why SMI Matters for Your Health
SMI exists primarily to screen for sarcopenia, the progressive loss of skeletal muscle that accelerates with aging, inactivity, or chronic illness. But it’s relevant well before old age. In one study of adults visiting a health promotion center, 60.5% of participants had low skeletal muscle mass, and many were not elderly. A low SMI was associated with higher risk of sarcopenia even in people who otherwise considered themselves healthy.
Low muscle mass doesn’t just affect strength. People with low SMI but normal body weight (sometimes called “skinny fat”) show a higher prevalence of metabolic syndrome components like elevated blood sugar and triglycerides. Reduced muscle tissue means less metabolically active tissue to help regulate blood sugar and store energy, which can quietly raise your risk for type 2 diabetes and cardiovascular problems. Research has also linked muscle reduction to slower recovery from illness and higher overall mortality.
What makes SMI particularly useful is that body weight and BMI can mask the problem entirely. Two people at the same weight and height can have very different SMI scores if one carries significantly more fat and less muscle. SMI cuts through that ambiguity.
How to Improve a Low SMI
Resistance training is the single most effective way to increase skeletal muscle mass and raise your SMI. Exercises that load the arms and legs directly, like squats, lunges, presses, rows, and deadlifts, target the exact muscle groups that SMI measures. Two to four sessions per week with progressive overload (gradually increasing weight or volume) is the standard recommendation for building muscle.
Protein intake plays a critical supporting role. Research on post-exercise muscle building suggests that roughly 0.31 g of high-quality protein per kilogram of body weight per meal optimizes muscle protein synthesis. For a 75 kg (165 lb) person, that’s about 23 g of protein per meal. Accounting for individual variation, an upper target of around 0.39 g/kg per meal covers most people. Spreading protein across four to five meals throughout the day appears to be more effective for muscle building than loading it into one or two large meals, because your body can only use so much at once before the excess is oxidized rather than used for repair.
For someone weighing 75 kg eating four meals a day at the upper target, that works out to roughly 117 g of protein daily. This aligns with the commonly cited range of 1.2 to 1.6 g/kg per day for people actively trying to build or maintain muscle mass.
How to Use Your InBody SMI Score
Your SMI is most useful as a tracking tool over time, not as a single snapshot. Because InBody measurements can shift based on hydration, time of day, and whether you’ve recently eaten or exercised, comparing scans taken under similar conditions gives you the most reliable picture of change. Try to scan at the same time of day, in similar clothing, and with consistent hydration habits.
If your SMI is well above the cutoffs (7.0 for men, 5.4 for women), you have a comfortable muscle mass buffer. If you’re near or below those numbers, it’s worth treating that as a signal to prioritize resistance training and protein intake before muscle loss progresses further. The earlier you address a low SMI, the easier it is to reverse, since building muscle becomes progressively harder with age due to declining hormone levels and reduced sensitivity to the signals that trigger muscle growth.