Muscle mass is the total weight of muscle tissue in your body. It includes all three types of muscle: skeletal muscle (the muscles you can see and flex), smooth muscle (found in organs like your stomach and blood vessels), and cardiac muscle (your heart). When most people talk about muscle mass, though, they’re really talking about skeletal muscle, the type you build through exercise and lose as you age.
Skeletal muscle is the largest organ in the body by mass, and it does far more than move your limbs. Understanding what muscle mass is, how much you should have, and why it matters gives you a clearer picture of your overall health than body weight alone.
Muscle Mass vs. Lean Body Mass
These two terms get used interchangeably, but they aren’t the same thing. Lean body mass is everything in your body that isn’t fat: bones, organs, skin, water, and muscle. Muscle mass is just one component of lean body mass. So if a body composition scan tells you your lean body mass is 140 pounds, that number includes roughly 7 to 10 pounds of bone, several pounds of organ tissue, and a large amount of water on top of your actual muscle.
This distinction matters when you’re tracking progress. If you gain 3 pounds of lean mass over a few months, some of that could be water retention rather than new muscle tissue. Tools that estimate skeletal muscle mass specifically give you a more useful number than lean mass alone.
Why Muscle Mass Matters for Health
Skeletal muscle is responsible for about 80% of glucose uptake after a meal. When you eat carbohydrates, insulin signals your muscle cells to pull sugar out of your bloodstream, and that sugar gets stored or burned for energy. The more muscle tissue you have, the larger that “glucose sink” is, and the more efficiently your body clears sugar from the blood. This is one reason why losing muscle mass is closely linked to insulin resistance and a higher risk of type 2 diabetes.
Muscle also plays a direct role in bone health. According to the mechanostat theory of bone remodeling, bones adapt to the forces placed on them. Stronger, larger muscles pull harder on the bones they’re attached to, and that mechanical stress stimulates bone formation and helps maintain bone density. On top of that, contracting muscles release signaling proteins called myokines that act on bone, fat, and other tissues. Animal research has shown that molecules derived from skeletal muscle can actually promote bone and cartilage formation and improve fracture healing.
Beyond blood sugar and bones, muscle mass influences your resting metabolic rate (muscle burns more calories at rest than fat), your ability to recover from illness or surgery, and your physical independence as you age.
How Much Muscle Mass Is Normal
Muscle mass varies widely based on sex, age, height, and activity level, so there’s no single “normal” number. Researchers typically use the skeletal muscle mass index (SMMI), which divides your total skeletal muscle mass in kilograms by your height in meters squared. A large UK study of nearly 400,000 adults found that the average SMMI was 8.75 kg/m² for men and 6.9 kg/m² for women.
At age 50, the median SMMI is about 8.84 kg/m² in men and 6.82 kg/m² in women. For men, the 50th percentile declines steadily with age, dropping from roughly 8.98 to 8.31 kg/m² across the adult lifespan. Women’s median values stay remarkably stable, hovering between 6.78 and 6.80 kg/m² for most of adulthood, with slight declines appearing only after age 55 in women at higher percentiles.
In practical terms, skeletal muscle typically makes up about 30 to 40% of total body weight in healthy adults, with men at the higher end and women at the lower end.
Muscle Loss With Age
Starting around age 30, the body naturally loses about 3 to 5% of its muscle mass per decade. This gradual decline is called sarcopenia when it becomes severe enough to impair strength and physical function. The loss accelerates after 60, and by 80, many people have lost 30% or more of the muscle they had in their 30s.
Muscle loss doesn’t just mean looking less toned. It translates directly into reduced strength, slower walking speed, difficulty climbing stairs, and a higher risk of falls and fractures. Importantly, the relationship between muscle size and strength isn’t perfectly linear. Research comparing muscle cross-sectional area to force production found a significant positive correlation, but with wide individual variation. Two people with the same amount of muscle can differ substantially in how strong they are, likely because of differences in muscle fiber composition and how well the nervous system activates those fibers. This means that maintaining muscle quality through regular resistance training matters just as much as maintaining muscle quantity.
How Muscle Mass Is Measured
The gold standard for measuring muscle mass is dual-energy X-ray absorptiometry, commonly called a DXA scan. You lie on a table while a low-dose X-ray passes over your body, separating your tissue into fat, bone, and lean mass. DXA can estimate muscle mass for your whole body or specific regions like your arms, legs, or trunk.
Bioelectrical impedance analysis (BIA) is the more accessible option. These devices, which range from medical-grade machines to consumer bathroom scales, send a small electrical current through your body and estimate composition based on how quickly the current travels (muscle, which contains more water, conducts electricity faster than fat). BIA takes less than 20 seconds and doesn’t require a clinic visit.
The trade-off is accuracy. BIA readings correlate well with DXA, but BIA consistently overestimates muscle mass. In one study, BIA overestimated trunk muscle mass by an average of 3.6 kg in men and 2.3 kg in women compared to DXA. The correlation between the two methods was strong for men (0.77) but moderate for women (0.54). BIA is still useful for tracking changes over time, since the overestimation tends to be consistent. Just don’t compare a BIA reading directly to a DXA result.
Other methods include MRI and CT scans, which provide the most detailed images of individual muscles but are expensive and typically reserved for research. Underwater weighing and skinfold calipers estimate body fat percentage, from which lean mass can be inferred, but they don’t isolate muscle mass specifically.
Building and Preserving Muscle
Resistance training is the single most effective way to build and maintain muscle mass at any age. Lifting weights, using resistance bands, or doing bodyweight exercises like push-ups and squats all stimulate muscle protein synthesis. For most people, training each major muscle group two to three times per week is enough to see meaningful gains or slow age-related losses.
Protein intake is the dietary factor that matters most. Muscle tissue is built from amino acids, and your body needs a steady supply to repair and grow muscle fibers after exercise. Most research points to a range of 1.2 to 1.6 grams of protein per kilogram of body weight daily for people who are physically active. For a 70 kg (154 lb) person, that’s roughly 84 to 112 grams per day, spread across meals rather than consumed all at once.
Sleep and recovery also play a role. Growth hormone, which supports muscle repair, is released primarily during deep sleep. Chronic sleep deprivation has been shown to impair muscle protein synthesis and accelerate muscle breakdown. Getting seven to nine hours per night supports the biological processes that keep muscle tissue healthy and responsive to training.