Skeletal muscle is a specific type of tissue in your body, while muscle mass is a measurement of how much of that tissue you have. The distinction matters because health screens, body composition scales, and fitness trackers use these terms differently, and knowing what each one refers to helps you interpret your own numbers.
Skeletal Muscle Is a Type of Tissue
Your body contains three types of muscle: skeletal, cardiac (heart), and smooth (found in organs like your stomach, blood vessels, and intestines). Skeletal muscle is the one you can consciously control. It’s the tissue that moves your bones, stabilizes your joints, and powers every deliberate movement from walking to lifting a barbell. Tendons attach it to your skeleton, and nerves in your voluntary nervous system tell it when to contract.
Skeletal muscle makes up the vast majority of all muscle in the body. It accounts for roughly 30% to 40% of total body weight, while smooth muscle contributes only about 2% to 3%. Cardiac muscle adds even less, since the heart, powerful as it is, weighs well under a pound. So when people talk about “muscle” in a fitness or health context, they almost always mean skeletal muscle, even if they don’t say so explicitly.
Muscle Mass Is a Quantity, Not a Tissue Type
Muscle mass simply refers to the total weight of muscle tissue in your body. It’s a number, measured in kilograms or pounds, or sometimes expressed as a percentage of your total body weight. Most of the time, this number reflects skeletal muscle specifically, since skeletal muscle dominates the total. But depending on the tool doing the measuring, “muscle mass” can also include cardiac and smooth muscle bundled into the estimate.
This is where confusion creeps in. A body composition scale at home might display “muscle mass” as one number and “skeletal muscle mass” as a separate, slightly lower number. The difference is that the skeletal muscle reading excludes the small contributions of cardiac and smooth muscle. In practice, the gap between the two is small, but it explains why the numbers don’t always match.
Typical Skeletal Muscle Mass by Sex and Age
A large study published in the Journal of Applied Physiology measured skeletal muscle in 468 men and women between the ages of 18 and 88. Men averaged about 33 kg (roughly 73 pounds) of skeletal muscle, making up 38.4% of their body weight. Women averaged about 21 kg (46 pounds), or 30.6% of body weight. These are averages across a wide age range, and individual numbers vary considerably based on height, weight, activity level, and genetics.
Muscle mass declines naturally with age. This loss accelerates after about age 30 and picks up speed again after 60. When that decline becomes severe enough to affect strength and physical function, it’s called sarcopenia. Tracking your skeletal muscle mass over time gives you a clearer picture of this trend than tracking total body weight alone, because weight can stay stable even as muscle is quietly being replaced by fat.
How Each Is Measured
Two tools dominate body composition testing, and they approach skeletal muscle mass differently.
DXA scans use low-dose X-rays to separate your body into three compartments: bone mineral content, fat mass, and fat-free mass. Fat-free mass includes muscle, organs, water, and bone. To estimate skeletal muscle specifically, DXA measures the lean mass in your arms and legs (called appendicular skeletal muscle mass, or ASMM), since the limbs contain almost exclusively skeletal muscle with very little organ tissue. This limb-based number is the standard reference in clinical settings for diagnosing muscle loss.
BIA scales (bioelectrical impedance analysis), including many consumer smart scales, work differently. They send a tiny electrical current through your body and measure resistance. Muscle tissue conducts electricity well because it holds a lot of water, while fat resists it. The scale then plugs that resistance value into a prediction equation to estimate your muscle mass. The catch is that hydration status heavily influences accuracy. If you’re dehydrated or retaining extra fluid, the reading can shift significantly without any actual change in muscle tissue. BIA is convenient and affordable, but it’s less precise than DXA, especially for people at the extremes of body weight.
Neither method directly weighs your muscle on a scale. Both are estimates, and different devices use different equations, which is why stepping on two different BIA scales on the same day can give you two different muscle mass readings.
Why the Distinction Matters for Health
When your doctor or a health screening mentions “low muscle mass,” they’re almost always talking about skeletal muscle mass. That’s the tissue responsible for your mobility, balance, metabolic rate, and blood sugar regulation. Losing it increases your risk of falls, fractures, metabolic problems, and slower recovery from illness or surgery.
Total “muscle mass” as a raw number is less useful on its own. A person who weighs 250 pounds will naturally carry more absolute muscle mass than someone who weighs 130 pounds, but that doesn’t mean they’re healthier. That’s why clinicians often adjust the number for body size, dividing skeletal muscle mass by height squared to create a skeletal muscle index. This ratio lets you compare muscle levels across different body sizes. There’s no single universally agreed-upon cutoff for what counts as “low,” though. A recent systematic review found significant variation in the thresholds different research groups use, which means the specific number flagged as concerning can differ depending on which guidelines your provider follows.
For everyday purposes, the practical takeaway is straightforward. If you’re looking at a body composition report and it shows both “muscle mass” and “skeletal muscle mass,” the skeletal muscle number is the more specific and useful one. It tells you how much of the tissue you can actually build, maintain, and strengthen through resistance training and adequate protein intake. The broader “muscle mass” figure includes tissue you have no voluntary control over, which makes it a slightly blurrier measure of your fitness and metabolic health.
Building and Preserving Skeletal Muscle
Because skeletal muscle is the only type you can voluntarily control and deliberately grow, it’s the only type that responds to exercise. Resistance training, whether with weights, bands, or bodyweight, is the most effective way to increase skeletal muscle mass. Endurance exercise like running or cycling supports cardiovascular health but has a limited effect on building new muscle tissue.
Protein intake matters too. Muscle tissue is in a constant cycle of breakdown and repair, and dietary protein provides the raw material for that repair. Spreading protein across meals throughout the day supports this process more effectively than loading it all into one sitting. The combination of consistent resistance training and sufficient protein becomes increasingly important after your 40s, when the body’s natural muscle-building signals start to weaken and preserving what you have takes more deliberate effort.