Muscle mass refers to the total weight of skeletal muscle in your body. It makes up roughly 40% of total body weight in healthy adults and plays a central role in movement, metabolism, and long-term health. While your body contains three types of muscle tissue (skeletal, cardiac, and smooth), “muscle mass” in everyday conversation almost always means skeletal muscle, the tissue you can voluntarily control and build through exercise.
Muscle Mass vs. Lean Body Mass
These two terms get used interchangeably, but they’re not the same thing. Lean body mass includes everything in your body that isn’t fat: skeletal muscle, yes, but also organs, bones, tendons, ligaments, and water. Muscle mass is just one component of lean body mass, though it’s the largest one. When a body composition scan gives you a “lean mass” number, that figure will always be higher than your actual muscle mass because it includes those other tissues. Knowing the distinction matters if you’re tracking progress, because changes in hydration alone can shift your lean mass reading without any real change in muscle.
How Much Muscle Is Normal
Muscle mass varies significantly by sex and age. Data from a large study of over 18,000 adults provides useful reference points. For men aged 18 to 29, the median skeletal muscle mass is about 34.7 kg (76.5 lbs), peaking around 36.8 kg (81 lbs) in the 30 to 49 range before declining. By the 70s, the median drops to around 30.5 kg, and by 80 and older, it falls to roughly 26.5 kg.
For women, the median at ages 18 to 29 is about 24.6 kg (54 lbs), peaking near 25.2 kg through the 40s, then gradually declining to around 23 kg in the 70s. These are population medians, so your individual number depends heavily on your height, frame, activity level, and genetics. The trend line matters more than any single number: sustained declines over time signal a problem worth addressing.
Why Muscle Mass Matters for Longevity
Low muscle mass is tied to a meaningfully higher risk of dying from any cause. A 2023 meta-analysis of prospective studies found that people in the lowest muscle mass category had a 57% higher risk of all-cause mortality compared to those with normal muscle mass. That risk was even more pronounced in people who were also overweight or obese. Among those with a BMI over 30, low muscle mass was associated with more than 2.5 times the mortality risk.
This isn’t just about strength or fitness. Muscle acts as a metabolic reservoir. It stores glucose, helps regulate blood sugar, and provides amino acids your body draws on during illness, injury, or surgery. People who enter a hospital stay or a cancer diagnosis with more muscle tend to recover faster and tolerate treatment better. In older adults, low muscle mass also increases fall risk and fracture rates, creating a cascade of complications that erode independence.
Muscle and Bone Health
Your muscles and bones are in constant communication. When muscles contract, they pull on the bones they’re attached to, and that mechanical stress stimulates bone to maintain or increase its density. Stronger, larger muscles generate more force, which encourages more bone formation. This is one reason weight-bearing exercise protects against osteoporosis.
The relationship goes beyond simple mechanics. Skeletal muscle functions as an endocrine organ, releasing signaling proteins called myokines during contraction. These molecules travel to bone tissue and promote growth and repair. Animal research has shown that compounds derived from skeletal muscle can actually stimulate cartilage and bone formation, and can improve fracture healing when applied to injury sites. Maintaining your muscle mass is, in a very real sense, maintaining your skeletal integrity.
How Muscle Affects Your Metabolism
You may have heard that muscle “burns more calories than fat at rest.” That’s true, but the effect is smaller than many fitness sources claim. Research using body imaging techniques estimates that each pound of fat-free mass burns about 6 to 7 calories per day at rest. Muscle specifically accounts for roughly 6 to 10 calories per pound per day. That means gaining 10 pounds of muscle might increase your resting calorie burn by 60 to 100 calories daily, roughly the equivalent of an apple.
That doesn’t mean the metabolic benefit is trivial. Those extra calories add up over months and years. More importantly, muscle tissue is where most of your glucose is stored and burned during activity. Greater muscle mass improves insulin sensitivity and helps your body handle carbohydrates more efficiently, which reduces the risk of type 2 diabetes over time. The metabolic value of muscle goes well beyond the simple calorie math.
What Causes Muscle Loss
Muscle mass begins declining at around age 30, with losses of 3 to 8% per decade. After 60, the rate accelerates. This age-related loss, called sarcopenia, is driven by several overlapping factors.
Hormones play a major role. Testosterone is the primary driver of muscle protein synthesis, and it works in a dose-dependent way: more testosterone generally supports more muscle. Growth hormone contributes indirectly by stimulating the liver to produce a secondary signal that activates muscle-building pathways. Both hormones decline with age, and that decline creates what researchers call “anabolic resistance,” where the same exercise stimulus produces less muscle growth in an older person than a younger one.
Inactivity accelerates the process dramatically. Bed rest studies show measurable muscle loss in as little as five days of immobility. Inadequate protein intake compounds the problem, especially in older adults who often eat less overall. Chronic inflammation, poor sleep, and certain medications can also contribute.
How Muscle Mass Is Measured
The most common methods are DEXA scans and bioelectrical impedance analysis (BIA). DEXA uses low-dose X-rays to distinguish between bone, fat, and lean tissue, and is generally considered the clinical gold standard. BIA, which is the technology in most smart scales and handheld devices, sends a small electrical current through your body and estimates composition based on how quickly the signal travels (muscle, being full of water, conducts electricity faster than fat).
BIA is convenient but less precise. When compared against DEXA, BIA measurements of total body lean mass show a margin of error around 8%, which sounds acceptable. But for specific body segments like arms or legs, error rates climb to 20 to 30%. Hydration, recent meals, and even skin temperature can all shift BIA readings. If you use a BIA scale at home, the best approach is to measure under the same conditions each time (morning, fasted, same hydration) and focus on trends over weeks rather than trusting any single reading.
Building and Maintaining Muscle
Resistance training is the single most effective tool for building and preserving muscle at any age. The stimulus of lifting heavy loads, or working muscles close to fatigue, triggers muscle protein synthesis and signals the body to add or maintain tissue. Two to three sessions per week targeting the major muscle groups is sufficient for most people to see measurable results within 8 to 12 weeks.
Protein intake is the nutritional foundation. Most adults need 0.8 to 1.6 grams of protein per kilogram of body weight daily. If you weigh 70 kg (about 154 lbs), that translates to roughly 56 to 112 grams per day. Older adults benefit from the higher end of that range, with recommendations of 1.2 to 1.6 grams per kilogram to offset age-related anabolic resistance. People who exercise intensely may need 1.4 to 2 grams per kilogram, and those trying to lose weight while preserving muscle could go as high as 2.3 grams per kilogram.
Spreading protein across meals matters too. Aiming for 20 to 40 grams per meal stimulates muscle protein synthesis more effectively than loading all your protein into a single sitting. Combining adequate protein with consistent resistance training is the most reliable way to protect against age-related muscle loss, regardless of your starting point.