What is skeletal muscle mass?

In one line

Skeletal muscle mass is the total weight of the muscles attached to your bones, the ones you move on purpose.

What is skeletal muscle mass?
The number you look for on a body composition sheet
After a body composition test, the result sheet shows your skeletal muscle mass in kg. It is a number people often check to see if training has added muscle. But which muscles in the body does it mean?
In one line
The weight of the muscles that move your bones
Skeletal muscle mass is the total weight of all your skeletal muscles. Skeletal muscles are attached to bones and let you move your arms, legs and body when you choose to. Arm, leg, chest and back muscles that you build with training are all skeletal muscles.
Three kinds of muscle
Skeletal, cardiac and smooth
The body's muscles come in three kinds. Cardiac muscle makes up the wall of the heart, smooth muscle makes up the walls of the stomach, gut and blood vessels, and skeletal muscle is attached to bones. Cardiac and smooth muscle work on their own, and only skeletal muscle moves when you choose.
What skeletal muscle does
Moves, holds and makes heat
Skeletal muscle moves the body, holds your posture and keeps joints steady. It also shields the organs in the belly from outside blows. Working muscle makes heat, and shivering in the cold is skeletal muscle making heat.
How much of the body is it?
About 40% of body weight
Skeletal muscle makes up about 40% of body weight. For a person who weighs 70kg, that is about 28kg. More than half of all the protein in the body is in skeletal muscle.
How is it measured?
MRI is the reference; gym machines estimate
The reference way to measure skeletal muscle is a whole-body MRI scan, a hospital imaging test, that works out the volume of the muscles. Body composition machines at the gym send a weak current through the body and put that reading, plus height, age and sex, into a formula to estimate skeletal muscle mass. In research, such formulas were built to match MRI results.
Age and skeletal muscle
It slowly shrinks with age
Skeletal muscle slowly shrinks as people age. When both muscle amount and strength drop a lot, it is called sarcopenia. Skeletal muscle mass is also measured to judge sarcopenia.
A common myth
If skeletal muscle mass jumps in a few days, did you gain muscle?
That number alone does not tell you. A body composition analyzer does not measure muscle directly; it puts the resistance of a weak electric current into a formula to estimate skeletal muscle mass. This method is very sensitive to the water in your body. So you compare numbers measured under the same conditions (in the morning after waking, before eating).
Summary
The weight of the muscles you move on purpose
Skeletal muscle mass is the total weight of the muscles attached to bones that move your body. The skeletal muscle mass a machine shows is an estimate, so you compare numbers measured under the same conditions. Related terms: sarcopenia, body fat percentage, body composition analysis.