What is muscle protein synthesis (MPS)?

In one line

Muscle protein synthesis is the body's work of linking amino acids together to make new protein for muscle.

What is muscle protein synthesis (MPS)?
The 'MPS' that keeps coming up with protein supplements
In protein supplement talk and workout videos you often hear about 'raising MPS'. MPS stands for 'Muscle Protein Synthesis'. Here is, step by step, how muscle is newly made inside the body.
In a word
Making new protein for muscle
The cells that make up muscle are packed with protein strands that produce force. The body makes a little of this protein anew every day. Making something new like this is called 'synthesis', and making new muscle protein is called muscle protein synthesis.
What it is made from
Amino acids linked one after another
A protein is a chain of small pieces called 'amino acids' linked one after another. When you eat the protein in meat or eggs, the body breaks it down into amino acids. It then links those amino acids back together, one at a time, to make the proteins it needs.
Muscle is always changing
It is built and taken apart at the same time
While muscle protein is being made, some of it is also being taken apart and removed. Taking protein apart like this is called 'breakdown'. Because synthesis and breakdown never stop, about 1-2% of muscle protein is replaced with new protein each day.
The difference decides gain or loss
'Protein balance': synthesis minus breakdown
Whether muscle grows or shrinks depends on synthesis minus breakdown. This difference is called 'protein balance', and when it stays positive, muscle grows. Normally the plus after meals and the minus while fasting cancel each other out, so muscle mass stays the same.
How it is measured
Following a tagged amino acid
Muscle protein synthesis cannot be seen, so it is measured with a special amino acid. An amino acid tagged with slightly heavier atoms than usual is given through a vein. Then a tiny piece of muscle is taken to measure how much of that amino acid has gone into muscle protein.
After exercise
Synthesis rises sharply and stays up for two days
A single session of strength training raises muscle protein synthesis sharply. In people who did not usually strength train, 3 hours after exercise it was more than double the resting rate, and two days later it was still higher than at rest. But breakdown also rose at the same time.
After eating
Protein raises it too, but only briefly
Eating protein also raises muscle protein synthesis. After a protein drink, synthesis rose to about three times the resting rate by around 1 and a half hours, then went back to where it started. This happened even though plenty of amino acids were still left in the blood.
Exercise and protein together
It takes both to make the balance positive
With exercise alone on an empty stomach, synthesis rose but breakdown was larger, so protein balance stayed negative. When amino acids were given through a vein after exercise, synthesis rose about twice as much as when the same amino acids were given at rest. When exercise and protein come together, the balance turns positive and muscle protein builds up.
A common misconception
Does a bigger rise in synthesis right after exercise mean more growth?
It did not. In people just starting strength training, how much synthesis rose after the first session had nothing to do with how much their muscles grew over the following months. The rise in the first week coincided with the greatest muscle damage; from week 3, once the damage had dropped, synthesis matched muscle growth.
Summary
Making new muscle protein from amino acids
Muscle protein synthesis is the body's work of linking amino acids to make new muscle protein, and whether muscle grows depends on the difference between synthesis and breakdown. A single big rise in synthesis right after exercise does not by itself mean muscle growth. Related terms: amino acids, skeletal muscle mass.