What is a leg press?

In one line

A leg press is a machine exercise where you sit or recline with your back supported and push a footplate away with your legs.

What is a leg press?
The machine where you push a footplate away with your legs
Among the lower-body machines at the gym, there is one where you sit with your back supported and push a large footplate away with your legs. That exercise is the leg press. This post explains what the leg press is and which muscles it uses.
In a nutshell
Push the footplate away with your legs
A leg press is an exercise where you sit or recline with your back against the machine, put your feet on a footplate and push it away with your legs. The weight is attached to the machine, so the footplate moves only along a fixed path. That makes it a machine exercise.
Two kinds of machine
Push up at an angle, or push forward
One kind has a footplate that slides diagonally upward on rails; you recline underneath it and push it up. There is also a machine where you sit in a chair and push the footplate forward. Either way, the movement of pushing the weight away with your legs is the same.
Several joints together
The hip and knee straighten together
When you push the footplate, your hip joint (where the leg attaches to the trunk) and your knee straighten together. So the leg press is a 'compound exercise', one where several joints move together. The movement looks like the squat, where you sit down and stand up, but your body stays leaning on the machine.
The main muscle
The quadriceps at the front of the thigh
The quadriceps, the large muscle at the front of the thigh, straightens the knee and is made of four parts joined together. This is the muscle that works hard in the leg press. In a study where people did the leg press for three months, the quadriceps grew clearly.
One part that grows less
The rectus femoris within the quadriceps
Of the four parts of the quadriceps, the 'rectus femoris' runs from the knee all the way up across the hip joint. A study comparing the leg press with the 'leg extension', a machine that only straightens the knee, found the rectus femoris grew clearly only with the leg extension. Growth of the other three parts did not differ.
What if you turn your toes?
How hard the muscles worked did not differ
Some people push with their toes turned outward on the footplate. Measured with EMG (a recording of the weak electrical signal muscles give off when they work), how hard the thigh muscles worked did not differ whether the toes pointed straight or turned out.
A common misconception
Does the leg press only grow the front of the thigh?
No. In a study where people who had not been strength training did the leg press for three months, the gluteus maximus, the buttock muscle, also grew clearly. Relative to its own size, it grew by about three times the percentage of the front thigh muscle. One inner-thigh muscle also grew. The hamstrings at the back of the thigh grew only a little.
Summary
Push the footplate by straightening the hip and knee
A leg press is a machine exercise where you lean on the machine and push a footplate away with your legs, straightening the hip and knee together. It is not a front-of-thigh-only exercise: in a three-month study of people who had not been strength training, the buttock muscle also grew clearly. Related terms: squat, compound exercise, free weights and machines.