The second most useless piece of gym equipment ever made, with the exception of the Smith Machine.
The leg press, in its original version, was a "safer" version of an exercise people did back in the day in which they balanced a barbell on the soles of their feet while lying down and putting their legs straight up in the air, bending the knees all the way and then pressing back out. Any slippage meant the barbell at best would tumble to the floor next to your butocks and flat out break, worst case was a fractured pelvis from the bar hitting you. But like the sissy squat it was a misguided early attempt to find something, anything that would build massive leg muscles.
The first leg press machines were basically small platforms on rails that you could lie under, in the same position, but with the feet pressed on a platform it was safer on your ankles, and with the weight plates attached to bars welded to the platform, you could load the machine, get under it, and press without the risk of the barbell falling. You lost the advantage of a free weight stressing the small balancing muscles, but you were able to resist gravity as it came down, and in essence the weight on the bar was the weight you were moving.
And then some fellows decided that that wasn't really good enough, and decided to build what amounts to either a machine where you sit down with your legs straight out in front of you and press a platform attached to a cable, attached to pulleys, attached to weights - or what amounts to a rail mounted sled at a 45 degree angle. You sit basically facing upwards at that angle, place your feet on a platform attached to two rails going up that 45 degree angle, and press the weight up and forward.
The pushing the weight out in front of you with cables machine by definition doesn't offer much resistance - in theory it IS a leg press, but is the province either of rehabilitation exercise or people who are very weak due to age or infirmity. But the plate-mounted heavy steel leg press? That's the province of the gym poser.
Because the weight is in essence at a 45 degree angle, you're looking in terms of effort expended the weight on the machine X cosine 45. "I can leg press 600lb bro!" Well, assuming that you are moving the entire weight without support (you aren't) your effort there is 425lb. Stabilize the weight entirely, and it's nowhere near the effort to do a proper back squat with 425lb, as evidenced by the fact that any leg press hero who tries it typically gets pancaked by it.
Given the range of motion involved, you're usually doing, if you're lucky, about a quarter squat, meaning that you're probably capable of moving 200lb if it was over your shoulders, to parallel.
Now do you see why the gym hero much prefers the leg press? What would you rather do, quarter squat one 45lb plate a side and change, versus tying on a do-rag, and sitting there texting while clanging 6 45lb plates? Like a popcorn container that's a huge cone (looks enormous, holds two cups of popped popcorn) it's basically visually impressive and from an exercise perspective, totally pathetic.
Bonus points if you barely move the plate stack even six inches, rocking it back and forward in what amounts to a calf raise while wrapping your knees and "stabilizing" everything by pushing on your knees with your hands. Screaming and banging the plate stack back and forth to make EPIC MAN NOISES is strongly encouraged. If you're going to pose, go large.
It also allows you to obnoxiously place every single plate in the gym you can find on the machine, because YOU'RE SO HARDCORE FREAKING STRONG that there's no way you could possibly engage your THICK FAT LEGS with any less. Some guys even have their friends sit on the machines as well. And of course leave them there for others to retrieve, how dare anyone desecrate your MONUMENT TO TESTOSTERONE.
That being said, there have been genuine, epic leg presses. But these are not quite the same athletic feats as the back squat.
In order to move a weight the classic (e.g. back squat) way, you need the small balancing muscles, you have to have control of the weight, and you need a strong set of core muscles. Many people think that thick lever belt some wear is in order to hold the back together, and it is - but the secondary function is to give the abs something to push against, engaging a tight strong core. You cannot push a car with a rope, you need your body to be a column of pure strength from neck to toes. That's why back squats involving half as much weight are far more epic feats.
But ludicrous posing aside, the leg press has two hidden dangers, besides making people believe they are stronger than they are.
The first is that the human body cannot really deal with shear stress at the joints. And if you look at the leg press position, you have the lower back compensating for a load pushing against the pelvis at a 90 degree angle to the spine. That is a really, REALLY good way to mess up your lower back over time, especially since you never develop the lower back muscles by sitting down all the time. (Check out the spinal erectors on an elite powerlifter or weightlifter to see what the back squat does to the back). the thick muscles either side of the spine, above the buttocks.
And the second?
Watch this video at your peril. Most idiots try to do more weight by locking out the press, making the joints of the knee handle the stress. Sometimes the knee quits on you.
Seriously. at your peril. SNAP.
Leg press? If you're hurt, or as an accessory, and even then, go for range of motion. But don't do it to avoid the back squat. I don't want to hear your excuses about your bad back, your bad knees, whatever. YOU HAVE NO EXCUSES.