Quite often through these boards I often see the question how come I can leg press much more than I can squat.
The answer is pretty simple and can be explained by 3 reasons:
(1) When using a typical sled type leg press you are in fact only moving the equivalent of 0.707 x weight because the leg press itself takes some of the weight.
(2) When squatting you are actaully squatting the weight of the load PLUS YOUR OWN BODYWEIGHT. When you leg press you sitting down you you don't actually have to lift your own weight.
(3) The leg press moves in a fixed path of motion, so stabilizers are less involved meaning you can shift more weight. The same principle applies when you consider the weight of DBs you can press compared to say benching on a smith machine.
I have worked out a rough formula to equate the two. I was bored walking home from the gym as my ipod ran out of battery.
Max Squat =~ (Stabilisation Factor x ((Max Leg Press x 0.707)) - Bodyweight
I found that the Stabilisation Factor will probably lie between 0.8 and 0.9 for most people, and will depend on body your body mechanics and how developed various stabilizer muscles are. For me it is approx 0.85.
Max Squat =~ (0.85 x (340 x 0.707)) - 85
In theory my max squat should be about 119 (its 115kg)
Or looking at it from the point of view of the amount you can leg press
Max Leg Press =~ ((Max Squat + Bodweight) / Stabilisation Factor) / 0.707
If anybody has any critique of this it would interesting to find out, or indeed if people work out their max theorised Leg Press from there Max Squat etc.