// Exercises under the microscope: Leg press versus squat
by Tony Podpera
Many fitness enthusiasts who are keen to increase thigh size and strength have taken to the 45o leg press with gusto. Most gyms have at least one such machine, which is often loaded with plate after plate as individuals press their way to strong legs. They can certainly push much more weight on the leg press than they can squat! But there are three main reasons why squats, even if performed with a much lighter weight, will result in shapelier, stronger legs than using a leg press ever will.
1. Anchor pointsAn anchor point is a point which cannot move as an exercise is performed. A squat has one anchor point: your feet on the floor. A leg press has two anchor points: your feet on the platform and your back against the seat. The more anchor points a movement has, the more likely it is to force you into a particular plane of movement, thereby minimising (a) the involvement of support muscles, which is what makes an exercise more functional, and (b) your ability to make an exercise better target a particular muscle or muscle group by making small adjustments to your body position as you perform it (these are the main reasons that free weights are generally preferable for developing muscle size and strength).
This means that, even though you can push big weights on a leg press, it’s still a machine with all the limitations that that involves. Try this experiment: do a light leg press with your feet near the top of the platform. You will notice that as the platform lowers, your lower back will probably lift off the seat. Then try the same exercise with your feet near the bottom of the platform.
You will find that your back feels much less like peeling off the chair, but your heels will probably rise off the platform as it lowers (photo 1). Neither position is ideal but you can’t do much about it because your anchor points are fixed. Anchor points by themselves are no problem; however, an increase in anchor points usually has an adverse effect on the line of pull of an exercise, and a poor line of pull is a problem.
2. Line of pull
In the case of the leg press, this is more like a line of push, but the principle is the same. A line of pull refers to how well resistance is applied through a target muscle during a movement, based on the angle at which the working limb allows the muscle to contract against the resistance. For example, a biceps curl performed with hands on a barbell just outside hip width works well because the line of pull targets the biceps effectively. If the same movement is performed with a really wide grip then the biceps won’t feel as strenuously worked during most of the movement. This is because the line of pull is wrong; where this happens the weight will usually move but the target muscles won’t be doing as much work as they otherwise would. Good machines are designed to deliver a good line of pull for most people, but they are rarely ideal for any one person, unlike free weight movements. In a squat, for example, your feet can’t move but everything else can. You are free to stand up in the most natural biomechanical plane for you (photo 2). If your torso starts to tip forward too much, you can straighten up - a good line of pull is always achievable. With a 45o leg press, though, your feet and your back are anchored and the resistance travels along a defined track. When you perform the movement there will also be times when the thigh muscles aren’t working as maximally as they could, simply because the angles at which the legs bend and straighten are not angles at which they would naturally work.
3. Range of motionIt is evident that anchor points can not only affect the line of pull in a leg press but can also result in either the lower back peeling off the seat or in the heels lifting off the platform during the eccentric phase if a long range of motion is used. This sometimes encourages the use of a shorter range of motion. Another reason people opt for a shorter range of motion is to keep the movement in a range where they can feel the best line of pull. A third reason is that some people just think it’s important that they push big weights, and a short leg press is the only way they can do it. Whatever the reason, even with a heavy weight, a short leg press will not tax a strong set of leg muscles very much (photo 3).
Biomechanically, the final stage of straightening a healthy knee joint is very easy; the body will only incorporate as many motor units as it needs to in order to achieve a necessary contraction, and to straighten a knee, that’s not many. Indeed, piling more plates onto a leg press machine will probably result in a better biceps workout than a thigh workout! Another problem with a short range of motion is that you are never really sure when you have completed an effective repetition, because the moment the weight starts to get really heavy as the knee bends, the lifter starts to straighten it again. As soon as the leg muscles start to come under a load which could potentially force an adaptation (i.e. force them to get bigger or stronger) the individual typically stops the eccentric phase and starts the concentric phase. Additionally, short repetitions like this tend to get shorter as a set progresses.
Even in a machine like a 45o leg press, a long movement with a lighter weight will stimulate your thighs much more effectively than a short one. If your heels and lower back lift off the bench and the platform during a long leg press, it’s not necessarily a sign that you should do shorter range movements – it’s a sign that this exercise has design limitations which you should consider in the context of your workout.
What about the extra weight in a leg press? Doesn’t that count?Well, not much. One might think that if one can push four times the weight on a leg press than on a squat, more work is being performed, but this is not so. After all, the weight on a 45o leg press is being pushed on a lubricated or wheeled track at a 45o angle, which makes the weights significantly easier to shift. Compensating for this by piling weights onto a leg press is a legitimate thing to do, though. After all, it is still a good exercise, but should be performed through as full a range of motion as possible.
However, if you have no injuries which prevent you from squatting fully, and you really want strong, muscular legs, squats should be the basis of your workout. They allow the fullest range of motion and do so for more muscles than a leg press does, and they allow you to maximise the line of pull for the whole movement because there’s only one anchor point. Conventional gym wisdom even claims that squats actually offer a whole body workout. And when you consider that you need a solid core and strong shoulders just to hold the weight up and breathe, let alone squat it, there might just be some truth to that claim.
Tony is a Canberra-based fitness instructor. He began bodybuilding in the early 1980s and has advised clients on weight training issues for 20 years. He started teaching group fitness classes in 1994.
NETWORK MAGAZINE • SUMMER 2006 • PP51-53