Leg Press question

Bob314159

I have a book that says when using a leg press machine - not to extend leg completely. Why?

cgvet37

You do not want to lock out your knees, as it applies too much pressure.

darthsogogi

Bob, I believe that to be wrong and here's why: when you consider the leg press and the angle at which you're lifting the weight, a good portion of force pushing against your body is reduced. That's why most people should be able to leg press more than squat; even though there's more weight, the angle of the press machine reduces the total weight being applied to your legs.

Now, this wouldn't mean much except that you have very strong people who can easily squat upwards of 500lb. (225kg). If you consider their ability to stand up fully (perhaps with a slight torso lean), this is more force on their body than the same weight on the leg press. This is to say that if strong people can squat more weight than the average person can leg press, full leg extension on the leg press is fine.

Azdak

If you keep control of the movement the way you are supposed to--with your muscles-- then you can fully straighten your legs without "locking" the knees. Pay attention to what you're doing and use proper form and you'll be fine.

jessef593

It's too much pressure on the joint and even takes away from the effectiveness of the exercises since it removes the tension from your muscles. Also.. People have locked out their legs and then the weight has actually forced their legs to bend the opposite way. You can imagine why you'd want to avoid that.

oedipuss

Potential hyper-extension of the knee joint.... your posterior and anterior cruciate ligaments will hate you!
And what jessef593 says. ^^

Joreanasaurous

jessef593 wrote: »

It's too much pressure on the joint and even takes away from the effectiveness of the exercises since it removes the tension from your muscles. Also.. People have locked out their legs and then the weight has actually forced their legs to bend the opposite way. You can imagine why you'd want to avoid that.

There are youtube videos of this and it really isn't pretty.

ninerbuff

darthsogogi wrote: »

Bob, I believe that to be wrong and here's why: when you consider the leg press and the angle at which you're lifting the weight, a good portion of force pushing against your body is reduced. That's why most people should be able to leg press more than squat; even though there's more weight, the angle of the press machine reduces the total weight being applied to your legs.

Now, this wouldn't mean much except that you have very strong people who can easily squat upwards of 500lb. (225kg). If you consider their ability to stand up fully (perhaps with a slight torso lean), this is more force on their body than the same weight on the leg press. This is to say that if strong people can squat more weight than the average person can leg press, full leg extension on the leg press is fine.

Believe what you'd like, but you don't want to lock your knees out in a squat either. The tension should be on the muscle not on the joint. Locking your knees takes the tension off the muscle and transfers it to the joint.

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davidmcgahee

But have you noticed when your doing leg presses that your back tightens up as well? That's because your back takes some of the force as well I fully extend my legs.

Cherimoose

Bob314159 wrote: »

I have a book that says when using a leg press machine - not to extend leg completely. Why?

You should have no problem straightening your knees if you don't have pre-existing ligament injuries and aren't prone to hyperextending joints. There's always lots of tension on the knee joint at any angle on the legs press. People straighten their knees on deadlifts all the time with no problem.

darthsogogi

ninerbuff wrote: »

Believe what you'd like, but you don't want to lock your knees out in a squat either. The tension should be on the muscle not on the joint. Locking your knees takes the tension off the muscle and transfers it to the joint.

I don't want the point being that locking out your knees without muscle activation is okay, so that's not what I'm saying. We know that the quadricep is a knee extensor, so even fully contracting my quadricep sitting down will inevitably put my knee in to full extension. When using my hamstring to resist the quadricep (since it partially acts as an antagonist) to intentionally slightly bend the knee, I start losing quadricep contraction.

This example is to show that when you squeeze your quadricep as hard as possible, unless you have a current or previous injury, reaching terminal knee extension is normal. This means that for any leg exercise, the knee extension should be the effect of quadricep (and also the hamstring) contracting and not the factor for doing a leg exercise (i.e. you shouldn't perform a movement just to lock out your knees).

We can also apply this to an elbow and the tricep, the extensor for the arm, and determine if locking out during a bench press is bad for your elbows.

But ultimately, the force applied to your legs on a leg press will be less force applied than if you were to squat the same weight, and because we powerlifters lock out knees on squats and deadlifts, I fail to see a true safety concern. Obviously what we don't want is an entire dependence on your joints to absorb the force, and we also don't want hyper-extension like oedipuss mentioned.

Lock Out Joints or Not?
http://journals.lww.com/nsca-scj/Citation/2002/10000/Lock_Out_Joints_or_Not__.3.aspx

There also is an importance of terminal knee extension:
http://www.sethoberst.com/blog/the-importance-of-terminal-knee-extension