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Is Locking Out Your Knees on a Leg Press ACTUALLY Dangerous?

Worried that you're going to hurt yourself by accidentally locking out your knees?

Fit and muscular woman doing heavy leg press to build muscle

We've all heard it before.

"Bro! Don't lock out your knees on the leg press! Haven't you seen the videos?"

Everywhere in the fitness community, we hear people telling us not to lock out our knees on the leg press, in fear that we'll snap our legs backwards and end up in hospital.

Heck, we've probably even told you not to do so ourselves somewhere in our training programs!

But is it REALLY that dangerous?

In this article we'll take a look at why locking out your knees on the leg press might be dangerous, but also why it might not be, and why you might be better off locking them out instead!

Stay tuned.

Is Locking Out Your Knees on Leg Press Dangerous?

Well, there's no doubt that it certainly can be dangerous.

Chances are, you've seen one of those videos online of people locking out their knees on a leg press and snapping their legs back in the wrong direction.

Yes, those videos are certainly scary and make us think twice, three times or even four times about locking our knees out.

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However, it's important to realize that practically anything in life can be dangerous if you do it incorrectly.

And there's a very good chance the people in those videos weren't doing their exercises correctly.

Locking Out the Knees Can Be Dangerous If You Use Too Much Weight

If you're using more weight on the leg press, or hack squat, or other leg exercises than your muscles can actually handle, then locking out your knees can certainly be dangerous.

Even if you think you can actually lift the weight and are able to bang out the reps, you might be going too heavy.

Everybody locks out their knees in a squat or a deadlift but nobody bats an eye!

Powerlifter doing heavy sumo deadlift to build strength and muscle mass

It's because it's much harder to cheat and still technically be able to achieve and perform 'reps' with these exercises as opposed to hack squats and leg presses.

An example of this would be those that have to use their hands to push off their knees to lift off at the beginning of a set.

If your legs aren't physically capable of pressing the weight up from that position, then the weight is likely too heavy for you to be using.

And if you tried to lock out your knees on a leg press with that amount of weight (even if accidentally), you might hurt yourself.

But remember, it's important to remember that anything done incorrectly can be dangerous.

Driving a car can be extremely dangerous if done incorrectly, but we all do it everyday!

Cutting up a vegetable can be dangerous if we don't do it correctly, yet we all do it at times!

Bench pressing can be dangerous if done incorrectly.

The point is, when we're doing things that could be inherently dangerous, we need to make sure we're fully focused on what we're doing and aren't distracted or putting ourselves in unnecessary danger.

Loading up the leg press with too much weight is like putting a learner driver who's just started to get the hang of driving into a modified sports car with 700 horsepower.

Sports car doing stunts in a storm

Even though the learner could still technically drive the car (by driving very slowly), any small slip up and pressing the accelerator too hard could lead to big accidents.

The same goes for the leg exercises we're discussing today.

Using too much weight on the leg press can certainly lead to injuries if you lock your knees out.

However, if you're sensible with the weight you use, and you've consistently been training with a full range of motion and locking out your knees all this time, working your way up to heavier weights, then you will be fine to lock out your knees.

Locking Out Your Knees Can Be Dangerous If You Have Hyperlaxity or Hypermobility in Your Joints

Some people will find that they are excessively flexible and mobile in their joints.

For example, some people can bend their thumbs back to degrees that others find impossible, or hyperextend their elbows past the 'flat' level and into a position where their elbows are actually starting to become bent backwards.

Chances are, you've met someone like that before.

Or you might even experience it yourself!

It is quite rare, and according to NHS Inform, affects 1 in every 100-200 people.

Now, if you think you have hyperlaxity in your joints, or are excessively mobile, then it might be inherently more dangerous for you to lock out your knees, and in that case, it's better for you to stay away from doing it, even at lighter loads.

Your knees are already halfway capable of bending backwards, and doing it under heavy loads (usually upwards of 100kg or 225lb) on the leg press might be quite dangerous.

But other than that, there's no real other reasons why locking out your knees will be dangerous on a leg press, assuming you're being sensible with the weight you're using.

Why Might We Want to Lock Out Our Knees?

More Range of Motion

Locking out your knees allows you to achieve greater ranges of motion for your legs, which we know is directly correlated with muscle growth up to a point, as found in this study by Dr. Brad Schoenfeld.

"When assessing the current body of literature, it can be inferred that performing RT through a full ROM confers beneficial effects on hypertrophy of the lower body musculature versus training with a partial ROM."

While the difference in range of motion is not massive whether we lock out our knees or not, it still adds up over a set of 12 reps, done 3-4 times each workout, twice a week, 52 weeks a year.

So yes, achieving that extra range of motion is certainly beneficial if you want to grow big legs and achieve muscular hypertrophy.

What About 'Keeping Constant Tension'?

People will tell you that it's important to keep constant tension on your muscles by not locking out your joints, on most exercises.

Man doing heavy barbell overhead press to build his shoulder muscles

They'll tell you not to lock out your elbows on a dumbbell bench press, or not to lock out your knees on a Bulgarian split squat.

While this does increase the burning sensation you'll feel in your muscles during the set, it's not worth the trade off in proper/full range of motion you're foregoing by not locking out.

That burning sensation is not what drives the majority of your muscle growth.

The mechanical tension you place under your muscles, through a proper range of motion is what determines your muscle growth.

Taking that extra half a second of rest in between reps isn't going to render your set useless.

It's going to make you able to train harder and get more out of your sets in the long run.

So don't be so worried about keeping the constant tension.

Instead, worry about the quality and productivity of each individual rep you do.

If you haven't been locking out your knees in the past and are wanting to start doing so, make sure you start with light weights and work your way up. DO NOT try and do it immediately on the weights you're used to using, because that is simply stupid and could lead to injuries if you're already using more weight than you can handle.

To Wrap Things Up

All in all, whether you choose to lock your knees out of not on your leg exercises is completely up to you.

You'll still be able to see leg gains either way, but it's just a matter of whether or not you want to train as optimally as possible, in a way that's also safe and appropriate for you.

It is safe if you're training properly and don't have hyperlaxity, as long as you are sensible with how you train.

In fact, it can be beneficial if you can do it safely.

Remember, safety always comes first and if you're going to try and implement locking your knees out into your workouts, make sure to always start small and don't try anything stupid.

Just remember the car analogy.

Do YOU lock our your knees on a leg press?


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