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The #1 Knee Killing Exercise in the Gym – How leg extensions are destroying your knees…and your back!


It’s unfortunate that one of the most popular and common leg exercises done today is also one of the most damaging. In fact, leg extensions in a lot of ways causes more damage to the knees than any other leg exercise. That’s hard to believe isn’t it? In addition, performing the leg extension improperly (which is 99.99% of the way I see it being done) creates problems for another part of the body; a part of the body you think would be safe with this exercise. I’m talking about your lower back and even though leg extensions are a seated exercise with a “back support”, the lower back is taking the heat, and your knees are leading the way. So, exactly how are leg extensions causing knee and back injuries? Let’s start with the knees.

There are several things about the leg extension that pound, grind, pop, and damage the knees and I have listed them here:

· The Starting Position – From the moment you sit down in a leg extension machine, your knees are in a very vulnerable position. In this seated position with your feet tucked underneath at less than a 90 degree angle, your quad muscles are basically being slightly stretched and the split second you start the extension, your patellar tendon (that hard ½ inch wide tendon right below your knee cap) is taking the impact before your quad muscles do. This is the start of a very common injury in exercise; patellar tendonitis. From here, and as the range of the exercise increases, it only gets worse.

· Hyper Extending and Shearing the Knee – almost every single person I see doing leg extensions have absolutely no control over the movement and when they get to the top (legs completely extended and straight), there is a slight over extension where the knees are actually forced to hyper extend which means they are forced to bend the wrong way. This causes a shearing between the lower and upper leg bones and will end up damaging the knee cartilage and the ligaments in the knee. For example: Straighten and bend your index finger back and forth a few times; this is the exact same motion as the leg extension. Now straighten your finger, hold it straight, and use your other hand to lightly push the tip of the straight finger upward causing a slight hyper extension in the finger. Now imagine doing this with resistance and going much faster, over and over again. Can you see what I mean?

· Using Momentum – this is where it really gets ugly! If you take into account the starting position where the patellar tendon is compromised, add the hyperextension of the knee at the top of the movement, and throw in going way too fast and using momentum to move the resistance, you’ve got yourself a full blown knee destroying exercise. When you go too fast with any resistance exercise, your joints are impacted because of the pop and jerk at the top and bottom of every repetition. Think about the shock absorbers on your vehicle; they are there to absorb the impact but if you hit something really hard or drive like an idiot, those shocks are going to wear out and then other parts of your suspension start taking a hit. Your knees and all of your other joints for that matter are the same way and if you keep pounding them, you’ll be in the shop replacing all kinds of parts real soon. Speaking of other parts, there’s another problem caused by leg extensions that nobody talks about, but don’t worry; I’ve got your back.

How in the world can you injure your back doing a seated exercise with a back support? It’s really very common, but the problem is that nobody associates the leg extension with causing back problems. Creating a back injury from doing leg extensions is a chain of events, and I have listed them here.

· It Starts with Your Feet – It never fails; every time I see someone doing leg extensions, their feet aren’t even. They either have one foot flexed and the other extended, one foot pointed outward and the other straight, one pointed in and the other pointed straight, or any combinations of these. Here’s the deal; when you turn your feet in or out, that turn is mostly generated from your hips. That’s right! As a matter of fact, try turning your feet in and out without involving your hips. You can definitely do it, but it places undo torque on your knees and when you add resistance to it, it really turns up the heat. So, when your foot position doesn’t mirror each other during leg extensions, and all other exercises for that matter, it affects one side of your hips differently than the other. And when you do it often, it leads to your next problem; uneven hips.

· Imbalanced Hips – Your hips are the center of your body and one little imbalance within them can and most certainly will cause your entire body to move differently and in a very imbalanced way. When your hips are imbalanced, your hamstrings, calves, hip flexors, IT band, piriformis, and many other muscles in your lower body also become imbalanced. This makes everything you do including walking, sitting, standing, lying in bed, and of course exercising to become imbalanced and the longer it goes, the more serious it becomes. Next up…your lower back.

· Lower Back Pain and Disc Problems – When your hips are imbalanced, it means one side of your body is over compensating for the other. When this happens, there is undue torque and unnatural stress right in the center of your lower back and if it’s not corrected, back pain will ensue and that precious commodity known as a disc between your vertebrae will become stressed and compressed leading to a very uncomfortable situation, to say the least. This leads to more imbalance and the probability of bulging, rupturing, or slipping a disc become great and once this happens, you’ll deal with it for the rest of your life.

There you go; this is exactly how leg extensions destroy your knees and eventually can lead to an injured back as well. Can leg extensions be done safely? Yes, but you have to start the exercise with your legs at a greater than 90 degrees angle between your calf and hamstring, you have to not fully extend your legs at the top of the movement, and you have to go slowly with a pause at the top and bottom of every single repetition. When I say slowly, I mean taking 4-5 seconds for every single rep you do, and that goes for all resistance exercises. In addition, you have to make sure your feet mirror image each other through the full range of motion and keep both feet completely straight with toes pointing up. The problem is, I never see anyone doing the leg extension like this and it’s just a matter of time before they can’t do them at all. So the next time you’re in the gym, take notice of how people do the leg extension and know that banging, clanging, and clashing sound isn’t coming from the machine; it’s coming from knees and backs screaming for help.

Bobby Whisnand, “The Exercise Doctor”