Why Leg Raises Hurt Your Back

About 30 years ago we learned that straight legged sit-ups were bad for you because it caused back pain. But aren’t lying or hanging leg raises the same thing as the straight legged sit-ups – done in reverse? Aren’t you using the same muscles, whether you raise your legs and keep your upper body still or raise your upper body and keep your legs still?
Great looking abs are a goal for both men and women, but lying or hanging leg raises may be contributing more to your back pain then you think.
How Muscles Work
To understand why this exercise may not be so good for your back, you need to understand the basic bio-mechanics. Learn which muscles move what body part. Anatomy and physiology teaches you that a muscle attaches at two ends. When those two endpoints or attachment points move closer together you get muscle contraction. It is the contraction of the muscle that pulls the endpoints closer and moves your body. When the two end points move further apart, the muscle stretches and relaxes.
FYI…when one muscle contracts – an opposing or opposite muscle stretches or relaxes. Every muscle has an opposing muscle. For joint stability and staying pain free it is important to exercise both the contracted and opposing muscle, otherwise you get muscle imbalance.
Look at the bicep muscle, for an example. It attaches at the top of your shoulder and just below the crease of your elbow. When you bend your arm, or do a bicep curl, the two endpoints move closer together, and the muscle is in a state of contraction. When you straighten your arm the two points move further apart and the bicep muscle stretches or lengthens.
Every muscle in the body works on this same “pulley” principle, including your eye muscles. Look at your chest muscles. The pec’s attach along the side of your sternum and run diagonally into the top of your shoulder. Now imagine as you lower the bar when doing bench press, the two end points move further apart. The pec muscles are stretching, but as you push the bar off your chest, the two endpoints move closer together. This causes the pec muscles to contract.
Examine the Leg Raise….
The muscles primarily responsible for raising the legs are the psoas muscles, NOT the abdominal muscle. The common name for the psoas muscle is the ‘hip flexor.’ The hip flexor attaches at the upper/inside portion of your thigh muscle and sits beneath your abs and intestines. It runs up and attaches to your spinal column, more specifically your lumbar vertebra and disc. These muscles are commonly overlooked for being a primary cause of back pain, but as you start examining which muscles move your body, you will quickly see how they can be the problem.
The abdominal muscles attach at the bottom of your ribs and run down past your belly button and attach to the pubic bone. Remember a muscle contracts when the two endpoints come closer together.
To prove to you that lying or hanging leg raises don’t specifically work the abdominal muscles let me get you to lie flat on your back as if you were performing a leg raise. Place your right hand at the top of your pubic bone and your left hand on the bottom of your rib cage. Now slowly raise your legs up to the ceiling, no more than 90 degrees. Did your hands come closer together? I don’t think so!
If you keep pulling your legs past that vertical position (90 degrees) and move them closer to your head, you will notice that your pelvis will begin to rock up towards your head. You should then begin to feel your hands move closer together, this is because you are finally contracting your abdominal muscles. You are basically doing a reverse crunch.
Now, keep your hands in the same position and do a regular crunch. You will notice your hands move closer together, which tells us your abdominal muscles are doing the work, not your hip flexor.
Crunches for Great Abs
Hopefully, this will help explain why crunches and reverse crunches are some of the best exercises for your abs. Doing leg raises, or holding your feet six inches off the ground or having someone throw your legs back down after you raise them is only going to make your hip flexors tighter and put you at risk for back pain.
For years I have seen patients come to my office complaining of back pain. One of the first questions I ask them is what kind of abdominal workout they do? If they say, lying or hanging leg raises or some other type of similar movement…..I help educate them on how that specific movement could be contributing to their back pain and how to properly target their ab muscles, without hurting their low back.
In Summary…
When you do leg raises or hanging leg raises, the first 90 degrees of the leg raise is activating the hip flexors. As you go past 90 degrees, the abdominal muscles will finally begin to contract. Unfortunately most people only do their leg raises within that first 90 degrees of range of motion, which is only working your hip flexors and will eventually make you susceptible to low back pain.
It doesn’t matter if you bend your knees, or put your hands behind your back. The muscles responsible for raising your legs those first 90 degrees are your hip flexors, NOT your abs. Grab an Anatomy & Physiology book and take a look at where these muscles attach to better understand what we are talking about. The last thing you want to do is slow your progress down with an injury.