I just can’t stay silent anymore. Every day at the gym I watch one member after another doing a straight leg raise, a roman chair leg raise or some variety of “sit up” (all hip flexion activities) , with the intention of working their abdominal muscles. Please stop this madness. It accomplishes nothing positive and can lead to potential back problems.
“But Dr Dave the body builder web site says its one of the most effective ways to work my abs,” Ya, if you are in that one-half of one percent fitness level you may get away with it.
First let’s look at the anatomy of what’s going on here, (caution technical jargon) hip flexion is primarily activated by two muscles, the iliacus and the psoas, collectively called the iliopsoas. They have a common insertion, the lesser trochanter of the femur, with the iliacus origin being the iliac fossa (pelvis) and the psoas origin being the lumbar vertebrae. The Sartorius, gracilis and rectus femoris provide hip flexion to a much lesser extent. Now, let’s look at the abdominal muscles, the rectus abdominis, transverse abdominis and the internal/external obliquus. Although each of these muscle groups have different functions for simplistic sake we will refer to them collectively as ‘abdominal muscles.’ With the origin being the anterior crest of the pubic bone and the insertion being the ribs and costal cartilage. Functions of the abdominal muscles are to compress abdomen, force expiration, rotate and flex pelvis and vertebral column.
Now we have all the technical jargon out of the way let’s take a look at what a leg raise is. You lie on your back and lift the legs by flexing the hips, nothing to do with the function of the abdominal muscles right? If we go back the attachments of the psoas muscle we see it attaches to the leg and the lumbar spine, by flexing the hips we are pulling the spine forward, increasing the lumbar curve or putting the spine into what is called hyperlordosis. In order to counteract this your abdominals have to contract to prevent your lumbar spine from being pulled too far forward, thus your abdominals are a secondary muscle and only engaged to prevent injury.
The first question we have to ask is why do we need to exercise our hip flexors anyway? The majority of us sit most of the day, our hips are at a 90-degree angle, therefor shortening the hip flexors. Thus when we stand our pelvis is tipped due to the shortness of the muscles with the resulting increased arch of the lumbar spine. Combine this with generally week abdominal muscles and short tight lumbar spines muscles, a hyperlordosis or excessive arch in the lower back is a time bomb waiting to explode.
This accentuation of the lumbar curve and the pressure it puts on the vertebrae can result in degenerative arthritis, inflammation, poor biomechanics and pain. So why do these dreadful hip flexion exercises? Start doing activities which elongate muscles and improve flexibility like the hurdler stretch, lunge stretch or my favorite the scorpion stretch.