This is a short note, but I would still like to put some light on a common problem, not only among bodybuilders but everybody. Perhaps even more with the couch potatoes because of their many hours of bad posture in front of the tube. But anyways...
When you're standing up straight and flexing your hip joint, i.e., raising your thigh in front of you, you're using a whole bunch of muscles. Some cross more than one joint and becomes multifunctional, but some are basically there for one purpose only. One of these muscles is M.Psoas Major, a strong little dude, too, often refered to at Ilio-psoas (two synergetic muscles doing roughly the same thing, kind of like your bicep and your brachialis).
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Sure, the picky person points out that it does a light rotating movement as well, but the main thing about it is flexion in the hip joint. So what about it? Well, time for another round of anatomy...
The M.Psoas Major
The M.Psoas Major attaches to bones, like most skeletal muscles, creating a pulling motion and thereby forcing the joint to move. The key to why this is so important is WHERE it attaches - namely starting from the vertebrae of the lumbar spine and ending in a spot on the upper part of the femur bone! It crosses the hip joint, and when contracting it flexes the hip, BUT... Unless you actively flex your abs and resist it, the pulling force from the muscle attachment will make your lower-back arch increase greatly!
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The Pulling Force From The Muscle Attachment
Will Make Your Lower Back Arch Increase Greatly.
Needless to say, this is putting great stress on the disks of the spine. So what happens if you're stiff? What if your M.Psoas Major gets a bit shortened from exercise and not properly stretched afterwards? Right - the shortened muscle is constantly pulling your poor lumbar spine into an exaggerated arch, creating discomfort and possible long-term injury. The more you pull your leg backwards, the more you're pulling the muscle, and as it's short and stiff the one thing that can move - the attachment to the spine - gets increasedly pulled.
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And what about running? If your hip-flexors are stiff when walking, what happens when you're running - and moving your legs even further back? Right - the arch increases every time you move the leg back. And when it's at it's most arched (worst!) position, you hit the ground with your other foot, sending hundreds of pounds of shock through your body.
By now I think we can conclude that keeping that little bandit in shape is a pretty good idea, so the only question is how. Easy. Stretching will most likely do the trick. More specifically, I'll narrow in on a special kind of stretching, "PNF" or "Contract-release" stretching, which works on just about all muscles. It's very easy on the muscles and joints, yet yields good results.
The M.rectus Femoris
Apart from stretching the Psoas M., I also stretch the M.rectus femoris (also causing increased arch, just like the Psoas M.), I stretch the hamstrings without pain and finally, and target the calves efficiently.