Maybe not today. Or tomorrow. Or very soon. But eventually.
During the recent Level 1 Teacher Training in L.A., an excellent question was posed by one of the outstanding Yoga Tune Up® Teacher trainees: I have fabulously tight psoas. Wouldn’t warming up by marching make my [bad] psoas worse, given the additional hip flexion? Actually, the trainee posed the question much better and funnier. But I don’t know shorthand and didn’t take a recorder. Also, if you don’t like this blog, you can blame her.
Seriously though, this same question bothered me when I was first told that squatting is one of the best things a person can do for his/her health. Wouldn’t this extreme hip flexion shorten my dysfunctional psoas more? Wouldn’t it tighten my already tight calves and hamstrings? (We’ll put aside the fact that I was doing the squat in a less than optimal fashion – that might be another blog.) What I was missing was that an active contraction of a muscle has a profoundly different effect on the tissue than when the muscle is passively shortened.
To clarify, a “tight muscle” may not mean the same thing to all people. A muscle can become passively shortened due to habits like sitting (for the psoas) and/or walking around in high heels (please don’t do this) (for the soleus). This is not the same thing as a muscle that has been through so much “exercising” it’s become hypertonic. Neither condition is good for skeletal muscles, but they are very different conditions from one another.
As with the brilliant YTU Teacher Trainee referenced above, the psoas of people who live in cultures with rampant usage of chairs, sofas and cars, are very likely to be have been passively shortened (i.e., shorter than optimal length). For people who also incorporate a fair amount of running and cycling, the psoas is also going to be hypertonic – unless its owner spends an equal amount of time in real hip extension (as opposed to fake hip extension – but that’s also another blog).
So what causes a muscle to passively shorten?
While this can happen to other muscles, the one that has the international spotlight lately, and this blog, is the psoas. If you have chronic lower back pain, if you have trouble standing or walking upright, or can’t sleep on your back without a bolster under your knees, you can thank your poor psoas. You have two of these skeletal muscles, one on each side of your pelvis and their geometry is determined by the angle of your thighs relative to your lumbar spine – and how many hours each day you spend sitting or curled up in a fetal position because your back hurts. Go ahead; count the number of hours your hips are flexed at a ninety degree (or smaller) angle each day. If it’s a lot, and for too many people it’s probably close to eight hours of at least their waking hours, the psoas will adaptively shorten.
The human body tries to be efficient. It will stop supporting tissues and structures which its owner does not use. Why waste the energy and resources of keeping a muscle at its optimal length if a person almost never keeps it or uses it at that length? Having excess length in a muscle is not efficient – consider how sloppy a yacht’s sail looks (and how ineffective it becomes) when the ropes holding it up are too loose. The body does not want sloppy muscles. So it will shorten muscles by removing sarcomeres at the end of muscle fibers. Slack in a muscle triggers the muscle fibers to shorten while a pull (stretch) on a muscle stimulates the addition of sarcomeres to the ends of its myofibrils. 
 P. E. Williams & G. Goldspink, Changes In Sarcomere Length And Physiological Properties In Immobilized Muscle, Journal of Anatomy, 127 (Pt 3), 459–468 (1978).