By Alexa Kim
During a recent Level 1 Yoga Tune Up Teacher Training in L.A., an excellent question was posed by one of the trainees: “I have fabulously tight psoas. Wouldn’t warming up by marching make my [bad] psoas worse, given the additional hip flexion?”
This got me thinking as 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.)
What I soon discovered 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. This active contraction during the pose is what makes all the difference.
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 (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 referenced by the YTU Trainee 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 hip extension.
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 that 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 muscle proteins at the end of muscle fibers. Slack in a muscle triggers the muscle fibers to shorten while a pull or stretch on a muscle stimulates the addition of muscle proteins to the ends of the muscle fibers. 
Why make a passively shortened muscle shorter by actively contracting it?
I’m so glad you asked. First of all, how much a muscle contracts is a major determinant of its size and rate of cellular regeneration. Decrease muscle activity and the rate of protein degradation will accelerate, resulting in muscle atrophy. Increase muscle activity and boost the production of new proteins.
Second, you never want to stretch a muscle when it’s cold. Concentric muscle contractions before static stretching minimize stretch-induced force deficits. In plain-speak, marching in place prior to stretching prevents the muscle from weakening after a static stretch. 
Lastly, to keep a muscle healthy, its nerve must supply both impulses (contraction instructions) and certain chemical factors. A muscle that is deprived of electrical or nervous stimulation will degenerate. On the other hand, an active muscle can increase short-term blood supply by telling its capillaries to open and implement longer term changes by causing the proliferation of capillaries.  This is why muscles also need to warm up prior to an extensive workout, and particularly before “cardiovascular” work. If the heart rate goes from resting to aerobic, the heart pumps a much larger quantity of blood – that needs to go somewhere. If the capillaries in the muscles are not open to receive the additional blood flow, it’s like turning on a fire hose that’s connected to a cocktail straw. This is not kind to blood vessels. And guess what happens to blood pressure if the muscles are cold and heart rate goes up?
If you’ve been guilty of inflicting long passive stretches or hundred-yard dashes on cold muscles, relax. Muscles can heal themselves and grow stronger with a sensible and healthy practice that incorporates a balanced systematic approach to stretching, strengthening, and cardiovascular exercise (which is why I’ve been so attracted to studying YTU). Active muscles that are kept at their optimal lengths move more blood, oxygen and nutrients – and increase one’s basal (resting) metabolic rate. This leads to better cellular regeneration and burning more calories while you sleep. The body is pretty spectacular.
 P. E. Williams & G. Goldspink, Changes In Sarcomere Length And Physiological Properties In Immobilized Muscle, Journal of Anatomy, 127 (Pt 3), 459–468 (1978).
 Whaaaaaaat?!?! Stretching weakens muscles? Yes, under certain conditions. Static stretching of cold muscles has been shown to damage muscle fibers. Eccentric stretching under heavy load conditions has also been shown to damage muscle fibers. And overstretching the muscle-tendon complex can increase joint instability and increase the risk of injury. Anthony D. Kay & Anthony J. Blazevich, Concentric Muscle Contractions Before Static Stretching Minimize, But Do Not Remove, Stretch-Induced Force Deficits, Journal of Applied Physiology, 108: 637–645 (2010).
 Steven Vogel, Prime Mover, A Natural History of Muscle (2001)
Alexa Kim teaches because she loves thinking about the complex and simple ways of the human body. The body’s needs for optimal health are incredibly simple; how the body compensates for damaging habits is extraordinarily complex. She loves assisting students distill functional movement to its most simple – so that students can restore, strengthen and protect their own bodies in any situation. She is certified to teach Yoga Tune Up, is a certified Restorative Exercise Specialist, and a Healthy Foot Practitioner.