Lately, I've been instructing students to "soften" their faces while practicing, especially during difficult poses (especially during Gomukhasana...which all my students seems to dislike. A lot. Judging by how much face-softening I have to remind them to do). I've found it a really useful reminder to myself, if I find myself straining during a pose; first I check my shoulders and then I check my face. Inevitably one, or both, places are tight and unnaturally tensed.
Thinking of unnaturally tensed, got me to thinking about the fight-or-flight response, and it's opposite the rest-and-restore repsonse. The Sympathetic Nervous System is what controls the body's response to danger or stressful situations; blood pressure rises, heart rate increases, breath gets more rapid and shallow, muscles tense. In theory, this is a good thing, especially if you need to outrun a predator, because the body is prepared to act quickly and intensely (more blood rushing to the muscles, muscles ready to work hard, mind focused). However, nowadays, this system is triggered by stressors that don't necessarily required a mighty jump or speedy run--deadlines, excessive responsibilities, irritating co-workers.
The Paraympathetic Nervous System is supposed to counteract the Sympathetic by slowing the heart rate, dropping blood pressure, calming the mind and relaxing muscles so the body can rest and be restored. Unfortunately, if the body is overloaded by stress, the Sympathetic mode never completely shuts down. The demands on the body become too great, and the results can include a compromised immune system, slow-to-heal muscles, high blood pressure, insomnia, etc etc.
Yoga, of course, is a great jump-starter of the Parasympathetic, but only if you let it. When I see yogis straining and grunting as rivers of sweat pour down their muscles, I pause. Of course, with enough work and sweat, yoga will give you a hard bod, but is that really the point? To me, yoga is slow and the work is very subtle and deep. The hardest exercise is keeping your thinking mind at bay so that you don't strain and overdo while trying to achieve some elusive, physical goal. The purpose is quieting and centering...that's what makes it yoga and not Pilates.
So, I Set That Woman Down and I tell myself to soften my jaw (or my tongue--such an evocative instruction). The pose doesn't become easier, but suddenly I can feel myself settle and open. Nobody is falling in love with Cow's Head, but, with the reminder about the face, I can see the elbows stretch away a little bit more, necks lengthen and the side ribs lift.
Let's save the Sympathetic response for when that saber-toothed tiger attacks...