I like to think that, as a yoga teacher, I'm helping my students improve their body awareness. They should know by feel the difference between slumped posture and spacious posture. Be able to feel the firmness of muscles contracting to support their bodies in yoga poses. Know how to release those muscles when the work is finished and it's time to relax.
You probably have similar aims for your teaching. We're all trying to guide our students into growing into healthier bodies, to name just one of yoga's benefits. However, do you know how to teach and describe the actual process of muscles contracting and letting go, so that your words confirm the students' physical experience? If a teacher tells a student to relax a muscle while that muscle actually has to contract in a pose, the student will be kinesthetically confused. They'll think, consciously or unconsciously, that a contracting muscle is what "relaxed" feels like.
Maybe you've had this experience: You approach students whose shoulders are elevated halfway to their ears, ask them to relax their shoulders, and they reply, "They are." That's a perfect illustration of kinesthetic confusion.
What Is Muscle Contraction?
Let's clarify what happens when a muscle contracts. Your brain sends a message, via nerve fibers, to tell a specific muscle to contract. The muscle responds by trying to pull the bones it attaches closer together (muscles never "push" bones apart). During this process, the muscle is working and burning calories, which is why you get warm while exercising. The muscle feels firm or hard to the touch, and it's trying to shorten. Your brain is asking for just the right intensity of contraction to do the job at hand. The intensity of contraction is determined by the percentage of the muscle's fibers that are contracting. One hundred percent contraction is a cramp, and while you're alive the percentage never falls all the way to zero.
For example, imagine you're going to lift a five-pound dumbbell, starting with your arm straight at your side, then bending your elbow to bring the dumbbell near your shoulder. The primary muscle to do the job is the biceps, on the front of your upper arm, which flexes (bends) the elbow as it contracts. As you begin to lift the dumbbell, your biceps will contract and shorten to bend your elbow, with just the right percentage of fibers contracting to lift the weight smoothly against the pull of gravity. If too many muscle fibers are called upon, you'll probably lift the weight with a jerk; if too few are activated, you won't be able to lift it very far, if at all.
Contractions Come in Threes
There are three types of muscle contraction that work to lift, position, and stabilize our bodies in relation to the constant pull of gravity: concentric, isometric, and eccentric. As you bend your elbow to lift that dumbbell, the biceps is working (it feels hard to the touch and is burning calories) and it's shortening, which is the definition of a concentric contraction. In an isometric contraction, the muscle is working but not changing length: In the process of bending the elbow to lift the weight, you'd simply stop with the dumbbell partway up, holding the position so the angle of elbow flexion doesn't change. The third type of contraction is called eccentric, which means that the muscle is working, but it's lengthening. To put the dumbbell back down by your side, the biceps lengthens (the elbow is moving from bent to straight) to control the descent of the dumbbell against the pull of gravity.
Yoga uses a wonderful variety of concentric, isometric, and eccentric contractions in asana practice, which makes our muscles strong and well-trained in sophisticated movements. Gravity is always pulling on our bodies, so when we hold poses, our muscles are contracting isometrically to hold our body parts in place so we don't fall to the floor. Just listen to your quads as you hold Virabhadrasana (Warrior) I or II, your shoulders in Adho Mukha Vrksasana (Handstand), or your back muscles in Salabasana (Locust), and they'll tell you how hard they're working.
Your muscles are also working, but in concentric or eccentric contractions, to take you in and out of poses and through the constant movement of flowing sequences. Come back, for example, to Virabhadrasana II. The action of the quadriceps is to extend, or straighten, the knee. Moving into the pose to the right, the right quads are contracting eccentrically (lengthening) as your knee moves from straight to bent. The quads then contract isometrically while you hold the pose, and then concentrically as you straighten the knee to come out of the pose.
On the other hand, when a muscle relaxes, its activity level falls very low. It burns few calories, which is why you'll cool off when resting, and the muscle will feel soft to the touch.
Provide Support to Relax
It's important in yoga pose instructions to be clear that a muscle can't relax when it's working to move, support, or stabilize a body part. In other words, the neck muscles can't relax when they're supporting the head in sideways standing poses such as Trikonasana (Triangle Pose). If you really do want your student's neck to relax in Trikonasasa—if there is a neck problem, for instance—guide her to rest her head at an appropriate height, perhaps on a well-placed table. Only when a part is supported can the supporting muscles let go and relax.
Your abdominals can't relax when they're holding up your torso in Navasana (Boat Pose). Your buttocks can't completely relax as they help lift up your pelvis and tailbone in Setu Bandha Sarvangasana (Bridge Pose). And your hamstrings can't relax if your torso is unsupported (your hands don't reach the floor) in Uttanasana (Standing Forward Bend), because they're helping support your pelvis and torso against the pull of gravity via their attachments to the ischial tuberosities (sitting bone). To help your student in Uttanasana, put a yoga block under his hands to allow the tight hamstrings to begin to relax.
So, teachers, give some thought to how the pull of gravity affects the weight of arms, legs, head, and torso in yoga poses. Don't deepen your students' kinesthetic confusion by telling them to relax the very muscles that are holding them in the pose. If a body part is dangling in air or held up away from the earth, chances are very good that a muscle is contracting to keep it there.