ANATOMY BASICS

Muscles can also contract and lengthen at the same time, which is called eccentric contraction (“e” refers to movement away or out, as in “exit” or “egress”). A yoga example of this is moving into Virabhadrasana II, when you bend the knee to the proper align- ment directly over your ankle. The quadri- ceps muscles are lengthening since the knee is bending, but they are also contracting to control the rate of bending. If they released totally, we’d fall down! Another example is lowering the arms slowly in order to move in tandem with the breath; our muscle contrac- tion slows the pace of movement. Eccentric contraction is usually done while resisting another force, such as gravity. Concentric and eccentric contraction are both types of isotonic contractions, mean- ing that the length of the muscle changes while the tone may not (“iso” means “one” or “same”). Eccentric contraction generally will strengthen the muscles more quickly. Have you noticed how much work the quadriceps muscles do when you climb down a moun- tain? Not only are your thighs tired from the climb up, but the eccentric contraction required on the way down is hard work as well! When the weight goes onto the forward foot, the quadriceps work to control the rate of descent in an eccentric contraction. Once you arrive at the bent-knee posi- tion in Virabhadrasana II, you hold it with isometric contraction, in which the muscle does not change length while contracting (“iso” means “the same,” and “metric” means “length”). This type of contraction is also done while resisting another force, which

could be gravity or another limb of the body. For example, the arm and the leg pressing against each other in Ardha Matsyendrasana (Seated Lord of the Fishes Twist) create isometric contractions in the muscles of both limbs. Much of the muscular effort we use in yoga is isometric contraction. A muscle that produces a given action is called the agonist or prime mover, and the muscle that produces the opposite action is called the antagonist or opposing muscle. For example, when you bend your elbow, the biceps at the front of the arm is the agonist, and the triceps in the back (whose job is to straighten the elbow) is the antagonist. Here are some pairs of agonists and antagonists: Muscle Pair: Biceps and triceps Function: Bending and straightening the elbow Muscle Pair: Hamstrings and quadriceps Function: Bending and straightening the knee Muscle Pair: Gastrocnemius and tibialis anterior Function: Plantar flexion and dorsiflexion of the ankle Muscle Pair: Rectus abdominis and erector spinae Function: Flexing and extending the spine Muscle Pair: Supinators and pronators of the forearm Function: Turning the palm up and down

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