Typically, agonist/antagonist relationships involve muscles contracting and relaxing on opposite sides of a joint, creating a biomechanical Yin/Yang. One muscle contracts to move the joint in a certain direction while another opposes that movement and stretches during this action. For example, when the knee extends, the contracting quadriceps are the agonist muscles and the stretching hamstrings are the antagonists. Similarly, when the knee flexes, the hamstrings are the agonist muscles and the quadriceps are the antagonists. Joint movement in response to muscle contraction is a biomechanical event that is coupled with a physiological event—reciprocal inhibition. When the brain signals an agonist muscle to contract, it simultaneously signals the antagonist muscle to relax. This is a physiological Yin/Yang. Understanding the major agonist/antagonist relationships is one key to doing yoga poses well. Accordingly, it is important to learn the muscles and their actions. We illustrate these relationships for you throughout the Mat Companion series.
FIGURE 1 In Utthita Trikonasana, contracting the quadriceps extends the knee and aligns the femur with the tibia at the joint. This maintains joint congruency and helps prevent injury. In addition, actively engaging the forward-leg quadriceps (the agonist muscle here) signals the hamstrings (the antagonist muscle) to relax. Relaxing the antagonist muscle allows you to move more deeply into the pose. This is an example of reciprocal inhibition. Try this yourself and experience how the stretch feels different when you strongly contract the forward-leg quadriceps.
FIGURE 2 In Uttanasana, the rectus abdominis flexes the trunk and signals its antagonist muscles, the erector spinae and quadratus lumborum, to relax. Engage this muscle in forward-bending poses to deepen the stretch of the antagonist back extensors.
FIGURE 3 In Utthita Parsvakonasana, the psoas is the agonist muscle that flexes the hip and tilts the pelvis forward (anteversion). When the psoas contracts, the brain signals its antagonist muscle, the gluteus maximus (the main hip extensor), to relax into the stretch.
