Every action has an equal and opposite reaction. Muscular contraction and gravity create opposing forces across the joint surfaces, known as joint reaction forces. It is important to spread these forces over the greatest possible joint surface area.
Joint congruency refers to the fit of a joint's articular surfaces. A joint is congruent when its surfaces fit together perfectly. Movement out of congruency focuses stress on a small surface area. A large force focused on a small area of articular cartilage can injure it, eventually causing degenerative changes.
Some yoga postures have the capacity to sublux or take a joint into an incongruent position. Avoid this by using the joints with a greater range of motion while protecting those joints with limited range of motion.
Joint reaction forces - applied
For example, the ball and socket joint of the hip has greater range of motion than the hingejoint of the knee. Lotus posture (or padmasana) requires a large amount of external rotation of the hip joint to bring the foot into position on the opposite leg. Obtaining this external rotation from the knee joint creates incongruency because the knee is a hingejoint with limited capacity to rotate. This incongruency can result in the abnormal distribution of joint reaction forces, injuring the intra-articular structures of the knee. Therefore it is essential to first obtain full range of motion of the ball and socket hip joint to protect the hinge knee joint, (see arrows)
Ligaments are fibrous connective tissue structures that link one bone to another at the joint. They serve to stabilize the joint while at the same time allowing mobility. Ligaments vary in size and shape according to their function. For example, the cruciate ligaments of the knee are short, stout structures that assist in maintaining the knee as a hinge. The sacroiliac ligaments are dense, broad and thick structures that limit movement of the sacroiliac joint. The shoulder ligaments are thin band-like structures that are confluent with the shoulder capsule, allowing for great range of motion.
Ligaments are non-contractile but actively participate in movement because they have sensory nerves that transmit information aboutjoint position to the spinal cord and brain.
Ligamentotaxis refers to the pull of Ligaments on the bones to which they are attached. This concept is used by physicians to set pull broken bones back into place and set them. It can also be used during yoga practice as illustrated here in the forward bend utthanasana. Here the weight of the upper body is transmitted to the pelvis via the ligaments of the back. This pulls the pelvis forward and lifts the ischial tuberosities, passively stretching the hamstrings. Similarly, ligaments have some capacity for elastic recoil. This recoil can be combined with the momentum of the body as it raises from postures such as back bend.
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