Liked on YouTube: Compression

Compression
From Uncovering the Treasure by Stephen Hwa, Ph.D. available on Amazon.com we take snippets of information about "Compression forces that exist in the body". "Our spine is constantly under compression forces due to the action of gravity on the body. I suspect the repetitive stretching of the spine by this move is one of the contributing factors to keeping the body's structure robust even in old age. The up and down energy flow is an important contributor during Form playing to maintaining continuous internal energy circulation." P. 59 "Not keeping head, neck, body, and the back leg at a straight line: A major problem for most new students especially those who have studied other styles of Tai Chi before. They will try to straighten the body perpendicular to the ground instead of leaning, or have a knee bent on the back foot, thus the body forms a crescent moon shape instead of a straight line. This weakens the forward fajin power similar to the problem with "heel not on the ground". Also the compression at the lower back is unhealthy to the lower spine. There are reports of a group of tai chi practitioners, practicing such style, all having lower back bone spur problems in their old age. The knee bend of the back leg puts uncomfortable strain on the Achilles tendon and calf muscle, and most likely will result in "heel not on ground"." P. 125 "Attention on the spine includes: protecting spine from excessive compression forces, giving the spine opportunity to stretch from time to time, strengthening the muscles and tendons around the spine through Form practice. Attention on the hip, knee, and ankle includes: avoiding overextension of the joints, such as the knee projecting beyond the toe and high kicks, avoiding torque or shear force in the ankle, knee, and hip joints by following rules such as "hand follows the foot, elbow follows the knee, and shoulder follows the hip" or "upper and lower body following each other"." P. 127 "The concave curvature in the lower back region in Picture X-1 indicates localized compression and stress concentration of the spine in the lower back region. The punch motion in the Form will create a reaction force adding more stress at that location. This situation will be substantially worse if the punch hits the opponent since the reaction force will be much higher. In fact, it may bend the body further backwards hurting the lower back and losing the power. A group of Tai Chi enthusiasts of this style in Shanghai practiced together regularly, including push hands and sparring. At old age, every one of them had lower back bone spur problems. In Picture X-2, the spine and back leg form a straight line to allow the reaction force transmitted along the straight path to be absorbed by the ground, without any stress concentrated along the spine. In addition, the energized abdominal muscles exert an upward stretch force on the spine which serves to minimize the compression stress along the spine."
via YouTube https://www.youtube.com/watch?v=MQ3Lw45NH1Y