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The radius is quite thick near the wrist but thin near the elbow, while the ulna is thin at the wrist and thick near the elbow. The angle of the interosseous membrane ensures that the weaker portions of each bone attach to the thicker portions of the other bone. And finally, the angle of the membrane is designed to work with the natural stress alignments along the arm when a punch is delivered, which is typically force generated along the line of the radius. The pressure on the radius will cause it to move back slightly, but the angle of the membrane works to keep the bone in place and transfers the forces without any play or slack. This means one should be careful not to punch strongly with the middle, ring, and little fingers because they direct the stress of the punch along the ulna.

During a full-twist punch, the ulna and radius wrap around each other, which means they collapse onto each other and the connecting membrane is folded between them. With the membrane slack, it cannot support the structure of the forearm bones, so they bend apart and shift upon impact, producing unequal forces in the wrist that results in a loss of punching power and a risk of injury at the weakest areas of the bones. Conversely, during a three-quarter-twist punch, the position of the bones maintains optimal tension in the membrane throughout the movement.

Muscle Factors

The muscles of the forearm are also important when punching. The ones that turn the forearm into the palm-up position are called supinator muscles; the ones that turn the palm down are called pronator muscles. The two oppose each other.

If the fist is turned fully palm upward, the supinators are completely contracted and the pronators are completely stretched. Likewise, when the palm is turned downward, as in the full-twist punch, the pronators are fully contracted and the supinators fully stretched. In the three-quarter-twist punch, the pronators and supinators are equalized, each being approximately in the middle of its respective range of motion. The result is muscular balance that contributes to the overall stability of the forearm. In contrast, at the conclusion of the full-twist punch, the supinators are fully stretched and unable to balance the contraction of the pronators. Since these muscle groups help support the wrist, their unbalanced state increases the likelihood of the fist buckling on impact.

Skeletal Factors

As mentioned earlier, the full-twist punch lines up with the center of the body, while the three-quarter-twist punch is aligned just inside the shoulder. The reason for the three-quarter-twist punch's alignment is skeletal mechanics.

If you watch a full-twist puncher strike a board, the person will align the fist and shoulder with the board. People instinctively know this is the way to get the most power into the punch. Thus, the off center alignment used in the three-quarter-twist punch is best for hitting with power. Its ideal skeletal and muscular alignment means that more energy may be transferred to the target with less likelihood of injury to the puncher.

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