The Motion of the arm and the shoulder when hammering involves osteokinematics or the movement between the bones at the synovial joints, which the most common movable joints in the body. It occurs around a center of rotation namely the joint axis. It is a fixed point that moves in a very specific way. The surfaces of the two bones at the joint are covered in cartilage. The thickness of the cartilage varies with each joint, and sometimes may be of uneven thickness. Articular cartilage is multi-layered. A thin superficial layer provides a smooth surface for the two bones to slide against each other. Arthrokinematics is the general term for the specific movements of joint surfaces. Joint surfaces move with respect to one another by simultaneously (1) rolling, (2) gliding, and (3) spinning Normal joint surface movement is necessary to ensure long-term joint integrity. (Joint surface movements are sometimes called joint plays motions or component motions).
There are four phases that we can consider when using a hammer in analyzing the movement of the arm and shoulder. The first phase is initiating an upward stroke or raising the hammer in preparation to strike. When raising a hammer in an upward stroke the position of the body must be stable, with the left shoulder a little lower than the right, hands slightly above and to the left of the top of the thrower’s head, hips squared forward. The thrower must also remain centered with as little sideways sway as possible.
The second phase is the deceleration and acceleration while lifting the hammer. You must have a good stability by positioning the leg in good position. The movement of the arm and shoulder must aligned in the axis of rotation and maintain the position until the hammer strikes the object. The muscular movement of the bone when lifting the hammer generates a force and produces a movement by contracting or pulling on the arm and shoulder. The individual muscle can only pull but they cannot push. These skeleton muscles are joined to the bone by tough connective tissue called tendons. The tendons are attached in such a way that they pull on the Bones and make them work like levers. These make s the hammer being lift at the highest point in preparing the hammer to strike. The movements of the Muscles and Joints enable the Bones to act as levers. The joint functions as the fulcrum or the fixed point around which the levers moves and the muscles provide the force to move the lever.
The third phase is the down stroke or swinging the hammer down. After reaching the apex. The arm with the hammer is ready to swing down and strikes the object. Accelerating the shoulder and drag the hammer towards the hitting object. Relaxing and effective movement of the arm gives the motion a perfect timing when striking the object. Usually there are several muscles surrounding each joint that pulls in different directions and the most skeletal muscles work in pairs. When one muscles or set of muscles contracts the other relaxes. When striking an object by using a hammer, the muscle on the upper arm does this dual action. The antagonistic muscle is the muscle that bends the joint and the extensor is the muscle that straightens the joint. When the bicep muscle contracts it bends or flexes the elbow joint. When the triceps muscle contracts it opens or extend the elbow joint. A controlled movement requires contraction by both muscles.
The fourth phase is when the moment hits the object. As you push the hammer down, the hammer moves faster and accelerates because of the force coming from the muscles and the weight of the hammer by which the gravity pulls. The cartilage fiber existing on the joints give another especial function and that is preventing from shear stresses. Deeper than that is an intermediate layer, which is mechanically designed to absorb shocks and distribute the load efficiently. The deepest layer is highly calcified, and anchors the articular cartilage to the bone.