The upper body should be erect, the lumbar spine against the support, and hips and knees bent at ninety degrees. Avoid having the backs of the knees pressed against the seat. Feet should be flat on the floor or on a foot rest, not dangling. The joints and soft tissues of the upper extremities are sensitive. Even simple tasks can add up to injury if they are done repeatedly over prolonged periods.
Safe alignment relies on a relaxed position of the joints.
Keep your shoulders relaxed at or near your sides. Problems arise when the shoulders are moved forward on a repeated or prolonged basis. Strain on the shoulder increases when the arm is held out or forward. The further the arm is moved away from the body, the greater the load on the shoulder joint. The elbow should generally be held at a right angle, or ninety degrees.