1.Elbow joint is a hinge joint
2.The elbow joint is not inherently stable. Greatest stability is achieved when bony surfaces are in closest contact, ie. when forearm flexed 90º, in mid pronation-supination. Hence this position is assumed naturally during fine manipulations of hands and fingers eg. in writing
3.The flexor group of muscles are more powerful than the extensors; when arm hanging loosely by the side, the elbow is slightly flexed. The power of the flexors is greater when the forearm is pronated, because the biceps is stretched, increasing its efficiency
4.Biceps works most efficiently between 80º and 90º, brachialis between 90º and 100º and brachioradialis between 100º and 110º.
5.Triceps is most efficient in 20º to 30º elbow flexion.Triceps more powerful when shoulder flexed and when both elbow and shoulder simultaneously extended from flexed position. Triceps weakest when elbow extended while shoulder being flexed.
6.THUS: maximum efficiency of extension when arm and forearm hang down with 20º to 30º between them. Maximum efficiency of flexion when arm and forearm stretched above shoulder