The upper limb is composed of three chained mechanisms, the shoulder girdle, the
elbow and the wrist. Considering bones in pairs, seven joints may be distinguished:
the sterno-clavicular joint, which articulates the clavicle by its proximal end onto
the sternum, the acromio-clavicular joint, which articulates the scapula by its acromion
onto the distal end of the clavicle, the scapulo-thoracic joint, which allows the
scapula to glide on the thorax, the gleno-humeral joint, which allows the humeral head to
rotate in the glenoid fossa of the scapula, the ulno-humeral and the humero-radial joints,
which articulate both ulna and radius on the distal end of the humerus, and finally the
ulno-radial joint where both distal ends of ulna and radius join together.
Considering translations negligible compared to rotations, each of them, except the
scapulo-thoracic joint, is usually assumed as ball and socket joint, allowing 3 degrees of
freedom (DOF) in rotation. The shoulder movements are usually referred to as ventral/dorsal,
cranial/caudal and axial rotations for the sterno-clavicular (3 DOF), as abduction/adduction,
flexion/extension and axial rotation for the gleno-humeral joint (3 DOF), as elevation/depression,
protraction/retraction, tipping forward/backward and medial/lateral rotations for the scapulo-thoracic
joint (5 DOF), and as flexion/extension and pronation/supination movements for the forearm joints (2 DOF).
To perform these movements, the upper limb is equipped with not less than 21
muscles actuators, among which some even divide in several bundles attached on
different bones. They can be divided in several groups according to the bone
they move and the DOF they control. Most muscles acting on the scapula insert
close to its medial border. This concerns the levator scapulae, the rhomboids,
the middle and the lower parts of the trapezius. The rotator cuff refers to the
group of muscles which covers the humeral head and control some of its rotations.
These are the subscapularis/teres major as opposed to the infraspinatus/teres
minor for controlling the axial rotations, and the supraspinatus/deltoideus which
handle the abduction. The other actuators of the humerus are the latissimus dorsi
and pectoralis major, which cooperate in its adduction, while they oppose each other
in flexion/extension and axial rotation.
Two prime antagonists groups of muscles control the flexion/extension movements of
the forearm: the brachialis and biceps brachii for the flexion as opposed to the
anconeus and triceps brachii for the extension. When the brachialis is inactive,
the biceps brachii also contributes for controlling the supination movement of the
forearm, together with the brachioradialis, as opposed to the pronator teres, which
controls the pronation. As muscles never work in isolation, natural movements always
involve the motions of all the bones. For a complete analysis, it is necessary to
consider the motion of the mechanism as a whole.
In practice, some muscles have very broad attachments while some others divide in several bundles attached on different bones. These may be modeled in dividing the muscles into several lines of action. The choice of the lines must be made on the basis of anatomical as well as mechanical considerations. Thus, we have assigned to the muscles mass and strength parameters, while their lines of action have been characterized by their activation parameter, their physiological cross-sectional area and the coordinates of their attachments. The topology is needed to compute the direction of the muscle force, which changes with the motion of the bones. The PCSA is a parameter useful for estimating the maximal force that the muscle or the bundle can bear.
