Veterinary Gross Anatomy

BIOMECHANICS OF FORE LIMBS

Architecture of thoracic/ pectoral/ forelimb

• In terrestrial mammals, the forelimbs bear the greater load both while rest and during locomotion. Their main function is to stabilise and support the trunk which is propelled from behind. To accomplish this basic function, the forelimb musculature is simply arranged but they are made stronger by tendinous insinuations. In many animals, the skeletal components of the fore limb is also reduced i.e. the scapula alone is well developed; the clavicle and corocoid are either absent or exist in vestigial form in certain species.
• The forelimb is attached to the cranial end of the bow and string apparatus i.e. the lateral wall of thorax only by muscles (Synsarcosis). This is done by the pectoral girdle muscles that join the forelimb to the trunk and named as synsarcosis which substitutes a conventional joint. The pectoral girdle muscles includes – trapezius, rhomboideus, brachiocephalicus, omotransversarius, deep pectorals, latissimus dorsi and serratus ventralis.
• When the animal is in standing position, some of the muscles of the pectoral girdle (serratus ventralis and deep pectorals) suspend the body between the fore limbs to which the weight of the head, neck and cranial part of the trunk is transmitted. The above said muscles form a cradle between the two scapulae which allows the thorax to raise and fall between the shoulders and to lean the animal towards one side without affecting the position of forelimb. These two muscles together with other pectoral girdle muscles stabilise the scapula against external forces and thereby prevent the rotation or displacement of scapula.
• During progression, the pectoral girdle muscles (serratus ventralis, trapezius and pectoral muscles) which are used to suspend the body between the forelimbs turn into antagonistic groups that control the swing of the limb; one group of muscles advances the limb (named as protractors) and the other group retracts the limb (named as retractors).
• In order to understand this, one must appreciate that the scapula is moving against the chest wall in two different ways. In one, the scapula is rotated around its transverse axis towards the proximal end. This movement is balanced by the opposing actions of trapezius and rhomboideus which are inserted on the lateral and medial surfaces of the scapula respectively. In the other movement, the whole scapula is moved on the thoracic wall. The scapula is moving downward and forward during protraction and upward and backward during retraction. This movement of the scapula on the lateral wall of the thorax is permitted by the looseness of the connective tissue between limb and trunk and also by the existence of the potential space, the axilla, corresponding to the human arm pit.
• When the animal is in standing position, the muscles of each limb helps to maintain the balance. Particularly in large animals, when the animal is standing for longer time, there is a possible chance that the muscles may become fatigue. To overcome this, in large animals the muscles are reinforced with adequate amount of fibrous tissue. For example, the thoracic portion of serratus ventralis is reinforced with more tendinous structure that provides great strength to the muscle.
• Apart from the muscles, the movement of the joint and ligaments associated with them also contribute in maintaining the equilibrium. In large animals, majority of the joint movement is restricted to flexion and extension except hip and shoulder joint. In some joints, presence of collateral ligaments restricts the movement.

Changes in forelimb during locomotion

• Stride: Stride is the fast walk. Each stride consists of three phases:
  • Thrust phase – this is initiated in the hind limb by the powerful contraction of all the hind limb muscles. This extends all the joints to create the thrust against the ground. Then the thrust is transmitted to the opposite forelimb via the trunk.
  • Swing phase – during this phase, the foot is off the ground by flexion of all the joints of the limb.
  • Stance phase – in this phase, the foot is in contact with the ground by the extension of the limb.
• During forward movement, the thoracic limb receives the thrust initiated by the opposite hind limb via the trunk. Subsequently the thoracic limb is raised off the ground from the starting point by the pectoral girdle muscles and the flexor muscle flexes all the joints of forelimb.
• The flexed limb is protracted by the brachiocephalicus muscle. At the same time, trapezius and omotransversarius rotates the distal end of the scapula cranio-dorsally and the caudal angle towards caudo-ventrally. Subsequently the stance phase begins, where all the forelimb joints are extended by the extensor muscles (biceps brachii, triceps brachii and extensors of forearm) of forelimb. Hence, the limb is straightened and moved forward from its starting point. Thus the limb becomes longer and reaches the ground further forward from its starting point.
• When one limb is under stance phase of the stride, the weight of the body is shifted to the other limb. Then the scapula is rotated in reverse direction by the contraction of rhomboideus and deep pectoral muscles.