Biomechanic and Prevention of Neck Pain

The science of the action of forces internal (or)external on the living body is called as Biomechanics. Means applying of mechanical advance in human body. First we must know about the center of gravity. The center of gravity is a hypothetical(imaginary or assumed )point at which all mass would appear to be concentrated and in this point only force of gravity would appear to act. If the object is symmetric the center of gravity is located in the geometric center of the object(Figure-1), but in asymmetric object the center of gravity will be located toward the heavier end at which the mass is concentrated and evenly distributed (Figure-2).Since the center of gravity is hypothetical point, it need not lie within the object being acted upon. If it is a single object center of gravity is thought of as the balance point(Figure-1).but the object is made by several segment each segment has its own center of gravity where the gravitational force act upon. For example while standing in normal position(anatomical position)whole segment of our body take as a single object the center of gravity present just anterior to the second sacral vertebra(present near to hip)(Figure-3) .According to the standing posture or position ( narrow base or broad base ) center of gravity may shifted up or down(Figure-4) .Take our body as several segment individual segments have different center of gravity as shown in the picture(figure-5) .Body weight and muscle strain depend upon the gravitational force. This gravitational force concentrate mainly at the center of gravity. Next we must understand Line of gravity: A vertical line drawn through the body’s center of gravity is known as line of gravity. As shown in the picture (figure-3) posterior and lateral(side) view line of gravity passes through the body. The vertebral column of a baby at birth exhibits one long curve that is convex posterior(figure-6) . This long curve modified in to several curve for the erect posture In adult the vertebral column exhibits regional curvatures. The thoracic, sacro- coccygeal curves are convex posteriorly and are called primary curves and curves seen in the cervical and lumbar regions have an anterior convexity and are called secondary curves(figure-6).According to the erect postural change during development (growth) line of gravity will fall. When the line of gravity passes directly through a joint axis no gravitational torque(force) is created around that joint If the line of gravity passes at a distance from the joint axis, a gravitational torque(force) is created. If the line of gravity fall in frond of the joint, Gravitational torque will act on the proximal parts or above parts of the joint and tends to bend it forward, This forward bend is counterbalanced by posterior or back side muscles[Example-Action of gravitational torque or force in the ankle joint(Figure-7)] , like that all gravitational torque act upon the body is counterbalanced by opposite side muscles and ligaments. If the line of gravity is away from the joint axis the magnitude of the gravitational force will be more, here the muscle activity and strain is more, If the line of gravity is close to the joint axis the magnitude of the gravitational force will be less, here the muscle activity and strain is less, So if we keep the part of our body close to the line of gravity we can avoid muscle strain, for example without muscle strain you can balance a book at the center of gravity with one finger but if you hold a book at its edge you can feel the strain(Figure-8).

Biomechanics of neck:

Anterior-posterior view or front view: In this view Normally the line of gravity passes in the center and bisects the body into two symmetric halves. As the line of gravity passes through the center of the neck(Figure-9) When the line of gravity passes directly through a joint axis no gravitational torque(force) is created around that joint so muscles and ligament activity in the two sides are equal. But in some abnormal condition like scoliocis, torticollicis the neck bend to one side, and the line of gravity is shifted to the bended side so the gravitational torque increased in this side, to counter balanced this pull or torque, opposite side muscle contract eccentrically in such condition stretching type of exercises to the affected side and concentric type of exercises to the opposite side must be advice Lateral view or side view: In the lateral or side view we can see two movements, one forward bending(flexion) and backward bending(extension).This two movement happened in the transverse axis. In the upper neck area ,Related to the head the line of gravity passes anterior(front) to this transverse axis(Figure-10) . So the gravitational force which tends to tilt the head forward, to counteract this force neck extensor muscles(back muscles of the neck) contract eccentrically ,So here the concentric type of exercises are advisable. In the lower neck also the line of gravity passes anterior(front) to the joint axis so advice also same.

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