16


PART I FOUNDATION CONCEPTS


whereas torsion twists a structure around a long axis. These forces or loads can be combined to produce forces in multiple planes around more than one axis.


INFLUENCE OF FORCES


The infl uence forces have on objects determines whether the object is balanced and in a state of equilibrium or in a state of fl ux and instability. Concepts related to an object’s center of gravity and its base of support play a role in the infl uence forces have on the object’s state of stability and equilibrium. By altering the body’s center of gravity and base of support, the body can change its function from one of producing large arcs of motion to one of providing stability and support.


Unloaded Tension Compression Center of Mass


Gravity is often referred to as the weight of an object, when actually the weight is the gravitational force acting on the object’s mass, where mass is the matter that makes up the object. An object is composed of the same mol- ecules and matter on Earth as on the moon, although it has a much greater weight on Earth because of the gravitational force on the object. Weight is defi ned as:


Weight = Mass × Gravitational Acceleration (32 ft/sec2


or 9.8 m/sec2 Bending Shear Torsion


Figure 1.14 Types of forces and loads encountered by body structures.


during movement and activity. The tension forces act in opposite directions to pull materials apart. However, structures debilitated by disease or immobilization may be incapable of resisting tension forces that elongate and overstretch tissues, and injury can result. The skel- etal system undergoes continuous compressive forces during weight-bearing activities of the extremities and spine. These forces act in directions toward each other, compressing structures. Compression forces from muscle contractions can also produce forces directed toward the joint tissue and surfaces. These forces approximate the joint surfaces, causing compression stresses. Axial com- pression describes compression forces along the long axis of a structure such as the spine. Skeletal systems that have undergone bone loss and become osteoporotic may be unable to withstand the compressive forces and can sustain fractures. Shear forces are parallel to each other but in opposite directions. Excessive shear forces can wear away joint surfaces and play a role in the develop- ment of degenerative conditions, such as osteoarthritis or degenerative disc disease. Bending occurs as both tension and compression bend an object around a point,


)


Weight is typically measured in newtons (N) or in pounds (lb) (1 newton = 0.225 lb).


Although gravity acts on each unit of mass of an object, it is typically depicted as being applied at the center of mass (COM) or center of gravity (COG) of an object. The COM represents the point around which all parts of the body segment or entire body are exactly balanced. If an object is symmetrical in shape, the COM is the geometrical center of the object. Placing a pencil on one’s fi nger at the pencil’s COM will result in the pencil balancing and not moving. In the human body, body segments and the body as a whole are more asymmetri- cal, resulting in various locations of the COM for each body part. The line of gravity (LOG) is represented as a vertical downward line, representing the force of gravity on the segment. In considering the entire body as a single object in the anatomical position, the COM lies approximately anterior to the second sacral (S2) ver- tebra. This location is an approximation, as variations in height, weight, and body proportions can alter each person’s COM. The COM also shifts as a person changes positions and contact with the supporting surface. The COM of the head, arms, and trunk is located just distal to the xiphoid process of the sternum and anterior to the border of the eleventh thoracic (T11) vertebra. The weight of the head, arms, and trunk can be calculated as approximately 65% of the body weight (Fig. 1.15).


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