Levers of the Body The action of the individual muscles can be understood only when the bones to which they are connected are considered as levers whose fulcra are fixed points upon which movement may take place. The muscles are the sources of power and usually work in groups rather than singly. To be effective in accomplishing work, a lever must be acted upon by two opposing forces; the one active, the other passive. Work may be defined as the movement of some object vertically through space. In foot-pounds, work is equal to the weight of the load in pounds multiplied by the distance in feet through which the load is lifted. In the normal human body, the active force resides in the muscles, the passive forces are essentially external, such as gravity, friction, cohesion, and other natural forces which oppose movement. The joints  It is through the skeleton that the body is given a somewhat variable but definite rigidity, without which it would be a shapeless mass incapable of performing any of its characteristic external movements. These are exhibited mainly by the head, the limbs, and the respiratory muscles. The movements may be relative to other parts of the body or may be general movements of the body as a whole, as in locomotion. The flexibility of the skeletal system, and hence the body, is due to the joints. These consist of the approximated surfaces of two bones or cartilages for the purpose of movement of one upon the other, subject, of course, to the restraints of the uniting structures. These opposed surfaces are held in position by ligaments and other supporting media, and normally move upon the surfaces of contact with a minimum of friction. This ease of movement is made possible by the presence of smoothly-surfaced hyalin-cartilage pads which cover the articulating surfaces of the bones. In other joints, as between the vertebræ, discs of white fibro-cartilage are inserted between the approximated bone surfaces. The synovial membrane which is attached at the edges of the hyalin cartilages completely encloses the joint. The inner surface of this membrane is lined with endothelial cells which secrete a clear, colorless, viscid fluid--the synovia. This fluid moistens and lubricates the articulating surfaces during movement.		Home Pace Motivation Measures of Endurance Computation of Dietary Requirement Work Efficiency Maximum Steady State Work Inefficiency of Anaerobic Work Estimation of Work Efficiency Efficiency of Grade Climbing Low Economy of Slow Work Maximum Rate of Work Levers of the Body Fatigue and Recovery General Fatigue Symptoms Anxiety State Organic Fatigue Cortical Motor and Sensory Centers Effect of Exercise on Mental Work Visual Fatigue Muscular Fatigue Fatigue Prevention and Recuperation