The forces and the moments acting on the joints of the upper and lower limbs were considered. Main forces considered in analysis are those produced by body weight, muscles, and externally applied contact forces. In the case of impact problems, the forces carried by ligaments must be considered. Static analysis is valid strictly for bodies at equilibrium or moving at constant speed. For a body to be in static equilibrium, the sum of the forces acting on the body must be equal to zero. In addition, to ensure rotatory equilibrium, the sum of the moments exerted by external forces must be equal to zero. When a limb undergoes a rotation, the forces and moments acting on it do not add up to zero. Nonetheless, for sufficiently slow limb movements, static analysis provides reasonable accurate results. The equations of static equilibrium are used to develop estimates for the unknown muscle forces acting on a joint.
The concept of physical stress was also introduced. Stress is a measure of force intensity on a surface area. It is associated with two directions: the direction of the unit vector that is normal to the surface area and the direction of the force applied to it. The component of stress that is perpendicular to the area is called the normal stress. The component tangential to the area is denoted as shear stress. A material will crack at a point at which the stress intensity reaches a certain point. Some materials have large resistance to compression (concrete) and others to tension (steel). The long bones of our skeletal system can withstand compression better than tension. The bones also fracture under sufficiently high levels of shear stress.
The growth and remodeling of bones, ligaments, and tendons are modulated by physical stress. Orthopaedic surgeons have devised methods to lengthen limbs and do other corrections by appropriately altering the state of stress in the bones and muscles involved.
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.