The focus of this chapter is to explore how various body structures work together to enable the body as a whole to either maintain a particular posture or to transition from one posture (or alignment) to another during movement. A brief review of the integrated mechanisms associated with postural control is included, and knowledge of individual joint and muscle structure and function is used as the basis for determining how each contributes to equilibrium and stability in an ideal posture. The internal and external forces acting on the body in relation to various postural orientations are considered, as well as how deviations in initial alignment can alter joint movement and function. This chapter begins with what is considered ideal alignment for various static positions before considering some common postural deviations, and discusses how these deviations may contribute to, or be the result of, various conditions encountered in physical therapy practice. Life span differences in postural control and balance will also be addressed.
Posture and Balance Defined
Posture is the orientation, or alignment, of the human body, and can be either static or dynamic. In static posture, the body and its segments are distributed in a manner that maintains the body in equilibrium. Standing, sitting, or lying are all examples of static postures with the body at rest. Dynamic posture refers to postures in which the body or its segments are moving—walking, running, jumping, throwing, and lifting are all examples of dynamic postures. All successful human movement is initiated from a static posture and is maintained through the integration of multiple systems.
The force of gravity directly impacts a person’s stability. In order for you to remain stable, your center of mass (CoM) and the vertical projection from this point, referred to as the line of gravity (LoG), must be maintained within your base of support (BoS; see Chapter 1). Stability is directly related to how easy or difficult it is to displace the CoM outside of your BoS, as once your LoG moves beyond your BoS, you may fall. You may increase your stability to avoid a fall by adopting a larger BoS or by lowering your CoM closer to the BoS. The multi-articulating components of the human skeleton can be arranged in an infinite number of positions. This rearrangement of body segments can influence the position of the body’s CoM and LoG and hence influence its stability (Figure 13–1).1,2
The center of mass (CoM) and base of support (BoS) impact stability. When the CoM is outside of the BoS, stability is affected.
Balance is the process by which upright posture is maintained. Postural control is responsible for achieving, maintaining, or restoring appropriate balance....