Understanding gait function

American Fitness, Jan-Feb, 2003

Walking involves the complex interaction of muscle forces on bones, rotations through multiple joints and physical forces acting on the body (Chambers and Sutherland, 2002). The walking gait maneuver is the body's natural means of locomotion. Together with balance and stabilization, this convenient mode of travel represents personal mobility and thus, autonomy.

Although many younger individuals take walking for granted, this activity of daily living represents a crucial threshold for the older adult. As our society places more emphasis on maintaining an independent lifestyle among an aging population, more attention must be placed on maintaining mobility.

Normal human gait repeats a basic sequence of limb motions that progress the body along a desired path while maintaining weight-bearing stability, conserving energy and absorbing the shock of floor impact (Perry, et al., 1996). A successful gait pattern depends on sufficient control at three levels: basic reflexive stride and support patterns, postural and equilibrium control as well as mechanisms which allow the body to adapt to unexpected changes in the environment (Spirduso, 1995). Although the process of aging appears to diminish the ability to maintain an efficient gait pattern, it is only partly responsible since various types of pathology alter mobility and muscular effectiveness. Some older adults maintain normal gait function into their ninth decades, suggesting disordered gait is not an inevitable component of aging (Bloem, et al., 1992).

The Relationship of Gait and Balance

The components of gait and balance are fundamental to physical function. In the broadest sense, balance involves the ability to control upright posture under a variety of conditions and sense stability limitations (Berg and Norman, 1996). Postural control can be defined as the process by which the central nervous system (CNS) generates the patterns of muscular activity required to regulate the relationship between the center of gravity (COG) and base of support (BOS) (Maki and McIlroy, 1996). To maintain stability, the body's COG must be positioned vertically over the BOS.

The area of sway is the latitude of stability in which the COG is within the BOS. Normal units of sway are approximately 12.5 percent for anterior/posterior sway and 16 percent for lateral sway (Sherlock, 1996). A study by Lucy and Hayes (1985) determined sway in the anteroposterior direction was 52 percent greater in subjects 70 to 80 years of age than in subjects 30 to 39 years old. Thus, postural stability, normally considered an automatic control process, requires more conscious attention in the elderly than in younger individuals (Spirduso, 1995).

Gait performance creates a continuous disturbance on the mechanical equilibrium, a perturbation. With each step, the mind processes internal and external information in reaction to these perturbations and engages specific muscles to anticipate them (Spirduso, 1995). Maki and McIlroy (1996) explain that there are two forms of external perturbation: mechanical and informational. With mechanical perturbations, the forces acting on the body either displace the COG beyond the BOS (e.g., a push or collision) or prevent the BOS from being aligned beneath the COG (e.g., a slip or trip). Perturbations or destabilizing forces can be imposed by the environment (e.g., impact from a swinging door or jostle from a crowd) or can occur during volitional movements (e.g., walking, rising from a chair or pushing a door). Informational perturbations change the nature of orientational information identified by the visual, vestibular and somatosensory systems. Therefore, there is a crucial relationship between the human gait pattern and the ability to maintain an upright posture. Together they are referred to as dynamic balance.

Gait Mechanics

Understanding the mechanics of the normal gait cycle is necessary to better comprehend assessment and training procedures. The following is Perry's (1992) explanation of the normal gait cycle with a corresponding graphic description in Figure 1. The mechanics of walking are referred to as the gait cycle (i.e., a sequence of events between two sequential contacts by the same limb). Stance and swing are the two phases that compose the gait cycle. The stance phase, which constitutes approximately 60 percent of the normal gait cycle, is the interval in which the foot of the reference extremity is in contact with the ground. The swing phase, which makes up the remaining 40 percent of the gait cycle, is the portion in which the reference extremity does not contact the ground.

[FIGURE 1 OMITTED]

Stance is further subdivided into three intervals according to the sequence of floor contact by the two feet. Initial double stance begins the gait cycle and is the time both feet are on the floor. Single limb support begins when the opposite foot is lifted for swing. Here, the word support is preferred over stance to emphasize the functional significance of floor contact by only one foot. During the single limb support interval, the body's entire weight rests on that extremity. The duration of single limb support is the best index of its support capability. Terminal double stance begins with floor contact by the other, foot (i.e., contralateral initial contact) and continues until the original stance limb is lifted for swing (i.e., ipsilateral toe-off).