By David A. Brown, PT, PhD
The ability to stand and remain upright during daily activities is something that most of us take for granted. However, after injury to the musculoskeletal system or nervous system, this seemingly simple function becomes a great challenge to overcome. The challenge is greatest when balance is required during mobility tasks such as walking, running, jumping, and riding a bicycle. In this article, we will explore contemporary ideas about balance and gait and examine how these newer ideas have resulted in technologies targeted at improving daily function and quality of life for people with gait and balance dysfunction. The construct of balance has been widely explored by biomechanists and motor control scientists. In simple terms, balance is the ability to maintain a stable, upright posture. To succeed, all segments of the body must maintain net forces in a state of equilibrium (ie, all forces must exactly counteract each other). In most real life situations, these segment's positions may be varying with respect to each other over time (dynamic balance), but as long as the net forces do not move the center of mass out of the base of support for more than brief periods, the body will not fall or collapse. Gait is a special case of dynamic balance where the forces accelerate the body through space in a way that allows the base of support to follow the movement of the center of mass. In other words, walking may be thought of as a series of brief falls that are caught by the extended leading leg during stepping. The understanding that balance is a dynamic activity that requires constant sensory feedback and updating of posture requires the clinicians to assist their clients with dynamic tasks rather than focusing on solely static standing tasks. For example, exercises targeted at fall prevention need to address recovery strategies, such as stepping forward or reaching forward to catch yourself, rather than exercises that solely focus on maintaining the center of mass within small sway regions. Therefore, the greatest challenge for the clinician who is assessing and intervening with gait and balance dysfunction is to help the individual regain a generalized control strategy for moving their body parts so that the center of mass is always within a safe zone of equilibrium, even during movement. With this framework, all body parts must be considered, including the head and neck, upper extremities, trunk, and legs. In addition, anticipatory movements, called anticipatory postural adjustments, are often necessary to allow the movement to occur without too great a disturbance to the equilibrium of the posture. Finally, since there is an almost infinite array of balance tasks associated with activities of daily living and, beyond this, athletic and recreational endeavors, the more generalizable the balance strategies, the more likely that a person will be able to enjoy life without falling. CONTEMPORARY CONCEPTS Although many new ideas have been tested recently, three important concepts have emerged in the last few years that promise to allow clinicians to effectively address the wide spectrum of balance and gait disorders. These concepts are: 1) use-dependent plasticity; 2) dual-task paradigm; and 3) implicit versus explicit learning. Use-dependent neuroplasticity. This concept is derived from many recent studies that investigate the recovery of the nervous system in both animal and human systems.1 A wide variety of neural and musculoskeletal injuries have been examined and the majority of these studies suggest that the nervous system has a great capacity to regain control of important functions. The two strongest factors that result in success for regaining function are that the exercise is goal-directed and repetitious. In other words, the person must be engaged in an activity that has a clearly understood desired outcome and they must be given ample opportunity to practice achieving the goal. For example, a clinician who is working with a client to achieve greater ability to recover from pushes or pulls to the body would pull the individual out of the base of support and force the person to step and recover balance. When this is repeated over and over again, the individual learns to adapt the step recovery pattern and may generalize this stepping strategy to real-world situations. Dual-task paradigm. This concept is derived from recent studies that demonstrate that conflict arises in motor control tasks that require two important goals to be achieved simultaneously. For example, walking down the hallway as fast as possible is a single, goal-directed task. However, walking down the hallway while simultaneously carrying a glass filled with water without allowing any spilling requires dual attention to achieve each goal simultaneously. Recent studies have demonstrated that these dual tasks result in severe compromise to the movement task and that practice is required in order to improve success with the tasks.2 The dual-task paradigm, then, requires clinicians to identify situations in which their clients need to attend to two tasks simultaneously. A common example is standing on a bus and reading the newspaper-while the person is attending to the text in the newspaper, the balance system is required to attend to the job of maintaining stability. Practice of these tasks allows the individual to develop the appropriate strategies for success in these real-world situations. The clinician may also assess the relative risk of these dual-task situations and counsel the individual to avoid engaging in some of these tasks, for safety's sake.
Figure 1. A limb-loaded pedaling system.
Figure 2. Kine-assist technology for gait and balance training.
