Creating New Balance

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November 2004

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.

Implicit vs explicit learning. This concept is derived from the field of cognitive psychology and the debate in that field about whether memory is stored in abstract form or a specific form. Implicit learning is thought of as a passive process where individuals are exposed to information, and acquire skill with that information simply through the exposure. Explicit learning, however, is a more active process that requires the individual to seek out the structure of information that is presented. It is proposed that implicit learning is the primary mode for acquiring motor skill in daily life, and perhaps after injury.³

For example, when an individual is learning to walk again after stroke, the clinician may take the approach of analyzing all of the components of the gait pattern, identifying those that are dysfunctional, and go about teaching an individual to focus on explicitly understanding and correcting these dysfunctional patterns. In contrast, the client may be exposed to important tasks that implicitly require corrective strategies in order to succeed, such as walking over an obstacle course to implicitly develop dynamic balance strategies during walking.

NEW TRAINING TECHNOLOGIES
The above concepts have necessitated the development of new technologies that can enable clinicians to implement these approaches in a clinical setting. I have been involved with the development of three technologies that will allow clinicians to aggressively work on components of gait and balance, incorporating the new concepts described above, without imposing undue danger of falls. Although these technologies are in an early stage of adoption in the clinic, they are examples of the kind of technologies that may be widely available to clinics in the near future.

Limb-loaded cycling. Limb-loaded cycling offers a task-oriented locomotor training intervention to individuals who are unable to support their entire body weight. This novel type of cyclical leg exercise requires coordinated transfer of weight between the two limbs and demands well-timed lower extremity force production. The apparatus includes a releasable seat, such that it is allowed to slide along a linear track similar to a leg press machine (Figure 1). In order to prevent the seat from sliding forward, individuals are required to uphold a loading force, set at a specific percent of their body weight, throughout the pedaling cycle. In order to successfully complete the task, the individual is required to maintain their support leg in an extended, and loaded position. If unable to maintain the load, the seat slides forward.

Figure 2. Kine-assist technology for gait and balance training.

The load that is applied to the legs can be increased as an individual acquires greater strength and ability to transfer loads from one leg to another. This kind of task enables goal-directed, repetitive practice. It implicitly trains an individual to learn how to weight-shift and allows the client to practice both locomotion and weight support as a dual-task paradigm.

Kine-assist technology. Kine-assist technology is a new class of devices that are microprocessor controlled and motor actuated, and assist clinicians in maintaining patient safety while performing walking and balance exercises. One form of this new technology is composed of two major components: 1) a buggy; and 2) a smart brace (Figure 2).

The buggy is a motor-actuated mobile platform. The wheels on the mechanism provide forward and turning motion for the device in order to follow a walking person and turn around corners, as well as sidestepping. The motion of the buggy is not programmed-it moves in response to the motion of the patient. The smart brace is the mechanism that supports the person's trunk and pelvis and allows natural relative movements that occur in walking and balance exercises. It also allows for un-weighting so that the therapist can practice gait and balance under easier control conditions.

With the further development of this technology, clinicians will have the ability to have their clients practice overground, real-life mobility tasks while engaging in challenging balance exercises. The tasks will be goal-directed and repetitious, enable dual-task performance, and encourage implicit learning, all while maintaining the ability to catch a person if they lose control and quickly restore them to a position where they can continue to practice the task.

Simulated obstacle gait training. With simulated obstacle gait training, individuals walk on a treadmill at their normal walking speed and are held safely in place using an overhead harness.4 The harness does not necessarily suspend or support their body weight, but provides safety and arrested falls. A color video camera is directed at the individual's legs from the side. The individual wears a head-mounted display to view the camera's real-time image.

The computer introduces a stationary image of a rectangular object of a selected height and length to the video of the individual's feet. As the treadmill runs, the individual is instructed to step over the virtual object on each step. The computer can detect any intersection of the user's feet with the objects. A collision by the toe on the front edge of the object indicates that the individual did not lift the foot high enough, while a collision with the heel on the top of the object indicates that the individual did not step far enough. Throughout the sessions, a clinician can offer encouragement and advice to improve stepping performance.

Foot switches worn between the shoes and "booties" can track the stance and swing phases of each foot. When a collision with the virtual object occurs, a vibro-tactile feedback can be directed to the heel or toe of the colliding foot. An audio indication of the collision can also be presented to the individual. The computer system can measure the individual's progress, keeping track of the steps and the number of collisions that occur during the training sessions.

The visual, vibro-tactile, and auditory cues offer multiple ways of informing the subject of a collision thereby providing the individual with meaningful multi-modal sensory feedback, which, in turn, is utilized to implicitly train individuals to take bigger steps and learn to balance dynamically during walking. Their ability to view the side of their legs during walking gives the individuals a unique perspective of their efforts to negotiate the objects. The sensory feedback combined with the safety provided by the overhead harness affords individuals a safe means to try different strategies recommended by the therapist to improve their performance.

In summary, the direction of clinical gait and balance training has moved closer to interventions that target functional tasks under conditions that build in an acceptable and safe level of risk. New interventions are incorporating strategies of goal-directed movements, with a lot of practice, adding dual-attention components, and focusing on implicit learning. Newer technologies are affording the clinician the opportunity to challenge their clients to levels of higher performance without increasing the risk for falls. The future holds great promise for effective interventions that will give clients with gait and balance disorders the opportunity to develop strategies for moving safely and functionally through their real-world environment.

David A. Brown, PT, PhD, is assistant professor in the Department of Physical Therapy and Human Movement Sciences, Northwestern University, Chicago.

REFERENCE

Wolpaw JR, Tennissen AM. Activity-dependent spinal cord plasticity in health and disease. Annu Rev Neurosci. 2001; 24:807-43.
Woollacott M, Shumway-Cook A. Attention and the control of posture and gait: a review of an emerging area of research. Gait Posture. 2002;16:1-14.
Boyd LA, Winstein CJ. Implicit motor-sequence learning in humans following unilateral stroke: the impact of practice and explicit knowledge. Neurosci Lett. 2001;298:65-9.
Jaffe DL, Brown DA, Pierson-Carey CD, Buckley EL, Lew HL. Stepping over obstacles to improve walking in individuals with poststroke hemiplegia. JRRD. 2004;41:283-292.

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November 2004