The failure to maintain balance is a clearly identifiable event that does not require extensive instrumentation to be observed. The maintenance of balance is a far more subtle phenomenon that benefits from analysis derived from the use of transducers to monitor and record forces and motion. Laboratory balance assessment has benefited from established instrumentation and techniques to produce a robust body of literature.
For the purposes of this discussion, laboratory balance assessment differs from clinical balance assessment according to several broad criteria. Laboratory balance assessment generally depends on instrumentation, is designed primarily for research purposes as opposed to clinical decision making, and enables the capture and recording of balance factors not generally available by simple observation. Further distinction is based on the activities observed. The bulk of balance assessment and posturography literature that has used instrumented laboratory analysis has focused on standing or “static” balance. Research that considers balance in walking or “dynamic balance” often falls within the domain of “slips, trips, and falls” or understanding the dynamic effects of a condition or device. These elements constitute an important focus of clinical balance assessment.
Interestingly, innovation in laboratory balance assessment has focused not on the improvements in instrumentation but on the analysis and interpretation of the outcomes provided by the same instrumentation that has been used for decades. A potentially confusing condition has emerged as no single gold standard has been identified, even with respect to specific clinical conditions or devices.
Despite the quantity of different laboratory measurements of balance, assessments typically consist of the measurement of one or both of two key parameters: 1) interaction of the distal segment and the ground and 2) motion of the center of mass (COM) or limbs.
The ground interface is most often characterized by analysis of ground reaction forces or, more often, trajectory of the center of pressure (COP). The COP is the point of application of the resultant of the distributed ground reaction force vectors. Although laboratory assessments of balance tend to focus on the fairly easily obtained COP, it should not be observed without understanding the reasons of COP changes in the first place. Motion of the body, which generally drives changes in the COP, is typically lumped into measure of the motion of the whole body COM. The COM is the single point at which the body's distributed mass could be concentrated. COM can be estimated based on the motion of individual body segments or more directly approximated by the placement of sensors such as accelerometers or ataxiameters at the waist. Because the instrumentation used to obtain COP is quite different from that used to measure COM, investigation involving each parameter has tended to develop independently, with relatively few studies considering both. In particular, a number of general balance studies have built increasing complexity in the independent analysis of COP.
ASSESSMENT OF CENTER OF PRESSURE
At the stage of data collection and reduction, COP remains a simple parameter consisting of a time series of only two Cartesian coordinates. The planar trajectory of the COP during a static test for postural sway and balance control is commonly called a stabilogram (Figure 1).
Even without numerical analysis, the stabilogram provides a wealth of information on postural sway. The overall area covered by the COP indicates the ability of the subject to maintain a stable upright posture. The shape of the stabilogram might suggest that sway is greater in either the medial-lateral or the anterior-posterior direction. Long, straight segments on the plot reflect sudden perturbations or corrections to balance, whereas shorter segments indicate finer control, even if the overall plot covers a large area.
Given that each of these observations is clinically informative, a need to quantify them for research purposes is not unexpected. Consequently, parameters associated with the COP have proliferated. As far back as 1996, Prieto et al.1 identified 36 measures based on COP alone. In 2002, Chiari et al.2 identified more than 55 measures. These postural sway parameters range from straightforward measures, such as the range of the stabilogram in anterior-posterior or medial-lateral directions, to the complex, such as the stabilogram diffusion analysis.
Several parameters are basic measures associated with the stabilogram plot in the time domain. These include the total distance covered by the COP along each axis, the mean velocity of the COP, and the root mean square distance of each COP coordinate from the mean of all COP coordinates. These measures were common to several articles in the evidence report associated with this State of the Science Conference.3–7
Other analyses take advantage of a more holistic understanding of sway. Rather than considering the coordinate-to-coordinate movement of the COP, conversion of the stabilogram into the frequency domain allows quantification of how many of the movements were sudden and how many were gradual; these data are often coupled with the amplitude of those movements as a means of measuring stability.
Still more measures address the tendency of the COP to maintain its path or divert from it. These analyses rely on classical theories of Brownian motion, rooted in attempts to understand the stochastic, or random, motion of dust particles in a fluid medium. Such theories have been applied to disciplines as varied as stock markets and our current concern of posturography. Without an extended mathematical discourse, it is sufficient to say that many outcomes of these more esoteric analyses may have direct clinical application. For example, the relationship between the mean square displacement of the COP (d) and the time interval (t) has been expressed as
Where, H is the Hurst exponent.8 The Hurst exponent is a numerical means to indicate the likely tendency of an individual toward greater or lesser sway.9 When H = 0.5, movement is completely random and unpredictable. If H is less than 0.5, movement away from the mean tends to reverse and is, therefore, called “antipersistent.” Such COP movement results in a smaller stabilogram and reduced postural sway. Conversely, if H is greater than 0.5, movement is “persistent,” and past COP movement away from the mean will tend to continue. Persistent movement of the COP produces a larger sway area.
For researchers analyzing COP, the choice of parameter is important, and the sheer quantity of available analyses can be daunting. Bigelow and Berme10 sought to determine which conditions and parameters best differentiated between fallers and nonfallers. An eyes closed, comfortable stance condition was identified as the best differentiating condition, with an associated model including two traditional sway parameters, two fractal measures, and two personal characteristics. One parameter that was not sufficient to differentiate between fallers and nonfallers on its own but present in the best differentiating models in all conditions was medial-lateral sway velocity.
Masani et al.11 studied COP, COM, and electromyography and identified the importance of velocity information of the COM and COP in regulating balance, suggesting that proprioceptive and plantar cutaneous sensations play an important role in the velocity feedback mechanism. Although a straightforward measure such as velocity has established utility, others have used fractal measures such as stabilogram diffusion analysis to identify control strategies accounting for both open- and closed-loop feedback.12 In particular, Norris et al.13 found that such so-called statistical mechanics techniques were more sensitive in detecting fall risk and analyzing balance control.
In choosing parameters to analyze, researchers must not overlook test conditions and subject characteristics. Chiari et al.2 indicated the importance of anthropometric measures and attention to foot placement in analyzing postural sway results. They recorded shank, thigh, and trunk length, shoulder breadth, and chest, waist, and hip girth. Subjects were permitted to stand on the force platform with a self-selected foot placement, but the relative positions of the feet were recorded. Forty-four of the 55 stabilometric parameters showed some linear dependence on anthropometrics and foot placement. This dependence was greater with eyes closed.
RELATIONSHIPS BETWEEN CENTER OF MASS AND CENTER OF PRESSURE
Although COP is a focal point for much study, balance and postural control are ultimately about the motion of the body. In an effort to simplify analysis and prediction, motion of the body in quiet standing is often modeled by the single-point COM. One of the simplest models of postural sway represents the body as an inverted pendulum, with the COM swaying fore and aft about the COP.14 This model accurately predicts motion of the COP relative to the COM and highlights the physical requirement that, to reverse the direction of COM, which is fundamental to balance control, the COP must move in front of and behind the COM to decelerate it.15 Consequently, the COP “overshoots” the COM as the two displace together (Figure 2).
Just as changes in COP are evidence of corrections to maintain a stable COM, the COM itself reveals changes in the position of the trunk and limbs. Wu et al.16 used a full motion analysis system and a force platform to study quiet standing in children, focusing on orientation of each body segment and its effect on COM and, consequently, COP. Such research is useful in identifying stages of development in balance control through the lifespan.
COMPUTERIZED DYNAMIC POSTUROGRAPHY
A hybrid category of instrumentation that is used in laboratory studies and clinical balance assessment is computerized dynamic posturography (CDP). Systems such as the EquiTest (NeuroCom International, Clackamas, OR) selectively manipulate the visual field and ground surface to enable discrimination of the independent contributions of the visual, vestibular, and somatosensory systems to standing balance. CDP systems enable the independent motion of the standing surface and the surrounding environment. Forward sway can cause the floor to rotate, or the visual field (a wall in front of the subject) can be rotated even when the subject is not falling.
With this instrumentation, researchers can attempt to determine which of the body's balance systems are affected by conditions or devices. For example, if an individual is standing still and the wall in front begins to move unexpectedly, the visual system suggests that sway is occurring when the vestibular and somatosensory systems believe (correctly) that balance has not been perturbed. The amount of postural sway that follows these selective stimuli indicates which systems instigate and direct a motor response.
Rao and Aruin17 used CDP to investigate the role of ankle-foot orthoses (AFOs) in balance for individuals with peripheral neuropathy. They used a component of CDP called the Sensory Organization Test, which includes four conditions: standing with eyes 1) open and 2) closed, and standing with a fixed visual field while the ground surface rotated in reference to anterior-posterior body sway with eyes 3) open and 4) closed. These techniques validated the effects of the tested AFOs in reducing sway values across multiple conditions.
Laboratory balance assessments have contributed broadly to understanding of both normal and impaired balance. Perhaps because of the nature of the instrumentation, studies tend to focus on either the COP or the COM but not often on both. Because AFOs alter the mechanical and sensory link between the body and the ground, research in this area may benefit from examination of both the motion of the COM and the resultant deviation of the COP. More elaborate techniques using CDP may serve to further inform on the components of balance most affected by AFO interventions.
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