Dynamic instability during obstacle crossing following traumatic brain injury

Abstract

Patients with traumatic brain injury (TBI) complain of “imbalance” or “unsteadiness” while walking, despite a normal gait on clinical examination. Thus, the purpose of this study was to determine if it was possible to quantitatively assess dynamic stability that did not have an obvious neuromuscular origin in individuals following TBI. Ten patients with documented TBI and 10 age, gender, and stature-matched healthy individuals participated in the study. All subjects were instructed to perform unobstructed level walking and to step over obstacles corresponding to 2.5%, 5%, 10%, and 15% of their height. A 13-link biomechanical model of the human body was used to compute the kinematics of the whole body center of mass (COM). Subjects with TBI walked with a significantly slower gait speed and shorter stride length than their matched controls. Furthermore, subjects with TBI displayed a significantly greater and faster medio-lateral (M-L) COM motion and maintained a significantly greater M-L separation distance between their COM and center of pressure (COP) than their matched control subjects. These measurements indicate that subjects with TBI have difficulty maintaining dynamic stability in the frontal plane and have a reduced ability to successfully arrest their sagittal momentum. These findings provide an objective measurement that reflects the complaints of instability not observable on clinical examination for individuals who have suffered a TBI. This ability to identify any functional impairment after a traumatic brain injury that may affect patient safety is critical for prevention of re-injury during the recovery period.

Introduction

Traumatic brain injury (TBI) is one of the most challenging problems faced by the medical community. It is estimated that 5.3 million Americans, a little more than 2% of the US population, currently live with disabilities resulting from TBI [1]. Each year over one million people are treated for TBI and released from hospital emergency rooms. The cost of TBI in the US is estimated to be US$ 48.3 billion annually. After one traumatic brain injury, the risk for a second injury is three times greater, and after the second injury, the risk for a third injury increases by a factor of eight [1]. Therefore, the ability to identify any functional impairment that may affect safety after a TBI is critical to prevention of re-injury. The utility of neuropsychometric testing in evaluating cognitive impairment resulting from TBI is well established [2], [3], [4], [5], [6]. However, comparatively little information is available on the performance of dynamic motor tasks following TBI.

Many individuals with a mild or moderate TBI complain of symptoms long after their injury, even though their clinical examinations and measurable cognitive deficits are small [7], [8]. Approximately one third of these patients complain of sensorimotor problems, in particular, poor balance and poor coordination [9], [10]. These balance and coordination complaints may not be surprising since effective coordination and balance involves a complex interaction of the sensory, motor programming and musculoskeletal systems. Sensory systems monitor the location of the whole body center of mass (COM) relative to the base of support, provide information about vertical orientation, and supply environmental information, particularly regarding the support surface. Appropriate motor responses include an appropriate latency of onset as well as measured and coordinated force generation in activated muscles [11].

Biomechanical studies of individuals with TBI have, for the most part, been limited to postural sway during quiet standing or during standing with altered sensory inputs [12], [13], [14], [15], [16], [17], [18], [19]. Subjects with TBI exhibit an increased reliance on visual input and are not able to use their vestibular systems as effectively as uninjured subjects to resolve conflicts in information from the visual and somatosensory systems [16]. Geurts et al. [16] reported that subjects who had sustained a TBI (mild, n=13; moderate, n=2; and severe n=5) exhibited at least 50% more static anterior–posterior (A–P) and lateral sway than healthy controls at 6 months following injury. These same subjects showed no sensorimotor impairments in a standard neurological examination. Guskiewicz [20], on the other hand, reported little relationship between symptoms and measures of cognitive function and static postural stability during the first 2 days following a concussion. These data suggest that motor function may recover more slowly than cognitive function or may not be closely related to standard neuropsychological assessments.

Many studies have used the whole body COM motion and its interaction with the center of pressure (COP) as indicators to examine an individual’s dynamic stability. It has been demonstrated that COM motion is tightly regulated to move between the alternating COP of each supporting foot [21], [22], [23], [24], [25]. Furthermore, our previous studies demonstrated that elderly adults complaining of “dizziness” or “unsteadiness” displayed significantly greater and faster medio-lateral (M-L) COM motion than the healthy elderly while negotiating obstacles of different heights [26], [27]. Therefore, COM motion in the frontal plane could be a functional indicator of balance maintenance during walking, and with the addition of the obstacle it could be a more sensitive measurement of dynamic stability.

Given that most falls appear to stem from tripping over objects [28], [29], [30], [31], it is important to determine the influence of pre-existing brain injury on an individual’s balance control while interacting with environmental hazards, such as stepping over an obstacle during walking. Therefore, the purpose of this study was to quantitatively assess dynamic stability of individuals who have complaints of “instability” or “imbalance” following TBI despite having an apparently normal gait on clinical examination. We hypothesized that patients with TBI would demonstrate less stability, as indicated by greater and faster M-L motion of the whole body COM, when stepping over progressively higher obstacles than subjects without a similar injury.