Center of mass approximation during walking as a function of trunk and swing leg acceleration.

The 3D center of body mass (COM) trajectory provides us with a measure of movement performance and level of stability while walking. As an alternative to directly calculating the COM from motion trajectories and anthropometric data, we propose developing models to estimate the COM trajectory during walking on irregular surfaces. The inputs to the models were acquired via two accelerometers, one representing the trunk segment placed on T2 and the second representing the swing leg placed on the lateral malleolus. The subjects walked on a fixed surface and encountered an uneven, irregular surface, causing instability in the balance system. The results were encouraging, providing an estimate of the COM trajectory with a low error of 4.17 +/- 1.94%. The reasonable accuracy, portability, ease of use and low cost (compared with video motion analysis systems) of the accelerometers increases the range of clinical applications of the proposed method.

Estimation of the center of body mass during forward stepping using body acceleration.

The center of body mass (COM) calculation is a key factor in analyzing human postural control. In recent years there have been attempts to estimate the COM by body acceleration during standing. In this study, a parabolic model is used to estimate the COM trajectory during forward stepping, using the body segments' accelerations, which were measured by inexpensive and portable accelerometers placed on the trunk and swing leg. Paced and voluntary forward stepping was performed on different support surfaces and with different speeds of stepping. Forward steps were extracted by analyzing the ankle marker position in vertical direction. The model was calibrated by genetic algorithm and tested on forward stepping data using the leave one out method. The results are encouraging for the use of the proposed model as a mean to estimate the COM trajectory during forward stepping.

Application of nonlinear dynamics to human postural control system.

Instability, fear of falling and fall injuries are common problems for the older population and individuals with chronic disabilities. Development of screening tools and outcome measures of balance performance and fall risk has become of great interest for health professionals and researchers in the field of rehabilitation. Although linear modeling is desirable due to its simplicity, however, most physiological systems are too complicated to lend themselves to linear analysis because the human body is multi-segmental, and there are feedforward and feedback control schemes of mutual relationship between the segments. In this study, we investigated the use of nonlinear dynamic tools to extract characteristic features of postural sway. Among various methods of nonlinear dynamics, the Rényi fractal dimension and the Rényi spectrum are presented as quantitative descriptors. The center of foot pressure (COP) was recorded during four quiet standing tasks of increasing difficulty (eliminating vision and use of a compliant surface). The COP trajectories' Rényi dimension and Rényi spectra of young healthy subjects with no history of neurological disorder were investigated in comparison to that of elderly patients with balance impairment and history of frequent falls. Sway path, the common linear parameter of postural sway, was also used to investigate its distinction from non-linear parameters. The results of this study suggest the COP trajectories' Rényi dimension and sway path can be used as classifiers for balance disorders, and they provided different indications of postural control system characteristics between the two groups and different task demands.

Development of an interactive motivating tool for rehabilitation movements.

In this paper, an interactive tool, including three computer games controlled via the center of foot pressure (COP) trajectory biofeedback, was designed to aid in pressure balance for rehabilitating persons with balance disorders. The games interact in real-time with the Vista Medical Force Sensitive Applications software and pressure mat. The main goal of this research was to employ attractive and motivational learning techniques, using equipment that is available to a large population, to increase volume of exercise practice and to retain the patient's attention. Questionnaires regarding the motivational aspects of the games were administered to 15 subjects (7 patients). The results indicate that the tools were indeed attractive, motivational and an improvement to conventional exercise regimes.

Development of an interactive motivating tool for rehabilitation movements.

In this paper, an interactive tool, including three computer games controlled via the center of foot pressure (COP) trajectory biofeedback, was designed to aid in pressure balance for rehabilitating persons with balance disorders. The games interact in real-time with the Vista Medical Force Sensitive Applications software and pressure mat. The main goal of this research was to employ attractive and motivational learning techniques, using equipment that is available to a large population, to increase volume of exercise practice and to retain the patient's attention. Questionnaires regarding the motivational aspects of the games were administered to 15 subjects (7 patients). The results indicate that the tools were indeed attractive, motivational and an improvement to conventional exercise regimes.

Center of mass function approximation.

In this paper, a hybrid genetic algorithm sum-of-sines model is developed to estimate the resultant center of body mass (COM) trajectory. The COM is an important parameter to consider when evaluating or analyzing human postural control, and is indicative of the system's stability. However, currently available systems that calculate the COM are not readily available for clinical routine assessment, making it difficult to widely assess balance problems. The input to the genetic sum-of-sines model developed in this paper is acquired through two accelerometers; equipment that is inexpensive, easy to use and portable. The results indicate that the model developed in this paper shows promising results for obtaining COM estimates that have clinical applications.

Effects of varying acceleration of platform translation and toes-up rotations on the pattern and magnitude of balance reactions in humans.

Different movement synergies used to restore balance in response to sudden support surface displacements have been described, which include the ankle movement synergy and a number of multisegmental movement synergies. The purpose of this study was to extend the analysis of the effects of stimulus magnitude on the pattern and scaling of balance reactions to larger magnitudes of balance disturbances, and to other types of balance disturbances, in particular, forward translations (FT), backward translations (BT), and toes-up rotations (RT). In addition, we examined whether the timing and magnitude of center of body mass (CM) displacement is an invariant feature of corrective responses to varying magnitudes of balance disturbances. Thirteen healthy adults were subjected to FT, BT, and RT of varying acceleration/velocity. The balance disturbance induced by FT and BT was fundamentally different from that induced by RT. The balance requirement during FT and BT was to rapidly translate the CM forward/backward to the new position within the displaced base of support. For RT, the requirement was to minimize the backward displacement of the CM. As evidenced from the initial phase of ankle, knee, and hip angular displacements and anterior-posterior (A-P) center of foot pressure displacement, the magnitude of the balance disturbance increased with increasing platform acceleration/velocity. For FT and BT, the present findings are consistent with the view that trajectory of CM is a control variable, as the timing, peak magnitude, and time to peak CM displacement did not vary as a function of platform acceleration/velocity. However, for RT, the peak magnitude and time to peak CM displacement did increase with increasing platform acceleration/velocity. The results demonstrate that in response to FT, BT, and RT, stability was restored by distinct multisegmental movement synergies. The corrective response to FT consisted of early knee flexion then ankle dorsiflexion and hip extension. The corrective response to BT consisted of hip flexion and ankle plantar flexion. For RT early hip flexion and knee flexion was observed. All muscles recorded (tibialis anterior, soleus, gastrocnemius, hamstrings, and quadriceps) were activated within a range of 60 to 170 ms from onset of platform displacement. For FT, BT, and RT, the pattern and timing of angular displacements and muscle responses did not vary as a function of platform acceleration/velocity, while there was a significant effect of platform acceleration/velocity on the magnitude of the corrective response, that is, peak magnitude of corrective hip, knee, and ankle angular displacements and magnitude of muscle responses. The present findings indicate that multiple sources of spatial information are necessary for the selection and initiation of the appropriate corrective response to meet the requirements of the different balance tasks. The present results strongly endorse the concept of a postural control network for recovery of standing balance, as opposed to positive feedback through local segmental or long loop reflex circuits.

Comparison of different exercise programs in the rehabilitation of patients with chronic peripheral vestibular dysfunction.

The purpose of this study was to evaluate the effects of two exercise programs on balance performance in patients with chronic peripheral vestibular dysfunction and to assess whether these exercise programs induce adaptive modifications of the vestibulo-ocular reflex (VOR). Patients were randomly assigned to one of two groups. (1) Those in the Rehab (Reh) group (n = 11) received a comprehensive exercise program that consisted of balance retraining and goal-directed eye-head exercises under combinations of varied visual and somatosensory sensory conditions. Patients received 45-minute training sessions, three times per week for 12 weeks, and were instructed on a custom home exercise program. (2) Those in the Home group (n = 12) were instructed to perform the Cooksey-Cawthorne eye-head exercises at home, on a daily basis, for 12 weeks. In addition, after completion of the exercise program and a follow-up period, 7 of the participants in the Home group (here defined as the A group) chose to enter the Reh program (here defined as the B group). Balance performance was assessed by measuring the peak-to-peak magnitude and total amount of anterior-posterior body sway, and of horizontal (shear) ground reaction force during six test conditions, in which visual and somatosensory orientation cues were reduced or altered by rotation of the visual surround or support surface in proportion to the subject's sway, and in which vision was eliminated (eyes closed). The VOR response to step chair rotations of 60 degrees/s and 120 degrees/s, and the optokinetic reflex (OKN) response to 60 degrees/s constant velocity optokinetic stimuli were recorded. Left-right difference in VOR gain, VOR time constant, and OKN gain were determined. These tests were performed 1 day prior to start of treatment (TD 1), 6 weeks after start of treatment (TD 2), at the end of the 12-week treatment period (TD 3), and 5 months after end of treatment (TD 4). The findings revealed a significant improvement in standing balance performance under dynamic conditions for patients in the Reh program (Reh and B groups) but not for patients performing the eye-head exercise (Home or A groups). Thus, even in patients with chronic vestibular dysfunction, compensation for the loss or disruption of peripheral vestibular inputs can be effectively induced by exercises that provide sensory feedback appropriate for behavioral changes involving sensory substitution or sensory-motor reorganization.(ABSTRACT TRUNCATED AT 400 WORDS)

Convergent effects from vestibulospinal tract and primary cutaneous afferent fibers on motoneurons to proximal lower limb flexor and extensor muscles in humans.

In human subjects, the latency and magnitude of cutaneomuscular test reflexes, evoked by stimulation of the sural nerve in the contralateral and ipsilateral quadricep and hamstring muscles, were investigated during static tilts in the pitch axis. The ANOVA demonstrated a highly significant, tilt-dependent modulation of the magnitude of the test reflex for the initial phase of both ipsilateral quadriceps and ipsilateral hamstrings. These results are consistent with results from a similar study on human ankle muscles and provide further evidence that the tilt-dependence of these cutaneomuscular reflexes originates from activity in the otolith receptors.

Convergent effects from vestibulospinal tract and primary cutaneous afferent fibers on motoneurons to ankle extensor and flexor muscles in humans.

The latency and magnitude of cutaneomuscular reflexes, evoked by stimulation of the sural nerve in the contralateral and ipsilateral tibialis anterior and soleus, were investigated in normal human subjects during static tilts in the pitch axis. For all subjects the test reflex consisted of oscillating sequences of excitation and inhibition, each muscle exhibiting a characteristic pattern defined by its latency and sign of the initial phase. The latency of the various components and the sign of the initial phase did not vary with angle of tilt. However, the results of an ANOVA demonstrate a highly significant tilt-dependent modulation of the amplitude of the test reflex for the initial inhibitory phase of the contralateral tibialis anterior. We propose that this tilt-dependent effect on the crossed cutaneomuscular reflex originates from activity in the otolith organ receptors.