Venkata Gade

Cross-correlations of center of mass and center of pressure displacements reveal multiple balance strategies in response to sinusoidal platform perturbations

Compared to static balance, dynamic balance requires a more complex strategy that goes beyond keeping the center of mass (COM) within the base of support, as established by the range of foot center of pressure (COP) displacement. Instead, neuromechanics must accommodate changing support conditions and inertial effects. Therefore, because they represent bodys position and changes in applied moments, relative COM and COP displacements may also reveal dynamic postural strategies. To investigate this concept, kinetics and kinematics were recorded during three 12 cm, 1.25 Hz, sagittal perturbations. Forty-one individual trials were classified according to averaged cross-correlation lag between COM and COP displacement (lag(COM:COP)) and relative head-to-ankle displacement (Δ(head)/Δ(ankle)) using a k-means analysis. This process revealed two dominant patterns, one for which the lag(COM:COP) was positive (Group 1 (n=6)) and another for which it was negative (Group 2 (n=5)) . Group 1 (G1) absorbed power from the platform over most of the cycle, except during transitions in platform direction. Conversely, Group 2 (G2) participants applied power to the platform to maintain a larger margin between COM and COP position and also had larger knee flexion and ankle dorsiflexion, resulting in a lower stance. By the third repetition, the only kinematic differences were a slightly larger G2 linear knee displacement (p=0.008) and an antiphasic relationship of pelvis (linear) and trunk (angular) displacements. Therefore, it is likely that the strategy differences were detected by including COP in the initial screening method, because it reflects the pattern of force application that is not detectable by tracking body movements.

Using Virtual Reality Driving Simulators in Persons with Spinal Cord Injury: Three screen display versus head mounted display

Purpose: Virtual reality (VR) is a relatively new technology that is currently utilized in a wide variety of settings to test and train individuals in specialized skills. This study examines methods for improving driver retraining protocols for persons with spinal cord injury (SCI). Method: We compared a VR driving simulator, under two different display conditions, a head mounted display (HMD) and a three screen display (TSD) to identify the best method for retraining driving skills following SCI. Results: Although there was minimal evidence for driving performance difficulties in the HMD condition relative to the TSD condition (e.g. greater number of times for being off course and longer stopping latencies for the HMD condition), rates of simulator sickness did not differ between display conditions. Conclusions: Taken together, findings suggest that both the HMD and the TSD are reasonable simulator options for driver retraining in SCI. Implications for Rehabilitation Virtual reality (VR) may be a useful tool for improving driver retraining for persons with spinal cord injury (SCI). Both head mounted display (HMD) and three screen display (TSD) virtual reality systems are reasonable simulator options for driver retraining in SCI.

American Society of Biomechanics Clinical Biomechanics Award 2013: Tibiofemoral contact location changes associated with lateral heel wedging—A weight bearing MRI study

BACKGROUND Vertically open magnetic resonance imaging permits study of knee joint contact during weight bearing. Lateral wedging is a low cost intervention for knee osteoarthritis that may influence load distribution and contact. This study assessed the ability of feedback-assisted weight bearing magnetic resonance imaging to detect changes in tibiofemoral contact associated with lateral wedging. METHODS One knee in each of fourteen subjects with symptomatic knee osteoarthritis was studied, without specification of compartmental involvement. Knees were imaged during upright standing and at 20° knee flexion. Bilateral external heel wedges were used to provide non-wedged and 5° lateral wedging conditions. Computer modeling was used to measure the medial and lateral compartment contact patch center coordinates on the tibial plateau and the respective contact areas. FINDINGS Lateral heel wedging in flexion was associated with a significant anterior shift of the contact patch of the lateral femoral condyle. Changes with knee flexion were similar to previous reports: both medial and lateral contact centers moved posteriorly with flexion, and lateral condyle contact also moved laterally. Lateral condyle contact area significantly reduced with flexion, while lateral wedging did not significantly affect contact areas. INTERPRETATION In symptomatic knee osteoarthritis patients standing in knee flexion, weight bearing magnetic resonance imaging recorded an anterior shift of lateral condyle contact in response to lateral heel wedging. Future studies may investigate lateral wedging effects more specifically in candidates for this clinical intervention.

Validation of an MRI-Based Method to Measure Tibiofemoral Joint Cartilage Contact Area

Osteoarthritis is a progressive and debilitating joint disease that is the leading cause of physical disability in industrial nations around the world [1]. It not only negatively affects the comfort and functional activity of individuals, but as a result also taxes the health care system of these countries.Copyright

Investigating the Ability of Knee OA Patients to Maintain Targeted Knee Flexion Angles for Weight Bearing MRI

Knee osteoarthritis (OA) is the most common clinical presentation of osteoarthritis, and has been estimated to affect 12–16% of the population older than 60 years in the US [1]. A biomechanical risk factor that has been linked to knee OA is the changes in the local loading and the contact area between the cartilage surfaces [2]. Investigation of such factors requires precise measurement tools to determine knee joint positioning and contact areas. Clinically, MR images of the knee are most frequently acquired in the supine position; such images are not representative of the loading conditions experienced functionally in the weight bearing knee joint. With the advent of vertically open MRI (e.g. Upright MRI, Fonar Corporation, Melville, NY), it is now possible to scan the knee in fully upright weight bearing conditions representing truly functional positions. To measure sensitive variables such as joint positioning and cartilage contact, it is important to minimize subject movement in order to obtain high quality images. In MRI, increased scan times allow for data of improved signal to noise ratio and resolution; however, long scanning durations without subject movement are not feasible, particularly in individuals with symptomatic knee OA.Copyright

Initial electro-mechanical response to rearward perturbation

The objective of this study was to examine the combined electromyographic (EMG) and mechanical response to a rearward perturbation and to separate the response into three categories: preset properties of the muscle, reflex changes to the muscle, and active changes to the muscle. We hypothesized that an active response is required to maintain balance on a moving platform. Eleven healthy adult subjects stood on a platform oscillating at three frequencies (0.75, 1.0, and 1.25 Hz). Ankle extensor EMG activity and ankle moment were analyzed and compared for initial movement cycles. Timing of events in EMG and moment data were examined to separate observed changes into the three categories. Results showed an initial rise in ankle moment as the platform started to move backwards, followed by a more rapid reflex increase. After a slight drop, ankle moment again rose due to active response. By the third cycle of platform movement, the EMG and moment were synchronized with the platform movement, maintaining the body in a desired posture. Initial preset properties of the ankle extensor muscles combined with reflex activity were not sufficient to maintain balance. Following an initial reflex reaction, further active control was required to match the timing of the ankle moment and the platform motion and avoid a loss of balance. This study provides new insight for the rehabilitation of postural deficits.

Weight-Bearing MRI: Comparison of Knee Angle Set-Up Methods and the Effect of Real-Time Visual Feedback

Osteoarthritis (OA) is the most common form of arthritis worldwide, and is reported to be the leading cause of disability in the United States [1, 2]. With the invention of an upright MRI (Fonar Corp., Melville, NY), it is now possible to image the knee of an upright, weight-bearing patient in truly functional positions to provide improved techniques for early evaluation of knee OA. However, scan set-up in a weight-bearing MRI is challenging because of limited space. To get a subject to a targeted knee flexion angle (KFA) and to have them maintain this angle for the duration of a scan is difficult.Copyright

Upright Magnetic Resonance Imaging Tasks in the Knee Osteoarthritis Population: Relationships Between Knee Flexion Angle, Self-Reported Pain, and Performance

OBJECTIVE To characterize the ability of patients with symptomatic knee osteoarthritis (OA) to perform a weight-bearing activity compatible with upright magnetic resonance imaging (MRI) scanning and how this ability is affected by knee pain symptoms and flexion angles. DESIGN Cross-sectional observational study assessing effects of knee flexion angle, pain level, and study sequence on accuracy and duration of performing a task used in weight-bearing MRI evaluation. Visual feedback of knee position from an MRI compatible sensor was provided. Pain levels were self-reported on a standardized scale. SETTING Simulated MRI setup in a research laboratory. PARTICIPANTS Convenience sample of individuals (N=14; 9 women, 5 men; mean, 69±14y) with symptomatic knee OA. INTERVENTIONS Not applicable. MAIN OUTCOME MEASURES Averaged absolute and signed angle error from target knee flexion for each minute of trial and duration tolerance (the duration that subjects maintained position within a prescribed error threshold). RESULTS Absolute targeting error increased at longer trial durations (P<.001). Duration tolerance decreased with increasing pain (mean ± SE, no pain: 3min 19s±11s; severe pain: 1min 49s±23s; P=.008). Study sequence affected duration tolerance (first knee: 3min 5s±9.1s; second knee: 2min 19s±9.7s; P=.015). CONCLUSIONS The study provided evidence that weight-bearing MRI evaluations based on imaging protocols in the range of 2 to 3 minutes are compatible with patients reporting mild to moderate knee OA-related pain.