Here, we present a novel protocol to measure positional stability at key events during the sit-to-stand-to-walk using the center-of-pressure to the whole-body-center-of-mass distance. This was derived from the force platform and three-dimensional motion-capture technology. The paradigm is reliable and can be utilized for the assessment of neurologically compromised individuals.
The overall goal of this protocol is to measure dynamic positional stability during sit, to stand, and walk. A complex transitional movement that individuals with pathology often find difficult. The main advantages of this technique is that it utilizes the best available center of pressure and whole body center of mass estimations, or COP, BOM, to provide a composite measure of stability.
The implications of this technique can also inform clinical practice. As meaningful stability measurements could be deployed to predict recovery and evaluate treatments to effect recovery rates of sitting to walking. Demonstrating the procedure our participant will go through, will be Michael Attwaters, a graduate student from the laboratory.
Begin by the opening the proprietary tracking software on the computer. Set the capture frequency to 60 hertz, and set the 3D tracking parameters, which include a prediction error of 20 millimeters, a maximum residual of two millimeters, a minimum trajectory length equivalent to two frames, and a maximum frame gap of 10 frames. Then, identify each of the eight individual force platform components from each form platform amplifier into the respective analogue to digital converter.
Ensure all predetermined calibration settings from each force platform, scaling factors, and analogue channels have been specified. Additionally, configure the software to read the baseline force plate reading during the last 10 frames of capture. Select a multiplier of 17 for the motion capture frequency to ensure an adequete analogue sampling frequency of 1020 hertz.
In preparation for the calibration of 3D space position the L shaped reference structure on the floor in the capture volume, with the long axis of this structure pointing in the interior direction. Then, within the calibration settings page in the software select wand as the calibration type with a 750 millimeter length. And select coordinate system orientation with the positive C axis pointing upwards and the positive Y axis as the long arm.
Click okay. Click the calibration icon and set the intended length of the calibration capture to 60 seconds. Then, identify the file directory where the results will be saved.
Click okay to begin calibrating. Move the calibration wand within the capture volume, by rotating and translating the wand around the intended capture volume for 60 seconds. Then, check calibration results and accept calibration with individual camera residual errors of less than 1.5 millimeters.
Click okay. Finally, locate the force platforms in the calibrated 3D space by placing one, nine millimeter diameter passive retro reflective marker in each of the four corners of each platform. Obtain a five second recording and identify each platforms reference system and marker within the 3D space.
On the subject, use double adhesive tape and self securing bandages to adhere retro reflective markers to anatomical landmarks of the lower and upper extremities, trunk, head, and pelvic segments, in accordance with the chosen technical frame of reference. Then, instruct the subject to stand stationary in there center of the capture volume. Open the propriatory tracking software with the previously set sampling parameters.
Within the software, confirm visually that all markers are accounted for. Then, click the record icon to complete a five second static capture. Begin by removing all anatomical markers.
To set up the final feed position, ask the subject to sit on the stool with their feet on the individual force platforms, one and two. Adjust the shank position on the subjects dominant side 10 degrees posterior from the vertical using an extendable arm goniometer. Adjust the non-dominant foot equally in line and then use lock calipers to arrange the inter foot width to the predetermined biocromial distance accordingly, between the lateral foot borders.
Next, adjust the transverse plane orientation of each foot such, that each medial foot border is placed in line with the direction of travel. Then, use a marker to draw around the final foot positions on the surface of the removable force platform. Within the software, confirm visually that all markers are accounted for, then click the record icon to complete a 15 second dynamic capture.
Five seconds into the dynamic capture turn on the operator light switch which notifies the subject to rise from the stool and pause, step on to force platforms three and four, walk toward the light switch, stop, then turn the light switch off with their dominant hand. Finally, reset the light switch and check for marker drop outs by accounting for all markers during slow motion playback of the trial. Within the proprietary tracking software identify and label all the markers from static and dynamic trials, and crop unwanted capture by moving the time slides to the beginning and end of the task.
Utilize the automatic identification of markers, or aim functionality to label the markers so that the software consistently constructs and calculates the relative trajectory of a rigid body in 3D space. If a marker dropped out for more than 10 frames locate the missing trajectory in the unidentified trajectories panel. Finally, format and export all static and dynamic trials in C3D format for post-processing and biomechanics analysis software.
In the mediolateral plane, the whole body center of mass, or BCOM, follows a sinusoidal displacement after initiation of gait, and the center of pressure, of COP displaces further laterally during single limb stance during steps one and two. In the anteroposterior plane, the COP at seat-off starts in front of the BCOM. And while they both move forward during rising, their separation diminishes steadily before merging at upright.
The horizontal separation distance between COP and BCOM during the rise, pause, gait initiation, step one, and step two, shows the complex interaction of COP and BCOM displacement, providing an index of positional stability. Once mastered, this movement capture technique can be completed within a few minutes if it is performed properly, and local post-processing can be systematized to capture positional stability, COP BCOM measures in real time. After watching this video, you should have a good understanding of how to measure dynamic positional stability using COP BCOM distance during sit, to stand, and walk.
