Synchronization Of Video Kinematic And Analog Biomechanical Data Using The Motion Analysis System

A significant advantage of using integrated biomechanical data collection systems to study human movement is the simultaneous storage in a single computer of video-derived kinematic data and digitized records of analog data such as force and electromyographic signals. However care must be taken to assure that these records are properly synchronized by the respective data-logging parts of the hardware and software. This is particularly important when the data are combined to calculate dynamic values such as net joint moments. We have studied data synchronization using the Motion Analysis Corporation's Expertvision 60Hz video system (VP320) and have identified three important issues: 1) sampling rate; 2) video sampling initiation; and 3) intra-frame video/analog initiation synchronization. The first issue, sampling rate, arises since each data collection process assigns time values throughout the data based on a nominal sampling rate. Errors in this assumed rate will cause increasing cumulative synchronization error throughout the sampling interval unless appropriate corrections are appplied. We have found the analog data collection rates to be sufficiently accurate (within the lOO-2KHz range), but the video sampling rate deviates from the nominal 60Hz by 1-2.5%, depending on the hardware version. This error requires a software adjustment to the packaged calculation of video frame times. The second issue, video sampling initiation, occurs because the video collection triggering process starts by storing (as "Frame 1") the second video field to occur after the detection of the trigger signal. Since the A/D conversion of analog signals uses the same algorithm but (usually) a much faster sample rate, the analog sample begins nearly one video frame interval ahead of the video sample. This constant initiation time offset must be corrected before post-hoc data analysis. The third issue compounds the initiation correction procedure, as the trigger signal (and subsequent analog data collection) may occur at any point between two video frames, introducing an additional random offset ranging from 0-1 7ms. In many applications this amount of error is unacceptable, and correction in the form of external video synchronization is required. One simple way to accomplish this is to lead the video synch pulse into an unused A/D channel; another is to use a pulsed light in the video record which is also sampled by the A/D system. Synchronization errors are system-specific, and may be constant or variable. Collection of accurately synchronized video kinematic and analog data requires careful attention and post processing to correct these three sources of potential error

INFLUENCE OF EXTERNAL FORCES ON THE CONTROL AND PERFORMANCE OF A MINIMUM TIME SHOULDER FLEXION TASK

INTRODUCTION AND METHODS We have been using planar mathematical models to simulate the task of a rapid bilateral arm raise and to obtain minimum movement time solutions. Here we report the effect of gravity (G), ground reaction force (GRF) and center of pressure (COP) location on the solutions of a three segment model. We modeled the arms as a rigid segment, the head, torso, upper and lower legs as a second rigid segment, and the feet as a third rigid segment. The shoulder and ankle joints were modeled as revolute pin joints. A nonlinear rotational spring and damper restrained the movement of the ankle joint within physiological limits. Joint muscle torques were generated through two idealistic torque generators. Torque history values (for each joint) were controlled by eight evenly spaced nodes, while intermediate values were obtained by linear interpolation. The foot to ground interaction was modeled with the use of two 2-D springs (nonlinear vertically, linear horizontally) and dampers. One set was attached at the toes and one at the heel. Thus, the feet were free to move off and slide along the ground depending on the dynamics of the simulation. The initial position was quiet erect stance with the arms and feet perpendicular to it. A variable step integrator was used for the forward simulations. The parameterized torque histories were optimized using a nonlinear optimization algorithm. We compared solutions with G, without G (free floating or attached to the ground), and with the COP location at the initial and final state proscribed to be either below the ankle joint or at the middle of the feet. This adjustment of the COP was accomplished by defining the initial and final orientation of the body segment. The arms and feet initial and final orientation, as well as the anthropometric parameters and strength limits of the model were held constant for all solutions. RESULTS AND CONCLUSIONS The overall performance and &ody kinematics were very similar and well within human subject experimental results, but the 1G condition revealed higher maximum arm angular velocity (8radsec) than the OG conditions (7 radsec). Although the 1G solution used maximum ankle torque, the heel moved up minimally (< 6 mm) only early in the stopping phase. On the other hand, although the OG free floating solution used minimal ankle torque values, there was maximum dorsiflexion followed by maximum plantarflexion. The 1G condition revealed significant plantar- and shoulder-flexion torques at the end of the movement that the OG attached to the floor model did not have. Finally, the two COP conditions produced opposite ankle torque coordination, a variability also observed during human subject experiments

Family and Early Life Factors Associated With Changes in Overweight Status Between Ages 5 and 14 Years: Findings From The Mater University Study Of Pregnancy and its Outcomes

Objective To describe different patterns of overweight status between ages 5 and 14 y and examine the role of modifiable family and early life characteristics in explaining different patterns of change between these two ages. Design A population-based prospective birth cohort. Subjects A total of 2934 children (52% males) who were participants in the Mater-University study of pregnancy, Brisbane, and who were examined at ages 5 and 14 y. Main outcome measures Four patterns of change in overweight/obesity status between ages 5 and 14 y: (i) normal at both ages; (ii) normal at 5 y and overweight/obese at 14 y; (iii) overweight/obese at 5 y and normal at 14 y; (iv) overweight/obese at both ages. Results Of the 2934 participants, 2018 (68.8%) had a normal body mass index (BMI) at ages 5 and 14 y, 425 (14.5%) changed from a normal BMI at age 5 y to overweight or obese at age 14 y, 175 (6.0%) changed from being overweight or obese at age 5 y to normal weight at age 14 y and 316 (10.8%) were overweight or obese at both ages 5 and 14 y. Girls were more likely to make the transition from overweight or obese at age 5 y to normal at 14 y than their boy counterparts. Children whose parents were overweight or obese were more likely to change from having a normal BMI at age 5 y to being overweight at 14 y (fully adjusted RR: 6.17 (95% CI: 3.97, 9.59)) and were more likely to be overweight at both ages (7.44 (95% CI: 4.60, 12.02)). Birth weight and increase in weight over the first 6 months of life were both positively associated with being overweight at both ages. Other explanatory factors were not associated with the different overweight status transitions. Conclusions Parental overweight status is an important determinant of whether a child is overweight at either stage or changes from being not overweight at 5 y to becoming so at 14 y