Joel A. Cort

Trunk muscle contributions of to L4-5 joint rotational stiffness following sudden trunk lateral bend perturbations

The purpose of this research was to investigate the contributions of individual muscles to joint rotational stiffness and total joint rotational stiffness about the lumbar spines L(4-5) joint prior to, and following, sudden dynamic lateral perturbations to the trunk. Kinematic and surface EMG data were collected while subjects maintained a kneeling posture on a robotic platform, while restrained so that motions caused by the perturbation were transferred to the pelvis, causing motion of the trunk and head. The robotic platform caused sudden inertial trunk lateral perturbations to the right or left, with or without timing and direction knowledge. An EMG-driven model of the lumbar spine was used to calculate the muscle forces and contributions to joint rotational stiffness during the perturbations. Data showed 95% and 106% increases in total joint rotational stiffness, about the lateral bend and axial twist axes, when subjects had knowledge of the timing of the perturbation. Also, the contralateral muscles exhibited a significantly larger total joint rotational stiffness about the lateral bend axis, and earlier surface EMG responses, than the ipsilateral muscles. The results indicate that, when the timing of the perturbation was unknown, subjects relied more on delayed muscle forces following the perturbation to stiffen the L(4-5) joint.

Muscle Contributions to L4-5 Joint Rotational Stiffness following Sudden Trunk Flexion and Extension Perturbations.

The purpose of this study was to investigate the contribution of individual muscles (MJRSm) to total joint rotational stiffness (MJRST) about the lumbar spines L4-5 joint prior to, and following, sudden dynamic flexion or extension perturbations to the trunk. We collected kinematic and surface electromyography (sEMG) data while subjects maintained a kneeling posture on a parallel robotic platform, with their pelvis constrained by a harness. The parallel robotic platform caused sudden inertial trunk flexion or extension perturbations, with and without the subjects being aware of the timing and direction. Prevoluntary muscle forces incorporating both short and medium latency neuromuscular responses contributed significantly to joint rotational stiffness, following both sudden trunk flexion and extension motions. MJRST did not change with perturbation direction awareness. The lumbar erector spinae were always the greatest contributor to MJRST. This indicates that the neuromuscular feedback system significantly contributed to MJRST, and this behaviour likely enhances joint stability following sudden trunk flexion and extension perturbations.

Neuromuscular response of the trunk to inertial based sudden perturbations following whole body vibration exposure

The effects of whole body vibration exposure on the neuromuscular responses following inertial-based trunk perturbations were examined. Kinematic and surface EMG (sEMG) data were collected while subjects were securely seated on a robotic platform. Participants were either exposed to 10 min of vibration or not, which was followed by sudden inertial trunk perturbations with and without timing and direction knowledge. Amplitude of sEMG was analyzed for data collected during the vibration protocol, whereas the onset of sEMG activity and lumbar spine angle were analyzed for the perturbation protocol. Data from the vibration protocol did not show a difference in amplitude of sEMG for participants exposed to vibration and those not. The perturbation protocol data showed that those not exposed to vibration had a 14% faster muscle onset, despite data showing no difference in fatigue level.

An Estimation of Supporting Hand Forces for Common Automotive Assembly Tasks

This study was designed to determine the forces applied to supporting hands, by automotive assembly operators, during common one-handed tasks such as hose installations or electrical connections. The data were computed as a percentage of body weight and a repeated measures analysis of variance (ANOVA) (p<0.05) was conducted. Supporting hand forces were observed to range from 5.5% to 12.1% of body mass across a variety of tasks. The results of this study can be used when performing a biomechanical/ergonomic analysis to account for these supporting hand forces.

The evaluation of absolute position drift of inertial-based motion capture systems

Inertial based capture technologies allow for direct capture of motions within a manufacturing environment and, may be used with human simulation software to perform ergonomics analyses. However, these technologies are negatively affected by metal environments where “drift” has been shown to cause error in capture accuracy. Twenty participants completed four multi-task events simulating real work, in a laboratory while instrumented with inertial and optical based motion capture systems. Participants began and ended each event by performing a static T-Pose posture in a known location. Lower-leg 3D position data were extracted and the position difference from the start and end T-Poses were analyzed. Results indicate significant lower-leg position error relative to the starting location of the T-Pose to the end, with the inertial systems as compared to the optical based system. These errors can impact overall accuracy and representation of work within a human modeling environment.

The Relevance of Psychophysical Methods Research for the Practitioner

The use of a psychophysical methodology in conducting manual materials handling and upper extremity studies is well recognized, and the findings (e.g., the Snook and Ciriello studies and the Liberty Mutual tables) have extensive application in the assessment and design of a variety of tasks in industry. In particular, the psychophysical methodology is directed to the assessment of what workers can actually perform and as such has identified acceptable workloads for various working populations. In many cases the identification of these acceptable workloads has historically been very difficult to achieve with methods in other scientific disciplines (e.g., Biomechanics, Physiology, Epidemiology). The purpose of this discussion panel will be to explore the questions on the usefulness and continuing relevance of the psychophysical methodologies to address the needs of the practitioner community. Each panelist will explore the applications of psychophysics in several areas of research and practice. The continuing relevance and directions of psychophysical research will be explored in discussion with the panelists and audience.