A Biomechanical Model of the Hand and Wrist

Research Trainee: David Christian Grieshaber, PhD Student, Dept. of Industrial and Operations Engineering at the University of Michigan

Faculty Researcher: Thomas J. Armstrong, PhD, Professor of Industrial and Operations Engineering and Biomedical Engineering at the University of Michigan

Evaluating posture and assessing the force required to perform a manual task is a significant aspect of studying the biomechanical and epidemiological aspects of upper extremity cumulative trauma disorders (UECTDs). Biomechanical and kinematic modeling are methods that can be used to predict postural and strength capabilities for a given set of task-related constraints. These models will aid in proactively describing stressful or potentially stressful tasks and work situations. This study was undertaken to verify a kinematic model of the hand, and to evaluate intra-subject variability in power grasp posture. Joint angle measurements of the MCP, PIP, and DIP of the second phalange were from nine subjects (six males, three females) were recorded using motion tracking techniques. An NEC, TI-23A CCD camera was used to receive infrared light reflections from markers that were placed on the MCP, PIP, and DIP joints, and the tip of the distal phalanx of the second phalange. A cylindrical dynamometer was used as the grasping object. The cylinder was elliptically shaped and had a long axis diameter of 4.5 cm, and a short axis diameter of 3.2 cm.

27 angle measurements were compared to the model predictions for each joint. For 25 of the 27 joints measured, the model predicted hyperflexion of the respective joint. For the MCP joint, the mean difference between actual and predicted was -20.5 degrees (hyperflexion), with a standard error of 1.8. For the PIP and DIP, the mean and standard error were -11.5 (3.9) degrees and -8.9 (4.2) degrees, respectively. The mean standard deviations for all subjects were relatively small at 1.11 for the MCP joint, 1.99 for the PIP joint, and 5.34 for the DIP joint. Individual standard deviations ranged from a low of 0.00 to a high of 5.09 degrees. This was measured over ten trials for each subject. The location of the object in the hand significantly affected the model prediction angles.

The current model was able to predict gross postures of the hand in power grasp with reasonable accuracy and precision. This study is the first step in developing a model capable of evaluating the postural and biomechanical aspects of grasping work objects. Future research will focus on developing research and algorithms to accurately predict how people grasp objects of varying geometry. Special attention needs to be given to algorithms for placing the object in the hand, quantifying skin deformation as a measure of the force of gripping and predicting contact between the hand and non-cylindrically shaped objects.

 

Publications resulting from this project:
Grieshaber DC, Armstrong TJ. Insertion loads and forearm muscle activity during flexible hose insertion tasks. Hum Factors. 2007;49(5):786-796.

 

Research trainee’s current position:
David Christian Grieshaber received his PhD in 2007 and is currently an Assistant Professor in the College of Applied Science and Technology at Illinois State University.