A study of the difference between nominal and actual hand forces in two-handed sagittal plane whole-body exertions

Abstract

Given a task posture, changes in hand force magnitude and direction with regard to joint locations result in variations in joint loads. Previous work has quantified considerable vertical force components during push/pull exertions. The objective of this work was to quantify and statistically model actual hand forces in two-hand, standing exertions relative to the required nominal horizontal and vertical hand forces for a population of widely varying stature and strength. A total of 19 participants exerted force on a fixed handle while receiving visual feedback on the magnitude of force exerted in the required horizontal or vertical direction. A set of regression equations with adjusted R² values ranging from 0.20 to 0.66 were developed to define actual hand force vectors by predicting off-axis forces from the required hand force magnitude. Off-axis forces significantly increase the overall magnitude of force exerted in two-hand push/pull and up/down standing force exertions.

Statement of Relevance:This study quantifies and statistically models actual hand forces in two-hand, standing exertions. Inaccuracies in hand force estimates affect the ability to accurately assess task-oriented strength capability. Knowledge of the relationship between nominal and actual hand forces can be used to improve existing ergonomic analysis tools, including biomechanical simulations of manual tasks.

Keywords:

• force direction
• hand force
• lifting
• push/pull
• standing exertions

Acknowledgements

This work was supported by the partners of the Human Motion Simulation Laboratory at the University of Michigan, including General Motors, Ford Motor Company, International Truck and Engine Corporation, the US Postal Service, and the US Army TARDEC. The authors acknowledge the substantial contributions of Megan Haubert to the data collection process.