Determination of Lateral Pinch Force Requirements for Six Common Activities of Daily Living

Niels Smaby, MS; B. Baker; M. Elise Johanson, MS PT; Joseph D. Towles, MS; Wendy M. Murray, PhD

Objectives: Restoring hand function after cervical spinal cord injury is critical in helping an individual regain functional independence, and can be accomplished via functional neuromuscular stimulation as well as mechanical surgical interventions, such as tendon transfers. However, there are limited data that quantitatively define the pinch requirements for completing functional tasks. The objective of this study was to quantify three important requirements for completing functional tasks that require lateral pinch: pinch force magnitude, pinch opening, and the accuracy with which the force must be located and directed.

Methods: We identified 6 activities of daily living (ADLs) to study. The tasks included: inserting and removing a key from a lock, inserting and removing a plug from an outlet, zipping and unzipping a zipper, pushing the button on a T.V. remote, stabbing food with a fork, and inserting and removing an ATM card. For each task we measured the forces imparted to the object while a robotic manipulator performed the task. The forces the robot produced to complete each task were converted into lateral pinch forces via static mechanical analysis that took into account the coefficient of friction between human skin and different materials. Further geometric and mechanical analysis was completed to estimate the accuracy with which the force had to be applied. The pinch opening requirement was defined as the thickness of the object.

Results: The pinch force requirement for the six tasks ranged from 1.7 N to 24.9 N; all but the plug and the zipper required 8.3 N or less. The pinch opening requirements ranged from 0.7 mm to 22.9 mm; all but the remote and the plug required pinch openings of less than 2.4 mm. The pinch accuracy requirements showed force location requirements ranging from 6.2 mm to 23.7 mm, with all but the ATM card requiring better than 8.3 mm. The directional pinch accuracy was more limited by the frictional coefficient than by object and finger geometry. The directional accuracy for all but the plug in/out task (which was 16°) was 24°.

Conclusions: We have identified three basic pinch force elements required to complete six ADL's with a lateral pinch (without compensatory strategies). A review of the literature suggests active tendon transfers restore approximately 20N of pinch force (typically measured at about 20 mm of pinch opening). This would predict that, with active transfers, pinch force magnitude should be sufficient to complete all but one of the ADLs. However, our analysis indicates that these forces must be produced at very narrow grasp openings (< 2.5 mm) which may be difficult for tetraplegic individuals. In addition the thumb force must be located with greater than 8.3 mm of accuracy for the majority of the ADL's. Consequently a poor ability to position the thumb can be a critical cause of task failure. We propose that these requirements are as important as adequate pinch force magnitude to successful task completion. Thus, future work is directed towards testing these abilities in tetraplegic individuals.

Funding acknowledgment: This study was funded by the VA Rehabilitation Research and Development Service, Project #B898-3RA, and from the sponsorship of a summer scholar program by the Paralyzed Veterans of America.