Development of a hand dynamometer for the control of manually applied forces

The aim of the study was to develop a prototype tool capable of providing instantaneous feedback about manually applied forces, and to use it to determine (1) whether force constancy can be maintained during different application postures and (2) whether sensitivity to stiffness using the tool is different from sensitivity when assessing stiffness with the hands.

Subjects were students and staff in a health sciences faculty. A series of grade II and IV mobilizing forces were delivered using a mobilizing tool, a modified JAMAR dynamometer fitted with a molded handle. Forces were executed without feedback at application points on a plinth that were near, midway, and far from the body. Thereafter, discrimination between elastic stiffness was determined with hands alone or using the tool.

Force was maintained at grade II when the point of application was moved away from the body, but attempts to replicate grade IV mobilization forces in similar postures showed a decrease despite increased effort. Variation in force produced was substantial both within and between subjects. Stiffness discrimination was not significantly different whether hands or the tool was used.

There is substantial variation in manually applied forces that could be controlled if instantaneous force readout was available. Reports of hand injury in manual therapists motivate further development of devices that maintain sensitivity and allow for control of applied force and spread the load over a greater surface area on the hands.