Precision-grip force changes in the anatomical and prosthetic limb during predictable load increases

This study examined precision-grip force applied to an instrumented test ob ject held aloft while the weight of the object was predictably varied by tr ansporting and placing loads (50, 100, or 200 g) atop the test object. Tran sport of the loads was performed either by the subject or the experimenter. Grip force was examined in four non-amputee control subjects and in the an atomical and prosthetic hand of a subject with a prosthetic device. As subj ects transported the load, anticipatory grip-force changes occurred in the anatomical hands and prosthetic hand, which were scaled in relation to the load. When the experimenter transported the load to the anatomical hands of control subjects or the prosthetic user, anticipatory increases in grip fo rce occurred that also were scaled in relation to load. However, when the e xperimenter transported the load to the prosthetic hand, anticipatory grip- force adjustments were absent. During the phase in which the load was being assumed by the postural hand, grip forces in the anatomical hands and pros thetic hand were further scaled to load demands. Ability to adapt grip forc e in the prosthetic hand during this phase suggested that the subject was u tilizing sensory information from the residual limb to adjust grip force. T hus, while anticipatory changes precede the process of adaptation to load c hanges, actual sensory consequences resulting from added weight remain nece ssary to fully adapt grip force to load demands, even for the prosthetic us er.