Many young females have difficulty opening jars. Although previous studies have attempted to clarify the body posture effect during such a task, the experiments therein focused on a single digit or actions with unnatural finger positions and were further restricted to upper extremity postures. A further study is required to investigate the fingers' coordination, as well as each subject's natural and self-selected upper extremity posture and finger grasp location when opening a jar. This study focused on the forces and coordination of the right hand fingers during a jar opening movement under both vertical and free-way opening postures. A jar simulator was set up to record the forces applied by finger groups (the thumb, the index-middle finger group, and the ring-little finger group) of the right hand. A self-selected finger position and free-arm posture of each subject were allowed. Results show that the force vectors of the finger groups were all in the counterclockwise direction for both postures. The total force and overall torque of the right hand decreased in the vertical opening posture. The thumb produced greater tangential and resultant forces in the vertical opening posture. Despite normal forces being 1.82-3.54 times the tangential forces for both postures, no difference was found for the normal force to tangential force ratio for each finger group between the two opening postures. The index-middle finger group had similar torque contributions for both postures. The torque contribution of the thumb increased (26% and 21% for vertical and free-way posture, respectively), while the ring-little finger group torque contribution decreased (35% and 42% for vertical and free-way posture, respectively) in the vertical opening posture. As such, the free-way opening posture is the better strategy for young females to open a jar.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Mechanical Engineering