The objective of this study was to identify the impact of modifying the object width on muscle and joint forces while gripping objects. The experimental protocol consisted to maintain horizontally five objects of different widths (3.5, 4.5, 5.5, 6.5, and 7.5 cm) with a thumb–index finger grip. Subjects were required to grasp spontaneously the object without any instruction regarding the grip force (GF) to apply. A biomechanical model of thumb–index finger pinch was developed to estimate muscle and joint forces. This model included electromyography, fingertip force, and kinematics data as inputs. The finger joint postures and the GF varied across the object widths. The estimated muscle forces also varied significantly according to the object width. Interestingly, we observed that the muscle force/GF ratios of major flexor muscles remain particularly stable with respect to the width whereas other muscle ratios differed largely. This may argue for a control strategy in which the actions of flexors were preserved in spite of change in joint postures. The estimated joint forces tended to increase with object width and increased in the distal–proximal sense. Overall, these results are of importance for the ergonomic design of handheld objects and for clinical applications.
Laurent Vigouroux, Mathieu Domalain and Eric Berton
Jérémy Rossi, Benjamin Goislard De Monsabert, Eric Berton and Laurent Vigouroux
The objectives of this study were to investigate the effect of handle shape on the grip force distribution in the hand and on the muscle forces during maximal power grip tasks. Eleven subjects maximally grasped 3 handles with different external shapes (circular, elliptic, and double-frustum). A handle dynamometer, equipped with both a force sensor and a pressure map, was used to record the forces exerted at the hand/handle interface. The finger and wrist joint postures were also computed from synchronized kinematic measurement. These processed data were then used as input of a biomechanical hand model to estimate muscle forces. The results showed that handle shape influences the maximal grip force, the grip force distribution, and the finger joint postures. Particularly, we observed that the elliptical shape resulted in a 6.6% lower maximal grip force compared with the circular and double-frustum handle. Concomitantly, the estimated muscle forces also varied significantly according to the handle shape, with up to 48% differences for the flexor digitorum superficialis muscle for example. Interestingly, different muscle coordination strategies were observed depending on the handle shape, therefore suggesting a potential influence of these geometrical characteristics on pathological risks such as tendonitis.