Grip Stiffness Research Articles

Key Insights into Hand Biomechanics: Human Grip Stiffness Can Be Decoupled from Force by Cocontraction and Predicted from Electromyography.

We investigate the relation between grip force and grip stiffness for the human hand with and without voluntary cocontraction. Apart from gaining biomechanical insight, this issue is particularly relevant for variable-stiffness robotic systems, which can independently control the two parameters, but for which no clear methods exist to design or efficiently exploit them. Subjects were asked in one task to produce different levels of force, and stiffness was measured. As expected, this task reveals a linear coupling between force and stiffness. In a second task, subjects were then asked to additionally decouple stiffness from force at these force levels by using cocontraction. We measured the electromyogram from relevant groups of muscles and analyzed the possibility to predict stiffness and force. Optical tracking was used for avoiding wrist movements. We found that subjects were able to decouple grip stiffness from force when using cocontraction on average by about 20% of the maximum measured stiffness over all force levels, while this ability increased with the applied force. This result contradicts the force–stiffness behavior of most variable-stiffness actuators. Moreover, we found the thumb to be on average twice as stiff as the index finger and discovered that intrinsic hand muscles predominate our prediction of stiffness, but not of force. EMG activity and grip force allowed to explain 72 ± 12% of the measured variance in stiffness by simple linear regression, while only 33 ± 18% variance in force. Conclusively the high signal-to-noise ratio and the high correlation to stiffness of these muscles allow for a robust and reliable regression of stiffness, which can be used to continuously teleoperate compliance of modern robotic hands.

Direct Testing of Gradual Postpeak Softening of Fracture Specimens of Fiber Composites Stabilized by Enhanced Grip Stiffness and Mass

Static and dynamic analysis of the fracture tests of fiber composites in hydraulically servo-controlled testing machines currently in use shows that their grips are much too soft and light for observing the postpeak softening. Based on static analysis based on the second law of thermodynamics, confirmed by dynamic analysis of the test setup as an open system, far stiffer and heavier grips are proposed. Tests of compact-tension fracture specimens of woven carbon-epoxy laminates prove this theoretical conclusion. Sufficiently, stiff grips allow observation of a stable postpeak softening, even under load-point displacement control. Dynamic analysis of the test setup as a closed system with proportional-integrative-differential (PID)-controlled input further indicates that the controllability of postpeak softening under crack-mouth opening displacement (CMOD) control is improved not only by increasing the grip stiffness but also by increasing the grip mass. The fracture energy deduced from the area under the measured complete load-deflection curve with stable postpeak is shown to agree with the fracture energy deduced from the size effect tests of the same composite, but the size effect tests also provide the material characteristic length of quasibrittle (or cohesive) fracture mechanics. Previous suspicions of dynamic snapback in the testing of stiff specimens of composites are dispelled. Finally, the results show the stress- or strain-based failure criteria for fiber composites to be incorrect, and fracture mechanics, of the quasibrittle type, to be perfectly applicable.

Correction: Task Dependency of Grip Stiffness—A Study of Human Grip Force and Grip Stiffness Dependency during Two Different Tasks with Same Grip Forces

Correction: Task Dependency of Grip Stiffness—A Study of Human Grip Force and Grip Stiffness Dependency during Two Different Tasks with Same Grip Forces

Task dependency of grip stiffness--a study of human grip force and grip stiffness dependency during two different tasks with same grip forces.

It is widely known that the pinch-grip forces of the human hand are linearly related to the weight of the grasped object. Less is known about the relationship between grip force and grip stiffness. We set out to determine variations to these dependencies in different tasks with and without visual feedback. In two different settings, subjects were asked to (a) grasp and hold a stiffness-measuring manipulandum with a predefined grip force, differing from experiment to experiment, or (b) grasp and hold this manipulandum of which we varied the weight between trials in a more natural task. Both situations led to grip forces in comparable ranges. As the measured grip stiffness is the result of muscle and tendon properties, and since muscle/tendon stiffness increases more-or-less linearly as a function of muscle force, we found, as might be predicted, a linear relationship between grip force and grip stiffness. However, the measured stiffness ranges and the increase of stiffness with grip force varied significantly between the two tasks. Furthermore, we found a strong correlation between regression slope and mean stiffness for the force task which we ascribe to a force stiffness curve going through the origin. Based on a biomechanical model, we attributed the difference between both tasks to changes in wrist configuration, rather than to changes in cocontraction. In a new set of experiments where we prevent the wrist from moving by fixing it and resting it on a pedestal, we found subjects exhibiting similar stiffness/force characteristics in both tasks.

A tunable hand biofidelity-enhancing device for Hybrid III dummies

Hybrid III dummies used for collision testing in the automotive industry include passive hands that cannot produce any grasping force. Such capability would be useful in low-acceleration collisions or vehicle tipover investigations, since grasping force and hand stiffness may impact the dummy movements during impact. In the context of studying the need for restraining devices for drivers in forklift tilting accidents, a new hand gripper compatible with the Hybrid III dummy was designed to replicate hand grasp capabilities, namely its grip force and stiffness on a steering wheel. A theoretical model of the device was developed to figure out a set of configuration parameters that replicate human hand grasping force and stiffness. Extraction force, manually tuned by a compression spring, was analytically characterised and validated experimentally with a prototype on a uniaxial traction machine. The system exhibits a non-intuitive behaviour, both in terms of extraction force and axial stiffness, due in part to the non-conservative friction forces involved. Tuning simplicity, low cost and robustness were desirable design constraints given the intended usage.