Measurement of Spatial Stability in Precision Grip

Summary

The goal of this protocol is to measure the center of pressure (COP) replacement using a high spatial resolution sensor sheet to reflect the spatial stability in a precision grip. The use of this protocol could contribute to greater understanding of the physiology and pathophysiology of grasping.

Abstract

The purpose of the protocol is to indirectly evaluate the direction of the finger force during manipulation of a handheld object based on the biomechanical relationships in which deviated force direction causes center of pressure (COP) replacement. To evaluate this, a thin, flexible, and high spatial resolution pressure sensor sheet is used. The system allows measurement of the COP trajectory in addition to the force amplitude and its temporal regulation. A series of experiments found that increased trajectory length reflected a sensorimotor deficit in stroke patients, and that decreased COP trajectory reflects a compensatory strategy to avoid an object slipping from the hand grip in the elderly. Moreover, the COP trajectory could also be decreased by dual task interference. This article describes the experimental procedure and discusses how finger COP contributes to an understanding of the physiology and pathophysiology of grasping.

Introduction

Force control is the fundamental basis of precision grip. Compared with power grip, precision grip evaluates the minimal force output reflecting the ability to manipulate an object. Multiple sensorimotor systems contribute to precision grip. For example, during a grip and lift task, visual information enables the perception of the object’s size and shape. After the fingertips touch the object, tactile signals are delivered to the somatosensory cortex to adjust the precision grip force. Grip force (GF) is generated when the fingertips make contact with the object, and it increases during the lifting phase¹. When an object approaches the go....

Protocol

The series of studies in the present paper were approved by Gunma University Ethical Review Board for Medical Research Involving Human Subjects.

NOTE: Inclusion criteria for participants were the ability to understand the use of minimal force and the ability to perform the task with the thumb and index finger. Exclusion criteria were selected based on the purpose of the experiments.

1. Equipment preparation

1. Connect two sensor connector cables to the USB ports of a computer. Pull up the lever attached to the sensor connector and insert the sensor’s tab into the insertion slot. Return the attached....

Results

Several studies have introduced experimental protocols and two kinetic parameters (the COP trajectory and the GF) to measure finger force during manipulation of an object. In previous studies, it was found that the COP trajectory increased in stroke patients⁹. In cervical myelopathy patients, the GF correlated with the cutaneous pressure threshold and upper extremity function¹⁰. In healthy young subjects, the GF increased with cognitive interference¹¹

Discussion

This experimental procedure provides evidence that a flexible pressure sensor sheet could be useful for evaluating spatial stability during precision grip. Altered grip force direction represents grasping spatial instability such as a finger slip. However, existing load cell-type force direction instruments have a limitation in terms of ensuring a natural reach-to-grip movement. To resolve this technical problem, the COP trajectory of the area between the finger pulps and contact surface based on a biomechanical relation.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Alcohol swab			Wipe participant’s finger pulps
Compressor	Nitta Corporation		Apply pressure to the sensor seats
Computer
Controller of compressor	Nitta Corporation		Use to manupirate the compressor
Double-sides tapes			Use to attach the sensorseats to the iron cube
Iron cube			150-250g, 30×30×30 mm
Sensor connector			Connect the sensorseats to computer.
Sensor sheet			Pressure Mapping Sensor 5027, Tekscan, South Boston, MA, 50 USA
Setting stand			Set the iron cube on it during the measurement
Software; I-SCAN 5027, Ver. 7.51	Nitta Corporation
Table			Use for the measurement