An instrumented cylinder measuring pinch force and orientation

Journal of NeuroEngineering and Rehabilitation An instrumented cylinder measuring pinch force and orientation Daniel Bourbonnais 1 2 Victor Frak 0 1 2 Jean-Franois Pilon 2 Michel Goyette 2 0 Departement de kinanthropologie, Universite du Quebec a Montreal , QC, H3C 3P8 , Canada 1 Ecole de readaptation, Faculte de medecine, Universite de Montreal , QC, H3C 3J7 , Canada 2 Centre de recherche interdisciplinaire en readaptation du Montreal metropolitain (CRIR), site Institut de readaptation de Montreal , Montreal, QC, H3S 2J4 , Canada Background: The function of a cylinder allowing simultaneous measurements of the opposition axis of the index finger and thumb of the hand and the magnitude of pinch force is described. Methods: The apparatus is made of two half-cylinders that are bonded together through a 6-axis force/torque sensor and allows the measurement of 3D orthogonal forces and moments of force. The amplitude of the pinch force exerted on the cylinder by the fingers is defined as the resultant of the forces in the different axes. A software program was developed to measure the barycentre of the forces on the instrumented cylinder, allowing calculation of the angle of the opposition axis between the fingers and the location of the resulting pinch force on the cylinder, assuming that the pinch or grip forces are co-linear through the center of the cylinder. In order to assess the validity and reliability of the measurements, the cylinder was mounted on a milling table and seven calibrated weights (from 100 to 500 g) were successively applied perpendicularly to a 9*9 matrix of sites separated by 1 cm. With the exception of the extreme lateral parts of the cylinder, the dispersion of the calculated vertical position of the resulting force was always within 1 mm of the application point, suggesting a high reliability of these measurements. In addition, the errors in the angles of the applied force were calculated and found to be less than 2 degree with no clear patterns of variation across the different locations of the cylinder. Results: The usefulness of the cylinder is demonstrated by evaluating the pinch force and the opposition axis in six healthy subjects lifting the cylinder from the table using three different orientations of their right hand. The magnitude of the grip force was not significantly different across orientations (45, 22 and -22 degrees relative to the midline of the subject) suggesting that force grip is controlled. Conclusion: From these results, it has been concluded that the cylinder is a valid, reliable and precise instrument that may prove useful for evaluating opposition axis and grip force in healthy and pathological populations. - Introduction Grasping, holding and manipulating objects represent one of the most important functions of the hand. During the phase where the hand is brought into the vicinity of the object to be manipulated, the grasp aperture increases to reach a maximum before contact with the object and is adjusted precisely when the hand is close to the object [1]. In addition to controlling the aperture of the fingers, the orientation of the hand relative to the object is critical for effective manipulation. The grasp orientation as described by Napier [2] is determined from the configuration of the arm and the hand that the nervous system has to define in order to allow utilization of an object [3-5]. Moreover, the positions of the fingers on the object need to be determined by the nervous system to ensure secure manipulation. For example, pinching a cylinder requires the opposition of the index and the thumb to be approximately through the center of the object to ensure stability of the grip. The location, size and weight of the cylinders do not impact on the grasp orientation or the opposition axis, which remain stable with respect to an egocentric reference frame in right-handed individuals [6]. In addition to this spatial consideration, lifting and holding an object between the index finger and thumb requires fingertip shear forces to overcome the weight of the object and prevent it from dropping. The amplitude of the shear force is determined by the friction coefficient of the object and the amplitude of the grip force (GF, considered as the finger force acting perpendicularly to the object's surface). To avoid slipping and/or dropping of the object, the GF must therefore be modulated as a function of the friction coefficient and weight of the object [7,8]. Usually, subjects exert a slightly larger GF than the GF mechanically required to hold the object, providing a safety margin that allows small perturbations to be corrected without dropping the object [8]. Many studies have demonstrated the precise coordination between the GF and the shear forces during the manipulation of an object [8-11] but few have characterized their changes or magnitudes when the opposition axis or orientation of the hand is modified. Typically, the opposition axis is determined by the location of force transducers not allowing a person who lifts an object to select his/her preferred grasp orientation. This provides a rationale for developing an instrumented cylinder measuring both the amplitude of the pinch force and allowing self-selection or imposed orientation of the opposition axis. The precision of the measurements obtained from such an apparatus was investigated experimentally in the present study. Moreover, its usefulness is illustrated by exploring changes in GF across different configurations of the arm and hand while the individual is lifting the cylinder. Methods Experimental set-up An instrumented cylinder (diameter of 60 mm and a height of 100 mm) having dimensional characteristics similar to everyday-life objects such as a glass or a bottle was built to allow the opposition axis and force magnitude to be measured while it is manipulated. The external frame of the cylinder consists of two separate nylon 66 half-cylinders rigidly connected to a single 6-axis force/torque (F/T) sensor (ATI Industrial Automation, NC, USA, model Mini 40 SI-20-1 with a resolution in each channel: FX, FY = 1/100 N; FZ = 1/50 N; TX, TY, TZ = 1/4000 Nm). All forces and moments of force exerted by the fingers are measured through the transducer since the two half-cylinders are separated by a gap of 0.56 mm. One of the half-cylinders is press-fitted and locked with set screws on an adapter that was fixed on the sensitive side ("tool side" as defined by ATI, see Figure 1C) of a single F/T sensor, enabling external forces and torques to be recorded (Figures 1A and 1C). The adapter consists of a machined hat-shaped nylon plate permitting adequate transfer of the forces and moments from the half-cylinders to the F/T sensor (Figures 1A and 1C, hatched area). The forces exerted on the non-sensitive halfcylinder are considered equal and opposite to the ones recorded on the opposite half-cylinder, assuming that stable manipulation of the instrumented cylind (...truncated)