The effect of object size and shape on grip aperture and grip orientation

  • Marc R. Inberg

Western Sydney University thesis: Master's thesis

Abstract

The study investigated the effect on grip size of grasping objects with different size contact surfaces between the finger and thumb. People use information about size (spatial information) and shape (featural information) to form a grasp. Spatial information refers to subconscious calculations about the relationship between the object's dimensions and the grasper's co-ordinates in space. This includes the size of the contact surface relative to one's body location. Featural information refers to perceptual object traits such as surface stability, and appearances that affect estimated object mass and volume. Both types of information can affect planning a grasp on the object. The stage of the grasp used to measure the grip size is known as the maximum grip aperture (MGA) during the reach and it has proven to be variable based on which information is used to form the grip at this stage of the trajectory. The current study aimed to further investigate the effect that object size and shape have on grip size by introducing a set of objects that differed in shape and contact surface size: a triangle, trapezium and square. Subjects were instructed to grasp at the surfaces on the top and base of the object between the index finger and thumb. The three objects differed in their spatial extent along the top contact surface but had the same base width thus altering the precision required for positioning the index finger. The objects also differed in their location of the centre of gravity (COG), which varied along the vertical axis such that some objects had a lower COG, closer to the base, thereby shifting the location of the object's average distribution of weight. The study introduced an alternative measure to compare with the MGA; the end grip aperture (EGA) just before the digits make contact with the object. This measure may afford a different result to the MGA in terms of its size relative to the object information at that stage of the grasp. Grip orientation was also calculated at both stages of the grasp: grip orientation at maximum grip aperture (GOM) and grip orientation at end grip aperture (GOE). The results showed a significant difference in the MGA and EGA between the different objects. Grip aperture was shown to be largest for the square, and smallest for the triangle. There was no significant difference, however, in the GOM and GOE between the different objects. There was a significant difference between the two aperture sizes, MGA and EGA, and the two orientations, GOM and GOE, across all the objects. The MGA was larger than the EGA, and the GOE was larger than the GOM. The overall result suggests subjects used a larger grip size both during the reach and at the end of the grasp to execute a grip across a larger object with longer contact surfaces. In this instance, subjects may have used spatial information about the extent of the index finger contact surface and conformed a grip size to match this dimension.
Date of Award2016
Original languageEnglish

Keywords

  • human mechanics
  • mechanical ability
  • motor ability

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