08/06/04

1. Definition of prehension

          Prehension can be defined as "the application of functionally effective forces by the hand to an object for a task, given numerous constraints". This definition emphasizes the function and task. There are infinite number of ways to grasp an object by changing the kinematics and kinetics of the hand. We need to control 30 mechanical variables to hold an object stable, 6 for each digit: 3 components of acting force and 3 components of moments on the object. We can change the force sharing (how the forces are distributed among fingers. Each finger can have a certain proportion of the sum of all finger forces) among the involved digits. We can also change kinematics of prehension using different finger positions to grasp an object.

2. Classification of prehension

          There are many researchers who have classified prehension such as Iberal, Jocobson and Sperling, Kamakura, Cutkosky, cooney and Chao, Griffiths and Kapandji, Naiper, and so on. Schlesinger classified prehension postures into cylindrical, tip, hook/snap, palmar, spherical, and lateral prehensions as shown below.

   
                   Cylindrical Grip                                     Tip Grip                                                     Hook Grip

   
                  Palmar Grip                                               Spherical Grip                                          Lateral Grip

     1. A cylindrical prehension is used for a cylindrical object, such as for a hollow cylinder and coffee mug.
      2. A tip prehension is used to hold a small a sharp object, such as a nail or a piece of page.
      3. A hook prehension is used for a heavy object.
      4. A palmar prehension is used for a thin and relatively thick object.
      5. A spherical prehension is used for a spherical object, such as a ball or an apple.
      6. A lateral prehension is used for a thin and flat object.

2. Power vs. precision prehension\

          Naiper's classification for prehension is more useful when it comes to the functional aspects while Schlesingr's classification is more extensive. Naiper classified grips into 2 different categories: 'power grip' and 'precision grip'. Naiper defined the power grip as 1) the thumb is in the plane of the palm, 2) thumb's metacarpophalangeal and crpometacarpal joints are adducted, 3) finger's are flexed, laterally rotated, and inclined towards the ulnar side of the hand, 4) the fingers flex in oposition to the palm, witht the degree of flexion depending on object dimensions, and 5) the wrist is positioned with ulnar deviation, neural between extension and flexion. The precision grip occurs when 1) the thumb is abducted and medially rotated at the metacarpophangeal and the carpometacarpal joint. 2) the fingers are flexed and abducted at the metacarpophalangeal joints, producing a degree of axial rotation in the digits. 3) The wrist is dorsiflexed, positioned between ulnar and radial deviation. 4) The object is pinched between the finger and the opposing thumb, especially between the thumb and index finger which are used to hold a small object.

        The thumb position mainly decides whether a grip is under the category of precision or power. When the thumb is more adducted, then the grip becomes close to precision and vice versa. When the grip has the least precision and the most power with large adduction of the thumb, it is called 'coal hammer' grip.

   
                    Power Grip                                           Precision Grip                                     Coal Hammer Grip

2. Opposition

          The most important movement of human hand is opposition. Oppsition is a movement by which the pulp surface of the thumb is placed squarely in contact with or diametrically opposite to the terminal pads of one or all of the remaining digits (Naiper 1986). Iberall, Bingham, and Arbib (1986) used the term opposition to describe three basic directions along which the human hand can apply forces.

     1. Pad opposition: occurs between hand surfaces along a direction generally parallel to the palm. This usually occurs between volar surfaces of the finger and thumb, near or on the pads. An example is holding a pen or a needle.
     2. Palm opposition: occurs between hand surfaces along a direction generally perpendicular to the palm. An examples is grasping a hammer.
     3. Side opposition: occurs between hand surfaces along a direction generally transverse to the palm. An example is holding a key between the volar surface of the thumb and the side of the index finger.

           Pad Opposition                                                 Side Opposition                                 Palm Opposition

The blue arrows shown in the figures indicates the force actions of virtual fingers in opposition.