Palmar prehension

Carrying a brief case is a good illustration of this kind of prehension. Keller et al. found that palmar prehension or tridigital pinch was the most frequently used prehensile pattern for static grasping whereas lateral prehension is used most often for dynamic grasping. 

The finding by Keller et al. (1947) that palmar prehension was the most frequently used pattern and reduction of most prosthetic terminal devices to a single DOF has meant that most prosthetic hands incorporate palmar prehension as the dominant grasp pattern. The persistence of this pattern, combined with a wide width of opening in prosthetic hand designs and its general acceptance over the years tends to support this compromise (Heckathome, 1992). 

A study done at Ae University of California at Los Angeles (UCLA) (Taylor, 1954) on human prehension force indicated that adult males could produce maximum mean forces of 95.6 N (21.5 Ibf) of palmar prehension, 103 N (23.2 Ibf) of lateral prehension, and 4(X) N (90 IbO of cylindrical grasp. In light of another (unpublished) UCLA study that showed that forces up to 68 N (15 Ibf) were needed for carrying out activities of daily living, Peizer et al. (1969) proposed that 68 N (15 IbO be a minimum standard for the maximum prehension force for electric prehensors (Heckathome, 1992). 

FIGURE 32.18 Schematic of the prehension patterns of the hand as defined by Keller, Taylor, and Zahn (1947) (al) palmar prehension (three-jaw chuck), (o2) palmar prehension (two finger), (b) tip prehension, (c) lateral prehension, (<0 hook prehension, (e) spherical prehension, (/) cylindrical prehension. In a handlike prosthesis, it takes two to four independently controlled degrees of freedom to implement these prehension patterns. In a non-hand-like device, a single-degiee-of-free-dom device such as a split hook can be used. 

Palmar prehension is used to grasp heavy, relatively large objects. The entire hand is wrapped around the object. The thumb opposes the force of the other four fingers. 

Peckham and colleagues [24] at Case Western Reserve University used chronic percutaneous stimulation of forearm muscles to provide hand grasp, including both palmar and lateral prehension and release, in C5 quadriplegic patients. 

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