Chapter 2. Prehension

Chapter 2 - Prehension 27
the button on a spray can, and cutting with scissors. Landsmeer provided
an extensive anatomical analysis of precision handling and suggested
that a key characteristic of it was the fact that the metacarpophalangeal
joints are in extension while the interphalangeal joints are in
flexion.
An important aspect of the human hand is the interplay between the
thumb, index finger, and middle finger. Due to the thumb’s placement
on the hand, its six degrees of freedom, and its ability to oppose the
other fingers, a posture called a tripod or three jaw chuck can be
formed. This is a powerful gripping mechanism used in tools such as
drills for holding drill bits. Included in Schlesinger’s full classification
and other classifications as noted in the table, the tripod can be
used to grasp a piece of chalk between the thumb, index, and middle
fingers. Additionally, a variation of this can be formed using a fourth
contact point. This dynamic posture, used in writing with a pen or
pencil, has been called the external precision grip or writing gri~
(Patkin, 1981), dynamic tri~od [Wynn-Parry, 1973), and tri~od &D-variation
1 (Kamakura et al., 1980). As seen in Figure 2.4b, the object
is held by the pads of the thumb, index, and middle fingers, with
an extra contact made by the thumb cleft or side of the index base.
The relative lengths of these digits and the complex interactions of
their degrees of freedom give rise to the thumb’s ability to oppose the
fingers and thus impart fine motions to the object. Using the cleft of
the thumb as an extra contact point counteracts torques being applied
to the object, as with the pen or pencil being applied to a piece of paper
in writing.
In terns of dynamic grasps, Elliott and Connolly (1984) noted that
care has to be taken in distinguishing anatomical features from functional
features of the hand. They argued that Landsmeer’s extension
to Napier’s classification did not clarify this difference, and therefore,
in their taxonomy, they distinguished two functions: palmar grips (for
immobilizing the object) and digital manipulative patterns (of independently
coordinated digit movements). Anatomically, palmar grips are
usually power grips, but digital manipulative patterns include more
than precision handling as it was anatomically defined by Landsmeer.
Elliott and Connolly identified three basic classes of manipulative
movements: simple synergies, reciprocal synergies, and sequential
patterns. Simple synergies are where ‘all movements of the participating
digits are convergent flexor synergies’ and include pinch (between
thumb and index pads), dynamic tripod (holding a pen), and squeeze
(squeezing rubber bulb or syringe). Reciprocal synergies include
twiddle (roll small object to and fro between thumb and index), rock