hand is capable of hold
ing and manipulating
objects in various ways. This capability is often referred to as prehension
, which has the same root as the words comprehend and hinder. Grip
s used for holding on to objects fall in to two categories, Power
Power grips are used when the hand itself isn't doing the primary movement involved in an action, but the arm or body is. That is to say, one uses power grips for things like swinging a bat or shifting a standard transmission. Power grips hold by having the fingers press the object against the flat of the palm, or against themselves. The thumb often doesn't need to oppose the fingers for these grips to work, although it is used in some to maintain stability. There are four types of power grip:
- Spherical Grip: Used for holding ball-shaped objects. For small objects (say, a tennis ball) the fingers splay out and curl around enough to press the object against the palm. In larger objects (a softball), the fingers press the object against the fingers or thumb on the other side. For very large objects (a basketball), this is as much friction as it is actual grip, and may be broken easily.
- Lateral Grip: You might use this grip to hold on to a tree trunk. It involves the fingers pressing in unison against the palm, with the object in between. This is what's commonly thought of as the power grip.
- Hook Grip: This grip doesn't involve the palm or thumb at all. It requires the fingers to be curled around the object, and the object to be pressed against the joints between the fingers and the hand. This grip is used to hold on to things like monkey bars or a water-ski line.
- Cylindrical Grip: Much like the hook grip, but uses the palm and thumb for added stability. This grip is used for holding things like baseball bats.
Precision grips are used when fine manipulation is desired, manipulation that will probably be done by slight movements of the fingers and wrist. They grip by opposing the fingers and thumb against one another in various ways. Precision grips come in three flavors:
- Tip-to-tip Prehension: Also known as the pinch grip. Accomplished by opposing the index finger and thumb to hold on to something very small or delicate. With the range of movement left open to the fingers and wrist while using this prehension, the object can be moved in essentially any direction.
- Lateral Prehension: Unlike the lateral power grip, this grip is between the thumb and the side of a finger, or between the sides of two fingers. It is used when the wrist is doing the primary movement. Turning a key in a lock is a use of lateral prehension.
- Palmar Prehension: Gripping is done by two or more fingers opposed to the pad of the thumb. Holding on to a pencil to write is an example of palmar prehension. Palmar prehension is a combination of tip-to-tip and lateral prehensions.