Tool Module: The Hand

The human hand, a late product of evolution, represents the pinnacle of fine movement and co-ordination. Australopithecus, human ancestors who lived 4 million years ago, could not oppose their thumbs to their forefingers, just as chimpanzees cannot do so today. Only in Homo sapiens has the thumb become longer and opposable to all the other fingers, enabling us to grasp smaller objects even more precisely. Data obtained from fossil hands indicate that this precise grasping ability first developed in the human species Homo erectus about 1.8 million years ago. The agility of the human hand comes not only from its ability to oppose the thumb to the other fingers, but also from the fingers' ability to move independently of one another. This flexibility contributes greatly to the variety of prehensile (grasping) movements we can make: we do not grasp the handle of a coffee cup the same way we do the handle of a hammer, or the barrel of a ballpoint pen. It is hard to imagine the complexity of the problems that your brain must solve to co-ordinate the simple act of picking up an object. First it must bring your hand close to the object. Then it must decide what shape your hand should assume, how it should be oriented, and how much force it should apply to the object to prevent it from slipping through your fingers. And to co-ordinate the many muscles contributing to this movement, the nervous system must process visual inputs about the object's orientation, size, shape, weight, expected texture, and location in space.

Study of Hands Nicolas de Largillière (1656-1746)

By analyzing films of many movements, researchers have been able to characterize the major steps in the action of picking up an object. Depending on the subject and the circumstances, the movement of the hand toward the object takes 400 to 800 milliseconds (ms). It begins with a rapid acceleration that peaks at about 150 ms and produces the maximum velocity 100 ms later. A deceleration peak then occurs, followed by a phase of low, declining velocity, until the hand stops near the object. The strong initial acceleration enables the hand to cover most of the distance rapidly (the hand spends nearly half of the total movement time in the immediate vicinity of the object).

The process of actually grasping the object is highly complex, because it involves many degrees of freedom: whether the fingers are flexed or extended, the position of each of the phalanges, the distance between the fingers, the amount that the thumb must rotate to achieve opposition, and so on. Note that during the approach movement, the fingers spread apart until they reach a maximum span that is wider than the object. Then they reverse this movement and close on the object gradually.

After this "visual" phase, a "tactile" phase ensues. Actually, this tactile phase begins even slightly before the fingers make contact with the object. The force with which they grasp it is calculated according to the object's weight and expected texture. For example, the fingers will apply more force to a smooth object than to a rough one. The pressure that the hand exerts on an object when it makes contact is therefore determined from visual indications that evoke past knowledge of this object or similar ones that is stored in memory.

The ascending force used to pick the object up is not applied until about 100 ms after contact. If the weight of the object is increased while it is being grasped (for example, if someone pours water into a glass while someone else is lifting it), the grasping force and the lifting force will increase synchronously in response to tactile signals transmitted by cutaneous receptors in the finger pads.

Movement of the hand, drawing a circle (1887)
Eadweard Muybridge (1830-1904)

Most researchers who have studied this complexly co-ordinated process that lets us pick up objects without dropping them agree on the following general facts:

- The actions of moving the arm and preparing the hand to pick up the object start together: the fingers start to open as soon as the arm starts accelerating.

- The halt in the arm's movement is not triggered by contact with the object but is instead pre-programmed; by the time the hand makes contact with the object, the arm's velocity is close to zero.

- The fingers begin to close on the object at the start of the low-velocity phase of the arm movement.