How do hands work?

The human hand is made up of a total of 27 individual bones: 8 carpal bones (in the base of the hand), 5 metacarpal bones (in the middle part of the hand) and 14 phalanges (finger bones) are connected by joints and ligaments. About one quarter of all our body’s bones are found in our hands. The hand can be divided up into three different areas based on the joints:

• Carpus (wrist bones)
• Metacarpus
• Fingers

The wrist is made up of two parts of a joint that work as one functional unit. It allows us to flex (bend) or extend (stretch) our hands. We can also tilt our hands sideways, towards our little finger or thumb.

The eight carpal bones are held together tightly by ligaments, and are more or less fixed in place. They are positioned in two rows of four carpal bones each. Together with the radius bone in the forearm, two of the carpal bones (the scaphoid bone and the lunate bone) form the lower part of the wrist joint, which is very important for hand movements. The ulna bone in the forearm is separated from the carpal bones by a cartilage disc. The other part of the joint is located between the two rows of carpal bones.

After the second row of carpal bones comes the metacarpus. This middle part of the hand consists of five long metacarpal bones. You can feel them quite clearly on the back of your hand. One of the carpal bones and the long thumb bone come together to form the basal joint of the thumb. Known as the carpometacarpal joint, it enables the thumb to be particularly flexible.

The freely movable part of our hand is made up of five digits (four fingers and one thumb). Each finger has three individual bones, and the thumb only has two. The fingers have three joints each, which can only be bent and stretched in one direction. The thumb is the only digit that can twist, thanks to the saddle-shaped carpometacarpal joint.

There are over 30 muscles in the hand, working together in a highly complex way. Movements of the hand are mostly started by muscles in the forearm. Only the thin tendons of these muscles are found directly in the hand: the extensor tendons used for stretching the hand run through the back of the hand to the tips of the fingers, and the flexor (bending) tendons run through the palms to the fingers.

There are short muscles between the individual metacarpal bones of the hand. They allow us to spread our fingers (abduction) and then pull them back together (adduction). They also help to bend and stretch the fingers.

Two groups of more powerful muscles in the hand itself make up the thenar eminence (at the base of the thumb) and the hypothenar eminence (controlling the movement of the little finger). Among other things, the thenar muscles enable the thumb and the tips of the four fingers to touch each other (opposable thumb). A separate muscle (the adductor pollicis) is used to pull the thumb towards the palm. The hypothenar eminence muscles are mainly used for sticking out the little finger and pulling it inwards again, and for tightening the skin that covers the hypothenar eminence.

The lumbricals of the hand are four thin, worm-like muscles that help bend the metacarpophalangeal joints and extend the fingers.

Some parts of the long flexor and extensor tendons of the forearm muscles are surrounded by protective layers called tendon sheaths. Tendon sheaths contain a fluid that acts as a lubricant. This allows the tendons to slide smoothly through the sheaths, without friction.

Muscle tendons, nerves and blood vessels running from the forearm to the hand pass through a tunnel-like passageway on the palm side of the wrist. Known as the carpal tunnel, this passageway is made up of strong connective tissue and carpal bones.

Our hands can grasp and move objects in two different ways: with a power grip or precision grip. The object's size, shape, weight and ease of handling determines which of these two approaches is used. The power grip is better suited for large, heavy objects, and the precision grip is used for small, delicate objects.

The power grip is used to do things like carry heavy bags or hold on to a handle. In the power grip, the object is held in the palm of the hand, and the long flexor tendons pull the fingers and the thumb so that they can tightly grasp the object. This grip is made possible by the four other fingers flexing (bending) and, most importantly, the ability of the thumb to be positioned opposite the fingers. With the hand in this position, larger objects such as a stone or a heavy bottle can be held and moved in a controlled way. The heavier the weight and the smoother the surface is, the more strength is needed to hold and move the object.

The precision grip is important for moving small and delicate objects, for example when writing, sewing or drawing. When using the precision grip, the thumb and the index (“pointer”) finger work like tweezers: The thumb is opposite one or more fingertips, allowing the hand to grip even very small objects – like pencils or delicate instruments – in a controlled way.

The hand is supplied with blood through two main arteries: one on the same side as the thumb, and one on the same side as the little finger. These two arteries each curve around in an arch shape where the palm is, so that the hand is supplied with oxygen-rich blood through a double loop. These loops have branches of blood vessels leading off into the individual fingers. Each artery is accompanied by veins and nerves: In total, each finger is supplied by four bundles of nerves and blood vessels.

The muscles and skin of the hand are supplied by three nerves:

• the radial nerve (nervus radialis),
• the median nerve (nervus medianus), and
• the ulnar nerve (nervus ulnaris).

The ulnar nerve moves the muscles of the hypothenar eminence, the muscles between the bones of the metacarpus, the muscle that pulls the thumb inwards (adductor pollicis) and two of the lumbrical muscles between fingers. This nerve also picks up sensations from the area below the little finger and the side of the ring finger closest to it.

The median nerve is responsible for the movement of the thenar eminence muscles and the rest of the lumbricals. It is also responsible for the perception of skin sensations on the palm, thumb, index finger, and middle finger, as well as sensations on the side of the ring finger closest to the middle finger.

The radial nerve activates the finger extensors and the muscles in the wrist that are responsible for extending the hand. It also carries sensations from the skin on the back of the hand and the back of the thumb to the brain.

The fingers on each hand are bent and stretched about 25 million times over the course of a lifetime. Our hands also have very sensitive “antennae” for receiving information from the environment: There are a total of 17,000 touch receptors and free nerve endings in the palm. These pick up sensations of pressure, movement and vibration. So it is with good reason that the sense of touch is often associated with the hands. The skin on our fingertips is especially sensitive to touch.