Introduction to the Upper Limb

Anatomical basics

The hand and forearm are made up of 29 independent bones, 38 muscles, and 3 major nerves. The muscles form the active unit, but there are many passive structures, like tendons, ligaments, joint capsules, and pulleys. All these components work with the brain, as a unit, performing tasks and reacting within fractions of a second.

The human arm is built like a tree, branching out and becoming more complex as you get closer to the tips. The biggest and strongest bone of the bone of the arm, the humerus, connects the arm to the main body at the shoulder joint. Two bones, the radius and ulna, form the skeleton of the forearm and can be manipulated to work against each other. The twisting of the forearm, the act of facing ones palms up (supination) and facing ones palms down (pronation) is done by rotating the radius around the ulna. The ulna forms a hinge joint (like a hinge in a door) with the humerus, at the elbow, and therefore cannot rotate. Running between the radius and the ulna is a tendon-like membrane (interosseous membrane) that distributes forcess between the two bones. For example, when doing push-ups the radius bears 80% of the load while the ulna bears the other 20%.

The wrist (carpus) is the connection between the radius and ulna and the bones of the hand. The wrist is an egg-shaped joint that allows motion in four directions – flexion, extension, and side flexion to the thumb (radial deviation) and to the side of the fingers (ulna deviation).

The eight wrist (carpal) bones are located in two rows of four that form parallel arches. Many short and tight ligaments connect these carpal bones. The apex of these arches is on the dorsal the arch is supported by a very strong palmar ligament called the flexor retinaculum, with the carpal tunnel being the space formed within that arch.

Running inside this carpal tunnel are the nine flexor tendons of the fingers and the median nerve. Though all eight carpal bones have multiple joint facets with their neighbours, they have a limited amount of movement, the flexion and extension of the hand is done at the wrist joint and not in the small joints between the carpal bones.

The five middle hand bones, known as the metacarpals, form the main part of the hand (i.e. the palm and back of the hand). These bones are attached to the second row of carpal bones in the wrist via extremely tight ligaments, Starting with the index finger metacarpal, which has a range of motion of 2-3 degrees of mobility, the range of motion increases in each joint down to the little finger, which has a range of motion of 15-30 degrees. This range of motion allows the hand to forma bowl that can be used for holding large round objects, like a ball, or making a cup to gather water.

The base finger joints form the connections between the middle hand bones (metacarpals) and the thumb and four fingers. The index, middle, ring and little fingers are made up of three different bones. These bones, known as phalanges, are the base (proximal) phalanx, the middle phalanx, and the tip (distal) phalanx. The base finger joint, in addition to allowing flexion and extension movement, also allows small side to side movements through the flexing of small muscles in the hand (this for example prevents water slipping through the fingers when ‘cupping’ described above.

The thumb is actually quite different from the long fingers.

For starters it lacks the middle phalanx, which makes it shorter. However, the joint where the thumb meets the hand, the carpo-metacarpal joint, commonly known as a saddle joint has a far greater range of motion than the equivalent joints in the four fingers. In conjunction with the small muscles in the hand, this saddle joint allows the thumb to touch the other finger tips (opposition).

Opposition is essential for much of how we utilise our hands, as when using a pen, opening a door, or eating with knife and fork. This movement sets us apart from other primates.

The middle and end finger joints, known as the proximal and distal interphalangeal joints allow only motion in the flexion and extension directions. Over extension and side-to-side motion is prevented by a very tight palmar joint capsule (the volar plate) and by collateral ligaments on the sides of the finger.

The joint capsule that encloses the joint has two basic functions. It serves as a stabiliser that protects the joint against outside forces that would over stress the joint in any direction. Also, the joint capsule produces a fluid known as synovial fluid that helps move nutrients into the cartilage and keeps the joint lubricated. This process takes place though flexing of the joint. The more flexing, the more fluid is created. However, overuse of the fingers can produce an excess of synovial fluid, which then stretches the joint capsule, which is then transferred to the brain via nerve endings as pain.


The biggest and most important muscles for flexion and extension of the fingers are in the forearm; these muscles are responsible for both the high range of motion of the hand and fingers as well as those motions that require a lot of power. These muscles span much of the arm, with some originating as far away as the upper arm and relaying their power all the way down through the elbow, wrist, and finger joints to the fingertips. The lower part of the forearm, where the arm is harder and thinner, is made up of tendons and bone. The tendons for the flexor muscles form roughly in the middle of the arm and lead down the fingers. When flexing the wrist joint on the palmar side some of the tendons can be identified independently. Add some finger movement to the fingers and one can see the forearms relaying the movement of the finger through these long tendons. All put together, there are over 40 muscles on the forearm contributing to the control of the hand and fingers.

Next to the flexor and extensor muscles there are muscles responsible for the internal and external rotation of the forearm (supination and pronation). The muscles that control supination lie in the extensor group while the muscles for pronation and on the underside or underneath the flexor muscle group. The biceps muscle is also a strong supinator of the forearm.

The origin of the flexor and extensor muscles is partly in the forearm and partly in the upper arm on the medial and lateral sides of the humerus (the medial and lateral epicondyle). The epicondyles are bony prominences that grow away from the bone to increase the mechanical advantage of the muscles. The inside (medial) epicondyle is often referred to as the ‘Funny Bone’.

The flexors are at work when making a fist or doing any kind of gripping. The extensor muscles are mainly used to counteract the motions of the flexors, as in opening the hand from a hold or releasing a fist.

Each finger has one superficial and one deep flexor muscle for its operation. Since those muscles are separate units, each finger actually has two flexor tendons. The tendon of the superficial flexor forks in the finger and attaches at two points on the middle phalanx. The tendon of the deep flexor (profundus) runs through the attachments of the superficial flexor and attaches to the distal phalanx. The division of the superficialis and profundus makes it possible to separately move the middle and last digit of the finger.

Flexors strongest with wrist extended, as in this position the fingers naturally want to form a hook, a position that creates a pre-tension of the finger flexors and allows the extensors to apply some force into the hand.

As the extensors weaken, the wrist goes into a more neutral position requiring even more force from the flexors.

Poor balance between extensors and flexors can account for insertional point tendonitis e.g. lateral epicondylitis of the extensor muscles (‘tennis elbow’).

The movement of the fingers is a complex and highly coordinated act using many different muscles and tendons. To flex a straightened finger the simplest of motions, the intrinsic muscles must begin by flexing the base joint slightly. After this initial action of the intrinsic muscles, the superficial and profundus flexors can begin working simultaneously to complete the movement.

In the functional (muscle with tendon) the weakest points are in the tendon and tendonous origin of the unit (where it meets bone). When activated, the muscles get a very high level of blood flow, while the tendon receives very little blood flow. Also, as a muscle grows through training, either by endurance or power training, the number of small blood vessels delivering nutrients to the muscle actually increases in the muscle body. This is known as hypertrophy.

The tendons and ligaments do not grow at the same rate and do not have the same level of vascular flow, so they do not get the same level of nutrients. The result can mean a stronger muscle relative to the tendon.

Another problematic area is the insertion point of the tendon. Because different materials are being attached together, and as the stresses created by the muscles are focused into a smaller point, these areas tend to suffer more injuries than the rest of the system. A tighter and less limber muscle will add to the potential for overuse syndromes.

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