Anatomy of Grip
The human hand is a miracle of evolution. Our elongated thumb is able to oppose our fingers, and hence manipulate objects and instruments with a far greater degree of precision than primates and other animals. The function of the hand is to grip, grasp and form precise movements, e.g. writing and sewing. This article will look at the anatomy of grip, including the different types of grip and the anatomical structures involved. In addition we will look at general movements, e.g. striking a ball with a baseball bat, carrying shopping bags, etc., that will place the types of grip into a real life context. Finally, we will discuss wrist biomechanics, clinical conditions affecting grip and functional relevance.
Components and actions
In order to understand the components of grip, it is best to break down the movements of the individual fingers into component parts.
The flexor retinaculum (the roof of the carpal tunnel) prevents bowstringing of the tendons of the long finger flexors, and the annular and cruciate pulleys of the fingers have a similar function, i.e. to keep the tendons close to bones to enable effective flexion.
Powerful grip is possible when the wrist is held in some degree of extension. Any muscles that lie in the extensor compartment of the forearm, and cross the wrist, will be able to contribute to wrist extension. These muscles are:
- extensor digitorum (forms the extensor hood of each finger)
- extensor indicis (an extra tendon for the index finger)
- extensor carpi radialis longus
- extensor carpi radialis brevis
- extensor carpi ulnaris
All these muscles are innervated by the radial nerve (or its branches), which arises from the posterior cord of the brachial plexus, runs in the spiral groove of the humerus with the profunda brachii artery, and divides within the supinator muscle into a deep and a superficial branch.
The carpus is also designed to increase the tension in the tendons of the fingers (both extensors and flexors) and optimise these movements. The proximal row (lunate, triquetrum, pisiform) are functionally separate from the distal row (trapezium, trapezoid, capitate, hamate), with the scaphoid bridging the two rows on the radial side. Flexion initially occurs between these two rows, and eventually the scaphoid acts as the bridge between the two rows, so that in the final stage of flexion the whole carpus is flexing. The same occurs in extension, which is the more functionally useful position for grip.
Finger Flexion and Extension
Finger flexion is required to varying degrees in all the types of grip described below. Wrist flexion does not play a role in any form of powerful grip. The proximal interphalangeal joints of fingers 2-5 are flexed by the flexor digitorum superficialis (FDS). This muscle is innervated by the median nerve. The tendon of FDS runs deep to the flexor retinaculum within the carpal tunnel, and divides to attach onto the lateral aspects of the middle phalanges. This creates a space called Camper’s chiasm at the level of the proximal phalanges, through which the single tendon of flexor digitorum profundus (FDP) passes. The tendon of FDP also runs in the carpal tunnel, but inserts into the distal phalanx of each finger. It causes flexion of the proximal and distal interphalangeal joints. The lumbrical muscles (which arise from the tendons of the flexor digitorum profundus muscles, and insert into the radial aspect of the extensor hood of each finger), also cause metacarpophalangeal joint flexion, as they pass anterior to the joint.
Extension of the fingers has less of a role to play in grip than finger flexion. Metacarpophalangeal extension is caused by extensor digitorum. Extension of the interphalangeal joints is caused by the lumbrical muscles, as they insert into the extensor hood of each finger.
Flexion of the thumb is performed by two major muscles: flexor pollicis longus that flexes the interphalangeal joint, and flexor pollicis brevis that flexes the first metacarpophalangeal joint. Thumb extension and abduction occur due to extensor and abductor pollicis longus, extensor and abductor pollicis brevis. Opposition occurs due to opponens pollicis while adduction is performed by adductor pollicis.
Types of Grip
This grip is formed by full flexion of the fingers into the palm, and flexion of the thumb, to lie outside the palm. The hammer grip is useful when considered in conjunction with the ulnocarpal joint. The ulna bone does not directly articulate with the carpals. Instead it articulates with the triangular fibrocartilaginous complex (TFCC). This complex is composed of a pad of triangular fibrocartilage in association with the ulnocarpal ligaments and radiocarpal ligaments. The TFCC covers the head of the articulating surface of the ulna, transmits the load of the ulnocarpal joint to the forearm, and enables the rotation of the radius and the ulna in pronation and supination by supporting the ulnar region of the carpus. The hypothenar eminence (flexor digiti minimi, abductor digiti minimi and opponens digiti minimi) contracts and produces the diagonal shaped palmar gutter in which sticks, hammers and bats lie. The palmaris brevis muscle lies over the hypothenar eminence, and toughens it when we are striking objects, and therefore protects from trauma.
When someone strikes an object with a hammer, they forcibly extend their elbow (with the contraction of the triceps, mainly the lateral head, which is largely composed of type 2b fibres and hence contracts with power). In the terminal part of elbow extension before the object is struck, there is a short quick period of ulnar deviation i.e. adduction. This is a whip like movement that causes powerful striking of an object. If the wrist was not able to deviate in this way, the force of the hammer strike would be far less, and repetitive hammer striking would place extreme load on the carpus, yielding bone degeneration and injury within a few hammer strikes. The TFCC dissipates this load, and therefore protects the carpus from the force of the hammer strike.
Baseball batter grip
This is another form of grip that relies on the diagonal shaped palmar gutter in which an object lies. Thenar and hypothenar eminences provide the borders to this gutter. Two hands tightly grip the handle of the bat, with the thumb lying parallel outside the gripped hand. When a baseball bat is used to hit a ball, the entire arm works as a unit. If we imagine a baseball batter at the mound, he stands with his elbows and knees bent, and the bat held behind his head, ready to be swung forwards to strike the ball.
The abdominal muscles (external oblique and rectus abdominis) are tense, and the legs are poised providing a firm foundation for the upper limbs and trunk to rapidly twist and extend into the swing of the bat. The shoulder joint moves next, with the rotator cuff muscles (infraspinatus, supraspinatus, teres minor and subscapularis) contracting to keep the head of the humerus within the glenoid fossa. The acromial fibers of deltoid are forcibly contracting, to generate velocity. Next the triceps contracts, forcibly extends the elbow and with the movement of the shoulder, generates an increase in velocity. The whip like movement of the arm is completed with the ulnar deviation (adduction) that is the final element of the bat swing. The various regions of the upper limb are able to generate their own moments or torque forces (force around a pivot), which accumulate during the bat swing and are able to generate the fastest hand movement before the ball is struck.
Precision grip (tip to tip)
The tip to tip connection of the fingers is caused by flexion of the fingers, with flexion and opposition of the thumb. This grip can be used for manipulation of small objects, e.g. sewing. When an individual is sewing, they hold the needle between the pulp of their index and thumb. The needle is passed through the cloth and is brought through the other side after forming a loop or stitch. The high level of dexterity required for this task is an example of the minute level of control the human hand possesses.
If we look at the cortical homunculus representing the primary motor cortex, the hand occupies a disproportionately large area. It forms an inverted Omega-shaped area (Omega sign), and is easy to locate anterior to the central sulcus that separates the primary motor cortex from the primary sensory cortex. The pulp of the thumb comes into contact with the pulp of the finger. The highly sensitive region of the distal third of the digit provides a highly accurate localization of objects, e.g. rolling an object between our fingers. If we imagine a blind person reading braille, with practice they are able to read at the same pace as people with sight. The pulp of the finger is so sensitive; they are able to delineate the raised dots on the surface of the text, with amazing speed and precision. Just like the primary motor cortex, the hand occupies a disproportionately large area of the primary sensory cortex.
This kind of grip can be used for holding keys. The key is held between a flexed thumb and the radial surface of the middle phalanx of the index finger. This is a surprisingly powerful grip, and depends on the function of the elongated human thumb. The thumb in our species is highly mobile, strong and long. We are therefore able to do a great deal more with our hands when compared to primates. The muscles that move the thumb are divided into short and long muscles. If we study the muscles of the thenar eminence, we have the abductor pollicis brevis, the flexor pollicis brevis and the opponens pollicis. The adductor pollicis adducts the thumb but is not a muscle of the thenar eminence. The first three of these muscles are innervated by the recurrent motor branch of the median nerve (C5-T1). This is therefore the branch that defines the amazing ability of our hand.
The thumb is not involved in this grip. The other fingers are flexed, and usually carrying a weight, e.g. shopping bags. The corresponding metacarpophalangeal joints are flexed, with the proximal and distal interphalangeal joints also flexed to create a hook.
Tripod (pen) grip
This is the grip we use when holding a pen. The thumb is brought into opposition with an index and a middle finger that are flexed at the metacarpophalangeal joint, slightly flexed at the proximal interphalangeal joint, and extended at the distal interphalangeal joint. The ring finger and little finger are also flexed at the same joints, and lie against the paper when the hand is writing to provide support and a foundation for the movements of the middle and index fingers. Our ability to write has advanced our civilization greatly.