The physics of anatomy

Chapter Five The physics of anatomy

Joint movement


In Chapter 4, we encountered a wide variety of joint types and also explored some ways in which the basic templates are modified from joint to joint to meet the specific needs of regional biomechanics. Now, we shall explore these variations and the strictures they place on movement in more detail.

Whilst we do this, there are two fundamental principles that we should keep in mind.


The image of ligaments acting like a sheet of powerful elastic to limit the motion of a joint and render it stable is one that is frequently presented to the student; in fact, the integral strength of individual ligaments does very little to support most joints. Stability actually comes from the far more powerful muscles and tendons, though the ligaments do have a crucial role to play.

Ligaments have a number of neurological structures embedded within them that fall under the broad heading of mechanoreceptors. Stretching of the ligament causes these receptors to be stimulated and send information to the central nervous system.

Some receptors such as Ruffini end organs and the receptors found in capsular ligaments and muscle spindles deliver continuous information about the relative position of muscles and joints and are used to facilitate balance, coordination and joint position sense. Other receptors such as Pacinian corpuscles are very much more rapidly adapting and react to stimuli with a sudden burst of high-frequency impulses, which quickly die away even when the stimulus is maintained. These signals are conducted to the postcentral gyrus of the central cortex via large-diameter, myelinated neurons that have high conduction velocities. Association fibres then run to the motor cortex; this means that if the ankle starts to invert signals run very rapidly to the central control systems, which send equally rapid messages to the evertor muscles of the ankle, causing them to contract and maintain the body’s balance.

Kinematic chains

The human body can be regarded mechanically as a series of rigid members or links (bones), connected by kinematic pairs (joints). A kinematic chain consists of a series of two or more members connected by a joint.

Functionally, there are two types of chain:

Closed loop chain

By contrast, if both ends of the chain are fixed, then individual kinematic pairs cannot move independently of each other; by moving one element of the chain, all other elements must move to a greater or lesser extent. There are many examples of closed loop chains in the human body including the thoracic spine and ribs; the pelvis; and the bones of the cranium. When both feet are on the ground, the lower extremities and pelvis form a closed loop chain – if you flex your knee, it is not possible to do so without moving both feet, the contralateral knee, both hips and the pelvis and lumbosacral junction.

Degrees of freedom

As we have seen previously, all joint motion can be described by rotation about three orthogonal axes (X, Y and Z) and translation in three orthogonal planes (XY, XZ, YZ). There are therefore three possible rotations and three possible translations; each one of these represents a degree of freedom.

For most joints, in the human body, translations are negligible and do not need consideration. Although a few joints do have pure translatory movements, this is usually very limited and almost all gross movement is by rotation alone.

When movement is limited to rotation about one axis, as it is in the phalanges of the fingers and toes, a joint is termed uniaxial: that is, it has just one degree of freedom. If independent movement can occur around two axes, it is biaxial, with two degrees of freedom. The first carpometacarpal joint is a good example of a biaxial joint: it can flex and extend; it can also adduct and abduct.

The shoulder and hip are both good examples of triaxial joints, being able to rotate about all three axes (flexion–extension; abduction–adduction; internal rotation–external rotation). Although these ball and socket joints can rotate about many chosen axes (and, for this reason, they are often termed multiaxial), for each position there will still be a maximum of three orthogonal axes, which means that there will still be three degrees of freedom.

As a rule, each class of synovial joint has a set number of degrees of freedom and these are given in Table 5.1. Plane (gliding) joints can translate, although these movements are usually heavily curtailed by ligamentous attachments. Usually plane joints such as the zygapophyseal, intertarsal and intercarpal joints have two degrees of translational freedom, although some authorities declare joint gapping to represent a third degree of freedom.


The study of movement occurring intrinsically between the articular surfaces of joints is called arthrokinematics. All joint surfaces are either concave or convex (even ‘plane’ joints are not completely flat) in a reciprocal fashion. The most obvious example of this is the spherical head of the femur sitting in the cup-shaped hemispherical concavity of the acetabulum.

Roll and glide

So far, we have considered joint movement in terms of pure rotation; of course, although this model is entirely acceptable from a functional standpoint, it does not reflect how joints actually work. If a joint were to rotate and only rotate, it would simply roll out of its socket and dislocate (Fig. 5.1A).

Obviously, this does not happen; instead, a combination of the ligamentous attachments, the line of force of the articular muscles and the lubricating effect of the synovial fluid allow the joint to roll and simultaneously slide (or glide), so that the joint moves in the desired fashion without losing its articular integrity (Fig. 5.1B).

A useful analogy is to consider a car tyre: as the tyre rolls it rotates along the road. If the surface is made slippery by ice or mud then, when the brakes are applied, the car skids: now it is continuing to travel along the road even though the tyre is stationary; this is gliding, which is a translatory movement. If the two movements are combined, for instance if the car is attempting to accelerate on ice and the wheels are spinning, then we have a combination of the two movements: the wheel is rotating forwards and translating backwards at an equal rate, the net effect is that the car remains stationary.


There is an additional intraarticular motion that we need to consider: spin. This is rotary motion of the distal part of the kinematic chain about the longitudinal axis of the proximal part – a concept better understood if visualized (Fig. 5.2).

Spin can occur alone or in conjunction with roll and glide: in pronation–supination of the forearm, it occurs independently; in flexion–extension of the knee it accompanies roll and glide and, as we shall subsequently discover, plays an important part in the knee’s stability.

When roll, glide and spin act together, it can have an interesting effect on joint motion. For example, we can externally rotate the shoulder by the simple action of resting our palm against the thigh and then turning the palm outwards so that it faces forwards; this motion is a combination of roll and glide. We can achieve the same effect by starting in the same position and then flexing the shoulder to 90°, horizontally abducting it by 90° and then adducting it back to our thigh (Fig. 5.3). The translation of the glide component, in conjunction with the rotation of the roll and spin components has caused an external rotation of 90°; the palm is now facing forwards, it is no longer resting against our thigh even though no specific rotation has taken place.


Assessing the ‘normal’ movement of a joint is an essential tool in the diagnostic armoury of the musculoskeletal clinician – it has a high level of inter- and intra-examiner reliability, particularly in these modern days of sophisticated goniometric measuring devices.

However, this begs the question of what constitutes normal: a general decline in flexibility of joints occurs with age, although this is much more noticeable in weight-bearing joints than in those of the upper extremity and, throughout life, females have greater flexibility than males.

This means that, say, right cervical rotation of 95° in a 21-year-old female and 55° in an 81-year-old male may both be perfectly normal. It is therefore a much more useful measurement when compared contralaterally (if the pensioner’s left cervical rotation was 70°, the right would no longer be regarded as normal) and for assessing outcome measures (does the range of motion equalize following treatment).

It is also important to differentiate between active range of motion and passive range of motion, which can be an important clinical indicator. For example, patients with shoulder pain often complain that their shoulder starts to hurt at around 80°–90° of abduction; however, when the physician moves their arm passively, no pain is felt. This difference between pain with muscular effort and no pain on passive joint motion strongly suggest a musculotendinous aetiology for the problem, most commonly a rotator cuff impingement syndrome.


In the same way that roll, spin and glide can be combined to produce new movements, flexion–extension and abduction–adduction can also be combined to produce a new type of movement: circumduction.

Circumduction comprises successive flexion, abduction, extension and adduction; it occurs when the distal end of a long bone circumscribes the base of a cone that has its apex at the joint in question, it is the action that we would make if we held our finger at arm’s length and used it to draw a circle in the air. Circumduction is also possible in the metacarpophalangeal joints, wrist, elbow, ankle, knee, hips and in the trunk and cervical spine.


The summative movements of the human body are part of what define us as human: although we have many similarities to other primates, the human skeleton has many distinctive differences to those of chimpanzees, gorillas or other primates. It is also distinct from other humans – homo sapiens neanderthalensis had easily recognizable morphological differences: a larger cranial capacity with many other differences in the skull’s anatomy; a longer, bowed femur with a shorter tibia and fibula; a more gracile pelvis; wider collar bones and ribs; and larger knee-caps.

These differences, presumed to have arisen from adaptation to living in Northern Eurasia during the last period of glaciation, suggest that Neanderthals would have had different biomechanical features to homo sapiens sapiens, modern man. Their thicker bones and shorter levers would have given them greater strength whilst their broad chests and shorter stature gave them a smaller surface area to volume ratio, more efficient at retaining body heat.

However, their joints would have been less flexible, their gait less efficient and their physiology required a high level of protein – Neanderthals were thought to be mainly carnivorous and thus more dependent on hunting for food. They suffered from many of the same conditions as we do today: gum disease, osteomyelitis, bone tumours and degenerative joint disease (osteoarthritis) – their heavier build and more robust morphology with an associated increase in joint loading made them particularly susceptible to this last condition.

One other interesting development in the evolution of human joints is the change in the sacroiliac joint, probably the most common source of low back pain. Although orthopaedic texts frequently suggest that this is as a result of the change from quadrupedal to bipedal status; in fact, it is much more to do with the broadening of the pelvis required for successful parturition of babies with large cranial vaults.

Successful parturition is a far more powerful selective force than locomotive considerations; its effects on reproductive success are immediate and profound. Humans have evolved circular pelvic midplanes to produce an adequate birth canal by modifying pelvic form in order to accentuate the area of the midplane; its original australopithecine form would be wholly inadequate by modern requirements.

The morphological changes that were effected between Australopithecus afarensis and Homo sapiens cannot have reflected improved mechanics: had the long femoral neck and pronounced lateral iliac flare been retained in the descendants of A. afarensis (along with a reduction in only the relative interacetabular distance in the pelvis), the mechanical advantage to the abductors in modern humans would now be far greater than it actually is. Therefore, the reduction of these mechanical benefits must have been the selective advantages in increasing the dimensions of the birth canal rather than a response to any change in gait pattern – in other words, our big brains are what give us our bad backs.

Joint features

Most of our consideration of joints so far has been generalized; however, there are specific joints that have special or unusual features or are of particular clinical interest and, in this section, we shall consider them in turn.

The lower extremity

As we have already seen, the lower extremity when in normal stance acts as a closed loop kinematic chain with each element interdependent to a greater or lesser degree.

The foot

As entire books have been written about the foot and an entire profession (podiatry) is dedicated to assessing and treating the pathomechanics of the area, it is self-evident that comprehensive analysis of the area is beyond the scope of this text: a quarter of all the bones in the body are found in the foot. However, there are certain key biomechanical points that need to be grasped if the fundamentals of lower kinematic chain function are to be understood.

The foot can be regarded as two connected closed loop kinematic chains. The forefoot has one longitudinal arch, situated medially, and two transverse arches. The more posterior of these crosses the longitudinal arch to produce the typical biconcave appearance of the posteromedial forefoot; it is this that is reduced in pes planus (flat feet). The anterior transverse arch lies underneath the metatarsal phalangeal joints; collapse of this is closely associated with pain in the area: metatarsalgia. The forefoot can pronate or supinate (to a limited extent) with respect to the hindfoot (talus and calcaneus).


The knee is the largest joint in the body. It is a remarkably stable joint; this ability to bear several times the body’s own weight arises from what happens when the joint is fully extended. To understand this, there is a new concept that needs to be introduced, the close-packed position (Fig. 5.4). The fit of reciprocal convex–concave surfaces is precise only at one end of the most common excursion of the joint, which is the key feature of the close-packed position. In all other positions the surfaces are not fully congruent, and the joint is said to be loose-packed.

As the knee approaches full extension, the femur rotates internally with respect to the tibia; the last 10° of extension is accompanied by approximately 6° of rotation. This rotation seats the femoral condyles in the rings of the medial and lateral menisci, also bringing the ligaments to tension.

So stable is this position that the knee cannot be flexed again unless the tibia is externally rotated by the action of the popliteus muscle; hypertonicity in this muscle can significantly disrupt normal knee mechanics and contribute to meniscal damage. Because of the natural valgus deformity of the knee, known as the Q-angle (10° in men, 15° in women), the medial meniscus has a higher degree of loading and is more vulnerable to damage and degeneration.

The upper extremity

We have already dealt with several joints in the arm in some detail; however, there are a couple of other wrinkles to the limb that we have not yet covered.


The shoulder is the most complicated joint in the body. It is in fact a complex of three joints (glenohumeral, acromioclavicular and sternoclavicular) and one articulation (the movement of the scapula across the superior seven ribs). It also contains over two dozen bursae, a similar number of ligaments and is controlled by a dozen or so muscles. Somewhat surprisingly, the whole complex is uniquely controlled by a single nerve root (C5) – every other joint has dual supply; the hip is controlled by three contiguous levels.

We have already discussed the effects of spin in the glenohumeral joint; however, there is another aspect of spin that probably passes unnoticed so automatically does it occur. When we fully abduct our arm, the greater tuberosity of the humerus (into which insert the tendons of supraspinatus, infraspinatus and teres minor) approximates to the acromion process and would, as the arm reaches 90° of abduction, impinge upon these tendinous insertions as they pass through the sub-acromial tunnel.

This does not happen, because the arm automatically spins through 180° of external rotation so that the greater tuberosity is moved out of the way – try performing the action without rotating the arm (use caution!) and see what happens.

The spine and pelvis

The spine and pelvis provide the conduit for the central nervous system and its synaptic connections with its counterparts in the peripheral nervous system. The dura mater anchors into the internal structures of the superior three cervical vertebrae and into the second sacral tubercle; it is also attached to the vertebrae in between by means of small, cruciform dentate ligaments.

The spine also acts as a shock absorber, cushioning forces transmitted upwards from the lower limbs and downwards from the body’s weight and weight bearing. These forces are absorbed and redistributed by several means:

The role of the normal spinal curves is to increase the resistance of the spinal column to compressive forces. This resistance can be quantified:


where R is the resistance to compression and Nc is the number of curves.

This means that a spine with no curves would have a relative resistance of one; a neonate, who has a single full-spine C-curve would have twice the resistance; however, an adult spine with its four curves has 10 times the compressive resistance of a straight column.


Apr 4, 2017 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on The physics of anatomy
Premium Wordpress Themes by UFO Themes
%d bloggers like this: