1 Basic sciences
All voluntary movement, including walking, results from a complicated process involving the brain, spinal cord, peripheral nerves, muscles, bones and joints. Before considering in detail the process of walking, what can go wrong with it and how it can be studied, it is necessary to have a basic understanding of three scientific disciplines: anatomy, physiology and biomechanics. It is hoped that this chapter will provide the rudiments of these subjects for those not already familiar with them, review the topic for those who are, and also provide a convenient source of reference material.
It is not the intention of this book to teach in detail the anatomy of the locomotor system, which is well covered in several other books (e.g. Gray’s Anatomy, 2009). The notes which follow give only an outline of the subject, but one which should be sufficient for an understanding of gait analysis. The anatomical names for the different parts of the body vary somewhat from one textbook to another, and as far as possible the most common name has been used. The section starts by describing some basic anatomical terms and then goes on to describe the bones, joints, muscles and nervous system. Although the arteries and veins are essential to the functioning of the locomotor system, they will not be described here since they generally affect gait only indirectly, through their role in providing oxygen and nutrients for the nerves and muscles and removing waste products.
The anatomical terms describing the relationships between different parts of the body are based on the anatomical position, in which a person is standing upright, with the feet together and the arms by the sides of the body, with the palms forward. This position, together with the reference planes and the terms describing the relationships between different parts of the body, is illustrated in Figure 1.1.
The anterior surface of the body is ventral and the posterior surface is dorsal. The word dorsum is used for both the back of the hand and the upper surface of the foot. The terms cephal (towards the head) and caudal (towards the ‘tail’) are sometimes used in place of superior and inferior.
Most joints have their largest amount of movement in the sagittal plane, although the coronal and transverse planes can be very important clinically. The directions of these motions for the hip and knee are shown in Figure 1.2 and for the ankle and foot in Figure 1.3. The possible movements are as follows:
1. Flexion and extension take place in the sagittal plane; in the ankle these movements are called dorsiflexion and plantarflexion, where the foot (distal segment) moves up or down relative to the tibia (proximal segment), respectively
3. Internal and external rotation take place in the transverse plane; they are also called medial and lateral rotation, respectively, the term referring to the motion of the anterior surface of the distal segment relative to the proximal.
1. Varus (adducted) and valgus (abducted), which describe an angulation of a joint towards or away from the midline, respectively, when viewed in the coronal plane. Therefore knock knees are in valgus and bow legs are in varus.
2. Pronation and supination, which are internal and external rotations of the hand about the long axis of the forearm. Pronation of both hands brings the thumbs together, supination brings the little fingers together (aide memoire: you can hold soup in your hands if the palms are upwards). Internal and external rotation of the foot about the long axis of the tibia has also been used to describe pronation and supination of the foot and ankle complex.
Terminology in the foot is often confusing and lacking in standardisation. This book has adopted what is probably the commonest convention (Fig. 1.3), in which the term pronation is used for a combined movement which consists primarily of eversion but also includes some dorsiflexion and forefoot abduction. Similarly, supination is primarily inversion, but also includes some plantarflexion and forefoot adduction. These movements represent a ‘twisting’ of the forefoot (distal segment), relative to the hindfoot (proximal segment). However, some authorities regard pronation and supination as the basic movements and eversion and inversion as the combined movements. Increasingly the foot is being modelled in multiple segments, most commonly into three or four segments although more have been used. This requires further referencing of the relative movement of the different segments, which will be dealt with later in the chapter.
It could be argued that almost every bone in the body takes part in walking. However, from a practical point of view, it is generally only necessary to consider the bones of the pelvis and legs. These are shown in Figure 1.4.
The pelvis is formed from the sacrum, the coccyx and the two innominate bones. The sacrum consists of the five sacral vertebrae, fused together. The coccyx is the vestigial ‘tail’, made of three to five rudimentary vertebrae. The innominate bone on each side is formed by the fusion of three bones: the ilium, ischium and pubis. The only real movement between the bones of the pelvis occurs at the sacroiliac joint and this movement is generally very small in adults. It is thus reasonable, for the purposes of gait analysis, to regard the pelvis as being a single rigid structure. The superior surface of the sacrum articulates with the fifth lumbar vertebra of the spine. On each side of the lower part of the pelvis is the acetabulum, which is the proximal part of the hip joint, being the socket into which the head of the femur fits.
The femur is the longest bone in the body. The spherical femoral head articulates with the pelvic acetabulum to form the hip joint. The neck of the femur runs downwards and laterally from the femoral head to meet the shaft of the bone, which continues downwards to the knee joint. At the junction of the neck and the shaft are two bony protuberances, where a number of muscles are inserted – the greater trochanter laterally, which can be felt beneath the skin, and the lesser trochanter medially. The bone widens at its lower end to form the medial and lateral condyles. These form the proximal part of the knee joint and have a groove between them anteriorly, which articulates with the patella.
The patella or kneecap is a sesamoid bone; that is to say, it is embedded within a tendon, in this case the massive quadriceps tendon, which below the patella is known as the patellar tendon. The anterior surface of the patella is subcutaneous (immediately below the skin); its posterior surface articulates with the anterior surface of the lower end of the femur to form the patellofemoral joint. The patella has an important mechanical function, which is to displace the quadriceps tendon forwards, thereby improving its leverage.
The tibia extends from the knee joint to the ankle joint. Its upper end is broadened into medial and lateral condyles, with an almost flat upper surface which articulates with the femur. The tibial tubercle is a small bony prominence on the front of the tibia, where the patellar tendon is inserted. The anterior surface of the tibia is subcutaneous. The lower end of the tibia forms the upper and medial surfaces of the ankle joint, with a subcutaneous medial projection called the medial malleolus.
The fibula is next to the tibia on its lateral side. For most of its length it is a fairly slim bone, although it is broadened at both ends, the upper end being known as the head. The broadened lower end forms the lateral part of the ankle joint, with a subcutaneous lateral projection known as the lateral malleolus. The tibia and fibula are in contact with each other at their upper and lower ends, as the tibiofibular joints. Movements at these joints are very small and will not be considered further. A layer of fibrous tissue, known as the interosseous membrane, lies between the bones.
The talus or astragalus is the upper of the two bones in the hindfoot. Its superior surface forms the ankle joint, articulating above and medially with the tibia and laterally with the fibula. Below, the talus articulates with the calcaneus through the subtalar joint. It articulates anteriorly with the most medial and superior of the midfoot bones – the navicular.
The calcaneus or os calcis lies below the talus and articulates with it through the subtalar joint. Its lower surface transmits the body weight to the ground through a thick layer of fat, fibrous tissue and skin – the heelpad. The anterior surface articulates with the most lateral and inferior of the midfoot bones – the cuboid.
A joint is the region where two bones come in contact with each other. From a practical point of view, they can be divided into synovial joints, in which significant movement can take place, and the various other types of joint in which only small movements can occur. Since gait analysis is usually only concerned with fairly large movements, the description which follows deals only with synovial joints. In a synovial joint, the bone ends are covered in cartilage and the joint is surrounded by a synovial capsule, which secretes the lubricant synovial fluid. Most joints are stabilised by ligaments, which are bands of relatively inelastic fibrous tissue connecting one bone to another. Fascia is a special type of connective tissue, being a continuous web of tissue found throughout the human body.
The hip joint is the only true ball-and-socket joint in the body, the ball being the head of the femur and the socket the acetabulum of the pelvis. Extremes of movement are prevented by a number of ligaments running between the pelvis and the femur, by a capsule surrounding the joint and by a small ligament – the ligamentum teres – which joins the centre of the head of the femur to the centre of the acetabulum. The joint is capable of flexion, extension, abduction, adduction, and internal and external rotation (Fig. 1.2).
The knee joint consists of the medial and lateral condyles of the femur, above, and the corresponding condyles of the tibia, below. The articular surfaces on the medial and lateral sides are separate, making the knee joint, in effect, two joints, side by side. The femoral condyles are curved both from front to back and from side to side, whereas the tibial condyles are almost flat. The ‘gap’ this would leave around the point of contact is filled, on each side, by a ‘meniscus’, commonly called a ‘cartilage’, which acts to spread the load and reduce the pressure at the point of contact.
4. The anterior cruciate ligament (ACL), in the centre of the joint, between the condyles; it is attached to the tibia anteriorly and the femur posteriorly. It prevents the tibia from moving forwards relative to the femur and also helps to prevent hyperextension of the knee as well as excessive internal rotation of the tibia
5. The posterior cruciate ligament (PCL), also in the centre of the joint, is attached to the tibia posteriorly and the femur anteriorly and prevents the tibia from moving backwards relative to the femur and also helps to limit external rotation of the tibia.
The anterior and posterior cruciate ligaments are named for the positions in which they are attached to the tibia. They appear to act together as what engineers call a ‘four-bar linkage’, which imposes a combination of sliding and rolling on the joint and moves the contact point forwards as the joint extends and backwards as it flexes. This means that the axis about which the joint flexes and extends is not fixed, but changes with the angle of flexion or extension. Pollo et al. (2003) challenged this description, saying that it only occurs in the unloaded knee and that during walking, the tibia moves backwards relative to the femur as the knee flexes.
In the normal individual, the motions of the knee are flexion and extension, with a small amount of internal and external rotation. Significant amounts of abduction and adduction are only seen in damaged knees. As the knee comes to full extension, there is an external rotation of a few degrees: the so-called automatic rotation or ‘screw-home’ mechanism.
The patellofemoral joint lies between the posterior surface of the patella and the anterior surface of the femur. The articular surface consists of a shallow V-shaped ridge on the patella, which fits into a shallow groove between the medial and lateral condyles. The principal movement is the patella gliding up and down in this groove, during extension and flexion of the knee, respectively. This causes different areas of the patella to come into contact with different parts of the joint surfaces of the femur. There is also some medial-lateral movement of the patella.
The ankle or talocrural joint has three surfaces: upper, medial and lateral. The upper surface is the main articulation of the joint; it is cylindrical and formed by the tibia above and the talus below. The medial joint surface is between the talus and the inner aspect of the medial malleolus of the tibia. Correspondingly, the lateral joint surface is between the talus and the inner surface of the lateral malleolus of the fibula.
The major ligaments of the ankle joint are those between the tibia and the fibula, preventing these two bones from moving apart, and the collateral ligaments on both sides, between the two malleoli and both the talus and calcaneus, which keep the joint surfaces in contact. The ankle joint, being cylindrical, has only one significant type of motion – dorsiflexion and plantarflexion – corresponding to flexion and extension in other joints.
The subtalar or talocalcaneal joint lies between the talus above and the calcaneus below. It has three articular surfaces: two anterior and medial and one posterior and lateral. Large numbers of ligaments join the two bones to each other and to all the adjacent bones. The axis of the joint is oblique, running primarily forwards but also upwards and medially. From a functional point of view, the importance of the subtalar joint is that it permits eversion/inversion (abduction and adduction or a valgus/varus motion) of the hindfoot. When performing gait analysis, it is usually impossible to distinguish between movement at the ankle joint and that taking place at the subtalar joint and it is reasonable to refer to motion taking place at the ‘ankle/subtalar complex’. This motion in normal individuals includes dorsiflexion/plantarflexion, hindfoot abduction/adduction, and internal/external rotation about the long axis of the tibia.
The mid tarsal joints lie between each of the tarsal bones and its immediate neighbours, making for a very complicated structure. The movement of most of these joints is very small, as there are ligaments crossing the joints and the joint surfaces are not shaped for large movements. As a result, the mid tarsal joints may be considered together to provide a flexible linkage between the hindfoot and the forefoot, which permits a small amount of movement in all directions.
The tarsometatarsal joints, between the cuboid and the cuneiforms proximally and the five metatarsals distally, are capable of only small gliding movements, because of the relatively flat joint surfaces and the ligaments binding the metatarsals to each other and to the tarsal bones. There are also joint surfaces between adjacent metatarsals, except for the medial one.
The metatarsophalangeal and interphalangeal joints consist of a convex proximal surface fitting into a shallow concave distal surface. The metatarsophalangeal joints permit abduction and adduction as well as flexion and extension; the interphalangeal joints are restricted by their ligaments to flexion and extension, the range of flexion being greater than that of extension. In walking, the most important movement in this region is extension at the metatarsophalangeal joints.
No description of the anatomy of the foot is complete without a mention of the arches. The bones of the foot are bound together by ligamentous structures, reinforced by muscle tendons, to make a flexible structure which acts like two strong curved springs, side by side. These are the longitudinal arches of the foot and they cause the body weight to be transmitted to the ground primarily through the calcaneus posteriorly and the metatarsal heads anteriorly. The midfoot transmits relatively little weight directly to the ground because it is lifted up, particularly on the medial side. The posterior end of both arches is the calcaneus. The medial arch (Fig. 1.6) goes upwards through the talus and then forwards and gradually down again through the navicular and cuneiforms to the medial three metatarsals, which form the distal end of the arch. The lateral arch (Fig. 1.7) passes forwards from the calcaneus through the cuboid to the lateral two metatarsals.
Muscles are responsible for movements at joints. Most muscles are attached to different bones at their two ends and cross over either one joint (monarticular muscle), two joints (biarticular muscle) or several joints (polyarticular muscle). In many cases the attachment to one of the bones covers a broad area, whereas at the other end it narrows into a tendon, which is attached to the other bone. It is usual to talk about a muscle as having an ‘origin’ and an ‘insertion’, although these terms are not always clearly defined. Ligaments and tendons are obviously similar and frequently confused. As a general rule, ligaments connect two bones together, whereas tendons connect muscles to bones.
The following is a brief account of the muscles of the pelvis and lower limb, including their major actions. Most muscles also have secondary actions, which may vary according to the position of the joints, particularly with biarticular muscles. The larger and more superficial muscles are illustrated in Figure 1.8.
1. Psoas major originates from the front of the lumbar vertebrae. Iliacus originates on the inside of the pelvis. The two tendons combine to form the iliopsoas, inserted at the lesser trochanter of the femur; the main action of these two muscles is to flex the hip.
5. Quadratus femoris, piriformis, obturator internus, obturator externus, gemellus superior and gemellus inferior originate in the pelvis and insert close to the top of the femur; they all externally rotate the femur, although most also have secondary actions.
Internal rotation of the femur was not mentioned in the above list; it is achieved as a secondary action by gluteus medius, gluteus minimus, psoas major, iliacus, pectineus and tensor fascia lata (described below).
1. Rectus femoris originates from around the anterior inferior iliac spine of the pelvis and inserts into the quadriceps tendon; it flexes the hip, as well as being part of the quadriceps, a group of four muscles which extend the knee.
2. Tensor fascia lata originates from the pelvis close to the anterior superior iliac spine and is inserted into the iliotibial tract, a broad band of fibrous tissue which runs down the outside of the thigh and attaches to the head of the fibula. The muscle abducts the hip and the knee.
3. Sartorius is a strap-like muscle originating at the anterior superior iliac spine of the pelvis and winding around the front of the thigh, to insert on the front of the tibia on its medial side; it is mainly a hip flexor.
4. Semimembranosus and semitendinosus are two of the hamstrings; both originate at the ischial tuberosity of the pelvis and are inserted into the medial condyle of the tibia; they extend the hip and flex the knee.
5. Biceps femoris is the third hamstring; it has two origins – the ‘long head’ comes from the ischial tuberosity and the ‘short head’ from the middle of the shaft of the femur. It inserts into the lateral condyle of the tibia and is a hip extensor and knee flexor.
1. Vastus medialis, vastus intermedius and vastus lateralis are three elements of the quadriceps muscle. They all originate from the upper part of the femur, on the medial, anterior and lateral sides, respectively. The fourth element of the quadriceps is rectus femoris, described above. The four muscles combine to become the quadriceps tendon. This surrounds the patella and continues beyond it as the patellar tendon, which inserts into the tibial tubercle. Quadriceps is the only muscle which extends the knee.
1. Gastrocnemius originates from the back of the medial and lateral condyles of the femur; its tendon joins with that of the soleus (and sometimes also the plantaris) to form the Achilles tendon, which inserts into the back of the calcaneus. The main action of these muscles is to plantarflex the ankle, although the gastrocnemius is also a flexor of the knee.
1. Soleus arises from the posterior surface of the tibia, fibula and the deep calf muscles. Its tendon joins with that of the gastrocnemius (and sometimes plantaris) to plantarflex the ankle. The soleus and gastrocnemius together are called the triceps surae.
2. Extensor hallucis longus, extensor digitorum longus, tibialis anterior and peroneus tertius form the anterior tibial group. They originate from the anterior aspect of the tibia and fibula and the interosseous membrane. The former two are inserted into the toes, which they extend; the latter two are inserted into the tarsal bones and raise the midfoot on the medial side (tibialis anterior) or lateral side (peroneus tertius). Tibialis anterior is the main ankle dorsiflexor; the others are weak dorsiflexors.
3. Flexor hallucis longus, flexor digitorum longus, tibialis posterior, peroneus longus and peroneus brevis are the deep calf muscles and all arise from the back of the tibia, fibula and interosseous membrane. The former two are flexors of the toes; the peronei are on the lateral side and evert the foot; tibialis posterior is on the medial side and inverts it. All five muscles are weak ankle plantarflexors.
2. Flexor digitorum brevis, abductor hallucis and abductor digiti minimi form the superficial layer of the sole of the foot; they flex the toes and abduct the big toe and the little toe, respectively.
The spinal cord is an extension of the brain and plays an active role in the processing of nerve signals. Like the brain itself, it consists of white matter, which is bundles of nerve fibres, and grey matter, which contains many cell bodies and nerve endings, where the synapses (connections) between nerve cells take place. The spinal cord lies within the spinal canal, which is formed in front by the vertebral bodies and behind by the neural arches of the vertebrae (Fig. 1.9). The vertebrae are divided into four groups: cervical (7 vertebrae), thoracic (12), lumbar (5) and sacral (5). It is usual to use the abbreviated names, for example, the fourth thoracic vertebra is known as ‘T4’.