Bone is a living vascular structure, composed of organic tissue i.e. cells, fibres, vessels, nerves (about 35% of a bone’s weight) and mineral calcium compound (about 65% of a bone’s weight)). Bone functions as a support structure, a site of attachment for skeletal muscle, ligaments, tendons and joint capsules, and a significant site of blood cell development for the entire body.
The skeleton provides the potential for human movement. The muscular system provides the force necessary for motion to occur. The potential for movement exists because the skeleton is made up of an intricate series of bones which articulate (join together) with each other at the joints. The relative range of motion at each joint is limited or restricted. The restriction may be the bony structure itself, as a result of the ligaments, which are attached to the bones or the muscles surrounding the joints. Each joint has a combination of these restrictions (bony, ligaments or muscular), but one type of restriction may predominate.
There is an inverse relationship between structural stability and mobility. As mobility increases, stability decreases and vice versa. The dancer may be unable to overcome existing restrictions, depending on their nature. For example, ligaments are pliable, but they are also very tough and inextensible. One should not want to stretch a ligament. Once a ligament has been lengthened, the joint which it supports no longer has its original stability. One can, however, increase mobility by gradual, developmental lengthening of muscle and tendonous tissue. Stability can also be increased by identifying, engaging and strengthening certain key muscles, which support joints or certain actions.
The foot is a mobile, weight-bearing structure. The ankle joint (hinge type joint ) between tibia, fibula and the talus basically permits only flexion (plantar flexion ) and extension (dorsiflexion ).
With excessive rotation of the joint, characteristic fractures and torn ligaments occur. The ankle has strong medial ligamentous and weaker lateral ligamentous support. (Lateral ligament strain and sprain are more common than medial.)
Because of the architectural efficiency of the two arches (long and transverse), it is possible to support the weight of the body and give impetus for locomotion. The great toe is designed to carry the majority of the weight because it has the largest metatarsal and sesamoid bones, which provide shock absorption for that area.
The relative length of the bones has an effect on the efficiency of the propulsive mechanism. The relative length of the ligaments (looseness or tightness) has an effect on the stability and range of motion of the foot. The relative width of the foot has an effect on the balance.
Pronation and supination are actions of the tarsus, which lead to problems with alignment. In dance terms, pronation is called bevelling the foot and supination is called sickling the foot. A sickled foot, when on releve or half toe, is a compensatory action to get the four lateral metatarsals on the floor to broaden the base of support. This increases the risk of severe sprain. Alignment of the tarsus is critical for the dancer. It takes stress off the knees, prevents injuries to the tarsus ankle region and ensures more efficient take-off and landings.
Another common problem associated with alignment of the feet is Morton’s Short Toe. This is where the weight is shifted into the second metatarsal, hallux valgus (misalignment of the first toe). This may be a result of a young dancer doing pointe work too soon, bunions and flat feet. These conditions all reduce the efficiency of movement and increase the risk of injury and experience of pain.
In the foot there are some individual differences, which are particularly important to dancers. The relative length of bones has an effect on efficient propulsion; the relative width of the foot has an effect on balance. The relative length of ligaments (looseness or tightness) has an effect on both stability and range of motion of the foot. The relative length of the metatarsals also affects the stability and alignment of the foot, particularly in releve. Morton’s Short Toe, where a short first metatarsal causes supination of the tarsus in order to get the four lateral metatarsals on the floor to broaden the base of support, increases the likelihood of ankle sprains.
Hallux Valgus is a condition in which the two phalanges of the first toe are angled towards the lateral side. This condition can be caused or exacerbated by pointe work (particularly in young dancers).
The alignment of the tarsus is critical for efficient and safe working of the foot. Two checks for alignment of the tarsus are the vertical path of the Achilles tendon and the aligning of the medial aspect of the first metatarsal (the tuberosity of the navicular and medial aspect of the calcaneous).
The structure of the lower leg contains two bones: the tibia and the fibula. The tibia, the larger of the two, is on the medial side and the fibula on the lateral side. The tibia is the primary weight-bearing bone of the lower leg. The fibula has no role in weight bearing and does not articulate with the knee joint; its role seems to be associated with expanding the bone surface for muscle attachments and forming the lateral completion of the ankle joint.
Strengthening exercises for muscles on the medial and lateral side of the ankle (tarsus) may alleviate misalignment due to the shape of the talus, which produces pronation or supination.
Another bony difference that can affect a dancer’s alignment is tibial torsion, a simultaneous bowing and twisting of the tibia increasing the likelihood of pronated feet.
Two common variances in knee structure are knock-knees and bowlegs. Knock-kneed dancers should be allowed to stand with their feet slightly apart in closed parallel so as to allow space for the knees without having to make compensations. They are more susceptible to injury on the medial side of the knee. Bowlegs tend to transfer weight onto the lateral side, which is more stable; it is not quite as risky as knock-knees, but the line of the leg may be distorted in parallel.
Another variation in knee structure is the placement of the tibial tuberosity (the site of insertion of the quadriceps) relative to the centre of the knee joint, most commonly lateral to the centre of the knee. The patella is pulled laterally and instead of riding in the patellar surface of the femur, actually rides over the lateral condyle of the femur. This irritation may cause a build-up of bony scar tissue, which results in grating during flexion and extension (chondromalacia).
Inflammation or irritation of the tibial tuberosity is called Osgood Schlatters disease, a condition common in adolescents as they grow.
The knee is structured by the articulation of the femur and the tibia and is encased in a joint capsule. The patella (kneecap) is encased in the tendon of the quadriceps, and while not technically part of the knee joint, protects the tissue within the joint anteriorly. The motion at the knee joint is minimal, the stability and restrictions of actions are provided by a complex bone and ligament structure.
There are six major ligaments, which support the knee joint: two collateral (on the medial and lateral sides), two cruciate (“crossing”) ligaments and the popliteal ligaments. Together with the menisci (medial and lateral cartilages), these structures provide the stability of the kneejoint.
When the knee is extended, the collateral ligaments are taut (tight), and when the knee is flexed, these ligaments are slightly slack. This explains the individual slight degree of inward and outward rotation that is possible in the flexed position, necessitating that care be taken with the rotation of the femur and alignment of tibia or fibula to ensure correct alignment in turned-out plies.
Rotary action on the knee joint places great strain on the collateral and cruciate ligaments and cartilage, thus increasing the likelihood of injury.
The pelvic structure is made up of the sacrum and the right and left os-coxae (hipbones); it is a complex three-dimensional structure. The os-coxae are made up of the ileum, the ischium and the pubis. The pelvis is actually shaped like a bowl, with the sacrum forming the back of the bowl, the ileum forming the sides of the bowl, the ischium forming the bottom and the pubis forming the anterior portion (which is lower than the back and sides). Functionally the pelvis is a solid unit, with movement of one part of the bowl causing movement of the whole bowl. Three joints allow movement of the pelvis: the articulation of the fifth lumbar vertebra with the sacrum, and the two hip joints where the femurs articulate with the two sides of the pelvis at the acetabula.
Pelvic alignment is fundamental to an individual’s efficient action at the hip joint and the lumbar spine. It can be assessed from two views: from the front and from the side. From the front the two spines of the ileum should be equidistant from the floor. From the side, the spines of the ileum and ischium, respectively should be on the same horizontal plane; and the pubic symphysis should be on the same vertical plane as the spine of the ileum.
Over the years, turnout at the hip (rotation of the femur in the acetabulum) has been over-emphasised. The “perfect” 180-degree turnout is generally unachievable for a large proportion of the population. Dancers with limited natural turnout, because of their bony structure, should not force their turnout. The more lateral the acetabular facing, and the greater the range of outward rotation, the more restricted is the turnout. The restriction of the rotation at the hip is also muscular in nature. Tightness of inward rotators restricts outward rotation, as does weakness of the outward rotator muscles.
The vertebral column structure is made up of the coccyx (actually a fusion of bones), the sacrum (also a fusion of bones), five lumbar vertebrae, twelve thoracic vertebrae and seven cervical vertebrae. The two uppermost vertebrae (the atlas and the axis) support the skull and are modified in shape to perform the task. The rib cage (twelve ribs on each side) articulates with the twelve thoracic vertebrae.
The natural curves of the spine allow the efficient bearing of weight above. (For example, the lumbar curve shifts the supporting column anteriorly so that it can efficiently support the weight of the rib cage and thorax above it.) A heavy individual will generally have deeper spinal curves and a slender person shallower curves.
Actions of the spine occur on the sagittal, frontal and transverse planes. Flexion, extension and hyperextension are movements on the sagittal plane. Right and left rotation are movements on the transverse plane. Right and left lateral flexion are movements on the frontal plane. The relative shape and size of the bony landmarks of the spine have an effect upon the possible actions in the three regions of the spine. The lumbar vertebrae allow flexion extension, hyperextension and lateral flexion but have minimal rotation. In the thoracic spine, the movements of lateral flexion and rotation are quite free. Flexion and extension are possible but extension is limited to 180 degrees (i.e. no hyperextension) because of the downward-projecting spinous processes. In the cervical spine, all movements are quite free, including flexion, extension, hyperextension, rotation and lateral flexion.
Spinal architecture is extremely complicated, incorporating bony, cartilaginous, ligamentous and muscular structures. The analogy of trying to balance a pole on its end is relevant. While the pole is vertical, it requires very little effort to maintain its balance. When the pole is tipped slightly off-centre, it requires effort to maintain balance. When the spinal curves are accentuated chronically or misaligned, the supporting muscular and ligamentous structures are under stress. There is potential for other postural problems, limited movement and risk of traumatic and chronic injury.
Variations in the spine and differences in movement potential occur because of the shape of vertebrae and the curves of the spine. Common deviations from ideal alignment include scoliosis (lateral flexion and rotation), lumbar lordosis (sway back), kyphosis (excessive flexion in the thoracic spine) and cervical lordosis (often accompanied by forward head).
Pain frequently occurs in the back and neck as a result of the strain on the musculature caused by the misalignment of the skeletal structure. Abnormal structural curve of the spine can also result in unequal pressure on the cartilaginous disks between the bodies of the vertebrae. This can cause a disk to be herniated (slipped disk) if the pressure squeezes the disk out to one side, placing pressure on surrounding nerve tissue and resulting in extreme pain and loss of mobility.
At the hip, several individual differences impact upon performance. Males tend to have an ischium which descends closer to vertical than females, making it more difficult for them to get up on the sit bones when sitting in parallel.
Natural turnout is related to the relative facing and depth of the acetabulum. The shallower and more lateral facing the acetabulum, the greater the degree of outward rotation.
Another important individual difference at the hip is the variance in angle found between the neck of the femur and the shaft. The wider the pelvis the smaller the angle (known as the Q angle); women have a smaller Q angle and this has an effect on the alignment at the knee and ankle.
The Penn Valley Community College site will help provide interactive information on anatomical terminology, the skeletal system and joints in motion as well as further links to additional web sites.
Refer to the following images.
In Core Performance you have studied
the skeletal system. A large part of the study of safe dance practice is
learning how to apply this knowledge of anatomy (to your own body) in technique
classes, thus enabling you to execute movement safely and
Choose a sequence of movement from your core performance dance.
Explain and demonstrate the muscular and skeletal actions that occur during the execution of the movements.
NB: Use appropriate anatomical terminology.
Back to Skeletal structure
Back to Individual differences