Читать книгу The Yoga Back Book: The Gentle Yet Effective Way to Spinal Health - Stella Weller - Страница 6
ОглавлениеBefore driving a car or using a piece of machinery, people usually acquaint themselves with the way it works. This allows them to operate it intelligently and therefore safely. The human spine, which is perhaps more sophisticated in its design than most machines, should be similarly considered.
A basic understanding of how the spine is constructed and how it functions is therefore a vital first step towards acquiring and maintaining a trouble-free back. This chapter represents that important first step. Spend a few minutes reading it from beginning to end. The time you invest in doing so will be rewarded; it will equip you to care for your back with insight and wisdom.
Functions and Structure
The spine supports the head and about 90 per cent of the weight of the human body in an upright position. It is mechanically balanced to conform to the stress of gravity and to permit movement from place to place, as well as to assist in purposeful movements.
The spine prevents shock to the brain and spinal cord during activities (such as running and jumping) through its curves and intervertebral discs, about which more information will be given later. It protects and houses the spinal cord. It provides attachment for many powerful muscles and it forms a strong posterior boundary for the body.
Known also as the spinal or vertebral column, the spine is composed of 33 bones (vertebrae; singular, vertebra) with a pad of cartilage, or gristle (intervertebral disc) in between every two bones (Fig. 1, Fig. 2 and Fig. 3). This disc buffers the vertebrae against shock during activities such as running, jumping or driving on bumpy roads.
Fig. 1. Intervertebral disc
The Discs
The disc has a tough elastic shell composed of crisscrossing fibres. Inside it (the nucleus) is a soft substance with the consistency of jelly. The upper and lower surfaces of the disc have a cartilaginous layer known as an end plate. This acts somewhat like a sieve between the disc and the bone.
Discs have no blood supply; they depend on a process of diffusion through the end plates for their nutrition. When we are resting or sleeping, the discs suck in water and other nutrients. When we move about or exercise, compression squeezes fluids out and expels wastes. A sensible balance of exercise and rest is therefore crucial in maintaining the health of the intervertebral discs.
Fig. 2. Vertebrae with intervening discs
All individuals can expect degenerative changes in the discs. Between the ages of 20 and 30, maximum development of the discs has occurred and the water content is maximal – about 80 per cent. This lessens with age. With a healthy lifestyle, however, it is possible to keep the discs from drying out, and to preserve a good balance between fibre and fluid, even into advanced age.
Spinal Units
You may also think of the spine as being composed of a number of functional units. Each unit (Fig. 2) consists of two segments: an anterior (front) portion which may be considered a hydraulic, weight-bearing, shock-absorbing structure comprising two vertebrae with a disc in between them, and a posterior (back) portion which may be thought of as a guiding mechanism. This includes three projecting pieces of bone: two projecting sideways and one rearward. You may be able to feel these as ‘buttons’ down the middle of your back. These processes provide for the attachment of ligaments and muscles.
In addition, the rear portion of the vertebra has two upper and two lower surfaces called facets. The lower facets of one vertebra glide along those of the vertebra below. The facets thus guide and limit the motion of one vertebra relative to its neighbouring vertebra.
The articulated posterior processes of the vertebrae form a canal which houses and protects the spinal cord. This is a bundle of nerve fibres connecting the brain with all parts of the body, and which carries messages to and from the brain.
On each side of the spine, between every two vertebrae, are tiny openings for the passage of nerves branching from the spinal cord.
Ligaments and Muscles
Strengthening the joints formed by the vertebrae and their intervening discs are ligaments – tough fibrous bands of tissue – running behind and in front of them along the entire length of the spine.
Reinforcement is also provided by the back muscles which help to control forward bending, and indirectly by the abdominal (‘tummy’) muscles which give a counterbalancing effect and help to prevent extreme backward bending.
It is worth noting that although the abdominal muscles are not directly attached to the spine, their strength and tone are crucial to the overall health of the back.
Connective Tissue
The space between the bones, tendons, ligaments and muscles of the back are filled with material known as connective tissue. Much of this filling is composed of a protein substance known as collagen. In fact, collagen accounts for about 30 per cent of total body protein. It is a sort of ‘cement’, which binds cells together. It is the key to the relationship between nutrition and spinal health. It acts as a transport medium, carrying nutrients from the bloodstream to the muscles and ligaments, and conveying wastes to the bowel and skin for elimination.
The chief functions of connective tissue in the back are: to bind tendons to bones and buffer muscles and ligaments to help to maintain resilience and strength; and to act as a medium of transport carrying oxygen and nutrients to spinal structures and removing wastes.
Spinal Curves
With your mind’s eye now, try to see the spinal column from the side (laterally). You will note four curves (Fig. 3): in the neck (cervical) region, one that is convex towards the front (lordosis); in the chest (thoracic) area, one that is convex towards the rear (kyphosis); at waist level (lumbar spine) another arch that is convex towards the front, and at hip level (sacral area) another arch that is convex towards the rear. These curves transect the plumb line of gravity in order to maintain a state of balance.
Fig. 3. Spinal curves
The Pelvic Ring
The spine is balanced on an undulating pelvic base known as the pelvic ring (sometimes pelvic girdle). (Fig 4.). This is the human body’s chief weight-transmitting structure, connecting the upper body to the legs. It consists of the sacrum at the back, and two innominate (hip) bones which are connected to the femurs (thigh bones). Together, these bones form five joints: two sacroiliac joints at the base of the spine; two hip joints where the hip bones connect with the legs, and the symphyis pubis where the two hip bones join in front.
A key feature of the pelvic ring is its strong ligamentous support, which is especially important in weight bearing.
During normal standing or sitting, the ligaments of the sacroiliac joints and the pelvic ring are somewhat loose. When weight bearing occurs, however, the pressure exerted down through the spinal column causes the sacroiliac ligaments to tighten. This tightening alters the position of the pelvic ring from a loose, or neutral structure to one that makes for greater stability.
A second key characteristic of the pelvic ring is that its degree of tilt determines the quality of the curves in the spinal column above. Any change in the angle of the sacral portion of the pelvis will influence the curves of the spine above, thus determining posture. If the pelvic ring is in a balanced position, the spinal curves will be proportionately balanced and the posture safe. If, however, the pelvic ring is abnormally tilted, poor posture will result and the spine will consequently be more vulnerable to injury and pain.
Fig. 4. The pelvic ring
Good posture and smooth movement and rhythm require good coordination between nerves and muscles, good flexibility of tissues and adequate functioning of all joints involved.
Muscular Supports
The chief back muscles are as follows:
The erector spinae muscles form two columns, one on each side of the spine. They extend the spine and keep the trunk erect.
The latissimus dorsi is a broad, flat muscle lying over the lower part of the chest and loins (between ribs and hip bones). It draws the upper bone of the arm (humerus) down and back, and rotates the arm inwards.
The gluteal muscles, which form the buttocks, raise the trunk from a stooping to an erect position. They are also involved in leg motions.
The sling muscles (hip flexors) connect the transverse processes of the spine on the inside (the projecting pieces of bone mentioned in the section ‘Spinal Units’), cross the pelvic ring and attach to the thigh bones just below the hips. They are very important in the maintenance of upright posture.
The lateral muscles are situated between the ribcage and the pelvic ring. They extend from the lowest ribs to the hip area and legs.
Secondary Supports
Muscles forming secondary supports to the spine are: the quadriceps (thigh muscles), which run down the front of the thighs and insert into the kneecaps or patellae, and serve to extend the knees; and the hamstrings, which are located at the back of the thighs, passing from the pelvic ring and inserting into the bones of the lower legs (tibiae and fibulae). The hamstrings flex the knees and extend the thighs.
Both sets of muscles – quadriceps and hamstrings – contribute to the tilt or balance of the pelvic ring and so are important to good posture.
The abdominal muscles, which will be dealt with in chapter six, basically extend from the breastbone (sternum) to the symphysis pubis, on each side of the midline. Although thin, these muscles give extremely important support to the spine. They operate at a distance and so provide leverage.
