D
dead space in human anatomy and physiology, refers to the respiratory passages (airways) leading to the alveoli of the lungs, so named because the air breathed in and out of this space does not reach the alveoli and so takes no part in gas exchange with the blood; dead space ventilation the volume of gas breathed in and out of the dead space per minute, normally about one-third of the total ventilation (minute volume) at rest, becoming a smaller fraction as tidal volume increases in exercise.
deceleration a change in motion of an object or body usually understood as the rate of reduction in speed (although it can also refer to a change in direction). A negative acceleration.
decompression illness the adverse effects of uncontrolled return to normal ambient pressure following exposure to high pressure when surfacing from a dive or, less commonly, exposure to rapid reduction in pressure in ascent from sea level in unpressurized aircraft. Symptoms range from pains in the joints, chest and back, weakness or sensory loss, to paralysis and loss of consciousness; severe neurological symptoms can be life-threatening. There are two main causes: (1) damage to the lungs by expansion of the gas in them if a diver does not freely exhale when surfacing. Gas can leak into the circulating blood (air embolus) or into the pleural cavity (pneumothorax); (2) release in the tissues (e.g. around joints or in the spinal cord) of bubbles of nitrogen that was dissolved in body fluids whilst at the higher pressure. Avoided or minimized by using computed tables to control speed of ascent and frequency of pauses, in relation to the duration and depth of the dive. All types of decompression illness require oxygen breathing as a first aid measure, and urgent treatment for all but the mildest by recompression to the initial higher pressure in a hyperbaric chamber, so that nitrogen is redissolved in body fluids, then more gradually released and exhaled as pressure is allowed to fall. See also diving.
deep friction massage the use of massage to the deep soft tissues, which aims to break up scar tissue.
defibrillation the application of a direct current (DC) electric shock to arrest ventricular fibrillation of the heart and restore normal cardiac rhythm.
degenerative disease disease characterized by deterioration in quality or function of a tissue or organ. In sport and exercise, acceleration in the deterioration of a joint (e.g. knee osteoarthritis) is seen following injury.
dehydration in general, the process of removal or loss of water from a substance or body. For related terms used in the context of human fluid balance, see also hydration status.
delayed-onset muscle soreness (DOMS) appears 24–48 h after exercise: may follow severe unaccustomed exercise, particularly that involving eccentric action of the muscles.
deltoid muscle the muscle which covers the shoulder joint. Arises from the shoulder girdle, in continuity from the outer third of the front of the clavicle, the acromion of the scapula above the shoulder and the spine of the scapula behind it. From here the fibres converge to be inserted into the ‘deltoid tuberosity’ on the outer side of the humerus. A powerful abductor of the arm.
density the ratio of mass to volume. Measured in kilograms per cubic metre (kg.m−3).
dental injury common in contact sports such as boxing and rugby.
deoxyribonucleic acid (DNA) a long macromolecule in the nucleus of animal and plant cells, which stores genetic information in its component genes, and instructions for their expression in controlling function and development. Each strand is closely linked to another in a double helix. There is also a small amount of mitochondrial DNA which carries only maternal genetic material, being transmitted in ova but not in sperm.
depression a mood disorder characterized by feelings of profound sadness. May be classified by severity, by the presence of somatic symptoms and by the presence or absence of psychotic symptoms. Cognitive symptoms include hopelessness, helplessness, guilt, low self-esteem and suicidal thoughts. endogenous depression not resulting from a reaction to a particular negative experience. reactive depression resulting from a reaction to a particular negative experience (such as bereavement, physical illness or loss of employment), also known as exogenous depression. Research has shown that a structured exercise programme can have a mood-enhancing effect similar (and complementary) to that seen with the use of antidepressant medication. See also antidepressants.
derivative the result of the calculation (usually with calculus) of the change of one variable with respect to another. Also alludes to the number of ‘steps’ of calculus required (e.g. acceleration is the second derivative of displacement with respect to time). See also differentiation.
depolarization reduction of the normal voltage difference between the inside and the outside of a cell. See also action potential, membrane potential.
desensitization (1) in psychology, a form of behaviour therapy in which an anxiety-provoking stimulus is repeatedly paired with a relaxation response, either in the imagination or in real life (the latter known as in vivo desensitization) in order to eventually eliminate the anxiety response and replace it with the relaxation response. In systematic desensitization a hierarchy of increasingly anxiety-provoking stimuli is established and each stimulus is paired with the relaxation response in turn, beginning with the least feared and working towards the most feared. Also known as reciprocal inhibition therapy; (2) in medical practice, treatment of a hypersensitive response to an allergen, by deliberate exposure to a very small, and then a gradually increasing, dose; (3) with reference to repeated dose of a drug, a progressive decline in its effect.
dexamethasone see glucocorticoids.
diabetes a condition characterized by excessive urine output and consequent excessive thirst and fluid intake. Most commonly refers to diabetes mellitus, a disease due to failure of pancreatic secretion of insulin, or to failure of its function in controlling glucose metabolism; the blood glucose level is raised (hyperglycaemia) and there is glucose in the urine. Diabetics who exercise need to achieve optimal blood glucose control and to be aware of the potential for exercise-induced hypoglycaemia.
diaphragm the musculotendinous partition between the thoracic and abdominal cavities, penetrated by the lower end of the oesophagus, aorta, vena cava and other vessels and nerves. Attached around its periphery to the ribcage and the vertebral column. The main muscle of breathing, controlled by rhythmic impulses from the brain stem via the phrenic nerves. Contraction flattens it, expanding the thorax, reducing the pressure inside the lungs and causing inspiration; in relaxation, it rises again to a ‘dome’, allowing passive expiration (this diaphragmatic or abdominal breathing is the normal pattern at rest: the abdomen protrudes as the diaphragm is lowered). Lavish blood supply and high oxidative capacity enable the diaphragm to sustain the major increase in the work of breathing during exercise, but at high intensity its demand for blood competes with that of the exercising muscles. Fatigue of the diaphragm (and of other respiratory muscles) has been shown to contribute to exercise limitation. Unique among skeletal muscles in maintaining its activity continuously for a lifetime under involuntary control, yet which can, within limits, be voluntarily overridden. See appendices 1.3 fig 4, 1.4 fig 1.
diastole the phase between beats when the heart muscle is relaxed, and the chambers are filling. See also cardiac cycle, electrocardiogram, venous return.
dietary fibre see non-starch polysaccharides (NSP).
dietary reference values (DRVs) in the UK a set of tables that estimate a range of nutritional requirements for different groups of healthy individuals in the population. Three values may be used: estimated average requirement (EAR) about half of a population group will need more than the EAR and half will need less; reference nutrient intake (RNI) an intake of two standard deviations above EAR; this is enough or more than enough for at least 97.5% of people in a group, including those with high requirements; lower reference nutrient intake (LRNI) an intake of two standard deviations below the EAR and sufficient only for a small number of people (about 2.5%) in a group who have low needs. See also balanced diet, energy requirement, recommended daily allowance; appendix 4.
Dietary reference values and intakes for UK adults
| Recommended DoH 1991 | National Diet and Nutrition Survey 2003 | |
|---|---|---|
| Energy intake (kcal) EAR | Mean ± SD | |
| Men | 2550 kcal/day | 2313 ± 582 |
| Women | 1940 kcal/day | 1632 ± 418 |
| % Total energy intake | ||
| Carbohydrate | ||
| Men | 50% | 47.7 ± 6.0% |
| Women | 50% | 48.5 ± 6.7% |
| Fat | ||
| Men | 35% | 35.8 ± 5.6% |
| Women | 35% | 34.9 ± 6.5% |
| Protein | ||
| Men | 15% | 16.5 ± 3.6% |
| Women | 15% | 16.6 ± 3.5% |
| Alcohol | ||
| Men | *5% | 6.5 ± 7.2% |
| Women | *5% | 3.9 ± 5.1% |
* Average intake, not recommendation
dietary supplementation see carbohydrates, ergogenic aids; appendix 4.4.
dietetics the interpretation and application of scientific principles of nutrition to feeding in health and disease.
diet-induced thermogenesis the energy required to digest and assimilate the food; measured as an increase in body heat production after eating. It typically represents only about 10% of total daily energy expenditure and is related to the type and amount of food ingested. Also known as thermic effect of food. Fats have relatively little thermic effect and proteins the most.
dieting any type of eating plan that aims at reducing body mass and body fat, and requires one to eat and drink sparingly or according to prescribed rules. Weight or fat reduction in athletes is generally motivated by a desire either to achieve a predesignated weight in order to compete in a specific weight class or category (e.g. horse racing, boxing) or to optimize performance by improving power to weight ratio (e.g jumping events, distance running).
difference threshold in psychophysics, the smallest difference between two sensory inputs that can be detected. Also known as difference limen, discrimination threshold or just noticeable difference (jnd). See also absolute threshold.
differentiation in mathematics, the use of calculus to compute the change of one variable with respect to another. Equal to the gradient (slope) of a graph of the one plotted against the other.
diffusing capacity of the lungs: the volume of a gas that moves across from the alveoli into the blood per minute, per unit partial pressure difference for that gas over the lungs as a whole. Depends, for any gas, on the total area and average thickness of the alveolar–capillary interface. Of most interest for oxygen, since it determines the efficacy of oxygen intake, but usually estimated in terms of the diffusing capacity for carbon monoxide which is more straightforward to measure. Increased in exercise as greater lung expansion both enlarges the area and decreases the thickness of the gas exchange surface.
digestion the processes which break down ingested food to substances that are absorbed from the alimentary canal into the blood or lymph. Catalysed by the digestive enzymes in the saliva, gastric juice, pancreatic juice and those secreted from the lining of the small intestine. These ‘juices’ together add up to a total of about 10 litres per day entering the gut. Apart from those that occur in the lumen of the gut, some further digestive processes are effected by the enterocytes, the cells in the lining of the small intestine. Depends also on other chemical processes, including for example emulsification of fats by bile from the liver.
digitization the conversion of an analogue variable to a digital one, i.e. a continuous variable into a set of numbers. Used in biomechanics to describe the derivation of co-ordinate data from video (or cine) pictures, e.g. plotting the co-ordinates of markers on the body.
disability inability to participate in activity at a standard level.
disc, intervertebral see intervertebral disc.
dislocation displacement of the articular surfaces of a joint, so that apposition between them is lost and the bony components no longer form a working joint. The cause can be congenital, spontaneous or traumatic, and dislocation may be recurrent. In sport, dislocations of fingers and the shoulder are the most common, as the result of collision with either an opponent or an object such as the ground or goal post. Replacement in position (‘reduction’) may be spontaneous but if not, it should be attempted early and only by a skilled operator.
displacement (1) change in position of a body or object, including size (magnitude) and direction of change, i.e. a vector quantity. linear displacement the distance and direction between the start and end point. Contains a measure of distance (e.g. metres) and a measure of direction (e.g. an angle in degrees to the horizontal) – effectively the distance and direction ‘as the crow flies’. angular displacement the angle between the start and finish positions in a rotational movement, including a direction (e.g. clockwise or anticlockwise). Measured in degrees or radians. (2) the volume of fluid (usually water) that is moved when a body or object is immersed in it. (Displacement must be distinguished from ‘distance moved’ which includes only magnitude and not direction.)
disposition a relatively enduring tendency to behave or respond to situations in a typical way. dispositional adj.
dissociative strategy in sport psychology, a strategy used by athletes in which they focus attention externally in order to distract themselves from feelings of pain or fatigue. Also known as dissociation. See also associative strategy.
distal (to) in anatomy, further away from some reference point. For example, in a limb, further from the trunk: the wrist is distal to the forearm; the abdominal aorta divides distally adj. into the two iliac arteries. Opposite of proximal.
distractor in experimental psychology, a stimulus that diverts a participant’s attention away from another stimulus that they are required to detect or respond to.
disuse atrophy loss of muscle mass due to inactivity. May follow a period of immobilization, e.g. bed rest or with a plaster cast. Prolonged disuse results in fibrous tissue replacing muscle tissue, limiting the extent of full rehabilitation.
diuretics substances contained in food and drink (e.g. caffeine), or given as medication, that increase the secretion of urine by the kidneys (diuresis). Used in medicine as a treatment for high blood pressure. In sport the use of diuretics for two main purposes is banned: as a means of losing fluid, and thus weight, in sports such as boxing and weight lifting, which have weight categories or in an attempt to increase the production of urine and thus the excretion of a banned drug, to avoid detection. See appendix 6.
diving (1) diving in swimming pools carries the potential hazard of neck injury, if the dive is too steep relative to the depth of the water; (2) in scuba diving the diver breathes compressed air from a cylinder, through a face mask and a closed system of tubes (‘self-contained underwater breathing apparatus’). Pressure increases by 1 atmosphere (1 bar) per 10 metres of depth. Below 30 m the ‘intoxicating’ effect of nitrogen narcosis is a danger, avoided by the use of helium–oxygen mixtures. Hypothermia is also a hazard in cold climates; (3) in breath-hold diving the duration of immersion can be increased if the dive is preceded by vigorous hyperventilation. This depletes carbon dioxide in the body so that it will take longer to rise to the level which would normally trigger the break-point, but oxygen in the lungs and the blood can be depleted to the point of threatening consciousness. See also barotrauma, cervical spine, decompression illness.
dopamine a neurotransmitter, especially within the brain, in pathways related to the co-ordination of movement and to behaviour and emotion; deficiency of dopamine in the brain is associated with Parkinson’s disease. Its secretion from the brain into the hypophysial portal blood vessels inhibits prolactin secretion from the anterior pituitary. Also, in the adrenal medulla it is a precursor of adrenaline and noradrenaline, and is itself released as a neurohormone.
doping the use of banned substances or methods (as defined and listed by WADA) in sport to attempt to gain an unfair advantage. Considered to derive from the South African word ‘dop’ for a stimulant drink first given to racehorses. The WADA list is an agreed table of both synthetic and naturally occurring substances considered to offer an advantage when taken during training and competition. Sportsmen and women take performance-enhancing substances for a number of reasons. These include the physical effects of the drug itself (anabolic steroids will allow the athlete to train harder, faster and for longer), the pressure on the athlete to succeed (from coach, family, sponsors, media and general public) and the direct effect on athletes themselves (to boost confidence, lessen anxiety, etc.). See also banned substance; appendix 6.
dorsal at or towards the back of the body (opposite of ventral), referring to the embryological history of the surfaces. In the ‘anatomical position’ the palms face forwards and dorsal applies to the back of the hands and arms.
dorsiflexion movement at the ankle joint that points the foot up towards the leg, or movement of the toes that lifts them away from the sole of the foot (compare plantarflexion).
double-blind study comparison of the effect of a drug or other intervention in a group of subjects with that of a placebo (an inactive ‘fake’ substance or procedure) in a second similar group, when neither those taking part as subjects nor the investigators observing the effects are aware of the group to which any subject has been allocated, until after completion of the study.
Douglas bag method for measurements of pulmonary ventilation and respiratory gas exchange and hence estimation of energy expenditure (indirect calorimetry). The subject breathes via a mouthpiece or face mask and one-way valve, so that the expired gas is collected in a large bag over a recorded period of time; the volume is then measured and the gas analysed for oxygen and carbon dioxide content; the differences from inspired air allow calculation of the rates of O2 uptake and CO2 output. Described in 1911 by Oxford physiologist C.G. Douglas.
drag force a retarding force, acting opposite to the direction of motion of a body or object. Often caused by air resistance or friction. form drag the resistance force caused by the shape of a body or object which is moving through a fluid medium. surface drag the force, opposing the direction of motion, that is due to the interaction between the surface of an object and the medium through which it is passing (or that is moving past it). propulsive drag force the force used by sportspeople to propel themselves, usually by pushing against a fluid medium (e.g. swimmers or rowers).
drawer sign a clinical sign that describes the movement of the tibia relative to the femur and is positive if the relevant cruciate ligament is torn. The cruciates provide the primary stability of the knee joint. Disruption of the anterior cruciate will allow excess anterior movement of the tibia relative to the femur and posterior movement is a sign of posterior cruciate damage. For the anterior drawer test, with the subject’s knee flexed to 90°, the hamstrings relaxed and the foot stabilized, the examiner firmly grasps the leg below the knee and pulls it so as to move the tibia forward; for the posterior drawer test, with the knee flexed to 80° and the foot stabilized, the examiner attempts to move the tibia backwards. Excessive movement, compared to the other knee, indicates rupture of the ligament. See also knee injury.
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