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obesity a condition in which body fat stores are enlarged to an extent which impairs health. Develops when food intake is in excess of energy requirements. The most common nutritional disorder worldwide, and the incidence is increasing. Defined in terms of body mass index and circumference at the waist. See also body composition, body weight, waist-to-hip ratio.
oestrogens (estrogens) a group of steroid hormones produced mainly by the ovaries (predominantly in the first half of the menstrual cycle) and during pregnancy by the placenta; also in small amounts by the testes, and by the adrenal cortex in both sexes. Responsible for female secondary sexual characteristics and the development and function of the female reproductive system. Used in the combined oral contraceptive and in hormone replacement therapy. See also hormones, menstrual cycle.
oligosaccharides carbohydrates with 3–9 monosaccharide residues. The main dietary sources are vegetables, particularly seed legumes.
omega-3 fatty acids polyunsaturated fatty acids found in oil from oily fish and in certain plant/nut oils. The three major types contained in foods and used by the body are alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Fish oil contains EPA and DHA, while some nuts (English walnuts) and vegetable oils (canola, soybean, flaxseed/linseed and olive oil) contain ALA. Supplements are also widely available. Research indicates that omega-3 fatty acids reduce inflammation, improve plasma lipid profile and help to prevent certain chronic conditions such as coronary artery disease and arthritis; they are highly concentrated in the brain and have been reported to be important for cognitive and behavioural function.
onset of blood lactate accumulation (OBLA) see metabolic and related thresholds.
open kinetic chain exercise in which a distal body segment is free to move.
open-loop control in motor control, movement that is executed without regard to sensory feedback. Contrast closed loop control.
openness to experience one of the big five personality factors characterized by a tendency to be imaginative, curious, insightful and creative.
opto-electronic motion analysis equipment used to measure the motion of a body or object, using light (often infra-red) reflected from markers attached to its surface.
oral rehydration fluids see hydration status, sports drinks, water balance.
origin with reference to a skeletal muscle, the site of its attachment to bone which remains relatively fixed during its contraction compared to the site of its insertion. For example, in elbow extension, contraction of the triceps moves the forearm (site of insertion) while the upper arm and scapula (sites of origin) may remain still.
orlistat a drug which in conjunction with dieting has been proven to produce weight loss. It is a pancreatic lipase inhibitor which prevents fat breakdown in the intestine and therefore its absorption. About a third of ingested fat is passed through the bowel undigested when on a course of orlistat, reducing energy intake. Other beneficial effects include a lowering of serum cholesterol, reduction in blood pressure and better control of diabetes. Undesirable effects are abdominal discomfort, diarrhoea and anal leakage, and potential loss of fat-soluble vitamins. See also lipolysis.
ornithine dietary supplement which in combination with other amino acids arginine and lysine is claimed to increase muscle growth/lean body mass to a greater extent than strength training alone, but this has not been supported by properly designed trials. See also ergogenic aids; appendix 4.4.
orthoses from Greek ortho, to straighten. Custom-designed external devices used to control or counteract the effect of an actual or developing deformity. They include braces, splints, etc. In sport the term is most commonly used to describe foot-supporting insoles used to correct structural imbalance which may result in discomfort in the back, hips, knees or feet. Orthoses decrease the risk of further injury and make movement of the foot more efficient. orthotics the study and manufacture of orthoses.
Osgood–Schlatter disease osteochondritis/apophysitis of the tibial tubercle (tuberosity). Described by both Osgood and Schlatter (American and Swiss surgeons) in 1903. An ‘overuse’ injury, which produces pain (due to inflammation) at the attachment of the patellar tendon to the tibial tubercle at an age when this is not fully developed. Most common around puberty/adolescence (rapid skeletal growth), in boys more than girls, who take part in repeated or multiple sports, especially those with repetitive running (football, athletics) or with repeated knee bending and jumping (athletics, gymnastics). Some authors suggest up to 50% are precipitated by trauma. Symptoms include swelling, pain and tenderness on direct pressure, and pain (felt precisely on the tibial tubercle) during exercise and contraction of the quads. Heals spontaneously, with no individual treatment shown to be particularly helpful, except reduction in activity to levels where symptoms are acceptable. Rarely leads to problems in later life.
osmolality the concentration of osmotically active particles in a solution, expressed as the number of osmoles per kilogram (Osm.kg−) of solution. In blood plasma, osmolality (280 – 300 mOsm.kg−1) is very slightly less than osmolarity (in mOsm.L−1) because the presence of large molecules (e.g. lipids) adds to the volume that contains 1 kg of water (plasma is about 94% water). The two terms are often incorrectly used interchangeably; osmolality applies appropriately to body fluids. Measurement allows assessment of dehydration/overhydration.
osmolarity the concentration of osmotically active particles in a solution expressed in osmoles per litre (Osm.L−1) of the solution. Values for human body fluids (e.g. blood plasma) are usually expressed in milliosmoles per litre (mOsm.L−1). Compare osmolality.
osmole the amount of a substance in a solution that forms one mole of osmotically active particles, irrespective of their size, e.g. a single sodium ion contributes as much to the osmolality as a large protein molecule; and 1 mole of glucose, which does not ionize, provides 1 osmole, while 1 mole of sodium chloride provides 2 osmoles – one of Na+ and one of Cl−.
osmoreceptors cells located in the hypothalamus, sensitive to a change in the osmolality of their surroundings, which will reflect any such change in the plasma and body fluids as a whole. A rise in osmolality (tending towards dehydration) triggers increased production and release of antidiuretic hormone (ADH) from the posterior pituitary (a neuroendocrine secretion), causing the kidneys to retain more water. Conversely, a decrease in body fluid osmolality (due to high water intake) reduces ADH release, leading to increased and less concentrated urine output (diuresis).
osmotic pressure the ‘suction’ exerted by a solution of higher, upon one of lower, osmolar concentration, which moves water by osmosis in the direction that will equalize concentrations if the solutions are separated by a semi-permeable membrane; this allows the passage of water but not of the solute particles. Applies to movements of water across cell membranes, maintaining osmotic equilibrium between extra- and intracellular fluids.
osteoarthritis (OA) a condition of the joints where articular cartilage becomes worn, exposing the underlying bone. More than just ‘wear and tear’, it is an evolving process with much research aimed at slowing its advance. Known aetiological factors include increasing age, female sex, manual occupations, obesity, malalignment and injury in sport. Genetic factors have been implicated, though not yet identified. There is little evidence that exercise per se produces OA. Clinically OA presents with pain, stiffness, limitation of movement and joint deformity. X-rays show joint space narrowing, osteophytes, subchondral sclerosis and bony deformity. Most commonly affects the weight-bearing joints (hips, knees, ankles and feet) and hands. Exercise is used in treatment to maintain mobility, flexibility and muscle strength. Analgesia and modification of activity are advised in mild to moderate OA, and osteotomy or joint replacement for severe disease in suitable joints.
osteopathy an established clinical discipline (now regulated by a statutory body) concerned with the interrelationship between structure and function of the body. Osteopaths have a holistic approach, treating the whole person in the prevention, diagnosis and treatment of illness, injury or disease. Osteopaths in sport mainly treat mechanical musculoskeletal problems.
osteopenia lower than normal bone mineral density.
osteoporosis reduction in bone mineral density with ageing, particularly in women; onset and progress are mitigated by regular exercise, particularly weight-bearing exercise. Increases the likelihood of fractures, often with relatively minor trauma. See also bone scan, dual emission X-ray absorptiometry (DEXA).
osteotomy surgical division of a bone to achieve realignment. Used to encourage healing, e.g. in a poorly aligned healed fracture, or to reduce pain in osteoarthritis.
outcome expectancy in social cognitive theory, a person’s expectations about the consequences of an action.
outcome goal a goal that specifies the outcome of a performance, usually involving a comparison with others such as winning a race. See also performance goal, process goal.
ovary the female gonad, the site of production of ova; one on each side, in the pelvic cavity, close to the open end of each Fallopian tube. An ovum is discharged into one of these tubes en route for the uterus at ovulation in each menstrual cycle. The ovaries are also endocrine glands (under the influence in turn of anterior pituitary gonadotrophic hormones), secreting the female hormones oestrogen and progestogen. These have actions both widely in the body and on the reproductive organs at specific times in the menstrual cycle and during pregnancy. See also hormones; appendix 5.
overjustification effect an explanation for the observation that if individuals are rewarded for engaging in an inherently enjoyable or satisfying activity, they are subsequently less likely to engage in the activity when given the opportunity to do so in the absence of a reward. It is proposed that the reward comes to justify engagement in the activity instead of the initial inherent enjoyment and when removed, there no longer remains a reason for taking part. For example, rewarding children for engaging in sport could undermine their inherent interest in sport.
overlearning the learning of a task or action beyond the point of mastery so that it becomes automated. Typically acquired through repetitive drills.
overload principle a term used less now than previously, indicating the need to place increased demand upon a tissue (e.g. connective), organ (e.g. muscle) or system (e.g. cardiovascular) if its performance is to improve. The demand concerned may be for strength, endurance or (less commonly) speed. Must be distinguished from ‘overload’ in engineering, which is a load greater than a structure can bear.
overtraining training exceeding the body’s recovery capacity, indicated by excessive fatigue both physical and mental, and resulting in impaired performance. Also called staleness. Short-term overtraining is usually adequately countered by a period of reduced intensity or a few days’ total rest but if extended, it leads to the overtraining syndrome – a set of symptoms and signs, probably of neuroendocrine origin. The psychological aspect of fatigue now usually predominates, while physical symptoms often include increased BMR, protracted elevation of pulse rate after exercise, and negative nitrogen balance leading to weight loss. Recovery may take months or never be fully achieved. Compare the unexplained under performance syndrome (UPS) which may apply in some instances previously classed as overtraining. See also burnout.
overweight see body weight, body mass index, obesity.
oxidative phosphorylation oxidation of products of carbohydrate, fat, protein and alcohol metabolism to carbon dioxide and water with formation of ATP from ADP and inorganic phosphate (Pi), associated with the transfer of electrons from substrate via coenzymes to oxygen, taking place in mitochondria. See also adenosine mono-, di- and triphosphates (AMP, ADP, ATP).
oxidative stress general term used to describe imbalance between reactive oxygen species and antioxidants. Oxidative stress can damage a specific molecule or the entire organism and is known to be implicated in the pathogenesis of a wide variety of disorders, including coronary heart disease, cerebrovascular disease, chronic obstructive lung disease, some forms of cancer, diabetes, skeletal muscular dystrophy and others. Oxidative stress-induced damage in muscle could be one of the factors that terminate muscular effort, but consecutive exercise bouts seem to induce antioxidant adaptations.
oxygen consumption (
O2) the rate of uptake of oxygen in the lungs, usually expressed in litres per minute. Measured in indirect calorimetry as an estimate of metabolic rate, taking the ‘calorific value’ of 1 litre of oxygen as 4.8 kcal (for an average ratio of carbohydrate to lipid oxidation), therefore 1 L.min−1 as equivalent to 4.8 kcal.min−1, 20 kJ.min−1 or 333 watts.
oxygen cost the rate of oxygen usage for a particular task or work rate.
oxygen debt amount by which oxygen consumption during recovery from exercise exceeds resting level. Consists of an initial rapid phase lasting ∼1–2 min (formerly called the ‘alactic’ phase), in which muscle CrP and ATP stores are replenished, and a subsequent slow phase (∼1 h) in which lactic acid is oxidized (hence the former term ‘lactacid’ phase), temperature falls toward resting level and blood hormonal concentrations are normalized. Also known as elevated post-exercise oxygen consumption (EPOC) or recovery oxygen consumption.
oxygen deficit shortfall of oxygen consumed during activity below that required to supply all necessary energy aerobically. Postexercise restoration of this deficit constitutes part, but not all, of the oxygen debt.
oxygen delivery the rate of supply of oxygen by the arterial blood to body organs and tissues, expressed as cardiac output (L.min−1)×oxygen content of the blood (L.L−1), e.g. typical resting value would be 5 × 0.2 = 1 L.min−1. This is four times the typical oxygen usage at rest, since only a quarter of the oxygen in the arterial blood is removed by the tissues, reducing haemoglobin saturation from 100% to 75%. See also oxyhaemoglobin dissociation curve.
oxygen plateau the flattening out of oxygen consumption after the maximal value is attained in incremental exercise.
oxygen poisoning a hazard of exposure to high ambient pressure (typically in diving) when breathing high percentage oxygen. With 100% oxygen inspired oxygen pressure is ∼100 kPa on the surface at 1 atmosphere, and increases by 100 kPa for every 10 m depth under water. Susceptibility varies between individuals and with the level of physical work, but limits typically advised are ∼150–170 kPa, equivalent to breathing 100% oxygen at depths of 5–7 metres (or proportionately <100% mixed with nitrogen at greater depths). Toxic effects are mainly on the brain (causing epileptic fits) and on the lungs (cough, oedema, impairment of oxygen diffusion). Fits under water can be fatal and the more slowly developing pulmonary toxicity can be irreversible if severe. (Higher pressures than those advised above are safely used to treat decompression illness by 100% oxygen at rest.)
oxygen transport see oxygen delivery, oxyhaemoglobin dissociation curve, partial pressure.
oxygen uptake see oxygen consumption.
oxyhaemoglobin dissociation curve the graph which describes the relationship in the blood between partial pressure of oxygen (PO2) and the percentage saturation of haemoglobin; it can also show the equivalent oxygen content of the blood when haemoglobin is in normal concentration. With normal lungs, saturation in arterial blood is determined by the PO2 in the alveolar gas with which pulmonary capillary blood equilibrates. The S-shape of the curve has important physiological advantages, e.g. a relatively small decrease in PO2 encountered where blood flows through tissues causes a ‘steep’ removal of oxygen from the blood; but there needs to be a relatively large decrease in PO2 in the inspired air (and therefore in the alveoli, e.g. at altitude or in a confined space) before there is a serious decline in haemoglobin saturation. (See fig overleaf.)
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