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Biomedical subjects

R J Shephard

Publications and source records attributed to R J Shephard.

At least 541 records · Page 30Linked to original sources
PubMedDate not supplied

Adaptation to exercise in the cold.

The winter athlete has several potential tactics for sustaining body temperature in the face of severe cold. An increase in the intensity of physical activity may be counter-productive because of increased respiratory heat loss, increased air or water movement over the body surface, and a pumping of air or water beneath the clothing. Shivering can generate heat at a rate of 10 to 15 kJ/min, but it impairs skilled performance, while the resultant glycogen usage hastens the onset of fatigue and mental confusion. Non-shivering thermogenesis could arise in either brown adipose tissue or white fat. Brown adipose tissue generates heat by the action of free fatty acids in uncoupling mitochondrial electron transport, and by noradrenaline-induced membrane depolarisation and sodium pumping. The existence of brown adipose tissue in human adults is controversial, and although there are theoretical mechanisms of heat production in white fat, their contribution to the maintenance of body temperature is small. Acclimatisation to cold develops over the course of about 10 days, and in humans the primary change is an insulative, hypothermic type of response; this reflects the intermittent nature of most occupational and athletic exposures to cold. Nevertheless, with more sustained exposure to cold air or water, humans can apparently develop the humoral type of acclimatisation described in small mammals, with an increased output of noradrenaline and/or thyroxine. The associated mobilisation of free fatty acids suggests the possibility of using winter sport as a pleasant method of treating obesity. In men, a combination of moderate exercise and facial cooling induces a substantial fat loss over a 1- to 2-week period, with an associated ketonuria, proteinuria, and increase of body mass. Possible factors contributing to this fat loss include: (a) a small energy deficit; (b) the energy cost of synthesising new lean tissue; (c) energy loss through the storage and excretion of ketone bodies; (d) catecholamine-induced 'futile' metabolic cycles with increased resting metabolism; and (e) a specific reaction to cold dehydration. Current limitations for the clinical application of such treatment include uncertainty regarding optimal environmental conditions, concern over possible pathological reactions to cold, and suggestions of a less satisfactory fat mobilisation in female patients. Possible interactions between physical fitness and metabolic reactions to cold remain controversial.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Physiological↗
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Factors influencing the exercise behaviour of patients.

The sports physician faces major practical problems in stimulating the activity of sedentary patients, and also ensuring their subsequent adherence to a prescribed exercise regimen. Both individual and society bear some responsibility for poor exercise compliance. Patients are most likely to persist with programmes which are well-adapted to their physical, physiological, psychological and socioeconomic characteristics. The quality of leadership is also an important variable. Open-ended and structured questionnaires suggest that motivations to exercise include the improvement of health, the development of fitness, the control of bodyweight as well as an improvement in personal appearance, socialising, and simply a wish to 'feel better'. The main perceived reason why people 'drop out' of formal exercise classes is through lack of time, although often the individuals making this complaint appear to have substantial free time. Personality characteristics of the adherent depend on the type of programme that is offered and the degree of social support that is provided; extraversion, a high self-esteem and self-motivation are common findings. Modelling of exercise behaviour has drawn upon the concepts of Skinner, Becker and most recently Fishbein. Beliefs and their evaluation seem more influential than subjective norms, although exercise behaviour is also heavily influenced by past experience of physical activity. Programmes should be designed to maximise external reinforcement of the exercise habit until the early discomforts of training are overcome, and the patient discovers the internal rewards of a more active lifestyle.

Attitude to Health↗
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Exercise in coronary heart disease.

Population levels of habitual activity have probably contributed to both the recent epidemic of cardiovascular disease and its waning. Evidence supporting the exercise hypothesis can be drawn from comparisons of individuals with differing levels of occupational and leisure activity. Both suggest that regular, endurance-type activity may halve the incidence of cardiac morbidity and mortality. This is an important prophylactic benefit, although Bradford Hill's criteria of a causal association have yet to be fully satisfied. Following the onset of clinical disease, both uncontrolled and randomised controlled trials suggest that progressive exercise rehabilitation improves prognosis by a useful 20 to 30%, but formal statistical proof is again difficult for technical reasons. Although over-enthusiastic vigorous physical activity can cause an immediate rise of cardiovascular events, this disadvantage is substantially outweighed by long term gains from regular physical activity. Classical epidemiology has proven its case by the experimental step of removing exposure to the causal agent. It is difficult to carry out such an analysis linking physical activity with the recent epidemic of ischaemic heart disease, although the recent waning of the disease may be attributed in part to an increase of habitual physical activity in many western nations. Evidence linking exercise to the prevention of clinical disease ('secondary prevention') is derived from large scale surveys of groups with supposed differences in occupational activity, athletic participation, active leisure pursuits or overall lifestyle. The majority of occupational comparisons have shown advantages to active workers in terms of deaths from cardiac disease, sudden death, cardiac morbidity, ECG abnormalities, and cardiac abnormalities at postmortem. However, concerns have been raised with regard to the accuracy of job classification, the intensity of occupational activity relative to active leisure, the adequacy of disease classification, and confounding influences due to differences of social class, stress and potential alienation. Studies comparing athletes and non-athletes have been faulted on grounds of initial selection for sport by body-build and uncertainties regarding continuing differences of endurance activity between recognised university athletes and their classmates. In general, no advantage of life expectancy has been seen in athletes, Karvonen and associates reported a 4 to 5 year advantage of longevity in Finnish cross-country skiing champions, although this might be attributable to other facets of their lifestyle.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMedDate not supplied

Science and medicine of canoeing and kayaking.

Canoeing and kayaking are upper-body sports that make varying demands on the body, depending on the type of contest and the distance covered. The shorter events (500 m) are primarily anaerobic (2 minutes of exercise), calling for powerful shoulder muscles with a high proportion of fast-twitch fibres. In contrast, 10,000 m events call for aerobic work to be performed by the arms. Such contestants need a high proportion of slow-twitch fibres, and an ability to develop close to 100% of their leg maximum oxygen intake when paddling. In slalom and whitewater contests, the value of physiological testing is somewhat limited, since performance is strongly influenced by experience and the ability to make precisely judged rapid paddling efforts under considerable emotional stress. Paddlers face dangers from their hostile cold water environment; causes of fatalities (drowning, cardiac arrest, ventricular fibrillation and hypothermia) are briefly reviewed. Medical problems include provision of adequate nutrition and a clean water supply, effects of repeated immersion (softening of the skin, blistering, paronychial infections, sinusitis, otitis), varicose veins (secondary to thoracic fixation) and hazards of exposure to fibreglass and polystyrene in the home workshop. Surgical problems include muscle sprains and mechanical injuries (haemotomas, lacerations, contusions, concussion, and fractures).

Biomechanical Phenomena↗
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Carbohydrate and fluid needs of the soccer player.

Soccer is a game that demands a combination of repeated maximal sprinting wit with 10 to 11km of moderate running, sometimes performed under extremely warm conditions. Over the course of a match, there is partial to near complete depletion of glycogen reserves in the leg muscles (depending on the extent of initial reserves and the level of competition), with a resultant decrease in physical performance. Blood glucose levels also fall, sometimes to values likely to cause a deterioration of both tactical thinking and cooperation between players (3.0 to 3.8 mmol/L), while in tropical climates, fluid losses can amount to 4 to 5kg of bodyweight. The effectiveness of glucose solutions in correcting these problems is limited for 2 main reasons: concentrations greater than 2.5% slow the rate of gastric emptying and thus fluid absorption, while provoking a secretion of insulin with a resultant hypoglycaemia. Fructose solutions are less liable to increase insulin secretion, but they have an equal propensity for slowing gastric emptying; moreover, the ingested fructose is largely metabolised in the liver, without boosting blood glucose. However, glucose polymer preparations have a low osmotic pressure per unit content of glucose equivalent, so that substantial amounts of carbohydrate can be administered in this fashion before gastric emptying is inhibited. If polymers are given before and during a soccer game, they sustain blood glucose, sparing muscle glycogen stores and increasing game performance. If the concentration of polymer is too high, one possible complication is a movement of water from the plasma into the gut; nevertheless, with an appropriate choice of concentration (for example, 7% polycose, 360 mOsm/L, plasma volume is increased rather than decreased relative to that seen with administration of water. Probably because the intergame interval for competitive soccer players is short, replenishment of glycogen reserves proceeds quite slowly. Moreover, this process does not seem to be helped by ingestion of either glucose polymers or a high carbohydrate diet.

Animals↗
PubMedDate not supplied

Worksite fitness and exercise programs: a review of methodology and health impact.

PURPOSE: To examine the methodology of worksite fitness and exercise programs and to assess their effect on health-related fitness, cardiac risk factors, life satisfaction and well-being, and illness and injury. SEARCH METHODS: The 52 studies reviewed cover English-language literature for the period from 1972 to 1994, as identified by a search of the Cumulative Index Medicus, Medline, the Canadian Sport Documentation Centre's "Sport Discus," computerized bibliography, and my own files. Reports were divided into five controlled experimental studies, 14 quasi-experimental studies with matched controls (one reported in abstract), and 33 other interventions of varied quality. SUMMARY: Methodologic problems include difficulty in allowing for Hawthorne effects, substantial sample attrition, and poor definition of the intervention (exercise or broad-based health promotion). Findings are analyzed by specific fitness and health outcomes. Program participants show small but favorable changes in body mass, skinfolds, aerobic power, muscle strength and flexibility, overall risk-taking behavior, systemic blood pressure, serum cholesterol, and cigarette smoking. Claims of improved mood state are based heavily on uncontrolled studies. Quasi-experimental studies suggest reduced rates of illness and injury among participants, but seasonal and year-to-year differences in health weaken possible conclusions. CONCLUSIONS: Participation in worksite fitness programs can enhance health-related fitness and reduce risk-taking behavior, but population effect is limited by low participation rates.

Cardiovascular Diseases↗
PubMedDate not supplied

The cognitive profile of those who intend to exercise but do not.

The purpose of this study was to identify the cognitive profile of people who intend to exercise but fail to carry out this intention. A theoretical framework was adopted to study the attitudinal beliefs of these persons about exercise, their evaluation of the associated consequences, and their normative beliefs and motivation to comply with these norms. Subjects were classified according to the congruence between stated intention and self-reported exercise behavior 2 months later in this way: positive intention and congruent behavior (CONG+, N = 74). positive intention and incongruent behavior (INCONG-, N = 45). negative intention and congruent behavior (CONG-, N = 42). negative intention and incongruent behavior (N = 2, not analyzed). MANOVA analysis indicated little difference between the cognitive profiles of inactive and active positive intenders. Relative to the CONG+ group, the INCONG- group perceived that regular exercise would be "tiring" (P less than 0.001) and "time consuming" (P less than 0.001); they also placed less value on the consequence of "being healthy" (P less than 0.05). Both groups differed from the CONG- group. As would be expected, those with positive intentions to exercise identified more advantages to being physically active. It appears that sedentary positive intenders perceived the exercise behavior as physically demanding and had difficulty in reconciling the time demands of an exercise program with their weekly schedules. This observation suggests that these two beliefs should be considered for the promotion of physical activity as well as the investigation of influential social and environmental variables.

Adult↗