ISSN:0112-1642    

DOI:10.2165/00007256-199111010-00001

出版商:Springer International Publishing    

年代:2012

数据来源: ADIS

摘要:

SummaryThe importance of carbohydrates as a fuel source during endurance exercise has been known for 60 years. With the advent of the muscle biopsy needle in the 1960s, it was determined that the major source of carbohydrate during exercise was the muscle glycogen stores. It was demonstrated that the capacity to exercise at intensities between 65 to 75% V̇O2maxwas related to the pre-exercise level of muscle glycogen, i.e. the greater the muscle glycogen stores, the longer the exercise time to exhaustion. Because of the paramount importance of muscle glycogen during prolonged, intense exercise, a considerable amount of research has been conducted in an attempt to design the best regimen to elevate the muscle’s glycogen stores prior to competition and to determine the most effective means of rapidly replenishing the muscle glycogen stores after exercise. The rate-limiting step in glycogen synthesis is the transfer of glucose from uridine diphosphate-glucose to an amylose chain. This reaction is catalysed by the enzyme glycogen synthase which can exist in a glucose-6-phosphate-dependent, inactive form (D-form) and a glucose-6-phosphate-independent, active form (I-form). The conversion of glycogen synthase from one form to the other is controlled by phosphorylation-dephosphorylation reactions.The muscle glycogen concentration can vary greatly depending on training status, exercise routines and diet. The pattern of muscle glycogen resynthesis following exercise-induced depletion is biphasic. Following the cessation of exercise and with adequate carbohydrate consumption, muscle glycogen is rapidly resynthesised to near pre-exercise levels within 24 hours. Muscle glycogen then increases very gradually to above-normal levels over the next few days. Contributing to the rapid phase of glycogen resynthesis is an increase in the percentage of glycogen synthase I, an increase in the muscle cell membrane permeability to glucose, and an increase in the muscle’s sensitivity to insulin. The slow phase of glycogen synthesis appears to be under the control of an intermediate form of glycogen synthase that is highly sensitive to glucose-6-phosphate activation. Conversion of the enzyme to this intermediate form may be due to the muscle tissue being constantly exposed to an elevated plasma insulin concentration subsequent to several days of high carbohydrate consumption.For optimal training performance, muscle glycogen stores must be replenished on a daily basis. For the average endurance athlete, a daily carbohydrate consumption of 500 to 600g is required. This results in a maximum glycogen storage of 80 to 100 µmol/g wet weight. To glycogen supercompensate in preparation for competition, the muscle glycogen stores must first be exercise-depleted. This should then be followed with a natural training taper. During the first 3 days of tapering, a mixed diet composed of 40 to 50% carbohydate should be consumed. During the last 3 days of tapering, a diet consisting of 70 to 80% carbohydrate is consumed. This procedure results in muscle glycogen concentrations that are comparable to those produced by more rigorous regimens that can result in chronic fatigue and injury. For rapid resynthesis of muscle glycogen stores, a carbohydrate supplement in excess of 1 g/kg bodyweight should be consumed immediately after competition or after a training bout. Continuation of supplementation every 2 hours will maintain a maximal rate of storage up to 6 hours after exercise. Supplements composed of glucose or glucose polymers are more effective for the replenishment of muscle glycogen stores after exercise than supplements composed of predominantly fructose. However, some fructose is recommended because it is more effective than glucose in the replenishment of liver glycogen.

ISSN:0112-1642    

DOI:10.2165/00007256-199111010-00002

出版商:Springer International Publishing    

年代:2012

数据来源: ADIS

摘要:

SummaryEstimated maximum oxygen uptake of middle-aged nonelite road race entrants is around 45 to 50 ml/kg/min, which is 40 to 100% higher than values from the female general population. Endurance training, low bodyweight, and nonsmoking of runners explain part of, but not the whole, difference in aerobic capacity observed between athletes and the general population. Sedentary women can improve cardiorespiratory fitness through aerobic exercise programmes, and the women with the lowest level of initial fitness have the highest proportional improvement following training. Regularly exercising women have a significantly reduced risk of fatal and nonfatal coronary events, and low cardiorespiratory fitness is associated with an increased risk of death and nonfatal stroke. The influence of habitual running on the female blood lipid profile is not clear. Cross-sectional studies have found elevated HDL cholesterol concentrations in distance runners, but intervention studies on the effect of jogging on lipid and lipoprotein levels have provided equivocal results. A higher level of physical fitness is associated with a lower risk to subsequently develop hypertension. Experimental studies have shown that moderate intensity aerobic exercise (40 to 60% V̇O2max) is able to reduce blood pressure significantly in hypertensive subjects. An athletic lifestyle may be associated with a reduced risk of adult-onset diabetes mellitus (via an exercise-induced increase in insulin sensitivity), and with a reduced risk of cancers of the reproductive system, breast, and colon. Recreational running is also correlated with better weight control.Surveys of recreational and elite distance runners show a great variability in the prevalence of secondary amenorrhoea, between 1 and 44%. Environmental factors determining the risk of amenorrhoea in runners are low body fat content, mileage, and nutritional inadequacy, with low intakes of calories, protein, and fat. Amenorrhoeic athletes in their third and fourth decade have lower vertebral bone density, which is improved after resumption of menses but does not completely reach age-specific average values. Regardless of menstrual status, the effectiveness of exercise to maintain bone mass throughout life is an important issue. Habitual exercise is associated with increased bone density of the spine both in premenopausal and postmenopausal women. Several controlled training studies suggest that postmenopausal women may at least retard their bone loss with regular aerobic exercise.Running-related injuries and complaints are common in recreational joggers, even though the reported 1-year incidence, varying between 14 and approximately 50%, depends on injury definition. Mileage and a history of previous running injury are known risk factors. Overweight, irregular menses, and absence of oral contraceptive use have been identified as risk factors in single studies. Female gender itself does not seem to be a major risk factor of running injuries among habitually active subjects, but it may be a relevant factor for sedentary subjects taking up jogging. Regarding the effect of habitual running on the development of osteoarthritis in weight-bearing joints, available data suggest that reasonable recreational exercise, carried out within limits of comfort, putting joints through normal motions, without underlying joint abnormality, even over many years, is unlikely to lead to significant joint injury.Habitual exercise is associated with reductions in anxiety and depression as well as increased self-esteem. The latter is an empirically supported outcome of exercise, and programmes of aerobic exercise seem also to be effective in reducing state anxiety and symptoms of mild depression. The prevalence of anorexia nervosa among competitive distance runners is not higher than among the general population, but it is the best runners who are most likely to be anorectic. Nonsmoking is highly prevalent among runners, and habitual runners who smoke have a quit rate of roughly 75%, with the rate of smoking cessation being related to mileage. Compared with the general population, age-matched runners have significantly fewer medical consultations, and probably less missed work days.Little data on the health effects of recreational, in contrast to competitive, running is available, and most epidemiological studies on prevention through exercise suffer from methodological shortcomings that hamper the ability to evaluate the health potential of aerobic exercise in an unbiased way. Nevertheless, there is a broad consensus that an energy expenditure of at least 150 to 400 kcal/day (corresponding to jogging 2.5 to 6 km/day) at a moderate intensity, should be the goal for health-oriented exercise.

ISSN:0112-1642    

DOI:10.2165/00007256-199111010-00003

出版商:Springer International Publishing    

年代:2012

数据来源: ADIS

摘要:

SummaryThe increasing participation in the athletic forms of bicycling warrants expanded physician attention to the traumatic and overuse injuries experienced by cyclists.The modern bicycle consists of a frame with various components, including handlebars, brakes, wheels, pedals, and gears, in various configurations for the various modes of cycling. For high performance cycling the proper fit of the bicycle is critical. The most efficient method to provide an accurate fit is the Fitkit, but proper frame selection and adjustment can be made by following simple guidelines for frame size, seat height, fore and aft saddle position, saddle angle, reach and handlebar height. The human body functions most effectively in a narrow range of pedal resistance to effort. Riding at too much pedal resistance is a major cause of overuse problems in cyclists. Overuse injuries are lower using lower gear ratios at a higher cadence.Cycling injuries account for 500 000 visits per year to emergency rooms in the US. Over half the accidents involve motor vehicles, and road surface and mechanical problems with the bicycle are also common causes of accidents. Head injuries are common in cyclists and account for most of the fatal accidents. Despite good evidence of their effectiveness, victims with head injuries have rarely worn helmets. Contusions, sprains and fractures may occur throughout the body, most commonly to the hand, wrist, lower arm, shoulder, ankle and lower leg. The handlebar and seat have been implicated in a wide variety of abdominal and genital injuries. Abrasions, lacerations and bruises of the skin are the most common traumatic injuries. Trauma may be prevented or reduced by proper protective safety equipment and keeping the bike in top mechanical condition. Anticipation of the errors of others and practising and adopting specific riding strategies also help to prevent traumatic injuries.Management of overuse injuries in cycling generally involves mechanical adjustment as well as medical management. Neck and back pain are extremely common in cyclists, occurring in up to 60% of riders. Ulnar neuropathy, characterised by tingling, numbness and weakness in the hands is common in serious cyclists after several days of riding. Managing saddle-related injuries or irritations may also involve adjusting seat height, angle and fore and aft position in addition to changing the saddle. Padding in the saddle and shorts play an important part in saddle problems. Saddle-related problems include chafing, perineal folliculitis and furuncles, subcutaneous perineal nodules, pudendal neuropathy, male impotence, traumatic urethritis and a variety of vulva trauma. Improper fit of the bicycle may also lead to problems such as trochanteric bursitis, iliopsoas tendinitis, and ‘biker’s knee’ (patellofemoral pain syndrome). Foot paraesthesias, metatarsalgia and occasionally Achilles tendinitis and plantar fasciitis have also been reported in cyclists. Cyclists should take proper precautions against sun and heat injuries, especially dehyration.Cyclists may benefit from a variety of protective clothing and equipment, such as helmets, mirrors, eyewear, lights and reflective clothing and footwear.

ISSN:0112-1642    

DOI:10.2165/00007256-199111010-00004

出版商:Springer International Publishing    

年代:2012

数据来源: ADIS