Bengt Kayser

Exercise starts and ends in the brain

Classically the limit to endurance of exercise is explained in terms of metabolic capacity. Cardio-respiratory capacity and muscle fatigue are thought to set the limit and the majority of studies on factors limiting endurance exercise discuss issues such as maximal oxygen uptake (V̇O2max), aerobic enzyme capacity, cardiac output, glycogen stores, etc. However, this paradigm does not explain the limitation to endurance exercise with large muscle groups at altitude, when at exhaustion exercise is ended without limb locomotor muscle fatigue and with sub-maximal cardiac output. A simple fact provides a basis for an explanation. Voluntary exercise starts and ends in the brain. It starts with spatial and temporal recruitment of motor units and ends with their de-recruitment. A conscious decision precedes a voluntary effort. The end of effort is again volitional and a forced conscious decision to stop precedes it, but it is unknown what forces the off-switch of recruitment at exhaustion although sensation of exertion certainly plays a role. An alternative model explaining the limitation of exercise endurance thus proposes that the central nervous system integrates input from various sources all related to the exercise and limits the intensity and duration of recruitment of limb skeletal muscle to prevent jeopardizing the integrity of the organism. This model acknowledges the cardio-respiratory and muscle metabolic capacities as prime actors on the performance scene, while crediting the central nervous system for its pivotal role as the ultimate site where exercise starts and ends.

Physical activity and pregnancy: cardiovascular adaptations, recommendations and pregnancy outcomes.

Regular physical activity is associated with improved physiological, metabolic and psychological parameters, and with reduced risk of morbidity and mortality. Current recommendations aimed at improving the health and wellbeing of nonpregnant subjects advise that an accumulation of ≥30 minutes of moderate physical activity should occur on most, if not all, days of the week.Regardless of the specific physiological changes induced by pregnancy, which are primarily developed to meet the increased metabolic demands of mother and fetus, pregnant women benefit from regular physical activity the same way as nonpregnant subjects.Changes in submaximal oxygen uptake (V̇O2) during pregnancy depend on the type of exercise performed. During maternal rest or submaximal weight-bearing exercise (e.g. walking, stepping, treadmill exercise), absolute maternal V̇O2 is significantly increased compared with the nonpregnant state. The magnitude of change is approximately proportional to maternal weight gain.When pregnant women perform submaximal weight-supported exercise on land (e.g. level cycling), the findings are contradictory. Some studies reported significantly increased absolute V̇O2, while many others reported unchanged or only slightly increased absolute V̇O2 compared with the nonpregnant state. The latter findings may be explained by the fact that the metabolic demand of cycle exercise is largely independent of the maternal body mass, resulting in no absolute V̇O2 alteration.Few studies that directly measured changes in maternal maximal V̇O2 (V̇O2max) showed no difference in the absolute V̇O2max between pregnant and nonpregnant subjects in cycling, swimming or weight-bearing exercise. Efficiency of work during exercise appears to be unchanged during pregnancy in non-weight-bearing exercise. During weight-bearing exercise, the work efficiency was shown to be improved in athletic women who continue exercising and those who stop exercising during pregnancy. When adjusted for weight gain, the increased efficiency is maintained throughout the pregnancy, with the improvement being greater in exercising women.Regular physical activity has been proven to result in marked benefits for mother and fetus. Maternal benefits include improved cardiovascular function, limited pregnancy weight gain, decreased musculoskeletal discomfort, reduced incidence of muscle cramps and lower limb oedema, mood stability, attenuation of gestational diabetes mellitus and gestational hypertension. Fetal benefits include decreased fat mass, improved stress tolerance, and advanced neurobehavioural maturation. In addition, few studies that have directly examined the effects of physical activity on labour and delivery indicate that, for women with normal pregnancies, physical activity is accompanied with shorter labour and decreased incidence of operative delivery.However, a substantial proportion of women stop exercising after they discover they are pregnant, and only few begin participating in exercise activities during pregnancy. The adoption or continuation of a sedentary lifestyle during pregnancy may contribute to the development of certain disorders such as hypertension, maternal and childhood obesity, gestational diabetes, dyspnoea, and pre-eclampsia. In view of the global epidemic of sedentary behaviour and obesity-related pathology, prenatal physical activity was shown to be useful for the prevention and treatment of these conditions. Further studies with larger sample sizes are required to confirm the association between physical activity and outcomes of labour and delivery.

Physical activity: the health benefits outweigh the risks.

Purpose of reviewThis article will summarize the current findings on the effects of physical activity on human health and well-being. Recent findingsPhysical activity is associated with enhanced health and reduced risk of all-cause mortality such as cardiovascular disease, hypertension, type 2 diabetes, obesity, osteoporosis, sarcopenia, cognitive disorders, and some forms of cancer. Nevertheless, the effects of exercise with respect to potential health consequences are complex. When untrained or previously sedentary persons undertake vigorous exertion suddenly, the undesired side effects of injuries, dehydration or cardiac arrest are amplified. SummaryIt is reasonable to conclude that the risk exposure through physical activity is outweighed by its overall benefits, and health authorities strongly encourage participation in moderate intensity physical activity on a daily basis. In the future, the identification and characterization of particularly inactive sub-groups of the population may facilitate and optimize the planning of public health interventions.

Effects of recommended levels of physical activity on pregnancy outcomes

OBJECTIVE We sought to examine the relation between recommended levels of physical activity during pregnancy and pregnancy outcomes. STUDY DESIGN We conducted an observational study with energy expenditure, aerobic fitness, and sleeping heart rate measured in 44 healthy women in late pregnancy. Medical records were examined for pregnancy outcome. RESULTS Active women, who engaged in > or = 30 minutes of moderate physical activity per day, had significantly better fitness and lower sleeping heart rate compared to the inactive. Duration of second stage of labor was 88 and 146 minutes in the active vs inactive women, respectively (P = .05). Crude odds ratio of operative delivery in the inactive vs the active was 3.7 (95% confidence interval, 0.87-16.08). Birthweight, maternal weight gain, and parity adjusted odds ratio was 7.6 (95% confidence interval, 1.23-45.8). Neonatal condition and other obstetric outcomes were similar between groups. CONCLUSION Active women have better aerobic fitness as compared to inactive women. The risk for operative delivery is lower in active women compared to inactive, when controlled for birthweight, maternal weight gain, and parity. Further studies with larger sample size are required to confirm the association between physical activity and pregnancy outcomes.

Globalisation of anti-doping: the reverse side of the medal

Current anti-doping policy is sufficiently problematic to call for debate and change, say Bengt Kayser and Aaron C T Smith

Nervous system function during exercise in hypoxia.

Aerobic exercise capacity decreases with exposure to hypoxia. This article focuses on the effects of hypoxia on nervous system function and the potential consequences for the exercising human. Emphasis is put on somatosensory muscle afferents due to their crucial role in the reflex inhibition of muscle activation and in cardiorespiratory reflex control during exercise. We review the evidence of hypoxia influences on muscle afferents and discuss important consequences for exercise performance. Efferent (motor) nerves are less affected at altitude and are thought to stay fairly functional even in severe levels of arterial hypoxemia. Altitude also alters autonomic nervous system functions, which are thought to play an important role in the regulation of cardiac output and ventilation. Finally, the consequences of hypoxia-induced cortical adaptations and dysfunctions are evaluated in terms of neurotransmitter turnover, brain electrical activity, and cortical excitability. Even though the cessation of exercise or the reduction of exercise intensity, when reaching maximum performance, implies reduced motor recruitment by the nervous system, the mechanisms that lead to the de-recruitment of active muscle are still not well understood. In moderate hypoxia, muscle afferents appear to play an important role, whereas in severe hypoxia brain oxygenation may play a more important role.

Nutrition and energetics of exercise at altitude. Theory and possible practical implications.

SummaryAltitude exposure may lead to considerable weight loss. Most reports, showing weight losses of 3% in 8 days at 4300m and up to l5% after 3 months at 5300 to 8000m, appear to indicate that this weight loss is a function of both absolute altitude and the duration of exposure.Based on the available scientific evidence to date, it is concluded that altitude weight loss is because of an initial loss of water and subsequent loss of fat and muscle mass due to malnutrition. Up to 5500m, malabsorption of macronutrients does not occur. Up to altitudes around 5000m, weight loss from a reduction of fat and muscle appears to be avoidable by maintaining adequate dietary intake. Primary anorexia, lack of comfort and palatable food, detraining, and possibly direct effects of hypoxia on protein metabolism seem inevitably to lead to weight loss during longer exposures at higher altitudes. To minimise losses, it is advisable to acclimatise properly, reduce the length of stay at extreme altitude as much as possible and maintain a high and varied nutrient intake. With sojourns at intermediate altitude for training purposes, adequate energy intake should be maintained taking into account the decrease in aerobic training intensity and the increase in basal metabolic rate that ensue from the hypoxic environment.

THE DECREASE OF MAXIMAL OXYGEN CONSUMPTION DURING HYPOXIA IN MAN : A MIRROR IMAGE OF THE OXYGEN EQUILIBRIUM CURVE

1. Endurance athletes (E) undergo a marked reduction of arterial O2 saturation (Sa,O2) at maximal exercise in normoxia, which disappears when they breathe hyperoxic mixtures. In addition, at a given level of hypoxia, the drop in maximal O2 consumption (VO2,max) is positively related to the individual normoxic VO2,max. 2. These data suggest that the curve relating VO2,max to PI,O2 may be steeper and perhaps less curved in E than in sedentary subjects (S) with low VO2,max values because of the greater hypoxaemia in the latter, whence the hypotheses that (i) the relationship between VO2,max and PI,O2 may be set by the shape of the oxygen equilibrium curve; and (ii) the differences between E and S may be due to the different position on the oxygen equilibrium curve on which these subjects operate. These hypotheses have been tested by performing a systematic comparison of the VO2,max or Sa,O2 vs. PI,O2 relationships in E and S. 3. On ten subjects (five S and five E), VO2,max was measured by standard procedure during cycloergometric exercise. Sa,O2 was measured by finger‐tip infrared oximetry. Arterialized blood PO2 (Pa,O2) and PCO2 (Pa,CO2) were determined in 80 microliters blood samples from an ear lobe. The subjects breathed ambient air or a N2‐O2 mixture with an inspired O2 fraction (FI,O2) of 0.30, 0.18, 0.16, 0.13 and 0.11, respectively, VO2,max was normalized with respect to that obtained at the highest FI,O2. 4. The relationships between Sa,O2 or normalized VO2,max and FI,O2 (or PI,O2) had similar shapes, the data for E being systematically below and significantly different from those for S. Linear relationships between Sa,O2 and normalized VO2,max, statistically equal between E and S, were found. 5. We conclude that the relationships between either VO2,max or Sa,O2 and FI,O2 (or Pa,O2) may indeed be the mirror images of one another, implying a strict link between the decrease of VO2,max in hypoxia and the shape of the oxygen equilibrium curve, as hypothesized.

Carbohydrate Mouth Rinse Effects on Exercise Capacity in Pre- and Postprandial States

Background. Oropharyngeal receptors signal presence of carbohydrate to the brain. Mouth rinses with a carbohydrate solution facilitate corticomotor output and improve time-trial performance in well-trained subjects in a fasted state. We tested for this effect in nonathletic subjects in fasted and nonfasted state. Methods. 13 healthy non-athletic males performed 5 tests on a cycle ergometer. After measuring maximum power output (Wmax), the subjects cycled four times at 60% Wmax until exhaustion while rinsing their mouth every 5 minutes with either a 6.4% maltodextrin solution or water, one time after an overnight fast and another after a carbohydrate rich breakfast. Results. Mouth rinsing with maltodextrin improved time-to-exhaustion in pre- and postprandial states. This was accompanied by reductions in the average and maximal rates of perceived exertion but no change in average or maximal heart rate was observed. Conclusions. Carbohydrate mouth rinsing improves endurance capacity in both fed and fasted states in non-athletic subjects.

Does cerebral oxygen delivery limit incremental exercise performance

Previous studies have suggested that a reduction in cerebral oxygen delivery may limit motor drive, particularly in hypoxic conditions, where oxygen transport is impaired. We hypothesized that raising end-tidal Pco(2) (Pet(CO(2))) during incremental exercise would increase cerebral blood flow (CBF) and oxygen delivery, thereby improving peak power output (W(peak)). Amateur cyclists performed two ramped exercise tests (25 W/min) in a counterbalanced order to compare the normal, poikilocapnic response against a clamped condition, in which Pet(CO(2)) was held at 50 Torr throughout exercise. Tests were performed in normoxia (barometric pressure = 630 mmHg, 1,650 m) and hypoxia (barometric pressure = 425 mmHg, 4,875 m) in a hypobaric chamber. An additional trial in hypoxia investigated effects of clamping at a lower Pet(CO(2)) (40 Torr) from ∼75 to 100% W(peak) to reduce potential influences of respiratory acidosis and muscle fatigue imposed by clamping Pet(CO(2)) at 50 Torr. Metabolic gases, ventilation, middle cerebral artery CBF velocity (transcranial Doppler), forehead pulse oximetry, and cerebral (prefrontal) and muscle (vastus lateralis) hemoglobin oxygenation (near infrared spectroscopy) were monitored across trials. Clamping Pet(CO(2)) at 50 Torr in both normoxia (n = 9) and hypoxia (n = 11) elevated CBF velocity (∼40%) and improved cerebral hemoglobin oxygenation (∼15%), but decreased W(peak) (6%) and peak oxygen consumption (11%). Clamping at 40 Torr near maximal effort in hypoxia (n = 6) also improved cerebral oxygenation (∼15%), but again limited W(peak) (5%). These findings demonstrate that increasing mass cerebral oxygen delivery via CO(2)-mediated vasodilation does not improve incremental exercise performance, at least when accompanied by respiratory acidosis.