Effects of caffeine on time to exhaustion in exercise performed below and above the anaerobic threshold

Controversy still exists concerning the potential ergogenic benefit of caffeine (CAF) for exercise performance. The purpose of this study was to compare the effects of CAF ingestion on endurance performance during exercise on a bicycle ergometer at two different intensities, i.e., approximately 10 percent below and 10 percent above the anaerobic threshold (AT). Eight untrained males, non-regular consumers of CAF, participated in this study. AT, defined as the intensity (watts) corresponding to a lactate concentration of 4mM, was determined during an incremental exercise test from rest to exhaustion on an electrically braked cycle ergometer. On the basis of these measurements, the subjects were asked to cycle until exhaustion at two different intensities, i.e., approximately 10 percent below and 10 percent above AT. Each intensity was performed twice in a double-blind randomized order by ingesting either CAF (5 mg/kg) or a placebo (PLA) 60 min prior to the test. Venous blood was analyzed for free fatty acid, glucose, and lactate, before, during, and immediately after exercise. Rating of perceived exertion and time to exhaustion were also measured during each trial. There were no differences in free fatty acids or lactate levels between CAF and PLA during and immediately after exercise for either intensity. Immediately after exercise glucose increased in the CAF trial at both intensities. Rating of perceived exertion was singificantly lower (CAF = 14.1 + 2.5 vs PLA = 16.6 + 2.4) and time to exhaustion was significantly higher (CAF = 46.54 + 8.05 min vs PLA = 32.42 + 14.81 min) during exercise below AT with CAF. However, there was no effect of CAF treatment on rating of perceived exertion (CAF = 18.0 + 2.7 vs PLA = 17.6 + 2.3) and time to exhaustion (CAF = 18.45 + 7.28 min vs PLA = 19.17 + 4.37 min) during exercise above AT. We conclude that in untrained subjects caffeine can improve endurance performance during prolonged exercise performed below AT and that decrease of perceived exertion can be involved in this process.

Possível efeito da suplementaçäo de aminoácidos de cadeia ramificada, aspartato e asparagina sobre o limiar anaeróbio / Possible effect of the supplementation of branched chain aminoacids, aspartate and asparagine on anaerobic threshold

Recentemente o conceito "Limiar Anaeróbio" tem sido muito criticado. As prinipais criticas repousam sobre os mecanismos considerados para o aumento da concentraçäo de lactato sangüíneo, hipóxia muscular principalmente, e sobre a suposta relaçäo de causa-e-efeito entre os limiares metabólico e ventilatório. Apesar de criticado, o conceito Limiar Anaeróbio encontrou muitas aplicaçöes, e por esta razäo, vários estudos foram realizados para facilitar a sua determinaçäo näo invasivamente, a partir de parâmetros ventilatórios e da deflexäo da curva de freqüência cardíaca. Recentemente o uso de aminoácidos tem se difundido largamente entre os praticantes de atividades motoras, tornando-se objeto de estudo para vßrios pesquisadores. Foi proposto que a suplementaçäo de aminoácidos de cadeia ramificada, aspartato e asparagina promove aumento da resistência ao esforço físico prolongado, em decorrência do aumento do conteúdo de glicogênio muscular e síntese de oxaloacetato para manutençäo da atividade do ciclo de Krebs e do próprio metabolismo oxidativo. Com isto o transporte de glicose para o interior da célula muscular diminui, retardando a depleçäo de glicogênio muscular e a acidose metabólica, causas evidentes de fadiga. Em conseqüência, a oxidaçäo de AGLs durante o exercício contínuo moderado aumenta, retardando o acúmulo de lactato sangüíneo e muscular. Isto retardaria o estímulo metabólico para o processo de tamponamento pelo HCO3 e a conseqüente compensaçäo ventilatória para CO2, podendo dissociar os limiares metabólico e ventilatório. A dissociaçäo dos limiares reforçaria as críticas sobre o conceito Limiar Anaeróbio e poderia, inclusive, prejudicar a sua identificaçäo através de outros métodos, como por exemplo, a deflexäo da curva de freqüência cardíaca.

Acute increase in exercise capacity with milrinone: lack of correlation with resting hemodynamic responses

1-The acute administration of milrinone, a positive inotropic vasodilator agent, improves resting hemodynamic function and maximal and submaximal metabolic rsponses to exercice in patients with severe congestive heart failure. 2. to determine whether the improvement in exerecise capacity induced by milrinone administration can be predicted by its acute positive inotropic and/or vasodilator effects at rest, milrinone was adminstered intravenously (progressive doses of 1.5 to 75 microng/Kg) to 15 patients with heart failure (functional classes III and IV, New York Heart Association) at rest, and during maximal upright exercise testing on a cycke ergometer. Serum drug levels were matched for the resting and exercise tests. Drug administration for exercise tests was placebo-controlled and double-blind. 3. At rest, milrinone administration caused substantial decreases in right atrial pressure (-53%), left ventricular end-diastolic pressure (-30%), and systemic vascular resistance (-35%); and increases in cardiac index (+59%), peak positive dP/dt (+20%) and stroke work index (+51%). Administration of milrinone during exercise resulted in a 15% increase in peak oxygen uptake and a 16% increase in anaerobic threshold. However, none of the changes in resting hemodynamic function correlated significantly in magnitude with the changes in peak oxygen uptake and anaerobic threshold. 4. Thus, the acute improvement in exercise capacity that occus with milrinone is not predicted by the positive inotropic or vasodilator effects of the drug at rest