High-intensity endurance exercise

Although fatty acids are avidly utilized by skeletal muscle during exercise, their plasma concentrations do not differ significantly from those at rest. Their utilization is being balanced by production from adipocyte lipolysis, probably as a result of an elevated catecholamine level. 

Glucose uptake by skeletal muscle increases with intensity of exercise even though levels of circulating insulin decrease. Either this glucose transport is insulin-independent or exercise enhances insulin sensitivity. 

The increase in glucose uptake with increased intensity of exercise is matched by hepatic glucose production to the extent that arterial glucose concentrations are elevated by very intense exercise. Hepatic glycogenolysis probably is enhanced by a decrease in the level of insulin and by an increase in levels of glucagon and epinephrine. 

Exercise may lower insulin levels even though plasma glucose concentrations are increased. This effect may result from a-adrenergic inhibition of -cell secretion. 

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Fast-twitch muscle fibers have a high capacity for anaerobic glycolysis but are quick to fatigue. They are involved primarily in short-term, high-intensity exercise. Slow-twitch muscle fibers in arm and leg muscles are well vascularized and primarily oxidative. They are used during prolonged, low-to-moderate intensity exercise and resist fatigue. Slow-twitch fibers and the number of their mitochondria increase dramatically in trained endurance athletes. 

A number of studies have shown that caffeine may favorably affect long-term endurance performance (ref.3) but research results concerning high intensity, short-term exercise have been a bit mixed (ref.4). Still, it seems very likely from an analysis of the biochemical effects of caffeine that is has a beneficial effect on short-term fatigue and muscle fiber in high intensity, short-term exercise like weightlifting (ref.5 6). 

Caffeine (from guarana, tea, chocolate, coffee, or as a pure form) and increased attention/alertness, reduction in the perceived effort, and increase in endurance capacity and physical performance during short-term high-intensity exercise. 

E Kargotich, S., Goodman, C., Dawson, B., Morton, A.R., Keast, D., and Joske, D.J., Plasma glutamine responses to high-intensity exercise before and after endurance training. Res Sports Med, 13, 287, 2005. 

Failure to maintain hydration status has serious consequences for the active individual. A body water deficit of as little as 1-2% of total body mass can result in a significant reduction in exercise capacity. Endurance exercise is affected to a greater extent than high-intensity exercise, and muscle strength is not adversely affected until water losses reach 5% or more of body mass. Hypohydration greatly increases the risk of heat illness, and also abolishes the protection conferred by prior heat acclimation. 

Lambert, E.V., Speechly, D.P, Dennis, S.C., and Noakes, T.D., Eaihanced endurance in trained cyclists during moderate intensity exercise following 2 weeks adaptation to a high fat diet, Eur. J. Appl Physiol, 69, 2%1-lPii, 1994. 

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