Metabolic adaptations to exercise: a review of potential beta-adrenoceptor antagonist effects

Human skeletal muscle contains 2 muscle fiber types: slow twitch (type I) and fast twitch (type II). They have different profiles including their biochemical, metabolic, O2 diffusion, microcirculatory and neuromotor characteristics. The slow twitch fiber represents endurance, high combustive potential and recruitment during moderate activity; in contrast, the fast twitch represents explosiveness, force, high capacity for phosphate splitting and lactate formation, but is more fatiguable. A muscle rich in slow twitch fibers is confined to low peripheral resistance at rest and during exercise, higher exercise leg blood flow and higher maximal oxygen uptake (VO2 max). During graded exercise lactate has been shown to be a good marker for the metabolic and circulatory characteristics of the contracting muscle and the exercise intensity (W) eliciting a blood lactate concentration of 4 mmol/liter-1 [(WOBLA) from onset of blood lactate accumulation] integrated for peripheral metabolic, neuromotor and central circulatory potentials both in health and disease. It is well known that a blood lactate level greater than 4 mmol/liter-1 represents a major increase in sympathetic tone and is incompatible with endurance or prolonged exercise. With prolonged exercise and sympathetic regulation both circulation and metabolism adapt. Adipose tissue is stimulated and fatty acids are released. Muscle tissue lipoprotein lipase activity is enhanced; that is, there is increased utilization of blood triglycerides for local lipolysis in the capillary bed of the contracting muscle. Both mechanisms will increase fatty acid availability and induce a "glycogen-sparing effect" resulting in a reduced respiratory exchange ratio. Studies have shown that both the magnitude of the initial glycogen stores and these adaptive responses will determine performance time. With age, changes take place in heart rate regulation, neuromotor control and muscle fibers. Thus VO2 max is decreased, but partly compensated for by a fast motor unit and fiber loss leading to a muscle more rich in slow twitch fibers--an "endurance training-like effect." Relative endurance is also increased with age; however, lactate metabolism is still a critical feature. The OBLA concept describes capacity for both occupational and leisure-time physical activity.