Utilization of blood‐borne and intramuscular substrates during continuous and intermittent exercise in man.

Abstract

Substrate utilization in the legs during bicycle exercise was studied in 5 subjects when performing intermittent intense exercise (15 s work-15 s rest) as well as continuous exercise during 60 min, with an almost identical average power output and O2 uptake in both situations. Muscle biopsies were obtained from vastus lateralis at rest, during and after exercise in order to determine intramuscular lipid and carbohydrate utilization. The contribution from blood-borne substrates to total oxidative metabolism was determined by arterial-femoral venous (a-fv) differences for O2, FFA [free fatty acids], glucose and lactate and leg blood flow. Intermittent and continuous exercise revealed a similar glycogen depletion and the intramuscular lactate accumulation was rather small. A similar uptake of blood-borne substrate (FFA, glucose) was found in both situations whereas a release of lactate only was observed in intermittent exercise. ATP and CP [creatine phosphate] levels oscillated between work and rest periods in intermittent exercise but were not resynthesized to resting levels at the end of the rest periods. The mainly aerobic energy release during each work period in intermittent exercise was partly caused by myoglobin functioning as an O2 store; this factor was more important than ATP and CP or lactate level oscillations. The metabolic response to intermittent exercise was similar to that found in continuous exercise with approximately the same average power out-put and O2 uptake. This indicates that some factor in the intermediary metabolism, for instance citrate, functions as a regulator retarding glycolysis and favoring lipid utilization and an aerobic energy release in intermittent exercise.