Biochemical and histochemical alterations in skeletal muscle in man during a period of reduced activity.

This study investigated the hypothesis that the adaptations in skeletal muscle to prolonged exercise overload, involving high levels of adenosine triphosphate (ATP) resynthesis, result in a preferential adaptation to pathways involved in energy metabolism. The change in selected properties of skeletal muscle during a period of reduced activity was used as an indication of training-induced adaptations. Muscle biopsy samples from the vastus lateralis were analyzed 6 weeks and 18 weeks after a 5-month, intense, intermittent training program. Significant reductions occurred (p less than 0.05) in enzyme activities representative of the citric acid cycle (succinic dehydrogenase, SDH), beta oxidation of free fatty acids (3-hydroxyacyl CoA dehydrogenase, HADH), glycogenolysis (total phosphorylase, PHOSP), and glycolysis (phosphofructokinase, PFK). In addition, reductions in concentration (p less than 0.05) were also found for ATP, creatine phosphate (CP), and glycogen. With the exception of PFK, all enzyme changes and the high energy phosphates reached new stable levels by at least the 6th week of detraining. The absence of changes in muscle cell type and size during the detraining period supports the hypothesis that adaptations in energy potential of the muscle cell predominate in this type of high intensity overload situation.