Abstract
We compared the metabolic capacity of calf muscles in young normal subjects who performed
endurance training to highly trained track athletes, older subjects, and subjects
with several metabolic diseases. Metabolic capacity was measured as the rate of phosphocreatine
(PCr) recovery following submaximal exercise. PCr levels were measured using magnetic
resonance spectroscopy (MRS). Exercise consisted of repeated plantar flexion against
resistance. MRS calculated metabolic capacity agreed well with rates from muscle biopsies.
Short term endurance training resulted in a 14% increase in metabolic capacity. This
adaptation was small compared to the metabolic capacities of endurance athletes (70%
greater than control subjects), and changes due to aging (20% and 55% declines by
66 and 80 years of age, respectively). Exercise training is also associated with muscle
injury. Muscle injury was seen as a transient elevation of Pi/PCr at rest in response
to an acute bout of exercise. The elevation of resting Pi/PCr persisted during continued
training. Metabolic capacity as measured by the rate of phosphocreatine recovery increased
with endurance training and decreased with normal aging and metabolic disease. Small
levels of persistent muscle injury is a natural by product of strenuous endurance
training.
Key words
Phosphocreatine - humans - NMR - training - age - disease
