Measuring exercise-induced adaptations and injury with magnetic resonance spectroscopy.

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.