PHASES OF METABOLISM DURING PROGRESSIVE EXERCISE TO FATIGUE IN HUMAN SKELETAL-MUSCLE

The purpose of this study was to assess noninvasively both the ''oxidative potential'' and the sequence of metabolic events that occur during the transition from rest to fatigue in an isometrically exercising muscle. P-31-magnetic resonance spectroscopy was used to obtain continuous measures of intracellular phosphocreatine (PCr), inorganic phosphate (P(i)), and proton concentration ([H+]) in the tibialis anterior muscle of eight healthy human volunteers during a progressive isometric exercise protocol. The exercise protocol consisted of 2-min stages, with a duty cycle consisting of 4 s of contraction and 6 s of relaxation, beginning at 10% of the force from an initial maximal voluntary contraction (MVC) and increasing by 10% to a final load of 80% MVC. An MVC was performed at the beginning of each stage; a decrease in MVC indicated fatigue. The initial linear slope of the relationship between force and P(i)/PCr was used as an index of oxidative potential for the muscle. This initial slope ranged from 85 to 167% MVC/(P(i)/PCr), indicating substantial variability of oxidative potential in these subjects. The changes in P(i)/PCr and [H+] over time were best described with a bilinear fit of the individual data. The inflection point for each fit was defined as the point at which the slopes intersected. The P(i)/PCr inflection point occurred at a similar value of P(i)/PCr in all subjects [0.47 +/- 0.04 (SE)]. The P(i)/PCr inflection point occurred significantly earlier (7.8 +/- 0.7 min) than the [H+] inflection point (9.3 +/- 0.4 min, P < 0.01). The data describe three phases of metabolism during progressive exercise: 1) a highly oxidative phase, 2) an intermediate phase, and 3) a highly glycolytic phase, during which fatigue becomes significant. The length of the first phase was well correlated with the oxidative potential for each individual (r2 = 0.78), suggesting that both indexes reflect oxidative metabolism in the muscle. In conclusion, these results demonstrate the possibility of determining, using intermittent isometric contractions, the in vivo oxidative potential of a muscle as well as a means of detecting a series of metabolic phases that occur during progressive exercise to fatigue.