ATP production rates as a function of force level in the human gastrocnemius/soleus using 31P MRS

Net forward adenosine triphosphate (ATP) production rates were calculated from 31P nuclear magnetic resonance spectroscopy (MRS) kinetic data collected with 1-8 s time resolution during isometric voluntary contractions of the human gastrocnemius/soleus muscle group. Volume normalized muscle output (Newtons/ml) was then divided by the calculated net ATP use (mM/s) to estimate the metabolic economy (ME) (Newtons.s/mumol ATP). The ATP production rates from anaerobic glycolysis (An Gly) and creatine kinase (CK) reactions are approximately half of the oxidative phosphorylation ATP production rates (Ox Phos) at the end of 90 s of isometric contractions for a series of force levels. However, ME was independent of the force level in these exercises. The correlation between MVC of healthy, trained subjects, and the maximal cross-sectional area of the gastrocnemius/soleus shows an average of 21.2 +/- 4.6 Newtons/cm2 (mean +/- SD, N = 15). This was measured using a foot pedal with a transducer measurement point 10.6 cm above the heel. [ADP] versus Ox Phos ATP production rate fits a Michaelis-Menten kinetic control equation with an offset (underestimation of Ox Phos) of 0.3 mM/s and a Km = 27 microM for ADP and a Vmax = 1.0 mM/s. This suggests that [ADP] is the controlling factor for mitochondrial function at the end of a 90 to 120 s isometric contraction in normal subjects at any force level. Quantitative measurements of the phosphorus metabolite concentrations were obtained from 20 individuals and these data are also reported.