EFFECT OF MUSCLE GLYCOGEN-CONTENT ON EXERCISE-INDUCED CHANGES IN MUSCLE T2 TIMES

Effects of gastrocnemius glycogen (Gly) concentration on changes in tr ansverse relaxation time (T2; ms) were studied after 5-min plantar fle xion at 25% of maximum voluntary contraction (MVC). Gastrocnemius Gly, phosphorus metabolites, and T2 were measured in seven subjects by usi ng interleaved C-13/P-31 magnetic resonance spectroscopy (MRS) at 4.7 T and magnetic resonance imaging (MRI; 1.5 T). After baseline MRS/MRI, subjects exercised for 5 min at 25% of MVC and were reexamined (MRS/M RI). Subjects then performed similar to 15 min of single-leg toe raise s (50 +/- 2% of MVC), depleting gastrocnemius Gly by 43%. After a 1-h rest (for T2 return to baseline), subjects repeated the 5-min protocol , followed by a final MRI/MRS. After the initial 5-min protocol, T2 va lues increased by 5.9 +/- 0.8 ms (29.9 +/- 0.4 to 35.8 +/- 0.6 ms), wh ereas Gly did not change significantly (70.5 +/- 6.8 to 67.6 +/- 7.4 m M). After 15 min of toe raises, gastrocnemius Gly was reduced to 40.4 +/- 5.3 mM (P less than or equal to 0.01), recovering to 45.8 +/- 5.3 mM (P less than or equal to 0.05) during a 1-h rest. After the second 5-min bout of plantar flexion (reduced Gly at 25% of MVC), T2 values i ncreased by 5.0 +/- 0.8 ms (30.4 to 35.4 ms), whereas muscle Gly rose to 57.6 +/- 5.3 mM. We conclude that muscle Gly concentration per se d oes not affect exercise-induced T2 increases in the human gastrocnemiu s.