Effect of muscle glycogen content on exercise-induced changes in muscle T2 times

Effects of gastrocnemius glycogen (Gly) concentration on changes in transverse relaxation time (T2; ms) were studied after 5-min plantar flexion at 25% of maximum voluntary contraction (MVC). Gastrocnemius Gly, phosphorus metabolites, and T2 were measured in seven subjects by using interleaved 13C/31P 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/MRI). Subjects then performed approximately 15 min of single-leg toe raises (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 values increased by 5.9 +/- 0.8 ms (29.9 +/- 0.4 to 35.8 +/- 0.6 ms), whereas Gly did not change significantly (70.5 +/- 6.8 to 67.6 +/- 7.4 mM). After 15 min of toe raises, gastrocnemius Gly was reduced to 40.4 +/- 5.3 mM (P