GCMS Analysis. The enrichment of free L-[ring-13C6]Phenylalanine and 3-Methylhistidine in blood and tissue fluid will be measured by GCMS after addition of appropriate internal standard U-13C9-15N Phenylalanine , precipitation of blood and tissue proteins with sulfosalicylic acid, extraction with cation exchange chromatography, and tert-butyldimethylsilyl derivatization (t-BDMS). Correction for skewed isotopomer distribution and overlapping spectra will be performed as previously described. The incorporation of L-[ring-13C6]Phenylalanine, [methyl d3] methionine and [methyl d3] 3-methylhistidine in the mixed and myofibrillar muscle proteins will be measured after protein extraction and hydrolysis, amino acid extraction with cation exchange chromatography, t-BDMS derivatization, and GCMS analysis.