Purpose: In this study we sought to determinewhether a Titin peptide fragment can serve as a clinical biomarker for changes in muscle mass. Methods: Mass spectrometry was used to identify Titin fragment in urine. An antibody against this Titin sequence was raised and used to develop a competitive ELISA assay for measurement in serum. Rat tissue extractions in the presence or absence of a series of proteases of interest were used to identify its enzymatic origin. A rat model of dexamethasone (DEX) induced muscle atrophy and a human 56-day bed rest study with and without vibration therapy were used to assess biological and clinical relevance. Results: A technically robust ELISA measuring the Titin fragment was developed against a Titin peptide fragment identified in human urine. The fragment was shown to be produced primarily byMMP-2 cleavage of Titin. In the rat muscle DEX induced atrophy model, Titin-MMP2 fragmentwas decreased in the beginning of DEX treatment, and then significantly increased later on during DEX administration. In the human bed rest study, the Titin- MMP2 fragment was initially decreased 11.9 (±3.7) % after 1 day of bed rest, and then gradually increased ending up at a 16.4 (±4.6) % increase at day 47. Conclusions: We developed a robust ELISA measuring a muscle derived MMP-2 generated Titin degradation fragment in rat and human serum. Importantly, the fragment can be measured in serum and that these levels are related to induction of skeletal muscle atrophy.