The insulin-like growth factor (IGF-1) gene is involved in exercised-induced hypertrophy. IGF-1 can undergo alternative splicing to generate different products, like the mechano-growth factor (MGF) expressed in mechanically overloaded muscle. The aim of this study was to determine the correlation between different modes of contractions and MGF expression in time. Therefore mouse EDL muscles were isolated and lengthening and isometric muscle contractions were performed. mRNA expression levels were determined by qPCR for MGF and IGF-1, and for Proliferating Cell Nuclear Antigen (PCNA) and MyoD as specific markers for satellite cells activation. In addition Myogenin expression was determined as marker for early differentiation. Lengthening contractions for 15' induced MGF expression after 4 hours and IGF-1 expression after 6 hours. Isometric contractions did not induce MGF and only a small amount of IGF-1. PCNA and MyoD are expressed after lengthening contractions, indicating that satellite cells start to proliferate. Myogenin is not expressed within 10 hours after contractions. These results provide evidence that MGF expression is induced in muscles which are subjected to lengthening contractions and suggest that MGF is involved in the activation of satellite cells.