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Andreo Fernando Aguiarwere elevated by 100-400% 0–24 h postexercise, In addition, Bickel et al. (8) alsoobserved a transient increase in myogenin (~3-fold) and MyoD (83%) mRNAexpression in human muscle after a single bout of RT induced by neuromuscularelectrical stimulation. The results of these studies support the hypothesis that the MRFs,particularly MyoD and myogenin, may be involved in regulating hypertrophy inducedby acute RT.MyoD and myogenin have also been implicated in regulating muscle-fiber type;myogenin has been found to accumulate in slow-twitch and MyoD in fast-twitch fibers(27, 60). Moreover, Mozdziak et al. (40) showed that myogenin and MyoD mRNAlevels are more implicated with myosin heavy chain (MHC) isoforms compositionchanges than with muscle mass alterations. This hypothesis was confirmed byWilloughby and Nelson (62) that showed a positive correlation between the increase ingene expression of type I, IIA, and IIX MHC isoforms and MyoD and myogeninmRNA expression after a single session of RT. Although considerable evidence fromtime-course studies involving acute RT suggest that the MRFs may play an importantrole in controlling muscle hypertrophy and the fiber-type transition, remains unknownwhether the hypertrophic and phenotypic response of skeletal muscle induced by longtermRT could be associated with the increase in MyoD and myogenin mRNAexpression.In addition to the role of MRFs in skeletal muscle hypertrophy, IGF-I has beencharacterized as a strong anabolic agent by acting in two different ways during loadinducedmuscle hypertrophy, 1- by stimulating SC to proliferate and differentiate duringthe process of compensatory hypertrophy (1), and/or 2- by activating a cascade ofsignaling pathways via phosphatidylinositol 3-kinase (PI3K)/Akt/ mammalian target ofrapamycinm (mTOR)/ p70 S6 kinase (p70S6K), resulting in the downstream activationof targets which are required for protein synthesis (10, 16). The "mature" IGF-I genecomprises exons 3 and 4 of the IGF-I gene; during gene processing, alternative splicingoccur resulting in three IGF-I transcripts (IGF-IEa, IGF-IEb and IGF-IEc, also knownas MGF). Some authors have shown that IGF-I transcripts gene expression areupregulated following acute high-intensity resistance exercise, albeit with different timecourses (47, 9, 5). However, contradictory results have been observed in differentspecies and RT protocols (63, 39, 5, 23, 47). According to Psilander et al. (47) thedifferential expression of the IGF-I transcripts may be associated with different stimulusthat affect skeletal muscle. While discrepancy results have been found on IGF-I44