Background and Aim: Tendon is the extracellular matrix of the muscle that mechanically and structurally adapts to the mechanical load. However, the cellular and molecular mechanisms of this adaptation are not known yet. The purpose of this study was to investigate the effect of 6 weeks of resistance training on expression of two Transforming Growth Factor Beta 1 (TGF-β1) and myostatin genes in the tendon of fast- and slow-twitch muscles including Extensor Digitorum Longus (EDL) and Soleus (SOL).
Methods & Materials: Twelve male Wistar rats with 8 weeks of age were randomly divided into two groups of exercise (n=6) and control (n=6). The exercise group performed resistance training (Carrying weights with 40-160% body weight on the ladder) for 6 weeks, 5 sessions per week. Forty-eight hours after the last training session, all rats were sacrificed and the tendons of SOL and EDL muscles were extracted. The mRNA expression level of TGF-β1 and myostatin genes was assayed using real time polymerase chain reaction. Independent t-test was used for statistical analysis. 
Ethical Considerations: All experiments on animals were according to the ethical guidelines of Research Ethics Committee of Islamic Azad University, Central Tehran Branch (ethical code: IR.IAU.PS.REC.1398.296)
Results: The expression of TGF-β1 gene in EDL (P≤0.001) and SOL (P≤0.01) muscle tendons significantly increased, while the expression of myostatin gene in EDL (P≤0.001) and SOL (P≤0.05) tendons were significantly reduced. 
Conclusion: Resistance training appears to up-regulate the basal levels of TGF-β1 gene and down-regulate the basal levels of myostatin gene in tendons of fast- and slow-twitch muscles, where these effects are significantly more pronounced in the tendon of fast-twitch muscle.