Extended Abstract
Introduction
Tendon is the Extracellular Matrix (ECM) of muscle that mechanically and structurally adapts to the mechanical load [2]. While the mechanical and morphological changes in response to resistance training in tendons are well documented, less is known about the basic cellular and molecular mechanisms that regulate these responses. It seems that the messaging pathway of the Transforming Growth Factor Beta 1 (TGF-β1) plays a key role in adaptation of the tendon muscle to resistance training. Myostatin is a member of the TGF-β family, the expression of which negatively regulates skeletal muscle growth [5]. Both TGF-β1 and myostatin stimulate tendon fibroblast proliferation and type I collagen synthesis [6 ،4]. Based on laboratory studies on adult tendon fibroblasts and collagen synthesis studies in adult subjects in response to resistance training, it is thought that TGF-β1 and myostatin play an important role in the growth and adaptation of adult tendons to resistance training [11]. The aim of this study was to investigate the effect of 6 weeks of resistance training on the expression of TGF-β1 and myostatin genes in Tendons Of Soleus (SOL) and Extensor Digitorum Longus (EDL) muscles of Vistar male rats.
Materials and Methods
In this study, 12 adult male Wistar rats with 8 weeks of age were randomly divided into two groups: exercise (n=6) and control (n=6). The exercise group performed resistance training for 6 weeks (Table 1). Forty-eight hours after the last training session, all ranks were sacrificed. Then, the tendons of SOL and EDL muscles of their right foot were immediately and carefully extracted and stored at -80° C for subsequent measurements. Expression levels of TGF-β1 and myostatin mRNAs were assayed using RealTime-PCR. Independent t-test was used for statistical analysis. 
Results
The results showed a significant difference between the mRNA values of TGF-β1 and myostatin genes in EDL and SOL tendons of exercise group compared to the control group. MRNA expression of TGF-β1 gene in EDL (0.48±0.14 in exercise group vs. 0.14±0.04 in control group, P<0.001) and SOL (0.32±0.08 in exercise group vs. 0.17±0.14 in control group, P<0.01) muscles increased significantly, while the myostatin gene expression level in EDL (0.27±0.1 in exercise group vs. 0.56±0.07 in control group, P<0.001) and SOL (0.21±0.07 in exercise group vs. 0.29±0.05 in control group, P<0.05) muscles decreased significantly (Figures 1 and 2).

Discussion
The results of the present study showed that after 6 weeks of resistance training program, the mRNA expression of TGF-β1 gene in both EDL and SOL muscles increased significantly. On the other hand, it significantly reduced mRNA expression of myostatin gene in both muscles. Heinemeier et al. (2007) reported an increase in mRNA levels of the TGF-β1 gene and type I and III collagens in tendon and skeletal muscle following isometric, concentric and eccentric contractions by stimulating the sciatic nerve for 4 days [14]. Evidence suggests that TGF-β1 has been a major mediator in the induction of collagen synthesis in fibroblasts by mechanical load [14], and a similar role has been suggested for this gen in tendons [15]. A 245% increase in TGF-β1 mRNA expression in the EDL muscle tendon compared to an 81% increase in the SOL muscle tendon reported in the present study is likely to indicate a higher involvement of fast-twitch muscles in resistance training which leads to greater adaptations in collagen tissue and higher tolerance of the force exerted by the fast-twitch muscle to the tendon.

There is ample evidence that the regulation of myostatin is a characteristic of the type of muscle fibers, and is strongly associated with the myosin heavy chain IIb isoform [18]and the high concentration of myostatin protein in the contractile muscle. Has been [19], and high concentrations of myostatin protein have been observed in the fast-twitch muscle compared to the slow-twitch muscles [19]. These reports could justify the results of the present study regarding a 53% reduction in myostatin mRNA expression in the EDL muscle compared to a slight 28% reduction in its expression in the SOL muscle.
In overall, resistance training appears to positively regulate the baseline mRNA levels of TGF-β1 gene and negatively regulate the baseline mRNA levels of myostatin gene in fast- and slow-twitch muscles; where its effect was significantly higher on the fast-twitch muscle than the slow-twitch muscle.

Ethical Considerations
Compliance with ethical guidelines
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)
Funding
The present paper was extracted from the PhD thesis of the first author, Ghasem Mohammadnezhad, Department of Physiology, Faculty of Physical Education and Sports Science, Central Tehran Branch, Islamic Azad University.
Authors' contributions
Conceptualization: All authors; Methodology and Data Analysis: Ghasem Mohammadnejhad; Editing and Review: all authors.
Conflicts of interest
The authors declare no conflict of interest.


Acknowledgements
The authors would like to thank the Department of Physiology and Pharmacology of the Pasteur Institute of Iran, as well as the head of the Laboratory Animal Housing of Tarbiat Modares University for their cooperation in conducting this study.