Is there any stem cell or biogel method that exist for muscle and tendon lengthening?
Limb lengthening surgery is usually used to treat sequelae caused by bone disease, trauma, pygmyism or inflammation. I personally do not recommend to use surgical methods to increase height. Typically Limb Lengthening requires Achilles pre-lengthening surgery and a lot of postoperative rehabilitation. Moreover, Limb lengthening surgery may cause pain, infection, muscle atrophy, joint stiffness and delayed bone healing. There's a lot of related research cause this study is timely for some patients.

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Szoke, G., Lee, S. H., Simpson, A. H., &amp; Prescott, J. (2005). Response of the tendon during limb lengthening. The Journal of Bone and Joint Surgery.British Volume, 87(4), 583-587.