The effect of hypoxia on tendons: 
https://www.researchgate.net/publication/51692429_Hypoxia_A_critical_regulator_of_early_human_tendinopathy/citation/download

The effect of collagen intake on the Achilles tendon (and calf muscle):

https://www.mdpi.com/2072-6643/11/1/76/htm

The effect of collagen intake on tendons:

https://www.jstage.jst.go.jp/article/jnsv1973/51/3/51_3_169/_article/-char/ja/