[1] Current Concepts In Muscle Stretching For Exercise And Rehabilitation - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3273886/
[2] Proprioceptive Neuromuscular Facilitation (PNF): Its Mechanisms and Effects on Range of Motion and Muscular Function - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3588663/
[3] Stretching And Flexibility: Everything You Never Wanted To Know - https://web.mit.edu/tkd/stretch/stretching_toc.html#SEC19
[4] The Effect of Time and Frequency of Static Stretching on Flexibility of the Hamstring Muscles - https://www.researchgate.net/publication/15133359_The_Effect_of_Time_on_Static_Stretch_on_the_Flexibility_of_the_Hamstring_Muscles
[5] Effect of Active-Assisted Stretching of 30 Seconds and 60 Seconds in Muscle Force - https://www.ijmrhs.com/medical-research/effect-of-activeassisted-stretching-of-30-seconds-and-60-seconds-in-muscle-force.pdf
[6] Static Versus Dynamic Stretching Effect on Agility Performance - https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=1691&context=etd
[7] Stretching Scientifically: A Guide to Flexibility Training by Thomas Kurz - https://www.amazon.com/dp/B017DYMH4M/