Muscles are a grand amalgamation of thin actin, thick myosin, calcium ions binding to troponin, tropomyosin and Z-lines. They require ATP to be relaxed and use ATP when they undergo contraction.

Stretching helps realign and lengthen muscles to prevent them from being tight and easily injured.

How are muscle contractions triggered in the first place?

Motor neurons use the neurotransmitter acetylcholine to send messages to muscles, prodding them into action. This causes the in-flow of sodium ions, propagating the action potential onward through the muscles. In turn, this invokes the release of calcium ions inside the cells.

The calcium binds to troponin and the obstructive bits move out of the way, allowing actin and myosin to meet for a sweet, brief moment. This is cross-bridge cycling

References

1. Biology Stack Exchange. (2015). How is ATP involved in muscle contraction?. [online] Available at: https://biology.stackexchange.com/questions/31323/how-is-atp-involved-in-muscle-contraction [Accessed 26 Jan. 2018].
2. Anon, (n.d.). Muscle Fiber Contraction and Relaxation. [online] Available at: http://oerpub.github.io/epubjs-demo-book/content/m46447.xhtml#fig-ch10_03_01 [Accessed 26 Jan. 2018].
3. The MIT Tae Kwon Do Club. (2008). STRETCHING AND FLEXIBILITY – Physiology of Stretching. [online] Available at: http://web.mit.edu/tkd/stretch/stretching_2.html#SEC13 [Accessed 26 Jan. 2018].
4. Harvard Health Publishing. (2013). The importance of stretching. [online] Available at: https://www.health.harvard.edu/staying-healthy/the-importance-of-stretching [Accessed 26 Jan. 2018].