As you start learning more about the mechanics of the human body in regards to sprinting specifically...
What you'll quickly see is that rather than focusing on the flexibility of each muscle individually...
It's far more effective to focus on a chain of muscles that work together to accomplish one goal.
And the two chains to know are:
1. The ANTERIOR chain - which runs on the front half of body...
From the chest, down to the abs, hip flexors, and quads.
2. The POSTERIOR chain - which runs on the back half of the body...
From mid back, to the lower back, the glutes, hamstrings, and finally the calf.
It is these two chains specifically, that ultimately determine your range of motion with each stride...
And therefore has an enormous impact on running speed.