I was talking with a parent the other day about her daughter who plays soccer. With my track background, she was a little dubious I could help her daughter because she wanted to make sure she'd be fast for soccer since she wasn't a track runner.
What I explained to her is that at her age she just needs to get fast. The best way to do it is to be trained as a sprinter to make sure her running mechanics are correct.
Differentiating between soccer speed and track speed doesn't start becoming a factor until late high school at the earliest. At her age (which is 7th grade), she is still developing and trying to improve her overall athleticism.
Focusing on muscles specific to movements of soccer at this point in her career would develop imbalances that could lead to injuries down the road.
There was a great video on Youtube that has since been taken down. But, it was a research video examining Cristiano Ronaldo, one of the best soccer players in the world, and a Spanish national level sprinter.
The goal of the video was to watch them move in different obstacle courses and learn how their bodies reacted.
The sprinter was obviously faster when the two ran linear sprints. But, when they shifted to quick change of direction courses, Ronaldo was faster. They measured mph and other metrics, but that was the gist of it.
The difference is that these were fully mature/developed professional athletes. They had already developed their bodies to the point where, yeah, you can train them differently based on their sports.
Adolescents who aren't fully developed, which is basically up to about 17-18 years old, don't need to try and improve their (insert sport here) speed. They just need to improve THEIR SPEED and training like a track sprinter is the best way possible.
Why??
Sprinters can sprint fast because their running mechanics are typically very good. Everything is moving the way they should be moving. If we can get your athlete to run like that, then we know that athlete is mechanically pretty sound.
Then, and only then, can you consider shifting the training to adapt to the sport.
If the athlete is not mechanically sound and has imbalances, if you train them specific to the sport, you aren't getting them as fast as they could be. And, you are just reinforcing those imbalances to just move faster. Thus, increasing the chances for an injury to happen.
I don't need them to be good at track. I just need them to train and run like a sprinter. That is the way you are going to get your young athlete faster for their sport.
Trust me, if I have a 13 year old who can win a straight line race. Then he/she can probably win (or be at the top) for change of direction as well.
Work on the sprint speed first.