To improve an athlete's speed (the ability to run from point A to point B), strength and conditioning trainer Mike Boyle says the simple answer is to run fast (wind sprints); to improve agility (the ability to run quickly from point A to point B and back to point A), Boyle says athletes need to build leg muscle strength (bigger "brakes").
More Mike Boyle videos on MomsTeam:
Plyometrics: Good, But Don't Overdo It, Says Trainer Mike Boyle [1]
Taking Gym On The Road Is Easy [2]
Strength & Conditioning Coach: Bigger Better, Nutrition Key, Mental Toughness Essential [3]
Don't Specialize in Single Sport Too Early, Advises Trainer Mike Boyle [4]
Links:
[1] https://www.momsteam.com/node/4112
[2] https://www.momsteam.com/node/4115
[3] https://www.momsteam.com/node/3922
[4] https://www.momsteam.com/node/4113