If so please tell how long a session should be and how many times a week.

I am skeptical that running in a pool is the preferred method of improving your time in the 40 yard dash. I recommend traditional methods- weight training, stretching, and running.

20 minutes a session 3 times weekly!

Yes. It will increase the power you have on your initial burst, and long-stride, therefor bettering your 40 time. You can use soft ankle wieghts too if just the running in the pool gets easy or old. Keep your workouts fresh. That will keep you interested, which will get you to work out harder.

run with a parachute and running in a pool will increase your 40 time.

I would recommend running your 40 on dry land. Running in the water will definitely slow down your time.

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