The biggest issue I deal with all the time is the players have no idea how to gage intensity. If you tell them 90%, they will still try to run at 100% effort. They think that more effort means more speed.
With my athletes, I performed a simple 20 yard test with electronic timers (Brower). I had them run two trials over 20 yards with 4 minute breaks (this is after an extensive warm-up). I did not provide any intensity parameters. After they ran their two trials, I told them to run at 90% intensity. At least 90% of the players ran faster at 90%. I then told them to run at 85%. A further 60% ran faster times at this intensity.
Another test we did was have them run 10 x 20 yards through electronic timers. I gave them 3 minute breaks between runs and instructed them to run them all as consistently as possible. I did not give them any intensity guidelines except to tell them to run them all in the same time. I did tell them that running slower, sub-maximal runs would yield less consistent times.
The results were interesting. Our fastest guys did not have the most consistent times - mostly because they were trying to “pick” an intensity to run the 20’s easily (i.e. they got cute). Our more experienced guys - who are better all around athletes - had the most consistent times (all runs within 0.05 seconds) running each rep strong, but not recklessly. Our slower, less athletic guys had the worst times as far as consistency was concerned.
Both of the above scenarios are a good way to illustrate to players how intensity can be regulated properly to yield good, consistent results.
You cannot assume if you tell an athlete to run at a given percentage of their best time they will know how to gage their speed. You must conduct an exercise so that they fully understand the art of running at 100% of best time is best accomplished by running at 90-95% of full exertion. Once players understand how to manage intensity, workouts can be undertaken much more strategically - as outlined in James’ post.