Rest interval for sprints
When training for speed development, two things are particularly critical. First, speeds must be 90-100% of max velocity. Second, rest times must be sufficient to maintain those necessary intensities. If you short your athlete's rest times, their output will likely drop over subsequent reps. We recommend two simple methods of keeping rest time.
When measuring sprint reps/volumes by time, use work to rest ratio:
1:20 - 1:40 (work:rest)
When measuring sprint reps/volumes by distance, use time per distance:
1 min per 10 meters
The 1 min per 10 meters method is especially practical if you don't want to time every rep. It's also the author's opinion that the math is a bit easier...
Sprint intensity is measured as a percentage of velocity, not effort. If you’re observing your athlete and they appear to be giving 100% effort on every rep but they are under-rested, their velocities may not be reaching the stimulus threshold necessary for proper speed development. Even if they promise you they are "giving it all they've got," they are still no longer benefiting from the training.
At Athletic Lab, we will sometimes set up timing gates for our athletes. We are not doing this to test times necessarily, but rather to watch for significant drop-offs in velocity each subsequent rep. This method is useful because it can give us some flexibility to individualize our athlete's daily sprint volumes without having to guess. Athletes may drop off at different volumes which we will use to dictate how many reps they complete in the session. Consider that athletes may have a bad rep or two, so you should be looking for a consistent drop-off before cutting them off.
A Word of Caution
That said we definitely don’t recommend timing your athletes every session as there are disadvantages that can arise when an athlete becomes too aware they are being timed. To get around this, you can let your athletes know why you have the gates out but that you will not be sharing their times. There's a time to test, and even a time for having a bit of motivation or incentive by having your athlete aware of their times. This is not that time.