I have no idea what your fitness level is but I'll give you examples -
First warm-up for a few minutes and then when ready:
raise your speed to say 6.0 and go for 1 minute
then lower speed back down to 3.0 for 1 minute
then raise speed up to 6.0 again for 1 minute
then back down for 3.0 for 1 minute
You will do this until you have complete 8-10 sprints. Your last sprint should be feeling like you will hardly make it-- completely and totally exhausted by the last one. You may have to play around with your speed to find your point but its so great to be able to see your improvement and how quickly you can move up.
I just gave you example numbers but my high interval is at speed 9.6 and my low is at 3.5
Lastly, imagine you are a soccer player on the field and waiting for the ball and while you are running you are taking it for the team!!!
Have fun you will feel fabulous when you are done.
What is a sprint for me may not be a sprint for you. Your narrow perspective does not change my effort or benefit.
If it helps you gain a new perspective --I have cerebral palsy, with bone on bone knee arthritis. Sprinting on a treadmill, at a speed that challenges/is all out for me works for ME. Context is important.
His point is valid though - if you are incapable of sprinting then that's fair enough; I don't see how calling it sprinting when it isn't is doing you any favors. Most of what has been described in this thread is interval training (which is very effective, but not sprinting). Nobody can go flat out for a minute. Usain Bolt can do 100 meters in 9.58 seconds, but he will never do 628m = 100m x (60s / 9.58s) meters in a minute because he cannot maintain a sprint for that long.If it helps you gain a new perspective --I have cerebral palsy, with bone on bone knee arthritis. Sprinting on a treadmill, at a speed that challenges/is all out for me works for ME. Context is important.
Sprinting on a treadmill is not really possible. Firstly by establishing a speed you are limiting your effort, and secondly because it would be incredibly dangerous, as sudden failure would involve a nasty fall onto the moving equipment.
If you have been 'sprinting' on a machine, I would strongly encourage you to find a 75 to 100 meter stretch (I like a soccer field) and try a "true" sprint for comparison. Running is far and away the best way to push yourself to maximum intensity as it works the vast majority of your muscles, and is also fairly forgiving in the event of catastrophic failure (particularly on sand or grass).
In addition, using a fixed distance which is readily completable encourages you to truly try for speed & intensity. If you are doing interval training it is far too easy to lower your intensity to finish the whole 60 seconds instead of truly going all out for 20 and stopping because you can't continue. This is the difference between sprinting and interval training; the notion of duration as a goal and sprinting is largely incompatible. I do interval training in a 5m/1m format and the pacing is RADICALLY lower than sprinting, simply because it is impossible for me to maintain a true sprint for longer than about 15-20 seconds.
Sprinting is about intensity; your sprinting session ends when you can taste blood on your breath and you are literally ready to fall over.
In answer to the original question, there are two ways I am aware of to sprint at the gym: on a stationary bike, and using a jumprope (which should, incidentally, look something like this). However, as explained above, it is going to be much easier to drive yourself to maximum output by running.
My point is this-- what constitutes a sprint(all out exertion) for one person (moving at 6mph or any other speed for 10-30 seconds) should not be judged as "not a sprint" by anyone else. The method used to exert oneself--be it outdoor running, treadmill running or inclines, stationary bikes, etc. should also not be judged, by anyone other than the person involved, as right or wrong.
Sprinting, as I see it, is moving as fast as one is capable of for a very short period of time. Then repeated as many times as the person is able. The details and parameters of "the sprint" are person specific.