In the video that you posted it doesn’t look like he is pulling a heavy weight to me. Look how it’s moving around. Regardless, and I’m speaking on limited knowledge, sprinting with a sled (up to 30m) makes make sense because as Charlie Francis said; sprinting alone is a body weight exercise.
So think about it; how much stronger could you get in the gym if you could only use body weight exercises? From what I can see the sled pulls is working because once Asafa gets to about the 30-35m the race is pretty much over for the rest of the field.
More specifically what I’m trying to say is, the sled training/weight training has to be key ingredient to producing the powerful block clearance along with the powerful few first steps to separate yourself from the rest of the pack early.
Thanks. I just needed info on the loading of the sled. after looking at the video, it does look lighter than 75lbs though. Looks more like a single 25lb or 35lb plate. And very easy.