I hope this is a valid question as I am looking for ways and means to translate some of your (and others) ideas into day to day sprint work.
I think I understand the time element but the question I have revolves around if when the “ground” contact is made in the running cycle makes any difference? Or is it strictly a matter of the length of contact time?
So for example relatively speaking if you are towing a sled during an acceleration the ground contact should happen in relatively the same body position as during an acceleration without resistance.
If you are doing step-ups, does the fact that your foot contacts the box (functioning as the “ground”) relatively “earlier” in the running cycle than “normal” make any difference? Since from observation I would say the “ground” contact on step-ups is longer than average since it takes more time for the athlete to be in a “completed” state. And if I can train the athletes to reduce that “ground” contact time on step ups, are the percentages good it will translate to reduced contact times during runs as well?
Never thought of this before but I think towing a sled would work best on a slight downslope. In this way you would get the extra resistance with good movement specificity to actual early acceleration phase in a way that may not adversely affect ground force production time. Has anyone tried this? As for step ups, the force production time would be too different to try to influence ground contact time and so any benefit to sprinting may need to incorporate a transfer method like plyometrics. I’m sure rapid step ups alone won’t make a difference you could feel but step up with a clean/snatch would encourage rapid hip extension that could well make a difference.
As for your question on whether it is the time of the ground contact in the running cycle or the length of contact time, I think that it is both. Time in the running cycle is best explained by Mann or Mike on foot placement relative to COM but no data could be provided that would allow me to analyse this for the Newtonian model. Clearly from other studies it can be seen that the ability to run fast demands that ground contact time reduces to below 100 and 90ms (with respect to the leg length of the sprinter). However, ground contact time is due to a few factors such as the ability of the muscle to contract quickly, the amount of force produced during the contraction, and the momentum of the leg prior to impact to name a few. Add to this the fact that the analysis revealed similarity in time between each ground contact which made me think of one word called resonance. I’m yet to develop a relationship for that yet (it’s very hard). I think dbandre knows a lot more than me about resonance because it refers to the behaviour of springs.
Hope that helps you enough.