Previously I mentioned a little about resisted speed training (RST) and how the application of load has changed over the years. If you want to have a great read in this area I direct you to the work of Micheál Cahill who just completed a PhD on the topic. It's packed with interesting findings and practical applications. To our knowledge Micheál was one of the first practitioners/researchers to compare sled pushing to sled pulling.
One of my favorite things to come out of his PhD was the Figure shown on the slide. Basically it provides insight how we can use RST to develop horizontal force and sprint capability, much like our understanding of periodising strength in the gym. You can use the load-velocity decrement relationship to develop everything from technical competency (light loads) to strength-speed (heavy loads). How many of you combine your vertical gym training with more horizontally focussed sled training, for a more holistic approach to force development? Do you use sled pushing and pulling for different effects? Head to this resource on the Research in Action page, to really upsize your knowledge in this area, the entire PhD is included in the resource.