In America, the most popular of all high intensity sports would easily be Football. On the outside looking in, you would assume that the majority of all the conditioning would be an accumulation of sprints and repeated bouts until your legs could take no more. Although there is a time and a place for this sort of energy systems training, many people fail to see the importance of the aerobic energy system and it’s ability to improve the performance of an athlete in a high intensity sport. Remembering that football is aerobic/alactic in nature it is necessary to address the aerobic component of this sport to best prepare the athlete.
Why would you do aerobic work for Football?
1. Increased blood flow because of an increased cardiac output/hypertrophy(eccentrically).
2. Increases the density of capillaries and mitochondria especially in the localized muscles that are used during training. This is important to keep in mind when constructing what tool you use for energy systems development. You want to work similar muscle regimes, as you would use during sport to enhance the density to the muscles that need it the most.
Aerobic training does have its drawbacks. It has been shown to reduce the effectiveness of power development, which is a primal component of most high intensity sports. There is also evidence showing that there is a ceiling for how much of an influence an enhanced aerobic capacity has on your recovery from repeated bouts. Keeping this in mind, it would only be necessary to develop your aerobic capacity to a point that would make it optimal for recovery and limit its volume extensively during blocks of power development. As I mentioned earlier, be aware of the specific muscles in use for your sport. For example, due to football being an upright sport heavily reliable on the backside dominant musculature (Hamstrings, glutes) it wouldn't make a ton of sense to condition sitting down such as a bike (front side dominant).