I think it's safe to assume that everyone on this page has heard of the term cardio. For some of you it means dreaded countless hours upon hours of burning calories trotting along on a treadmill, but is there a better way? While you may think that more is better, but that might no necessarily be the case.
There are 3 main energy systems in the body, each with a different function. To effectively understand how to best train you have to understand each of these systems, and the conditions under which they function.
The ATP-PC System is the shortest term energy system in the body, mainly used for sprinting activities lasting under and up too 10 seconds. This is the main system used for things such as powerlifting or 100m sprints. While these energy stores run out quickly, they are also quickly replenished by the body.
The Anaerobic System is the next shortest term energy system in the body. It's used predominately in exercises lasting under 2 minutes. This system produces lactic acid as a by product, which for many people is the "burn" they experience when lifting weights. This system is used in weight training, and shorter distance running (1/2-1 mile)
The Aerobic system is the long term energy system in the body. Providing nearly 98% of the energy for a marathon runner this system is the one most commonly trained as "cardio". Sustained exercise for more than 2 minutes is training mainly this system. *note that these systems don't work independently, and cannot be trained as such. When describing activity it is not which energy system is working but which predominates.
While training that is mostly aerobic in nature has it's benefits, such as increases in mitochondrial growth, and better fat utilization it has drawbacks as well. After 45 minutes of sustained aerobic training the body begins to produce cortisol, which causes your body to break down muscle for energy. This is a widely overlooked NEGATIVE side effect to aerobic training. Cortisol also stops the production of testosterone which also negatively impacts muscle size and strength. If you're looking to gain muscle, and keep it on.. keep the cardio short and sweet!
Studies have shown * that anaerobic training has all the same benefits as aerobic training, but without all of the negative effects on muscle size! Doing high intensity interval training increases anaerobic, as well as aerobic capacities. The time of the individual events and the rest in between determines which systems are effectively used. As a basic rule of thumb, rest 5-6 times longer than your work periods. So for a 10 second sprint, you would rest 1 minute and repeat that. It is very important in any type of interval work that the power
output for each work segment of the interval is consistent. This means taking the full rest, even if you feel you do not need it.
*(Explosive-strength training improves 5-km running time by improving
running economy and muscle power. J Appl Physiol. 1999