Energy Systems - Basic Primer
Energy systems are the ways that our bodies convert substrates (think food) into energy (a compound called ATP - adenosine triphosphate) to power physical activity. For simplicity we're going to discuss energy systems in three categories.
The ATP/PC system uses the very limited supply of stored energy in the muscles. This makes the energy instantly available in a powerful and short burst, but it runs out quickly as well. Count on this burst lasting up to ten seconds, any longer and the energy will need to come from a different energy system, such as the glycolytic pathway also known as the lactic acid system.
Energy produced by the lactic acid system is drawn from carbohydrates. Sugar in the blood and muscle stores called glycogen are broken down into ATP (energy). This system can produce a fairly substantial supply of energy, but in order to create energy this fast the carbohydrates aren't broken down completely causing a build up of lactic acid. The build up of lactic acid will cause an individual to slow down and rely on the aerobic energy system after about two minutes.
Finally, the aerobic system can use the power-house of completely breaking down carbohydrates and fat for energy, which allows for a nearly endless supply of potential energy. It just takes longer to make this energy available, so the power (work per unit of time) output isn't as great in the short-term, but over the long term this is the primary energy system that fuels activity.
Training Energy Systems
Getting back to using this information to help plan your training program, you can do a needs assessment based on your goals.
Back to the example of the sub-optimal training recommended to a young hockey player. He was spending an inordinate amount of time developing his aerobic system, while neglecting the ATP/PC and lactic acid systems. By assessing the needs of the athlete it becomes clear that these systems should not be ignored.
Simple break down for performance training:
Look at the event you are training for and assess whether it requires all-out bursts for up to ten seconds (100M sprint), nearly all-out pushes from 10 seconds to 2 minutes, or anything that requires energy for more than 2 minutes at a time. Don't just think about how long the activity takes, but look at specific breakdowns of activity within the event. For instance if you play baseball the game can last well over an hour, but a lot of that time is spent sitting on the bench waiting to bat or standing in the field watching for the ball to come your way. When you are called on to bat or catch, a sudden high intensity burst is needed to perform optimally. In this case training the ATP/PC system would seem advisable because the actual activity bursts will rarely last longer than 10 seconds and the rest of the time during the game allows for ample recovery. Cross-country skiing on the other hand involves long periods of constant motion which requires energy that is sustainable for the event. In this case aerobic training would be more useful.
Aerobic training is healthy for the heart and does help almost all athletic endeavors to some extent. It is not a bad idea to always have some training in the aerobic system even for athletes who use more explosive bursts of energy in small amounts. This is because a well developed aerobic system will enhance recovery of the other energy systems.
Sample training plans:
ATP/PC - Sprint intervals
If you want to improve your efficiency in calling upon the energy stored in the ATP/PC system try doing sprint intervals lasting 6-10 seconds followed by recovery periods. One example would be to ride a spin bike with appreciable tension as fast as possible for 8 seconds and then to pedal with no tension at an easy pace for two minutes to allow your ATP/PC stores to replenish to some extent before another all-out bout.
Lactic acid system - Burn intervals
For this system you might ride a bike for 90 seconds as fast as you can maintain for that time. You won't be able to go as fast as you would for 8 seconds, but you can still go quite fast. You should feel a mean burn from the build up of lactic acid after completing an interval like this. You can also play with longer and shorter rest intervals from 30 seconds to 3 minutes. Shorter rest periods will give you less time to clear out lactate between bouts and may contribute to developing improved lactate clearance rates and build mental strength to push through the burn. Longer rests will leave you with more energy allowing each interval to be at a higher intensity over-all.
Aerobic energy system - Steady-state training
This is probably still the primary method most people think of for "cardio" training. This involves going for walks, jogging, biking, etc. where your workload is fairly constant and therefore your heart rate doesn't fluctuate much. You can build this system by starting at a specific workload (let's say jogging at 5 mph) and keeping it up for 30 minutes. Over time you can build this up until you have the endurance to go for a full hour or more. Eventually you can build up the intensity as well. It's not a bad idea to decrease your time back to 30 minutes when increasing intensity (going faster, steeper etc.) and then gradually building the time up again.
Fat loss training:
Okay so a lot of you reading might be more interested in how to use this information for burning fat. At first glance it looks like the aerobic system is the best bet for burning fat because it uses fat as an energy source instead of stored ATP or carbohydrate. However all three energy systems are always working together to some extent and all activities call upon carbohydrate, fat, and sometimes protein to some extent. The aerobic system is actually the one I'm using predominately as I sit here typing this article. I'm burning a higher percentage of calories from fat as I sit here typing than I do when I'm having a work-out. However I'm burning far fewer total calories from all sources. Essentially the main path to weight-loss according to the energy balance principle is to burn more calories than we take in. With this in mind any of the energy pathways can be highly effective for weight-loss depending on how long they are implemented.
Fat loss recommendations:
Short on time
You don't mind pushing hard - Try intervals in the lactic acid range with shorter rest periods to ramp up the total energy expenditure. A lot can be accomplished in 20-30 minutes of high intensity work. If you're new to exercise I'd suggest first building up an aerobic base of lighter - moderate intensity before undertaking this kind of training.
You prefer a moderate approach - Give steady-state aerobic training a go, you'll still burn more calories than you would otherwise. You can also try a more moderate approach to intervals where you push yourself kind of hard, but not to the extent that the lactic acid burn is forcing you to stop, just really breathing heavy.
Exercise is a priority and you carve out the time
You don't mind pushing hard - Intervals are still an excellent option. You can afford to take longer recovery periods with your greater time available which can allow each interval to be more consistently high power and less of a trudge.
You prefer a moderate approach - Again just get your body moving. Find an activity you like and stick with it.
In part three we'll take a look at flexibility