Energy systems show that training for muscle strength and size is inherently different compared to cardio for endurance. They each require different amounts of work, and each goal brings its own demands and changes. Each system provides energy depending on your intensity.
Think of intensity as power or how much work you do over a length of time. Imagine that you had to work as hard as possible over three durations: 15 seconds, 1 minute, and 3 minutes. Each would feel challenging in its own way even though 15 seconds has the most intensity by definition. 3 minutes will speed up your breathing and cause some burning in the muscles. 1 minute would feel similar but with that burn slowing you down. 15 seconds would make the muscles feel weak through the intense squeeze or tension that would be required.
There is some overlap. You can use intervals, with bursts of sprinting followed by short rests, to develop the systems used for both the 1 and 3 minute lengths. The 15 second duration though would train the muscles uniquely.
Three Energy Systems
Your body has three systems: two anaerobic and one aerobic.
Anaerobic:
- High Intensity: ATP-PC / Phosphagen / Phosphocreatine / Alactic System
- Mid Intensity: Glycolytic / Lactic Acid System
Aerobic:
- Low Intensity: Oxidative System
Anaerobic means without oxygen and aerobic means with oxygen. The anaerobic systems provide most of the energy when intensity goes above the maximal oxygen uptake a person can attain. This depends on intensity and not duration, as walking for 15 seconds would still use the aerobic system and not the anaerobic systems very much. From the body’s point of view, the aerobic system is best but requires more time to bring into action.
ATP-PC System
Anaerobic activities, like lifting and sprinting, divide into two areas. For short durations of less than 30 seconds, the ATP-PC system dominates. Challenging this system is most specific for strength and size.
This process relies on high energy phosphate. This lacks the disadvantage of the next system as it does not cause the build-up of lactate that interferes with muscle contraction. This allows the most tension, the main stimulus for strength and size, to generate.
ATP and creatine phosphate (PC) are limited and cannot supply energy for long.
Glycolytic System
This system lasts about 20-120 seconds. The energy comes from glycogen (stored carbs) in the muscles or glucose delivered in the blood. During glycolysis, lactic acid forms, which is the main limiting factor of this system. This disrupts muscle contraction in the short-term and causes the burning sensation you begin to feel with prolonged work. Glycogen depletion can also limit you but this rarely occurs with a sensible lifting program.
For lifting, you want to avoid venturing too far into this system. Instead, address it during intervals. You can think of this system as endurance for brief periods. Lactate may have an association with growth hormone released after training and create sarcoplasmic hypertrophy but these both remain speculative. The lactic acid instead limits your muscles from creating much tension.
Some authorities recommend training only for a duration fueled entirely in the ATP-CP range. This belief relies solely on energy systems and needs to be tempered by other considerations as well. The body can adapt to handle lactate better, so this limiting factor does not necessarily prevent strength and size from developing. Longer durations of work can be safer and allow better form. I still usually recommend that sets during lifting last no longer than 30-60 seconds, which seems to balance all the factors properly. You could extend this length with longer pauses between reps.
The Oxidative System
The oxidative system begins at roughly 110-150+ seconds. As long as you hit this minimum duration, you work the aerobic system. It provides limitless energy in theory if given enough time.
So with as little as 2 minutes of effort you can stimulate aerobic changes. The limiting factor comes from how much oxygen you can process and use. This elevates your heart rate.
The interesting conclusion is that all athletes will benefit from aerobic exercise. The aerobic system works at all times and mostly at rest. A better aerobic system may improve recovery from all types of exercise.
Interference
Too much development of one pathway may impair the others according to some research. For example, the utmost development of cardiovascular endurance may limit muscle size. This may come from competing adaptations.
This may occur due to overtraining from too much exercise though as opposed to any opposition. Changes seem to take place in different regions of the muscles. Slow-twitch fibers work mainly for endurance and fast-twitch muscle fibers work for strength and grow the largest. Fast-twitch tend to work anaerobically and slow-twitch work aerobically. It can also occur due to more practical reasons, such as athletes fatiguing due to heavier bodyweight or runners lacking strength because they are small.
Well-rounded improvement should work best. Long-distance athletes seem to benefit from strength training, as long as their distance work is prioritized. Likewise for explosive athletes benefiting from cardio. I would suggest ignoring interference. Lift and perform efficient cardio to the best of your ability. If you compete then you should focus on what you do best by nature.
Energy Systems Dictate Proper Training
You need both lifting and intervals for complete fitness. Many thought in the past that you needed longer durations for cardio such as 15-20 minutes but energy systems shows that even a tough couple of minutes work fine. Intervals have a greater capacity to raise the heart rate and improve muscular endurance while lifting creates tension for more strength and size. Lifting addresses the first energy system and intervals address the latter two. Energy systems show that specific training generates specific results.