Your aerobic fitness progression eventually will take you to specific training types that develop the muscles’ energy systems.
All force generated by muscle requires energy. All energy-requiring processes in the body use the energy stored in a molecule called adenosine triphosphate (ATP). ATP works like a battery storing a small amount of energy for future use. The metabolism of glucose and fat generates ATP, and muscles use ATP to generate the force that powers our activities.
The more efficient you are at delivering ATP to muscles, the fitter you will become.
Your muscles use three energy systems during exercise to provide ATP to generate the force required to meet the demands of your chosen activity.
Although no system works completely independently from each other, one or a combination of two will become dominant depending on the exercise’s duration and intensity.
1) The Immediate System
Muscles store a small amount of ATP that can be used immediately for a couple of seconds. Along with ATP, muscles also store something called phosphocreatine, which can rapidly generate ATP. This combination of ATP and phosphocreatine is known as the ‘immediate system.’ It is the dominant energy source during short durations (up to around 10-12 seconds) along with high-intensity efforts. Activities requiring short bursts of high energy: weight training, sprinting, and shot-put utilize the ‘immediate system.’
The availability of stored ATP and phosphocreatine becomes the limitation of the ‘immediate system.’
2) The Glycolytic System
For activities requiring high intensity for 30 seconds to 2 minutes, muscles use glycolysis, which rapidly converts glucose to lactate and does not require oxygen. After two minutes, the build-up of lactic acid becomes too high, and the muscles get that dreaded burning sensation and performance drops. Activities like hockey, CrossFit, and middle distance running and swimming (i.e. 800m run, 200m swim) predominantly leverage the anaerobic lactic system.
The accumulation of lactic acid, rather than a fuel shortage (glycogen = stored glucose), limits this energy system.
3) The Aerobic Energy System
Lower intensity activities utilize the slower aerobic system where glucose and fat combined with oxygen generate large amounts of ATP through oxidative phosphorylation in the mitochondria. Aerobic activity can be maintained for extended periods (hours). Activities like walking, jogging, running, hiking, cycling, biking, and swimming are predominantly aerobic.
In the aerobic zone, your body will preferentially use fat for fuel, requiring oxygen to generate energy through oxidative phosphorylation. Your breathing will be more rapid than at rest, but you will not be gasping for air. Your heart rate will be less than 65% of your maximum heart rate. Above 65%, your body begins to recruit glucose (from stored glycogen) for energy.
The aerobic system has no energy limitation as long as it is using fat. As you go faster, at intensities above 65% of maximal heart rate, your muscles will begin to use glucose from stored glycogen.
Glycogen along will muscle fatigue are the ultimate limiting factors for the aerobic system.
When you start exercising, we recommend that you spend most of your time in the aerobic zone.
As your pace quickens and you have mobilized glucose from glycogen, glycolysis may not keep up with the slower, aerobic, oxidative phosphorylation and lactate levels will begin to build.
The Aerobic Threshold is the point at which your energy is no longer provided exclusively from the aerobic energy system.
Training at a pace slightly slower than your aerobic threshold is ideal for developing your aerobic system. (Zone 2 training.)
As you develop the aerobic system, your pace will increase at the same intensity and heart rate, allowing you to go faster for longer before activating glycogen use (stored glucose).
For long-distance athletes, the more efficient their aerobic system, the faster they will go at higher intensities.
Even athletes in sports that do not predominantly use the aerobic system, like hockey or Cross-Fit, will find that improve aerobic endurance will increase their performance and improve their recovery.
After you cross the aerobic threshold, lactate levels gradually increase until a point at which your ability to recycle lactate becomes overwhelmed, and lactate levels rise exponentially - this is the anaerobic threshold.
At this point, you are no longer able to sustain a given workload, and fatigue sets in, which will limit your endurance ability.
The anaerobic threshold is often a good predictor of endurance sports performance because athletes with higher thresholds can race faster for longer and at a higher percentage of their VO2max.
You can train the lactic system, exercising at paces close to the anaerobic threshold.
Training to Increase VO2max
VO2max refers to the maximum volume of oxygen that your body can consume during exercise. Maximal oxygen uptake occurs at very high intensities of exercise.
VO2max is a good measure of aerobic fitness. Workouts designed to improve VO2max are typically very high in intensity and only last up to a few minutes at a time.
Training zones are a way of quantifying specific intensities during your training. Heart rate or perceived effort can be great ways to determine your training zones; however, more specialized aerobic performance testing can allow you to determine the exact intensities for each zone. Each zone has a different training purpose.
Zone 1 (Recovery Zone)
Zone 2 (Aerobic Zone)
Zone 3 (Junk Zone)
Zone 4 (Lactic Training Zone)
Zone 5 (VO2max Training)
For individuals looking to monitor intensity with heart rate training, identifying your maximum heart rate can be a great starting point. Here is an easy formula to estimate your maximum heart rate:
Max Heart Rate = 220 - your age
(For example, if you are 30 years old, your maximum heart rate would be close to 190 beats per minute (bpm) (220-30=190)).
Progressing your aerobic capacity will require thoughtful combinations of workouts with different intensities. This type of training is best done with an experienced coach or exercise professional.
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