Training intensity - thresholds and zones

As our understanding of the influence training has increased and we learned more about the physiological processes that are happening in our body, we started using that knowledge to improve athlete’s performances.

Today, even someone who isn’t to interested into the science of training has heard about different thresholds and training zones.

This will be the topic of today’s blog post.

Physiological thresholds

When engaged in any kind of activity, our body needs energy to function.

Yesterday, we scratched the surface by just briefly going into the concept of breaking down carbohydrates into energy and having the byproduct of lactic acid.

Today we will dive deeper into the molecules that are really responsible for keeping the engine going.

ATP

It might share it’s name with the Association of Tennis Professionals, ATP or Adenosine triphosphate is a truly remarkable molecule that is also called the molecular currency because it supports many processes in living cells, such as muscle contraction, nerve impulse propagation, and chemical synthesis.

Unfortunately for us, ATP is a scarce resource. At any give point in time, humas have only a few seconds of ATP when exercising and maximum effort.

For anything longer than a few seconds of activity we rely on getting the ATP through different processes.

Immediate Energy

ATP stored in muscles can provide energy for only about 1-2 seconds of maximal effort. After that, the body uses creatine phosphate (CP) to quickly regenerate ATP, which can last for another 8-10 seconds.

Short-Term Energy

For activities lasting from 30 seconds to around 2 minutes, the body relies on anaerobic glycolysis, which breaks down glucose without oxygen to produce ATP. This system is less efficient and produces lactic acid as a byproduct.

Long-Term Energy

For prolonged activities, the body primarily uses aerobic metabolism, which generates ATP through the oxidation of carbohydrates, fats, and proteins. This system can sustain energy production for hours, depending on the availability of these fuel sources and oxygen.

Anaerobic threshold

When ATP is created, lactic acid is put into our bloodstream and the hydrogen ions start messing up the things. A chain reaction is initiated which interferes with energy production and muscle contraction.

When lactic acid is building up quicker than it can be recycled, the athlete’s body has crossed the anaerobic or lactate threshold.

We can imagine the human body as a bath tub. Water coming into the tub slowly flows down the drain. This represents a regular person just going about their daily lives and their body being able to handle the recycling of ATP and clearing lactic acid. Once we have water flowing in at a higher rate (i.e. we increase the activity’s intensity) water will start to build up slowly and eventually it will spill over. That’s the point at which our body is no longer able to effectively remove lactic acid and fatigue will kick in very soon.

When crossing the anaerobic threshold, the athlete’s body needs energy quickly so it relies on generating energy without the use of oxygen. The consequence is that it has to use a less efficient way of generating ATP and because of that only two ATP molecules are generated from a single glucose molecule.

Aerobic threshold

Athletes engaged in an activity of moderate intensity where they stay slightly below the anaerobic threshold will slightly raise the lactic acid levels but the increase will not cause problems.

For endurance athletes, it’s important to pay attention to their nutrition because the energy reserves they have available are, generally, limited to 2 to 3 hours and if the activity is longer, they will have issues finishing strong.

One more thing to keep in mind is that both aerobic and anaerobic thresholds will vary greatly depending on many factors like, fatigue, environmental conditions, or different terrain. For multisport athletes, having different thresholds for different sports is perfectly normal, so it’s important to keep that in mind as well.

Since the athlete’s body isn’t under a lot of stress, it can still rely on using oxygen to boost efficiency of the ATP production. Through this process, up to a whopping 38 ATP molecules are created from a single glucose molecule which is perfect from long sustained energy demands.

Image from https://www.fasttalklabs.com

Intensity zones

Training the athletes body to be as efficient as possible is at the heart of any endurance sport. In order to do this they have to train at specific training intensities. Combined with training duration and frequency in a proper way, athletes can improve their performance by a wide margin.

Below, we will describe one of the most common ways of assigning intensity zones to different workouts and what benefits they provide to the athletes body.

Zone 1 - recovery

This represents the easiest workouts in your calendar. For new athletes this might be even taking the day off but more experienced athletes will benefit from these workouts as a way to boost muscle repair and recovery.

Zone 2 - extensive endurance

This “conversational” intensity zone is where most of the work for endurance athletes will take place. Lactic acid production is low enough to allow the body to clear them efficiently. Slow-twitch muscles become stronger and more efficient which helps conserve glycogen and glucose.

Zone 3 - intensive endurance

Workouts in this zone are used primarily in the early stages of the training cycle because it engages more fast-twitch muscles and teaches them to be more efficient as slow-twitch muscles. Once the initial adaptation period is done, this intensity zone is usually skipped for the next one.

Zone 4/5a - threshold

Work at this intensity levels are measured in minutes (compared to hours in zone 2). Slow-twitch muscles are highly stressed and a lot of the energy generation is done anaerobically. This in turn helps the body learn how to effectively remove lactic acid and tolerate it for longer.

The two zone designation is used here to distinguish between working slightly below or slightly above the anaerobic threshold level.

Zone 5b - anaerobic endurance

Interval training is now the typical type of training. Fast-twitch muscles are the main contributor so their growth and development is stimulated. An even greater emphasis is put on tolerating and removing lactic acid.

It’s important to leave enough room between sessions going into this zone because the risk of overtraining is now very high when done to often in succession.

Zone 5c - power

This zone has limited benefits for endurance athletes but can be quite beneficial for people that struggle with developing muscle mass.

Intervals are just a few seconds long with long rest in between and training sessions should leave at least 2 days of room if not more to prevent any chance of damage to muscle tissue.

The figure to the left illustrates how these intensity zones can be combined in a typical training cycle. The actual ratios will vary depending on your athletes performance and goals.