There is one aspect of CrossFit that has always fascinated me, and that is a coaches ability to come up with beautiful workouts that dramatically improve an athletes overall fitness levels. There are a lot of moving pieces and variables to consider, such as a time domains, sets, repetitions, and the most important part: the composition of movements. Even though CrossFit’s mantra is preparation for the unknown and unknowable, these workouts are not simply randomly selected movements over a random period of time. Instead, they are carefully selected and choreographed by coaches in an effort to capitalize on the athlete’s effort. Knowing this leads me to questions like:
- How do I choose the movements?
- Which time domains are best?
- How do I know a good workout from a bad one?
There may be some subjectivity to the third question, but there has to be some notion of comparison between workouts, so I’m not going to throw it out. In order to answer these questions, perhaps the first place to look is how the body responds to exercise.
The Three Metabolic Pathways
When you go from resting to exercise, your body beings to utilize energy known as Adenosine triphosphate (ATP) which fuels muscle contraction. The muscles throughout the body have ATP stores, but these stores are small and become exhausted as exercise increases. So to ensure that the muscles receive the right amount of ATP to keep contracting, three energy systems called metabolic pathways are activated.
The three metabolic pathways are the phosphocreatine (or phosphagen) pathway, the lactate (or glycolytic) pathway, the aerobic (or oxidative) pathway. Taken together, the first two are known as the anaerobic systems and the third is the aerobic system. At the start of exercise all three of these pathways are activated simultaneously, but one can dominate the others in ATP production based on the intensity of the movement being done and how long the movement is being done.
The aerobic system is the default system used during exercise, but it takes the longest to fully activate (seconds to minutes) and so the anaerobic system can rapidly (milliseconds) provide ATP to cover what the aerobic system cannot provide.
Lets take a look at each pathway in a little bit more detail.
Phosphocreatine (Immediate Energy)
There are times when we need immediate energy to fuel muscle contraction. For example, suppose you are laying on the couch and you need to get up to get the remote you forgot to grab before laying down. To get off the couch a burst of immediate energy is needed for your muscles to contract and lift you off of the couch. This energy transfer mush happen within seconds. So where does this energy come from?
Our muscles contain stores containing ATP and PCr. These stores contain just enough energy to fuel muscle contraction for short bursts; typically no more than 1 minute depending on the intensity. For example, walking at a casual pace for 1 minute, running at a marathon pace for 20 to 30 seconds, an all out sprint for 5 to 8 seconds, or a 1-rep max olympic lift.
Since the energy used to fuel muscle contractions from ATP is stored directly in the muscle, no lactate accumulation occurs. However, if the intensity stays high, and for a longer period of time, then lactate production starts which moves us into the lactate pathway.
Lactate (Short Term Energy)
As the time of intense exercise increases above 2 minutes the ATP+PCr stores become depleted. In order to keep up the demand for ATP the lactate pathway becomes the dominate pathway to resynthesizes ATP by rapidly breaking down anaerobic glycolysis. This process also begins to form lactate. An interesting point about this process is that it is done in the absence of oxygen; this is why we consider this process reserve fuel.
The formation of lactate while this pathway is active is the reason it’s called the Lactate metabolic pathway. Lactate used to have a bad rep, because it was thought to be the cause of things like muscle burn and upset stomachs during intense exercise. However, we now know this to be false. Lactic acid consists of both lactate and hydrogen ions called H+. The latter is responsible for the negative effects rather than lactate.
Lactate is honestly super cool! It can be viewed as a linkage between the anaerobic and aerobic systems. Lactate serves as a bridge between the glycolytic and oxidative metabolic pathways. In fact, lactate is produced in glycolytic cells, and then shuttled around to either nearby receiver oxidative cells or added to the blood. During intense exercise, lactate accumulates in the blood because it is being produced at a much higher rate than it can be consumed. However, during aerobic conditions (see the next section) lactate is greatest in consumer cells which implies that blood lactate is near zero. At a whole body level it has an important signaling effect and is a major energy source. It’s not a metabolic waste or dead-end.
The notion of lactate production vs consumption is extremely important when it comes to training, because it affects ones ability to generate max power, recover, and then generate max power again. We discuss this more in the section on training below.
Aerobic (Long Term Energy)
The anaerobic pathway generates energy only for a maximum time of 2 minutes. In addition, it generates very few ATPs, so for exercise that lasts longer than 2 minutes, the aerobic pathway dominates energy production. This is the pathway that is most concerned with oxygen.
As we exercise over a period of time, oxygen consumption begins to be used at an exponential rate. This oxygen is delivered to the active muscle groups so it can be used to synthesis ATP producing energy. As the respiratory system becomes more efficient at taking in oxygen, the cardiovascular system can deliver oxygen to working muscle groups and these active muscle groups can utilize this oxygen (the aerobic pathway). The better this system is, the better one’s aerobic capacity will be. The measurement of this process is called oxygen uptake.
An important concept of understanding aerobic capacity is the notion of steady rate. Generally speaking, this is the point of time the graph of an increasing function begins to flatten out. For example, if we graph along the x-axis duration of an exercise, and along the y-axis oxygen uptake, then the graph starts out increasing very sharply, but then over time begins to flatten out–this usually happens after a few minutes–this point is the steady rate of oxygen uptake.
During the oxygen uptake, steady rate lactate produced either oxidizes or is reconverted into glucose; hence, no appreciable blood lactate accumulation occurs. This is the point during the activity that energy required and ATP production is in balance.
Steady-rate oxygen uptake occurs when there is homeostasis between energy demands and energy production. At any given exercise intensity, homeostasis of oxygen uptake will occur. If we then increase the intensity, steady-rate oxygen uptake is not reached right away, taking time to return. Each time we increase the intensity, it takes time to reach a steady rate of homeostasis of oxygen uptake. If we keep increasing the intensity over time, then there will be a point where oxygen uptake reaches an all time max and no increase in oxygen uptake will be observed despite increasing the intensity of the work being done. This max is called the maximal oxygen uptake or $\VOmax$.
$\VOmax$ is an important measurement of aerobic power (capacity). It quantitatively describes ones ability to maintain intense exercise for longer than 4 to 5 minutes. Obtaining a high $\VOmax$ requires optimizing ventilation, hemoglobin concentration, blood volume, cardiac output, peripheral blood flow, and aerobic metabolism.
Muscle Fiber Differences
One interesting aspect of the metabolic pathways is their connection to muscle fiber type. There are two unique types of muscle fibers: fast twitch and slow twitch. Both types generate ATP differently.
Fast twitch–also called type $\rom{II}$–fibers have a very high contraction speed and are predominately used for explosive movements and generating power for short amounts of time. Thus, they generate ATP using the anaerobic pathway. That being said, fast twitch fibers are broken up into two categories: type $\rom{II}_a$ and $\rom{II}_x$. The later generate ATP using the aerobic pathway.
Slow twitch–also called type $\rom{I}$–fibers have a slower contraction speed, but are predominately used for steady-rate aerobic energy transfer. Because of their make up they generate ATP using the aerobic pathway.
We will discuss the role of muscle fiber type in training below.
Training and the Metabolic Pathways
The sport one plays is typically dominated by one of the two main metabolic pathways. For example, endurance athletes predominately use the aerobic pathway while olympic weightlifters make use of the anaerobic pathway. That being said, there are times when we might need to utilize the non-dominate pathway when competing; e.g, when a marathon runner is approaching the finish line and has been keeping pace with a competitor, now they need to all-out sprint to the finish line to overcome them and take the win. To be able to get that last-minute sprint, we need to be able to tap into our anaerobic pathway, which might be hard to accomplish without proper training when we are at the end of a long race and we’re tired.
Now CrossFit is slightly unique in that athletes need to be proficient in movements and workouts that utilize every possible combination of the metabolic pathways. This just is not going to happen without the proper training program and assessment.
The characteristics of each metabolic pathway tell us how we can train for them. Time and intensity give us very good constraints on how we can train within a particular metabolic pathway or a combination of them. The following table details the time and intensity for each of the metabolic pathways.
| Phosphocreatine | Lactate | Aerobic | |
| Time Domain (activation) | Short ~10 seconds | Medium ~120 seconds | Long >120 seconds |
| Intensity | Maximum-Intensity (~100%) | Medium-High-Intensity (70%) | Low-Intensity (40%) |
Training the Anaerobic Pathway. In order to train the anaerobic pathways, workouts must incorporate rest so that the working time is short and the intensity during work is high. This style of work-rest pattern is known as an interval.
An interval consists of work that must be completed in a short amount of time at a high intensity, approximately 70% to 100%, and typically only a few minutes long. Composing several intervals into a workout with rest before each interval is known as interval training. This style of training has two major benefits:
- Intervals can be used to train the anaerobic pathways.
- Intervals can also be used to train the aerobic pathway.
- They have been shown to increase aerobic capacity without loosing strength unlike long aerobic style workouts.
Strength training is also predominately anaerobic. Sets are typically short, the weight is usually above 70%, and we always rest between sets; thus, essentially doing intervals. If the weight is light, and we are doing large sets of reps with little rest, then we are most likely not training the anaerobic system.
Training the Aerobic Pathway. One particular popular and effective training method to increase aerobic capacity is to train in the 60% to 70% heart rate zone, typically called zone 2, over a long time period. Generally, this is at least 60 minutes. The movements used are typically running, bike erg, row, ski erg, and combinations of these movements. At first, zone 2 training can seem very boring and easy, but the better your aerobic capacity gets, the harder you will be able to work in zone 2.
We can also train the aerobic pathway in a high-intensity manner using weights, jump ropes, gymnastics, and other movements. We just need to ensure the workout is long, the reps are high, but the weight is low to moderate. This will ensure that the aerobic system dominates the workout. In any workout with a time domain that is longer than 2 minutes, the time after the 2 minute mark the aerobic system begins to dominate. Thus, by choosing a time domain that is at least 4 minutes long, then we can train the aerobic system and the anaerobic system equally. As time progresses longer than 4 minutes the aerobic system begins to dominate.
Training Protocol. To see improvements in both explosive power and endurance we must train both the anaerobic and aerobic pathways. This implies that a properly designed training protocol should include time spent training in both. Now, depending on ones goals, we might lean toward the majority of our training being in one versus the other, but neither should be excluded.
If ones training leans more heavily towards anaerobic than aerobic, then that is less of an issue, since as we stated above, we can train the aerobic pathway without sacrificing strength when training the anaerobic pathway. This is the reason CrossFit is mainly anaerobic. That being said, there should still be time spent training purely aerobic, because the low intensity aspect of it invites recovery, improves lactate to ATP production, and offers a mental break from having to work at a high-intensity day-in and day-out.
The following chart gives a very nice view of when each system is activated, and thus, can be used when designing workouts.
Across the x-axis is the duration of a workout and down the y-axis is percentage of work. As we can see, the contribution of the anaerobic system gets smaller as the duration increases, but never turns off.
An unbalanced training protocol can have physical effects and could hinder an athlete from progressing. We see this in muscle fiber type. Athletes who predominantly train in the anaerobic pathway have a higher concentration of fast twitch muscle fibers than athletes who train predominately in the aerobic pathway; vice versa, the latter have a higher concentration of slow twitch muscle fibers than the former. Neglecting to train the other pathways will leave holes and weaknesses that only training can cure.
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