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17.2 - Exercise Physiology 101

Dr. Dwayne after 15.3

17.1 tested your ability to pace yourself. Did you find your ideal pace, a point where you pushed your limits but didn’t leave yourself hands-on-hips gasping for air watching the clock tick down?

Perhaps a review of your energy systems will help you further understand the rationale behind your coaches’ programming (there is method to the madness) and tackle WODs more efficiently.

ATP - fuel the beast

Adenosine triphosphate (ATP) is the molecule your body uses to produce the energy required to lift heavy, run fast and jump high. Specifically, you convert the high energy ATP molecule into the lower energy molecule ADP (Adenosine triphosphate) releasing energy to power your muscles.

Your muscles need a constant supply of ATP to power themselves and, since they store very little ATP, you must be able to replenish ATP levels efficiently and effectively. Your body uses a combination of three systems to accomplish this:

Creatine Phosphate - rocket fuel

Creatine phosphate (CP), stored in your skeletal muscles, provides the phosphate required by ADP to return to the ATP form. Using CP is the fastest way to replenish ATP, however there are limited quantities of ATP and CP available in your muscles causing this system to fatigue rapidly.

The ATP-CP system is good for about 10 seconds of intense activity, making it ideal for activities such as single-rep heavy lifts and sprints. The ATP-CP system requires 3-5 minutes to recharge between uses. In other words, when your coach recommends 3+ minutes of rest between attempts, take 3+ minutes, the WOD was programmed to allow this system to fully recharge.


Glycolysis is the process of burning sugar (glucose) for energy. Blood glucose, or muscle glycogen (sugar stored in muscles), breaks down forming ATP and pyruvate (remember that name, it will be important later). The rate of ATP production from this system is slower than that of the ATP-CP system, however it is able to produce energy over a longer periods of time.

Glycolysis produces the ATP required for exercise lasting 30 seconds to 2 minutes. This system is ideally suited for work including higher volume rep schemes or a 500m row.

Aerobic - slow burn

The aerobic system uses a molecule called acetyl-CoA to power ATP production. Acetyl-CoA is produced during the breakdown of fats and sugars:

  • Lypolysis breaks fats into fatty acids used to produce acetyl-CoA

  • Pyruvate, from glycolysis (see I told you), can also be used to produce acetyl-CoA in the presence of sufficient oxygen

Aerobic metabolism is capable of sustaining production of large quantities of ATP over a prolonged periods of time, making it ideally suited for longer exercise sessions.

Contact Dr. Dwayne to learn more about exercise physiology in your training

  • Visit our clinics at CrossFit Connection and/or CrossFit Streets

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