What is Fitness? Part 3: Metabolic Pathways


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In our previous articles we have summarized CrossFit’s first 2 standards to evaluate fitness – the 10 General Physical Skills and capability at random physical tasks. Here we will discuss the 3rd and final standard in more detail: competency in the 3 metabolic pathways.


Our body uses a chemical known as ATP for energy. The way we use ATP can be categorized into 3 metabolic pathways. These pathways are the Phosphagen pathway, Glycolytic pathway, and Oxidative pathway. The Phosphagen pathway dominates the highest-powered activities, those lasting from 0 to 30 seconds. The Glycolytic pathway dominates moderate-powered activities, those lasting up to 4 to 6 minutes. The Oxidative pathway dominates low-powered activities, lasting anywhere from several minutes to several hours.




The first two metabolic systems, Phosphagen and Glycolytic, are considered “anaerobic” while the third, Oxidative, is considered “aerobic.” High and moderate powered efforts lasting up several minutes (approx., 4-6 minutes) are anaerobic, while low powered efforts lasting more than several minutes are aerobic. There are several examples of each in CrossFit. Phosphagen can be a max effort ranging from a 100m sprint to a 1RM Clean and Jerk. Glycolytic would be a workout like Fran or Grace, specializing in high intensity. These examples are considered anaerobic. The Oxidative (aerobic) system tends to be compromised of longer workouts with movements like biking, running, swimming, and jumping.


IMG_1070A max lift is an example of the phosphagen system


Anaerobic training – high and moderately powered, quick efforts – is unique in its capacity to dramatically improve power, speed, strength, and muscle mass. It also greatly benefits cardiovascular function and decreases body fat better than aerobic training, which keeps working up to 36 hours after training! In fact, anaerobic training will also help develop aerobic capacity, where the inverse unfortunately is not true. Many endurance athletes recently have implemented anaerobic (sprint/explosive) training, and seen immense improvements in their performances. Greater muscular development and function in a fatigued state helps simplify endurance events.


IMG_9307Explosive movements like box jumps are considered anaerobic


Aerobic training – lower powered, longer lasting efforts – benefits cardiovascular function and decreases body fat. It helps vastly improve 2 of the 10 General Physical Skills, Cardiovascular/Respiratory Endurance and Stamina, and is beneficial to many sports. Unfortunately as a sole focus, aerobic training is inferior to anaerobic training for an individual concerned with total conditioning and optimal health. This is evident at any elite level track meets. Pay attention to the physiques of the sprint athletes and the distance athletes. The difference you’ll notice is a direct result of training at those efforts.


IMG_7517A 10k bike is an example of aerobic training


Well rounded, overall fitness, which CrossFit encourages and develops, requires competency and training in each of these pathways. Balancing the stimuli and adaptations of these determine the metabolic conditioning or that we implement in CrossFit. It is important to be able to run 2 miles at a consistent and strong pace, while also being able to sprint 100m at a full effort. Each have their own benefits and place in a person’s metabolic abilities. The more you train with a constant variance, the more well-rounded, and optimally fit, you will become.