Training for broad, general, and inclusive fitness means preparing for everything from short, intense sprints to long-duration endurance challenges. A key part of this training philosophy lies in understanding how our metabolic pathways function to fuel different types of workouts.
Metabolism is the body’s process of converting food into energy, and CrossFit programming strategically develops all three of the body’s primary metabolic energy systems:
- Phosphagen (ATP-PC) System – Short bursts of high-intensity effort
- Glycolytic (Anaerobic) System – Moderate-duration, high-power output efforts
- Oxidative (Aerobic) System – Long-duration, lower-intensity efforts
Let’s dive deeper into each pathway and why training them all is essential for overall fitness.
1. Phosphagen (ATP-PC) System: Power & Explosiveness
The Phosphagen system (also called the ATP-PC system) is your body’s quickest source of energy, providing an immediate burst of power. It is used during short, high-intensity activities lasting about 10 seconds or less—think of a one-rep max lift, a 40-yard sprint, or an explosive box jump.
How It Works:
- This system relies on adenosine triphosphate (ATP) and phosphocreatine (PC) stored directly in the muscles.
- ATP is broken down to produce energy, and PC helps regenerate ATP rapidly for continued short bursts of effort.
- Because there’s a limited supply of ATP and PC in the muscles, this system depletes quickly and requires 2-3 minutes of rest to fully recover.
Training the Phosphagen System:
- Olympic lifts (snatch, clean & jerk)
- Short sprints (100m or less)
- Max-effort jumps (box jumps, broad jumps, vertical leaps)
- Max-effort strength work (heavy squats, deadlifts, presses)
By training this system, CrossFit athletes develop explosiveness and peak power output, which translates to faster acceleration and improved maximal strength.
2. Glycolytic (Anaerobic) System: Sustained High-Intensity Work
The Glycolytic system is your body’s primary fuel source for efforts that last between 30 seconds and 2 minutes—think of a 400-meter sprint, a fast-paced metcon, or a set of high-rep thrusters.
How It Works:
- This system breaks down stored glycogen (carbohydrates) to produce ATP rapidly without requiring oxygen (anaerobic process).
- Because lactic acid is a byproduct of this process, sustained efforts in this pathway can lead to muscle fatigue and the well-known “burning” sensation.
- Recovery from glycolytic efforts typically takes 1-3 minutes, making interval training highly effective for improving this system.
Training the Glycolytic System:
- High-intensity interval training (HIIT)
- Rowing, biking, or running intervals (200m–800m sprints)
- AMRAPs (as many reps as possible) lasting 1-3 minutes
- CrossFit benchmark workouts like “Fran” and “Helen”
Training the Glycolytic system enhances anaerobic endurance, helping athletes push harder for longer in moderate-duration WODs.
3. Oxidative (Aerobic) System: Endurance & Stamina
The Oxidative system is responsible for sustained energy production over long durations, typically for efforts lasting several minutes to hours—think of a long-distance run, row, or an extended hero WOD like “Murph.”
How It Works:
- The oxidative system primarily uses oxygen to break down carbohydrates, fats, and proteins to produce ATP.
- This process is much slower than the other two pathways, but it can generate large amounts of energy over time.
- While lower in intensity, training the aerobic system builds stamina, cardiovascular efficiency, and overall endurance.
Training the Oxidative System:
- Sustained, moderate-intensity efforts (running, rowing, cycling)
- Longer CrossFit workouts (20+ minutes in duration)
- EMOMs (Every Minute on the Minute) with moderate intensity
- Zone 2 training (maintaining a steady heart rate for endurance)
Building a strong oxidative system allows athletes to recover faster between workouts, sustain energy levels longer, and improve their overall work capacity.
Why CrossFit Trains All Three Systems
CrossFit programming is unique because it intentionally develops all three metabolic pathways, ensuring well-rounded fitness. By cycling through short, medium, and long-duration workouts, CrossFitters improve their ability to:
- Generate maximum power and strength in short bursts.
- Maintain high-intensity effort for longer periods without excessive fatigue.
- Sustain endurance and stamina for extended workouts.
In real-life scenarios, no single energy system works in isolation. Whether you’re lifting a heavy object, chasing after your kids, or running a 5K, your body transitions between these pathways seamlessly. Training across all three ensures you’re ready for anything life throws your way.
Unlock Your Full Potential at CrossFit St. Charles
Understanding and developing all three metabolic pathways is what makes CrossFit such an effective training methodology. Whether you want to improve strength, endurance, or overall work capacity, CrossFit St. Charles is here to guide you through expert coaching, smart programming, and a supportive community.
Check out this video from CF HQ to deepen your knowledge
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