2.1 Energy Systems, Workout Formats & Stimulus Selection

Energy Systems in Mixed Modal Training

At the core of CrossFit programming is a deep understanding of bioenergetics—how the body produces, utilizes, and replenishes energy during high-intensity functional movements. A Level 3 Certified CrossFit Trainer must not only understand the independent function of each energy system but also how they interplay during mixed-modal workouts. Programming without acknowledging energy system constraints often leads to "junk volume" or an improper stimulus that fails to drive adaptation.

The Three Primary Energy Systems

Human movement is powered by adenosine triphosphate (ATP). Because the body stores very little ATP, it must be continuously resynthesized through three distinct energy pathways.

1. Phosphagen System (ATP-PCr): This system provides immediate energy for maximal-intensity, short-duration activities. It relies on stored ATP and creatine phosphate (PCr) within the muscle. The phosphagen system dominates activities lasting 0 to 10 seconds, such as a 1-rep max deadlift, a 50-meter sprint, or a heavy clean and jerk. It operates anaerobically (without oxygen) and produces energy rapidly, but its stores are quickly depleted. Recovery of this system requires 3 to 5 minutes of rest, corresponding to a work-to-rest ratio of 1:12 to 1:20.
2. Glycolytic System: When high-intensity effort continues beyond 10 seconds, the body shifts to the glycolytic system, which breaks down muscle glycogen and blood glucose to produce ATP. This system is the primary driver for efforts lasting between 10 and 120 seconds, such as a 400-meter sprint or an unbroken set of 30 thrusters. A byproduct of this anaerobic process is lactic acid, which dissociates into lactate and hydrogen ions, leading to a drop in muscle pH and the familiar "burning" sensation that causes fatigue. Proper recovery for glycolytic efforts typically demands a work-to-rest ratio of 1:3 to 1:5.
3. Oxidative (Aerobic) System: For continuous efforts lasting longer than 2 to 3 minutes, the oxidative system becomes the primary source of ATP. This system requires oxygen to metabolize carbohydrates and fats. While it produces ATP at a slower rate than the anaerobic systems, its capacity is virtually limitless, making it the engine for endurance events like a 5K run or a long 20-minute AMRAP. Crucially, a highly developed aerobic system also facilitates the clearance of lactate and speeds up the replenishment of ATP-PCr stores between high-intensity anaerobic intervals. Work-to-rest ratios for aerobic training are typically 1:1 or 1:0.5.

Bioenergetics of Mixed Modal Workouts

CrossFit’s constantly varied, high-intensity functional movements blur the lines between these energy systems. A workout like "Fran" (21-15-9 Thrusters and Pull-ups) is a classic example of a glycolytic test. Elite athletes complete it in under 3 minutes, relying heavily on the glycolytic pathway and generating massive amounts of lactate. In contrast, "Murph" (1-mile run, 100 pull-ups, 200 push-ups, 300 squats, 1-mile run) is primarily an aerobic test due to its duration, though it involves local muscular endurance that heavily taxes the glycolytic system. Designing mixed-modal workouts requires an understanding of how fatigue in one system affects performance in another.

Common CrossFit Workout Formats

The format of a workout dictates its pace, strategy, and ultimate physiological stimulus.

• AMRAP (As Many Rounds / Reps As Possible): This is a time-priority format where the duration is fixed, and the athlete's work output is the variable. AMRAPs are excellent for testing pacing, mental fortitude, and aerobic/glycolytic capacity. Because the time domain is fixed, athletes learn how to manage their fatigue threshold.
• EMOM (Every Minute on the Minute): EMOMs are structured interval workouts where a specific task is completed at the start of every minute, with the remaining time in that minute used for rest. This format is ideal for skill acquisition under fatigue, pacing practice, and energy system specific training (e.g., maintaining phosphagen output with heavy lifts or managing glycolytic clearance).
• For Time (Task Priority): In a "For Time" format, the work is fixed, and the time to completion is the variable. This encourages athletes to push their intensity to the maximum, often testing their glycolytic threshold as they attempt to minimize transition and rest times.
• Chipper: A Chipper typically involves a single round of several different movements performed in succession, usually with high volume. Chippers test muscular stamina and mental resilience, often requiring athletes to break sets strategically to avoid early muscular failure.
• Tabata: Based on the research of Dr. Izumi Tabata, this format involves 8 rounds of 20 seconds of all-out work followed by 10 seconds of rest (4 minutes total). True Tabata intervals are designed to maximize both aerobic and anaerobic capacities simultaneously, though the 2:1 work-to-rest ratio makes complete recovery impossible, leading to a massive oxygen debt.

Targeted Physiological Stimulus

The effectiveness of a program hinges on clearly defining the intended stimulus for each workout. Is the goal to build absolute strength (phosphagen), improve lactate tolerance (glycolytic), or build a massive aerobic base? The stimulus dictates the loading, the volume, and the format. For instance, if the intended stimulus is a heavy, phosphagen-dominant effort, forcing an athlete into a 15-minute AMRAP of heavy deadlifts is counterproductive and dangerous; a 5x3 format with 3 minutes of rest between sets is appropriate. Mastery of programming involves matching the format, movements, and rep schemes to the precise physiological adaptation you are trying to elicit.