Intensity and Heart Rate Zones
Coach Kris Perry-Tedesco • November 1, 2024
The Three Energy Systems & The Balance of Bio Energenetics

Why do program 5 minute workouts vs. 30 minute workouts?
(and all the time domains in between)
Let’s break down the differences between these two CrossFit workouts by looking at the intensity, energy systems used, heart rate zones, and recovery needs:
1. Intensity and Heart Rate Zones
30-Minute CrossFit Workout at Average Zone 3:
Intensity: This workout would be a moderate-intensity session. Zone 3 (around 70–80% of max heart rate) represents a steady aerobic state.
Heart Rate: In Zone 3, your heart rate is elevated but stays within a manageable range, allowing you to sustain the activity for a longer period without burning out.
VS.
5-Minute CrossFit Workout at Average Zone 5:
Intensity: This workout is very high-intensity. Zone 5 (90–100% of max heart rate) pushes your heart rate to the max, close to its peak.
Heart Rate: In Zone 5, your heart is working at or near its capacity, delivering maximum oxygen to muscles.
It’s difficult to sustain Zone 5 for more than a few minutes without significant fatigue.
2. Energy Systems Used
30-Minute Workout (Zone 3):
Primary System:
The aerobic (oxidative) system dominates here, as it’s best for prolonged, lower-intensity exercise.
Support from Glycolytic System:
During intense parts of the workout (like short bursts within the workout), the glycolytic system may provide additional energy.
Benefits:
Builds endurance, improves cardiovascular health, and increases aerobic capacity.
VS.
5-Minute Workout (Zone 5):
Primary Systems:
The ATP-PC and glycolytic systems are the main sources of energy, providing quick ATP without oxygen for high-power output.
Limited Aerobic Contribution:
The aerobic system plays a minor role because there’s not enough time for it to be the primary energy provider.
Benefits:
Enhances power, speed, and anaerobic capacity, and boosts VO₂ max over time
3. Workout Goals and Training Effects
30-Minute Workout (Zone 3):
Goal: Build endurance and improve aerobic fitness.
Training Effect: Improves muscle stamina and efficiency in oxygen usage, which helps with longer WODs or endurance-based CrossFit events.
Recovery: Lower intensity means quicker recovery time, so you’re less likely to feel burned out the next day.
VS.
5-Minute Workout (Zone 5):
Goal: Boost anaerobic power, muscular strength, and explosive endurance.
Training Effect: Increases power output, speed, and lactate tolerance, which can improve performance in sprint-style WODs or competitions.
Recovery: Zone 5 intensity requires longer recovery, as it leads to higher muscle breakdown and increased lactic acid build-up.
4. Fatigue and Recovery Needs
30-Minute Workout (Zone 3):
Fatigue:
Moderate fatigue due to sustained effort, but manageable because it’s mostly aerobic.
Recovery Needs:
Generally shorter recovery, as Zone 3 intensity doesn’t cause as much muscle strain or lactic acid buildup as Zone 5.
VS.
5-Minute Workout (Zone 5):
Fatigue:
High fatigue due to intense effort, leading to “burnout” feeling in muscles and potential soreness.
Recovery Needs:
Requires more recovery time, potentially a day or two, due to the higher stress on muscles and cardiovascular system.
Summary
30-Minute, Zone 3 Workout: Steady aerobic effort, builds endurance, and has moderate fatigue/recovery needs.
5-Minute, Zone 5 Workout: High-intensity anaerobic effort, builds power and strength, and has significant fatigue/recovery needs.
Both workouts are valuable in CrossFit but serve different purposes:
One builds endurance, while the other enhances power and anaerobic capacity.
Let's dig a little deeper....
Bioenergetics is about how living cells manage energy, especially how they create and use it to keep the body functioning.
Think of it like a battery. Just as your phone or flashlight needs to be charged to work, your body needs energy to function. You get this energy from food, which your body converts into a molecule called ATP (adenosine triphosphate). ATP is like a tiny packet of energy that powers everything your cells need to do, like muscle movement, brain activity, and even breathing.
The balance of bioenergetics involves two main processes:
1. Energy Production
(Charging the Battery): Your body breaks down food (like sugars, fats, and proteins) in a process called cellular respiration. This happens in the mitochondria, the "powerhouses" of cells, where food is converted into ATP.
2. Energy Usage
(Using the Battery): When your body needs to do work—like contracting muscles when you walk or think—it "spends" ATP. This energy allows cells to do tasks, and then the ATP is depleted and needs to be replenished by breaking down more food.
The goal of bioenergetics is to keep these processes balanced so that your body has enough energy to function without running out. If your body produces too much ATP or too little, it can disrupt the balance and affect your health and energy levels.
Heart rate is closely tied to which energy system your body is using because different intensities and durations of exercise require different levels of oxygen and energy.
Here’s how heart rate connects to each energy system:
1. ATP-PC System
(Immediate Energy System):
Since this system is for very short, explosive bursts (like sprinting or lifting heavy weights), it doesn’t rely on oxygen and doesn’t demand much from the heart.
Heart rate may rise quickly at the start but won’t reach high levels because the activity is too brief for the aerobic system to kick in fully. So, you might feel a quick spike but not a sustained high heart rate.
2. Glycolytic System (Anaerobic System):
In this phase, your body still isn’t using oxygen fully, so the heart rate increases to deliver more oxygen to muscles in anticipation of greater demands.
As you push through moderate-duration, high-intensity exercise (like a 400-meter run), your heart rate climbs toward the upper end of your anaerobic zone (usually around 80–90% of your maximum heart rate) to help sustain this effort and deal with the buildup of lactic acid.
3. Oxidative System (Aerobic System):
For longer, lower-intensity activities, the oxidative system takes over, relying on oxygen to generate ATP. To provide enough oxygen, your heart rate will stay elevated but usually at a lower, steady rate compared to anaerobic activities.
For moderate aerobic activities, your heart rate may stay in the 60–80% range of your maximum heart rate. This lets you sustain the exercise for longer without fatiguing.
In essence:
Short sprints (ATP-PC): Short, quick heart rate rise but not sustained.
Moderate time/effort (Glycolytic): Higher heart rate (around 80–90%) to support anaerobic work.
Longer, steady activities (Oxidative): Moderate, steady heart rate (60–80%) for sustained aerobic energy.
The more intense the exercise, the higher your heart rate goes, reflecting the body’s need for faster energy replenishment and greater oxygen delivery to muscles.
Previous WODs

Work Stress and Exercise: How to Train, Eat, and Recover During Busy Weeks Have you ever arrived at the gym feeling like you've already completed a workout? Not because you already have. Because you've spent the entire day working, managing responsibilities, making decisions, dealing with unexpected problems, and trying to keep up with everything that needs your attention. By the time you finally have an opportunity to exercise, you're exhausted. And now you're supposed to work hard. Sound familiar? A recent national workplace survey reported that 75% of employees experience workplace stress . That's a significant number of people navigating demanding days while also trying to take care of their health. It raises an interesting question. How do we approach exercise, nutrition, and recovery when life is already asking so much from us? Your Workout Isn't the Only Thing Demanding Energy We often think about exercise as something separate from the rest of our lives. We go to work. We manage our responsibilities. Then we go to the gym. But our bodies don't experience those activities as completely unrelated events. Physical training creates demands that our bodies must adapt to. So do insufficient sleep, demanding work schedules, and other sources of stress. That doesn't mean all stress is bad. In fact, appropriately challenging exercise is one of the ways we become fitter. The important distinction is understanding that our capacity to recover and adapt can be influenced by what's happening outside the gym. Training Hard Still Matters At Warrior's Path CrossFit, we believe in challenging people. We want our members to become stronger, improve their cardiovascular fitness, learn new skills, and accomplish things they previously thought were beyond their abilities. Those improvements require effort. Sometimes significant effort. But there's a difference between applying an appropriate training stimulus and assuming that harder is always better. Imagine two athletes performing the same workout. One slept well, feels recovered, and is ready to challenge a previous performance. The other has been working long hours, sleeping poorly, and feeling unusually fatigued. Both can benefit from training. But the most appropriate approach may not look the same. That's where coaching becomes valuable. Not because we want to remove every challenge. Because we want the challenge to accomplish something useful. Stress Can Influence How We Eat Nutrition presents another interesting challenge. When life becomes demanding, eating behaviors can change. Some people find themselves eating quickly or skipping meals. Others may rely more heavily on convenient foods. Some notice changes in hunger, appetite, or their usual routines. And now we're seeing new research examining whether the timing of meals may also influence health. That's worth investigating. But research findings shouldn't automatically become another source of anxiety about food. A person working an overnight shift, for example, may have limited control over when meals fit into the day. Rather than immediately introducing another rule, we can explore what's happening. Are meals being missed? Is someone regularly arriving at workouts without enough energy? Are there opportunities to make food more accessible? What would help? Those questions can lead to useful behavior changes. Mindset Isn't Just About Pushing Harder Here's where I think we sometimes misunderstand discipline. When we're struggling, our first instinct may be to demand more from ourselves. Try harder. Be more organized. Wake up earlier. Stop making excuses. And sometimes greater effort really is necessary. But sometimes the problem isn't effort. It's that we're trying to manage too many demands without enough resources. An effective mindset involves recognizing that distinction. What deserves your attention? What can wait? Where would additional effort make a meaningful difference? Where might support be useful? These aren't questions about lowering expectations. They're questions about directing effort effectively. What Does Your Body Need From You This Week? Here's something worth considering. Before your next workout, take a moment to notice how you're feeling. Not to decide whether you deserve to exercise. But to understand what you're bringing into the experience. Are you energized? Tired? Mentally distracted? Excited to train? Struggling with recovery? That information can help guide a productive conversation with your coach. The same principle applies to nutrition and other healthy behaviors. You don't need to respond to every demanding week by abandoning your goals. And you don't need to prove your commitment by ignoring every signal that something needs attention. The goal is to keep developing your health while learning how to manage the demands that come with living. At Warrior's Path CrossFit, that's part of what professional coaching is about. We challenge people. We teach. We observe. We help them make informed decisions. Because the purpose of training isn't simply to see how much effort someone can tolerate. It's to help them become healthier, fitter, and more capable through the process. Editorial note: This article is intended as general education, not guidance for diagnosing or treating burnout, sleep disorders, or other medical conditions.
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