Glycogen, stored in the muscles and liver, serves as a crucial fuel source during exercise, especially during higher-intensity activities. As heart rate rises, so does glycogen usage. Knowing the specific heart rate zones that trigger glycogen burning for different age groups can help tailor workouts to meet endurance and energy goals.

What is Glycogen?

Glycogen is a stored form of glucose found primarily in muscles and the liver. During low to moderate-intensity exercise, fat is the primary fuel source. However, as exercise intensity increases, the body relies more on glycogen, which is rapidly converted into glucose to meet high-energy demands.

Heart Rate Zones and Glycogen Usage

The heart rate zones, relative to your maximum heart rate, affect how and when your body uses glycogen. Here’s how it breaks down by heart rate zones, each correlating with a different exercise intensity:

• Zone 1 (Very Light, 50-60% of Max Heart Rate): Minimal glycogen use. This is a fat-burning zone, typically for light activities like walking.
• Zone 2 (Light, 60-70% of Max Heart Rate): Low glycogen use, especially effective for endurance workouts. A mix of fat and glycogen is used.
• Zone 3 (Moderate, 70-80% of Max Heart Rate): Glycogen becomes an increasingly important fuel source.
• Zone 4 (High, 80-90% of Max Heart Rate): High glycogen consumption. Activities in this range—such as interval training—significantly draw on glycogen.
• Zone 5 (Very High, 90-100% of Max Heart Rate): This is the maximum effort zone where glycogen is the primary fuel source, depleted rapidly.

Calculating Maximum Heart Rate and Zones by Age

To determine heart rate targets for different ages, the formula “220 – age” is typically used to calculate Maximum Heart Rate (MHR). Then, multiply MHR by the target percentages for each heart rate zone. Here are examples for different age groups:

Example for a 25-Year-Old (MHR = 195 bpm)

• Zone 1 (Very Light, 50-60%): 98–117 bpm
• Zone 2 (Light, 60-70%): 117–137 bpm
• Zone 3 (Moderate, 70-80%): 137–156 bpm
• Zone 4 (High, 80-90%): 156–176 bpm
• Zone 5 (Very High, 90-100%): 176–195 bpm

Example for a 40-Year-Old (MHR = 180 bpm)

• Zone 1 (Very Light, 50-60%): 90–108 bpm
• Zone 2 (Light, 60-70%): 108–126 bpm
• Zone 3 (Moderate, 70-80%): 126–144 bpm
• Zone 4 (High, 80-90%): 144–162 bpm
• Zone 5 (Very High, 90-100%): 162–180 bpm

Example for a 55-Year-Old (MHR = 165 bpm)

• Zone 1 (Very Light, 50-60%): 83–99 bpm
• Zone 2 (Light, 60-70%): 99–116 bpm
• Zone 3 (Moderate, 70-80%): 116–132 bpm
• Zone 4 (High, 80-90%): 132–149 bpm
• Zone 5 (Very High, 90-100%): 149–165 bpm

Example for a 65-Year-Old (MHR = 155 bpm)

• Zone 1 (Very Light, 50-60%): 78–93 bpm
• Zone 2 (Light, 60-70%): 93–109 bpm
• Zone 3 (Moderate, 70-80%): 109–124 bpm
• Zone 4 (High, 80-90%): 124–140 bpm
• Zone 5 (Very High, 90-100%): 140–155 bpm

When Glycogen Depletion Happens

During prolonged or intense exercise, glycogen is depleted more rapidly. In Zone 4 (80-90% of MHR), glycogen use is high, and in Zone 5 (90-100% of MHR), glycogen depletion is at its fastest. At these intensities, the body relies primarily on glycogen, and performance begins to drop once glycogen stores are exhausted. For most people, glycogen depletion at high intensities can occur within 30–60 minutes, depending on fitness level and carbohydrate intake prior to exercise.

Maximizing Glycogen Usage and Recovery

1. Pre-Workout Carbohydrate Loading: Increasing carbohydrate intake 1–2 days before a workout maximizes glycogen storage.
2. During-Exercise Fueling: For workouts lasting longer than an hour, consuming 30–60 grams of carbohydrates per hour helps preserve glycogen stores.
3. Post-Workout Recovery: A combination of carbohydrates and protein within 30 minutes post-exercise, at a ratio of 3:1, helps replenish glycogen and aids in muscle recovery.

Practical Takeaway: How High Should Your Heart Rate Be?

For those looking to train glycogen use, heart rates should be in Zone 4 (80-90%) or Zone 5 (90-100%), depending on age. For example, if you’re a 40-year-old, aim to keep your heart rate between 144–180 bpm during intervals to maximize glycogen use, enhancing endurance and helping the body adapt to switching between energy sources effectively.