Muscle Glycogen Use Patterns During Exercise
Understanding Muscle Fiber Types and Glycogen
Before we dive into glycogen use patterns, let's quickly refresh our understanding of muscle fiber types:
- Type I (Slow-twitch): Oxidative, fatigue-resistant
- Type IIa (Fast-twitch oxidative): Mix of aerobic and anaerobic
- Type IIx (Fast-twitch glycolytic): Mainly anaerobic, powerful but quick to fatigue
Each fiber type has different glycogen storage capacities and utilization patterns during exercise.
Glycogen Use Patterns at Different Exercise Intensities
Low-Intensity Exercise (0-65% VO2max)
- Primarily utilizes Type I fibers
- Glycogen depletion is relatively slow
- Fat oxidation is the primary energy source
- Glycogen stores can last several hours
During a long, slow distance run at 60% VO2max:
- Type I fibers are predominantly active
- Glycogen depletion occurs gradually
- Athletes can maintain this intensity for 2-3 hours
Moderate-Intensity Exercise (65-85% VO2max)
- Recruits both Type I and Type IIa fibers
- Increased glycogen utilization rate
- Mixed use of fat and carbohydrates
- Glycogen depletion becomes more significant
This is typically the intensity zone where many endurance athletes train, as it provides a good balance between performance and sustainability.
High-Intensity Exercise (>85% VO2max)
- Recruits all fiber types, including Type IIx
- Rapid glycogen depletion
- Primarily carbohydrate dependent
