Key Takeaways
- Fatigue Index measures the percentage drop in workout performance across repeated efforts.[1]
- Lower FI indicates better recovery capacity between sets—the ability to maintain performance.[2]
- Three methods available: jump-based (Bosco), rep-based, and power decay for different training contexts.
- FI below 15% = excellent, 15–30% = good, 30–45% = average, above 45% = more recovery needed.
What Is the Fatigue Index?
The Fatigue Index quantifies how much your performance drops across repeated bouts of exercise. Originally developed by Bosco for repeated jump tests[1], the concept applies broadly to any measurable output—reps, power, or jump height. It's a practical tool for tracking workout performance decay and adjusting rest periods.
This is framed strictly as a workout performance tracker—not a clinical fatigue assessment. For recovery monitoring, see our Muscle Recovery Calculator. For training load management, try the Overtraining Risk Calculator.
Three Measurement Methods
- Bosco Jump Test: Perform repeated vertical jumps and record the best and worst heights. FI = (best − worst) / best × 100. Best for explosive power sports.[1]
- Rep-Based: Perform multiple sets at the same weight and record reps achieved per set. FI = (Set 1 reps − Last set reps) / Set 1 reps × 100. Most practical for strength training.
- Power Decay: Record peak watts and final watts from repeated bike sprints or intervals. FI = (peak − final) / peak × 100. Ideal for cycling and sprint-based training.[2]
Interpreting Your Results
A low FI means you maintain performance well across sets—either because you're well-conditioned, resting adequately, or the load is appropriate. A high FI suggests the load, volume, or rest periods may need adjustment.
Track your FI over time. If it decreases with the same workout protocol, your work capacity is improving. If it increases, you may be accumulating fatigue and need recovery. For volume tracking, see the Training Volume Calculator.
How to Use This Calculator
- Select your measurement method (jump-based, rep-based, or power decay).
- Enter your performance data for each method.
- Review your Fatigue Index percentage and classification.
- Follow the rest period recommendation to optimize recovery between sets.
- Retest periodically to track changes in your work capacity.
Frequently Asked Questions
Sources & References
- Stretch-shortening cycle fatigue: Interactions among joint stiffness, reflex, and muscle mechanical performance. Bosco C, et al.. International Journal of Sports Medicine (1992)
- Fatigue monitoring in resistance exercise. Research on performance decay tracking. European Journal of Applied Physiology (2013)
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NASM-certified fitness and nutrition coach with over 10 years of hands-on experience helping people build strength, lose fat, and live healthier lives. Specializing in gym-based workouts with a strong focus on lifting technique, biomechanics, and practical exercise science. Through FitLifeRegime, sharing the tools, tips, and insights that have worked for hundreds of clients — helping you start your own fitness journey with confidence and clarity.
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Certified Personal Trainer & Sports Nutritionist from the National Academy of Sports Medicine
Disclaimer: This tool is for informational and personal journaling purposes only. Results are estimates based on published formulas and may vary based on individual factors such as genetics, training history, and measurement technique.