Fatigue Index Calculation Formula
The Fatigue Index (FI %)quantifies the relative decline in work output during a maximal, continuous multi-repetition dynamic isokinetic test (typically 20 to 30 repetitions at 180°/s or 240°/s). It compares the work completed in the initial third versus the final third of the test:
Fatigue Index (%) = [1 - (Work in Last 1/3 Reps / Work in First 1/3 Reps)] × 100%
A lower Fatigue Index (e.g. 30–40%) indicates superior muscular fatigue resistance and aerobic/anaerobic capacity, whereas a high Fatigue Index (>50–60%) demonstrates rapid neuromuscular exhaustion under continuous velocity loading.