Autonomic nervous system fatigue is detected earlier in jump tests than in heart rate because it first impairs muscle activation. This system controls automatic functions like muscle response and heart rate. When fatigued, it can't sustain strong, quick muscle contractions, reducing jump performance.

Jump & reaction time tests are sensitive to nervous system changes, showing reduced muscle reaction and power. Heart rate, however, may not immediately reflect this fatigue, as heart control mechanisms compensate longer. Thus, early signs of nervous fatigue, like lower jump results, appear before heart rate changes.