This study aimed to investigate the effect of ground contact time on VO₂max and heart rate variability (HRV) during treadmill running at different speeds. Thirty male athletes (age: 24.6 ± 2.1 years) with at least three years of running experience participated in this quasi-experimental study with repeated measures. Participants ran on a treadmill at three speeds (8, 10, and 12 km/h) with a 1% incline. Ground contact time (GCT) was recorded using a force plate, VO₂max was measured using a metabolic gas analyzer (Bruce protocol), and HRV indices were recorded via electrocardiography. Data were analyzed using repeated-measures ANOVA, independent t-tests, and Pearson correlation coefficients. Increasing speed significantly decreased GCT (p = 0.001) and increased VO₂max (p = 0.001), while HRV decreased (p = 0.003). GCT and VO₂max showed significant differences among all three speeds (p < 0.05), but HRV only showed a significant difference between 8 and 12 km/h (p = 0.012). Athletes with higher training experience exhibited shorter GCT (p < 0.001), higher VO₂max (p < 0.001), and greater HRV (p = 0.003). Significant negative correlations were observed between GCT and VO₂max (r = -0.58) and HRV (r = -0.48). Reduced ground contact time is associated with improved aerobic capacity and better autonomic nervous system function. Concurrent monitoring of GCT and HRV may serve as an effective tool for evaluating training responses and preventing overtraining in endurance athletes.