Physiological Determinants of Performance in Vertical Kilometer Running

Abstract

Introduction & Purpose: Vertical Kilometer (VK) running is a growing discipline on extreme uphill terrain. On October 10th, 2025, both men’s (Rémi Bonnet, 27 min 21 s) and women’s (Axelle Gachet-Mollaret, 32 min 52 s) world records were broken in Fully, Switzerland—one of the steepest courses worldwide (1.9 km, 1000 m ascent, 52% slope). Although previous research has defined the energetic demands and optimal gradients for maximizing vertical velocity during uphill locomotion, the physiological determinants of elite VK performance and the transferability of laboratory treadmill tests to field performance remain poorly understood. This study examined the relationships between physiological parameters measured during a treadmill test and actual VK race performance in athletes. We hypothesized that V̇O_2max and maximal vertical velocity (vV_max) would strongly predict VK race time.

Methods: Eight athletes (Tier 3-5), (4 males, 4 females) from the Swiss national ski mountaineering team performed treadmill tests at 25% slope, followed by the Fully VK race 10–12 days later. Each athlete completed two 5-min submaximal bouts (4.0 km·h⁻¹ for men; 3.5 km·h⁻¹ for women) to assess VO₂ kinetics and vertical running economy, followed by an incremental test to exhaustion (0.3 km·h⁻¹·min⁻¹ increments; with poles). Breath-by-breath gas exchange (Quark CPET, Cosmed, Italy) was analyzed to determine ventilatory thresholds and maximal values. Pulse oxygen saturation (SpO₂) was measured continuously. Hemoglobin mass (Hbmass) was assessed by the optimized CO-rebreathing method (OpCo, Detalo Instruments, Denmark), and body composition was estimated from six skinfolds to calculate body fat percentage (BF%) and fat-free mass (FFM).

Results: V̇O_2max and V̇O_2max __FFM differed significantly between females and males (60.0 ± 3.1 vs. 73.7 ± 4.9 ml·kg⁻¹·min⁻¹, p < 0.001; and 69.7 ± 4.1 vs. 79.0 ± 5.5 ml·kg⁻¹·min⁻¹, p = 0.003, respectively). vV_max showed the strongest relationship with VK time (Figure 1). V̇O_2max was also highly correlated with performance. Minimal SpO₂ at exhaustion (SpO_2min) correlated positively with VK time, indicating that greater exercise-induced hypoxemia (EIH) was associated with poorer performance. Moreover, Hbmass was also highly correlated with VK time (r = -0.95, p = 0.011. Surprisingly, BF% did not correlate significantly with VK time (r = 0.67, p = 0.067).

Discussion: As expected, vV_max, V̇O_2max, and Hbmass were the strongest predictors of VK performance. In our study, EIH was associated with decreased performance, likely due to increased quadriceps fatigue, which contributes to earlier exhaustion and reduced endurance capacity (Romer et al., 2006). Normalizing V̇O_2max and Hbmass to FFM did not substantially change their correlation with performance, suggesting that oxygen transport efficiency is a primary determinant independent of body composition.

Conclusion: A treadmill test at 25% slope provides a valid assessment of VK performance determinants. Beyond aerobic power and maximal vertical velocity, EIH appears influential.

References

Romer, L. M., Haverkamp, H. C., Lovering, A. T., Pegelow, D. F., & Dempsey, J. A. (2006). Effect of exercise-induced arterial hypoxemia on quadriceps muscle fatigue in healthy humans. American Journal of Physiology. Regulatory, Integrative and Comparative Physiology, 290(2), R365-375. https://doi.org/10.1152/ajpregu.00332.2005