Time-resolved energy-share reconstruction of individual glycolytic flux kinetics reveals duration-dependent bias in elite sprint cycling.

Background

Maximal lactate accumulation rate (vLamax) is commonly used as a surrogate of anaerobic glycolytic power. Conventional protocols calculate vLamax as a mean rate over a fixed sprint window after subtracting an alactic phase, even though glycolytic flux rises, peaks, and declines within seconds. Fixed-duration protocols may therefore underestimate the instantaneous peak rate.

Methods

Seventeen elite track cyclists performed 12 s and 60 s maximal isokinetic sprints with continuous power, gas exchange, near-infrared spectroscopy, and blood lactate. Total metabolic energy (from dynamic gross efficiency), aerobic energy, and phosphocreatine energy were reconstructed, and the glycolytic residual gave vLa(t), vLamax, and time to peak.

Results

The indirect assessment yielded higher vLamax than the gold standard (0.91 ± 0.18 vs. 0.82 ± 0.17 mmol/L/s; p < 0.001), while predicted 12 s lactate accumulation matched measured values (7.67 ± 1.77 vs. 7.58 ± 1.60 mmol/L; p = 0.464). Peak vLa occurred at 9.1 ± 1.4 s, before the 12 s endpoint.

Conclusion

A single maximal sprint suffices to reconstruct the individual glycolytic flux trajectory (its onset, peak, and post-peak decline), and the cumulative lactate accumulation was independently validated against a separate 12 s sprint. On this basis, fixed-duration protocols appear to underestimate instantaneous peak vLamax by averaging across the post-peak decline (mean bias ~ 10%). Durations around 9-10 s may therefore offer a physiologically grounded compromise for vLamax assessment in elite cycling.