Heart Rate Acquisition and Threshold-Based Training Increases Oxygen Uptake at Metabolic Threshold in Triathletes: A Pilot Study

Abstract

Exercise intensity is a critical component of the exercise prescription model. However, current research employing various non-specific exercise intensity protocols have reported wide variability in maximum oxygen uptake (VO_2max) improvement after training, suggesting a present lack of consensus regarding optimal heart rate (f_C) training zones for maximal athletic performance. This study examined the relationship between percentage of time (%time) spent training between the metabolic (VO₂θ) and ventilatory thresholds (V_Eθ), and the resultant change in markers of aerobic performance. Thirteen (6 males) collegiate club-level triathletes were recruited for eight weeks of remote f_C monitoring during all running and cycling sessions. Participants donned a forearm-worn optical f_C sensor paired to a smartphone that collected and stored f_Cs. Subjects were categorized into Low and High groups based on %time spent training between the VO₂θ and V_Eθ. Significant increases were observed in relative VO_2max (P = 0.007, g = 0.48), VO₂θ (P = 0.018, g = 0.35), and V_Eθ (P = 0.030, g = 0.29) from baseline after eight weeks for both groups. A 95% bootstrapped confidence interval that did not include zero (-0.38, -0.03; g = 1.26) revealed a large and significantly greater change in VO₂θ in the High group (0.37 ± 0.15 L/min) versus the Low group (0.17 ± 0.14 L/min). No significant differences were observed in other variables between groups. Increasing triathletes' %time spent exercising between VO₂θ and V_Eθ may optimize increases in VO₂θ after eight weeks of training.

Keywords: Maximal oxygen uptake; athletic performance; exercise intensity; training zones; ventilatory threshold.

Figure 1

Panel A: Diagrammatic representation of oxygen uptake (VO₂) in red and carbon dioxide output (VCO₂) in blue as a function of work. Panel B: Diagrammatic representation of minute ventilation (V_E) as a function of VCO₂. VCO₂ was measured at the inflection point in V_E. Panel C: Diagrammatic representation of VCO₂ as a function of VO₂. Metabolic threshold (VO₂θ) was determined by measuring the VO₂ at which the first inflection point in VCO₂ occurred. Ventilatory threshold (V_Eθ) was determined by measuring the VO₂ at the corresponding value for VCO₂ derived from Panel B. Panel D: Diagrammatic representation of heart rate (f_C) as a function of VO₂. f_C at VO₂θ, which served as the boundary between Zone 1 and Zone 2, was determined by measuring the f_C at the corresponding value for VO₂ derived from Panel C. The identical process was employed to obtain the f_C at V_Eθ, the boundary between Zone 2 and Zone 3.