The effect of severe intensity bouts on muscle oxygen saturation responses in trained cyclists

Abstract

Near-infrared spectroscopy (NIRS) quantifies muscle oxygenation (SmO₂) during exercise. Muscle oxygenation response to self-paced, severe-intensity cycling remains unclear. Observing SmO₂ can provide cycling professionals with the ability to assess muscular response, helping optimize decision-making. We aimed to describe the effect of self-paced severe intensity bouts on SmO₂, measured noninvasively by a wearable NIRS sensor on the vastus lateralis (VL) muscle, and examine its reliability. We hypothesized a greater desaturation response with each bout, whereas, between trials, good reliability would be observed. Fourteen recreationally trained, and trained cyclists completed a ramp test to determine the power output (PO) at the respiratory compensation point (RCP). Athletes completed two subsequent visits of 50-minute sessions that included four severe-intensity bouts done at 5% above RCP PO. Muscle oxygenation in the VL was monitored using a wearable NIRS device. Measures included mean PO, heart-rate (HR), cadence, and SmO₂ at bout onset, during work (work SmO₂), and ΔSmO₂. The bouts were compared using a one-way repeated measures ANOVA. For significant differences, a Fisher's least square difference post-hoc analysis was used. A two-way repeated measures ANOVA was used using trial and bout as main factors. Intraclass correlations (ICC) were used to quantify relative reliability for mean work, and standard error of the measurement (SEM) was used to quantify absolute agreement of mean work SmO₂. Both PO and cadence showed no effect of bout or trial. Heart-rate at bout 2 (168 ± 8 bpm) and 4 (170 ± 7 bpm) were higher than bout 1 (160 ± 6 bpm). Onset SmO₂ (%) response significantly increased in the final two bouts of the session. Mean work SmO₂ increased across bouts, with the highest value displayed in bout 4 (36 ± 22%). ΔSmO₂ showed a smaller desaturation response during bout 4 (27 ± 10%) compared to bout 3 (31 ± 10%). Mean work SmO₂ ICC showed good reliability (ICC = 0.87), and SEM was 12% (CI 9-15%). We concluded that a non-invasive, affordable, wearable NIRS sensor demonstrated the heterogeneous muscle oxygenation response during severe intensity cycling bouts with good reliability in trained cyclists.

Keywords: cardiorespiratory fitness; cycling; exercise; exercise testing; muscle oxygenation; near-infrared spectroscopy; severe intensity bouts; wearable.

Figure 1

Severe intensity interval exercise protocol. Percent power output (PO) is calculated from PO detected at respiratory compensation point (RCP) (dotted line) from initial ramp test. Ten-minutes warm up at 40% RCP PO, followed by four severe intensity bouts of four minutes at 105% of RCP PO, one minute passive rest, and five minutes active recovery at 50% RCP PO.

Figure 2

Illustration of muscle oxygen saturation (SmO₂) at exercise onset (black arrow), mean work phase SmO₂ during the final two minutes of the severe intensity bout, and ΔSmO₂ calculation of the difference between SmO₂ at bout onset and the 30s average SmO₂ at the end of the effort (dotted line).

Figure 3

Representative dataset (n = 1) during a 50 min mixed intensity session. Power output (W) is represented on right Y axis by shaded bars, Heart rate (bpm) is presented on left Y axis by solid gray line, and muscle oxygen saturation (%) is displayed on left Y axis by gray dots and line.

Figure 4

Means for power output (W) (A), heart rate (bpm) (B), and cadence (rpm) (C) for each severe intensity interval (bars), with representation of individual responses across the four bouts from both sessions (dots + lines). *p < 0.05, ****p < 0.0001.

Figure 5

Means for muscle oxygen saturation (SmO₂) at bout onset (%) (A), mean work SmO₂ (%) (B), and mean SmO₂ amplitude (ΔSmO₂) (%) (C), for each severe intensity bout (bars), with representation of individual responses across the four bouts from both sessions (dots + lines). *p < 0.05, **p < 0.01.