Muscle oxygenation maintained during repeated-sprints despite inspiratory muscle loading

Abstract

A high work of breathing can compromise limb oxygen delivery during sustained high-intensity exercise. However, it is unclear if the same is true for intermittent sprint exercise. This project examined the effect of adding an inspiratory load on locomotor muscle tissue reoxygenation during repeated-sprint exercise. Ten healthy males completed three experiment sessions of ten 10-s sprints, separated by 30-s of passive rest on a cycle ergometer. The first two sessions were "all-out' efforts performed without (CTRL) or with inspiratory loading (INSP) in a randomised and counterbalanced order. The third experiment session (MATCH) consisted of ten 10-s work-matched intervals. Tissue saturation index (TSI) and deoxy-haemoglobin (HHb) of the vastus lateralis and sixth intercostal space was monitored with near-infrared spectroscopy. Vastus lateralis reoxygenation (ΔReoxy) was calculated as the difference from peak HHb (sprint) to nadir HHb (recovery). Total mechanical work completed was similar between INSP and CTRL (effect size: -0.18, 90% confidence limit ±0.43), and differences in vastus lateralis TSI during the sprint (-0.01 ±0.33) and recovery (-0.08 ±0.50) phases were unclear. There was also no meaningful difference in ΔReoxy (0.21 ±0.37). Intercostal HHb was higher in the INSP session compared to CTRL (0.42 ±0.34), whilst the difference was unclear for TSI (-0.01 ±0.33). During MATCH exercise, differences in vastus lateralis TSI were unclear compared to INSP for both sprint (0.10 ±0.30) and recovery (-0.09 ±0.48) phases, and there was no meaningful difference in ΔReoxy (-0.25 ±0.55). Intercostal TSI was higher during MATCH compared to INSP (0.95 ±0.53), whereas HHb was lower (-1.09 ±0.33). The lack of difference in ΔReoxy between INSP and CTRL suggests that for intermittent sprint exercise, the metabolic O2 demands of both the respiratory and locomotor muscles can be met. Additionally, the similarity of the MATCH suggests that ΔReoxy was maximal in all exercise conditions.

Fig 1. Representative data of the effects of inspiratory muscle loading (INSP) on mouth pressure (P_m) compared to control (CTRL) and work matched (MATCH) exercise conditions.

Fig 2. Representative data of the raw NIRS responses to repeated-sprint exercise.

(A) Concentrations of vastus lateralis total haemoglobin ([tHb]), oxyhaemoglobin ([O₂Hb]), and deoxyhaemoglobin ([HHb]), and (B) vastus lateralis Tissue Saturation Index (TSI) for the entire repeated-sprint protocol. (C) [tHb], [O₂Hb], and [HHb], and (D) TSI for the first 10-s sprint and the following 30-s of passive rest.

Fig 3. Total mechanical work performed during control (CTRL), inspiratory loading (INSP), and work match (MATCH) exercise conditions.

(A) Total mechanical work performed during each 10-s sprint. (B) Individual (grey lines) and mean total work (black line) completed over the entire repeated-sprint protocol. Values are presented as mean ± SD.

Fig 4. Sprint and recovery pulmonary oxygen uptake (V˙O2) expressed as a percentage of V˙O2peak for control (CTRL), inspiratory muscle loading (INSP) and worked matched (MATCH) exercise.

The symbols represent comparisons between INSP and CTRL (⁕), INSP and MATCH (#). The number of symbols; one, two and three denote likely, very likely and almost certainly respectively, that the chance of the true effect exceeds a small (-0.2–0.2) effect size. Vales are presented as mean ± SD.

Fig 5. Respiratory and locomotor muscle NIRS responses to repeated-sprints during the control (CTRL) inspiratory loading (INSP) and work matched (MATCH) experiment sessions.

(A) Respiratory muscle tissue saturation index (TSI_RM) (n = 8). (B) Respiratory muscle deoxy-haemoglobin (HHb_RM). (C) Vastus lateralis tissue saturation index (TSI_VL). (D) Vastus lateralis deoxy-haemoglobin (HHb_VL). Values are presented as mean ± SD. The symbols represent comparisons between INSP and Control (⁕), INSP and MATCH (#). The number of symbols; one, two and three denote likely, very likely and almost certainly respectively, that the chance of the true effect exceeds a small (-0.2–0.2) effect size. Values are presented as mean ± SD.

Fig 6. Mean locomotor muscle NIRS responses to repeated-sprints during the control (CTRL) inspiratory loading (INSP) and work matched (MATCH) experiment sessions.

(A) Sprint vastus lateralis tissue saturation index (Sprint TSI_VL). (B) Recovery vastus lateralis tissue saturation index (Recovery TSI_VL). (C) Sprint vastus lateralis deoxy-haemoglobin (Sprint HHb_VL). (D) Recovery vastus lateralis deoxy-haemoglobin (Recovery HHb_VL). Individual responses are represented by the grey lines and mean responses are represented by black lines. The symbols represent comparisons between INSP and Control (⁕), INSP and MATCH (#). The number of symbols; one, two and three denote likely, very likely and almost certainly respectively, that the chance of the true effect exceeds a small (-0.2–0.2) effect size. Vales are presented as mean ± SD.

Fig 7. Mean respiratory muscle NIRS responses to repeated-sprints during the control (CTRL) inspiratory loading (INSP) and work matched (MATCH) experiment sessions.

(A) Respiratory muscle tissue saturation index (TSI_RM) (n = 8). (B) Respiratory muscle deoxy-haemoglobin (HHb_RM). Individual responses are represented by the grey lines and mean responses are represented by black lines. The symbols represent comparisons between INSP and Control (⁕), INSP and MATCH (#). The number of symbols; one, two and three denote likely, very likely and almost certainly respectively, that the chance of the true effect exceeds a small (-0.2–0.2) effect size. Vales are presented as mean ± SD.

Fig 8. Standardised effects (Cohen’s d) with 90% confidence intervals for NIRS variables comparing inspiratory loading (INSP) to control (CTRL), and work matched exercise (MATCH) to INSP.

Shaded area indicates a trivial effect, and dotted lines thresholds for small, moderate and large effects. Abbreviations are: respiratory muscle tissue saturation index, TSI_RM; respiratory muscle deoxy-haemoglobin, HHb_RM; vastus lateralis tissue saturation index, TSI_VL; vastus lateralis deoxy-haemoglobin, HHb_VL; vastus laterals reoxygenation, ΔReoxy; vastus laterals reoxygenation rate, Reoxy rate; time to peak deoxy-haemoglobin, TTP_HHb.