The Influence of Respiratory Muscle Work on Locomotor and Respiratory Muscle Oxygenation Trends in Repeated-Sprint Exercise

This thesis investigated the role respiratory muscle work has on locomotor and respiratory muscle oxygen (O2) utilisation during multiple sprint work. To measure O2 delivery and uptake in real time, near-infrared spectroscopy (NIRS) can be used. However, there are inconsistent methods of smoothing and determining peaks and nadirs from the NIRS signal. Therefore, the aim of study 1 was to examine the effects of different methodologies commonly used in the literature on the determination of peaks and nadirs in the vastus lateralis deoxyhaemoglobin (HHbVL) signal. Means derived from predetermined windows, irrespective of length and data smoothing, underestimated the magnitude of peak and nadir [HHbVL] compared to a rolling mean approach. Based on the results, we suggest using a digital filter to smooth NIRS data, rather than an arithmetic mean, and a rolling approach to determine peaks and nadirs for accurate interpretation of muscle oxygenation trends. In the second study, the effects of heightened inspiratory muscle work on [HHbVL] and respiratory muscle deoxyhaemoglobin ([HHbRM]) trends were examined. In response to the heightened inspiratory muscle work, HHbRM was elevated across the sprint series. There were no clear differences in HHbVL trends between exercise conditions. The lack of difference in HHbVL between trials implies respiratory muscle O2 uptake does not limit locomotor oxygenation trends. Study 3 investigated the role of arterial hypoxemia on respiratory muscle oxygenation trends, and its implications on locomotor oxygenation. While exercising in hypoxia (14.5% O2), HHbVL was higher during the sprint and recovery phases of the repeated-sprint protocol compared to normoxia (21% O2). There were no clear differences in respiratory muscle oxygenation trends between conditions. The clear reduction in locomotor muscle O2 delivery (inferred from HHbVL) while respiratory muscle oxygenation was maintained, suggests preferential blood flow distribution to the respiratory muscle to compensate for arterial hypoxemia, which may explain in part compromise locomotor O2 delivery. The aim of the final study was to examine the role of respiratory muscle strength on locomotor and respiratory muscle oxygenation trends in repeated-sprint exercise. Inspiratory muscle training (IMT) was used to reduce the relative intensity of exercise hyperpnoea by strengthening the respiratory muscles. Repeat-sprint ability was again assessed in normoxia and hypoxia. After 4 weeks of training, there was a 35% increase of inspiratory muscle pressure in the IMT beyond the control group. Despite the substantial change in respiratory muscle strength, oxygenation trends were not affected in either normoxia or hypoxia. The findings of this thesis do not support the work of breathing as being a limiting factor in locomotor muscle oxygenation in normoxia. The intermittent nature of repeated-sprint activity is likely a key mediating factor for which O2 delivery can be maintained to both the locomotor and respiratory muscles. However, under conditions of arterial hypoxemia, locomotor muscle oxygenation may be compromised by preferential O2 delivery to the respiratory muscles.