Abstract
Abstract
The primary aim of this thesis was to describe the dynamic response characteristics of leg blood flow (LBF) and leg vascular conductance (LVC) during exercise and recovery, and to investigate the influence of muscle activation (sEMG) and the affects of ageing on these characteristics.
Two studies were performed as part of this thesis, the first in a group of thirteen healthy younger participants [chapters three, four and five] and the second in a group of ten healthy older participants [chapter six]. In study one, plantar-flexion exercise was performed by each participant across seven submaximal exercise transitions, and in study two across two submaximal exercise transitions. In both studies, leg blood flow (LBF), leg vascular conductance (LVC), mean arterial pressure (MAP), and surface electromyography (sEMG) responses were measured and fitted with empirical models.
In study one [chapter three], the main hypothesis tested was that the dynamic response structure of LBF and LVC to low, median and high intensity submaximal exercise is biphasic. This hypothesis was based on a common hyperaemic response structure presented in the literature. The hypothesis was tested by comparing the goodness-of-fit of six competing empirical models to the hyperaemic responses. It was found that, in contrast to the hypothesis, a quadphasic and not a biphasic model best represented the hyperaemic responses at all intensities. Whilst a biphasic growth structure was a fundamental feature of the response, at all intensities an early decay phase was present between the two growth phases in addition to a final ‘variant’ phase which presented as either a third growth or second decay phase. In study two [chapter six], the robustness of this newly found structure was further tested by assessing the hypothesis that the quadphasic model would also represent the hyperaemic response to exercise in an older group of participants. The hypothesis was supported across both low and median intensity submaximal exercise.
In study one [chapter four] and study two [chapter six], this newly identified quadphasic structure was utilised to investigate the systems response characteristics of LBF and LVC more accurately. Whilst the systems response characteristics had been described previously for a younger group, these were based on a biphasic model. Furthermore, no description of the response characteristics in an older group had ever been made. There were several key observations. First, phase 1, which presented as a growth phase immediately [time delay (TD) of ≈ 1-2 s] following the commencement of exercise, was the largest of the four phases in both studies. Phase 1 was linearly related to both exercise intensity and muscle activation (study one) and it was the only phase whose amplitude was reduced as a function of age, an effect that was independent of muscle activation (study two). The newly described phase 2, a decay phase, presented rapidly and transiently following phase 1 [TD ≈ 9-14 s]. It was linearly related to exercise intensity, and acted as a restraint on the initial hyperaemia. Phase 3 presented shortly after [TD ≈ 15-20 s] as a secondary increase in hyperaemia; but unlike the first growth phase (1), this increase was only loosely associated with exercise intensity. The final phase 4 was variable both in its magnitude (growth or decay) and its timing [TD ≈ 75-225 s]; however, during this phase a steady-state response was observed that remained linear as a function of intensity [LBF]. A link between muscle activation and the decay variant of phase 4 was postulated.
In study one [chapter five] the structure of the recovery responses of LBF and LVC from exercise were investigated. To date, the assumption had been made that the recovery response is biphasic, although this had not been experimentally verified. The hypothesis that the response would be biphasic was therefore tested, and found to be supported. This was the first demonstration of an assymetry between exercise and recovery responses. Assymetry was further observed when phase 1 of the exercise and recovery responses were compared. Phase 1 of recovery displayed a smaller amplitude than phase 1 during exercise. This smaller amplitude led directly to the mean response time during recovery being slower than during exercise.
In conclusion, a quadphasic response structure is a robust characteristic of the LBF and LVC responses during exercise both as a function of intensity and ageing. However, the structure during recovery is assymetric when compared to the response at exericise onset presenting as a biphasic response. A better understanding of the large, but transient, phase 1 of both exercise and recovery responses will provide significant insight into the control of BF and VC overall. In younger participants, this phase is the largest, linked tightly to muscle activation. With increasing age, during exercise, it is phase 1 that is reduced in amplitude, and so directly responsible for a reduction in BF and VC at steady-state. Presumably, this decrease in phase 1 contributes to the associated reduction in exercise performance seen as a function of age. Furthermore, in regard to recovery it is the smaller amplitude of this phase that causes a slower recovery in comparison to exercise onset. However, the importance of the two new phases (2 and 4) must not be overlooked. Phase 2 in regard to its ‘autoregulatory’ role between the feed-forward and feed-back action of phases 1 and 3. Phase 4 in regard to the control of the LBF response approaching steady-state which may have a link to muscle activation.