Respiratory mechanics during upper body exercise in healthy humans

Abstract

The physiological responses to upper-body exercise (UBE) are well established. Few published studies, however, have attempted to elucidate the mechanical ventilatory responses to UBE. There is empirical evidence that respiratory function may be compromised by UBE during which the ventilatory and postural functions of the ‘respiratory’ muscles may be exacerbated. Therefore, the aims of this thesis were: 1) to characterise the mechanical-ventilatory responses to UBE in healthy subjects; 2) to explore the putative mechanisms that underpin the respiratory responses to UBE; and 3) to assess whether the mechanical-ventilatory stress imposed by UBE induces contractile fatigue of the respiratory muscles. Compared to lower-body exercise (LBE; leg cycling) at ventilation-matched work rates, UBE (arm-cranking) resulted in constraint of tidal volume, higher respiratory frequency, and greater neural drive to the respiratory muscles. Furthermore, end-expiratory lung volume was significantly elevated during peak UBE compared to LBE (39 ± 8 vs. 29 ± 8% vital capacity, p < 0.05) and was independent of expiratory flow limitation. In assessing the influence of cadence on cardiorespiratory function and respiratory mechanics, submaximal arm-cranking at high cadence (90 rev.min-1) induced significantly greater cardiorespiratory stress, a trend towards elevated intra-thoracic pressures and significantly greater perceptions of dyspnoea than at low cadence (50 rev.min-1). Furthermore, there was a greater prevalence of locomotor-respiratory coupling at high cadences (p < 0.05), suggestive of greater antagonistic loading of the thoracic muscles, likely the result of static postural contractions. Finally, there was objective evidence of abdominal muscle contractile fatigue in response to severe- but not heavy-intensity UBE. Specifically, there was a 22% decrease in gastric twitch pressure from pre- to post-exercise in response to magnetic stimulation of the thoracic nerves (p < 0.05). However, there was limited evidence of exercise-induced diaphragm fatigue, as assessed using magnetic stimulation of the phrenic nerves (p > 0.05). In conclusion, mechanical-ventilatory function may be compromised during UBE due to complex interactions between thoracic muscle recruitment, central neural drive and thoracic volume displacement. This thesis presents novel findings which may have important functional implications for clinical populations who report breathlessness during activities of daily living that involve the upper-body, as well as