and requires the respiratory muscles to contract more force fully and to shorten more quickly. At rest, expiratory mus cles make very little contribution to breathing, but during exercise they contribute to raising VT and expiratory air flow rate. However, at all intensities of exercise the majority of the work of breathing is undertaken by the inspiratory muscles; expiration is always assisted to some extent by the elastic energy that is stored in the expanded lungs and rib cage from the preceding inhalation. This elastic energy is 'donated' by the contraction of the inspiratory muscles as they stretched and expanded the chest during inhalation. Recent studies have estimated that, during max imal exercise, the work of the inspiratory respiratory muscles demands approximately 16% of the available oxygen (Harms & Dempsey, 1999), which puts into per spective how strenuous breathing can be in healthy young people