This study was designed to investigate the factors which affect the safe and effective performance of manual hyperinflation by physiotherapists. To determine this, experienced and student physiotherapists were requested to deliver manual hyperinflations at tidal volumes of 1000mL or peak inspiratory pressures of 15cm H2O during a series of trials with variations in lung compliance. These hyperinflations were delivered into an artificial lung system. Subjects were blind to random changes in lung compliance and were only allowed the feel of the bag for feedback during testing. It was found that compliance had a significant effect on the tidal volumes and pressures generated, and both groups of subjects allowed peak inspiratory pressure to rise significantly when lung compliance was low. Experience, hand size and grip strength of individuals did not influence these results.