The level of minute ventilation, the frequency of breathing, a decrease in lung compliance, or an increase in airway resistance increase the work of breathing and energy demands on the muscle. A hyperstimulated central respiratory drive also imposes an increased inspiratory muscle work of breathing.
The greater the fraction of the maximum pressure that can be developed by the inspiratory muscles, the greater are their energy demands. This fraction can be increased by either increasing the pressure necessary to breath (e.g. increased airway resistance) or reducing the maximum force that the muscle can develop (hyperinflation, muscle atrophy). Hyperinflation, which is common in patients with chronic obstructive pulmonary disease (COPD), shortens the diaphragmatic fibers and thus reduces the maximum force of the diaphragm (maximum force is a function of initial fiber length).
For a given workload, the energy demands and oxygen cost of breathing increase as efficiency decreases. Efficiency decreases with airway resistance. In this condition, hyperinflation occurs, leading to a flatness of the diaphragm which, in turn, acts as a fixator and not as an agonist (quasi-isometric contraction). This requires energy expenditure without production of work.
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