Respiratory Muscle Function in a Large Cohort of Healthy Subjects: Can Strength Predict Endurance?

Abstract

involved a small number of patients with a wide age range. Although strength and endurance appear to be closely linked in many conditions, there are several circumstances such as COPD, asthma, and heart failure in which endurance cannot be accurately predicted from measurements of maximum respiratory pressures [3] . Thus, endurance measurement can be helpful to evaluate the respiratory muscle involvement in normal and pathological conditions in addition to measurement of muscle strength. The study by Reiter et al. [7] , published in this issue of Respiration, is the fi rst to evaluate inspiratory muscle endurance (Tl im ) using a fl ow-dependent resistive loading device in a large population of 68 healthy Austrian individuals with an age range from 17 to 75 years. The authors divided the subjects into fi ve age groups, and investigated Tl im and its relation to anthropometric, spirometric data and PI max . They found a high interindividual but low intraindividual variability for both Tl im and PI max . In accordance with previous studies, PI max was negatively related to age and positively related to weight and spirometric parameters. Interestingly, the authors found that Tl im was not related either to respiratory effort senRespiratory function depends on coordinated activity of the respiratory muscles which assure effective ventilation. Several conditions such as chronic obstructive pulmonary disease (COPD), neuromuscular diseases and congestive heart failure can impair respiratory muscle function, leading to reduced respiratory muscle strength and endurance [1] . Moreover, respiratory muscle training has been shown to signifi cantly improve respiratory muscle strength and endurance in patients suffering from COPD [2] . Therefore, measuring respiratory muscle function may be clinically relevant in all disease states involving respiratory muscles. The maximum static inspiratory pressure (PI max ) and the maximum static expiratory pressure (PE max ) generated at the mouth are simple techniques to estimate inspiratory and expiratory muscle strength, respectively [3] . Inspiratory muscle endurance can be measured with different tests: the most common techniques use resistive and threshold loading devices. Whereas normal values for PI max and PE max have been reported in adults as well as in children and elderly [4, 5] , normal values for inspiratory muscle endurance have not yet been developed. In the early 1980s, Nickerson and Knees [6] measured endurance as sustainable inspiratory pressure, using a threshold loading device, but the study