Tests of Respiratory Muscle Endurance

Muscle endurance is the ability to sustain a specific muscular task over time. It is a highly integrated and complex quality of a muscle or a group of muscles that is related to its resistance to fatigue. To a large extent, any measurement of endurance is task specific because different tasks result in varying recruitment patterns of motor units and synergistic muscle groups, each with varying endurance qualities. The wide variety of techniques that have been developed to measure endurance of the respiratory muscles differ largely on the type of task that is being performed. For each specific task, an endurance curve can be generated by plotting task intensity versus the time it can be sustained. Task failure is an event defined by the inability to continue performing the required task (Figure 1). At high levels of intensity, a task can be performed for only a few repetitions. As the intensity is decreased, each task can be endured for a longer time until a level can be sustained for an indefinite period (i.e., hours). The latter is referred to as the maximum sustainable task or load. Another estimate of endurance involves performing incremental increases in task intensity for a given time period until a peak intensity is identified, which is the maximum that can be maintained for a finite period of time (Figure 1). This intensity is not sustainable but may also be used to reflect endurance properties.

Although respiratory muscle strength and endurance appear to be closely linked in many conditions (1-3), there are numerous examples in which endurance would not be accurately predicted from estimates of maximum pressures or maximum venti-latory capacity. Furthermore, the characteristics of endurance curves for a given muscle may change with training, disuse, drug treatment, and so on. For example, in heart failure patients (4) or in normal subjects (5) following certain respiratory muscle training protocols, larger relative effects are seen on endurance compared with strength. Some patients with asthma show inherent elevations in endurance properties as a fraction of strength (6), as do patients with cystic fibrosis (7), suggesting these patients naturally train for endurance during periods of airway obstruction. In contrast, patients with chronic obstructive pulmonary disease (COPD) (8) or patients receiving acute steroid treatment (9) show marked reductions in endurance properties relative to strength. Therefore, endurance measurements can be useful in some clinical and investigative settings for evaluating patient populations and responses to treatment and rehabilitation.

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