We read with great interest the opposing view presented by Hulzebos et al. (2017), which provides evidence that skeletal muscle oxidative capacity is preserved in patients with cystic fibrosis (CF). It is clear that both of our positions acknowledge the strong importance of understanding how skeletal muscle function contributes to exercise intolerance in patients with CF. Nonetheless, Hulzebos and colleagues (2017) defend the viewpoint that a reduction in muscle mass, not muscle function, is the underlying mechanism of exercise intolerance in patients with CF.
Patients with CF spend less time performing moderate and/or vigorous physical activities compared to healthy controls, even during periods of enhanced wellbeing (Troosters et al. 2009; Ward et al. 2013). It is therefore logical to consider that physical inactivity can contribute to quantitative impairments of skeletal muscle (i.e. muscle deconditioning and smaller muscle mass), which can result in exercise intolerance (de Meer et al. 1999). However, reductions in muscle strength and peak power during exercise have been observed in physically active, competitive athletes with CF compared to their healthy peers (Selvadurai et al. 2003). Importantly, these results were supported by the presence of similar muscle and bone cross‐sectional area between patients and healthy controls, ruling out the possibility of skeletal muscle deconditioning in this athletic CF cohort.
The data that indicate a preserved skeletal muscle function in patients with CF are scant. In contrast, there are a handful of studies that provide compelling data to support an impaired mitochondrial oxidative metabolism in patients with CF compared to controls (de Meer et al. 1995; Selvadurai et al. 2003; Wells et al. 2011; Erickson et al. 2015), even when controlling for skeletal muscle cross‐sectional area (Selvadurai et al. 2003; Wells et al. 2011). It is likely that the disparity in findings can be explained by inclusion of a younger, healthier CF population (Werkman et al. 2016) and/or the evaluation of different muscle groups that are composed of different fibre‐type proportions (Decorte et al. 2017).
In conclusion, there are convincing data that document both qualitative and quantitative impairments of skeletal muscle in patients with CF. Results from different studies provide strong evidence to support the existence of mitochondrial dysfunction and altered skeletal muscle oxidative metabolism in CF, independent of muscle mass. Although it is reasonable to speculate that reductions in both muscle mass and muscle function contribute to exercise intolerance in CF, further investigation of the link between skeletal muscle function and exercise capacity in CF is certainly warranted.
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Additional information
Competing interests
None declared.
Linked articles This article is part of a CrossTalk debate. Click the links to read the other articles in this debate: http://dx.doi.org/10.1113/JP272486, http://dx.doi.org/10.1113/JP272505 and http://dx.doi.org/10.1113/JP273738.
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