Several studies, both in humans and in animal models, showed that mechanical ventilation rapidly induces atrophy of diaphragm muscle fibers [
1,
6‐
8]. Our results are in line with those studies as we found that 18 h mechanical ventilation provoked a 20% reduction of diaphragm fiber cross-sectional area. The capacity of a muscle to generate force strongly depends on the number of crossbridges that can be formed in parallel [
14]. As such, reduction of diaphragm muscle fiber cross-sectional area may explain reduced diaphragm force output when cross-sectional area and contractile protein content decrease proportionally. Yet, reduction of cross-sectional area only partially explains mechanical ventilation-induced diaphragm weakness, because several studies have shown that prolonged mechanical ventilation reduces diaphragm bundle or single fiber specific force-generating capacity, i.e., absolute force divided by cross-sectional area [
9‐
12,
25]. Our data are in line with these studies and show that fiber specific force-generating capacity is already reduced after 18 h of mechanical ventilation (Fig.
1). Moreover, we found that myosin concentration is reduced in diaphragm fibers from mechanically ventilated animals (Fig.
2), indicating that the severity of myosin loss exceeds the degree of fiber atrophy. This can explain the large decrease in contractile protein content recently observed in the diaphragm of mechanically ventilated humans [
26]. Notably, after correction of force for myosin concentration, no differences exist in force between control and ventilated diaphragm. This implies that contractile protein loss is a strong determinant of diaphragm weakness upon mechanical ventilation. In addition, this finding indicates that the remaining contractile proteins display normal maximal force-generating capacity. This is in contrast to the diaphragm in patients with COPD, where loss of force results from reduction in protein content and impaired function of the remaining contractile protein [
27,
28]. Contractile protein loss can result from both increased proteolysis and reduced synthesis. Indeed, previous studies have demonstrated that synthesis of contractile proteins is reduced [
29], and proteolytic systems like the ubiquitin–proteasome pathway, caspase-3, calpains, and lysosomes are activated in the diaphragm of mechanically ventilated rodents and humans [
6,
7,
12,
26,
30,
31].