Determinants of diaphragmatic injury.

Limb muscles can be injured during and after vigourous contractions. However, this injury is most evident under specific conditions. The strength and type of muscle contraction as well as the contractile status of the muscle are important determinants of injury. The initiating event leading to muscle injury is not clearly understood but there are several leading theories. The respiratory muscles are of obvious importance to survival, and fatigue or injury to them has been hypothesized to be prevented by various mechanisms. One such mechanism is reduced activation by the central nervous system. In this review information on the neural activation of the breathing muscles during inspiratory loading is discussed and reveals that neural activation to the diaphragm, the main inspiratory muscle, is high. Previous studies investigating the presence of muscle fatigue immediately after such inspiratory loading have shown little evidence of it. However, based on information from limb muscles, delayed or secondary muscle injury might occur and could produce deleterious effects on respiratory muscle function. Recent evidence shows that chronic low intensity inspiratory loading can produce diaphragmatic injury (Reid et al.) and secondary or delayed muscle injury can occur three days after an acute period of high intensity inspiratory loading. The results reviewed in this article suggest that the respiratory muscles, specifically the diaphragm, are not spared from injury or the results of muscle injury. Diaphragmatic function during the period of secondary muscle injury is markedly impaired and thus respiratory muscle injury is a phenomenon that warrants further investigation.