DETERMINANTS OF DIAPHRAGMATIC INJURY

Limb muscles can be injured during and after vigourous contractions. H owever, this injury is most evident under specific conditions. The str ength and type of muscle contraction as well as the contractile status of the muscle are important determinants of injury. The initiating ev ent leading to muscle injury is not clearly understood but there are s everal leading theories. The respiratory muscles are of obvious import ance to survival, and fatigue or injury to them has been hypothesized to be prevented by various mechanisms. One such mechanism is reduced a ctivation by the central nervous system. In this review information on the neural activation of the breathing muscles during inspiratory loa ding 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 load ing have shown little evidence of it. However, based on information fr om limb muscles, delayed or secondary muscle injury might occur and co uld produce deleterious effects on respiratory muscle function. Recent evidence shows that chronic low intensity inspiratory loading can pro duce diaphragmatic injury (Reid et al.) and secondary or delayed muscl e 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 f rom injury or the results of muscle injury. Diaphragmatic function dur ing the period of secondary muscle injury is markedly impaired and thu s respiratory muscle injury is a phenomenon that warrants further inve stigation.