During postnatal development the diaphragm also increases rapidly in bulk, its fiber composition changes, the fiber sizes change, and its oxidative capacity increases (Trang et al., 1992; Gaultier, 1995; Fratacci et al., 1996a). These changes are important, since they affect the relative fatiguability of the adult diaphragm compared to that of the infant diaphragm. Diaphragmatic fatigue is "the inability to maintain force or power output during sustained or repetitive contractions" and can result from faulty neurotransmission to the diaphragm, faulty response of the muscle to stimulation, or both (Fratacci et al., 1996b). This subject is very controversial in the literature on the postanatal development of the diaphragm. Some research has shown the neonate diaphragm to have a higher fatigue resistance than the adult diaphragm, yet other research has shown the exact opposite to be true.
Before detailing these conflicting studies, however, it will help to have a primer on muscle composition, using Swynghedauw's review article on skeletal muscle (1986). Skeletal muscle contains myosin, which is the most abundant protein in muscle and forms the thcik fibrils of muscle. It works with actin to cause the contraction and relaxation of the muscle. Myofibers express different myosin heavy chains (MHC), which have different isoforms with differing ATPase activites, and different MHC isoforms can be defined by their ATPase activity. These isoforms have the same function and genomic origin but have different structures. Studies on rats have shown the progression of the expression of different MHC isoforms: Embryonic MHC is expressed until 20 days postpartum; neonatal MHC is expressed from one day before birth until three to four weeks later; and fast, or adult MHC is expressed from fourteen days postpartum onwards. Similar expression patterns have been observed in sheep: Only fetal myosin is seen at G140 days (term=147 days); fetal and adult myosin is present in 5 day lambs; and only adult myosin is seen at 30 days (Finkelstein et al., 1992) In general, neonatal mysoin expression occurs much earlier in the diaphragm than in other skeletal muscles, which probably relates to the diaphragm's need to function immediately after birth (Kelly et al., 1991; Finkelstein et al., 1992). Other skeletal muscles are not vital and do not need to be very developed at birth.
These changes in MHC expression occur in tandem with changes in the muscle fiber types of the diaphragm. The classification of fiber types is as follows: type IIB is the white fast-twitch muscle that is glycolytic and has low fatigue resistance; type IIA is the red fast-twitch muscle that is oxidative-glycolytic and is highly resistant to fatigue; type I is red slow-twitch muscle that is oxidative and highly resistant to fatigue; type IIC is fast-twitch and fatigue-susceptible (Swynghedauw, 1986). In the rat there is a progressive increase in the number of type I fibers in both the costal and crural parts of the diaphragm between weeks one and eight after birth The adult distribution of fiber types is ~33% slow oxidative, ~37% fast glycolytic, and ~30% fast oxidative-glycolytic (Fratacci et al., 1996a).
Whether the neonate or the adult diaphragm is more resistant to fatigue remains hotly contested in the physiological journals. Human neonate diaphragms contain less than 10% fatigue-resistant type I muscle and relatively high percentages of type IIC muscle. Additionally, neonate diaphragms have low oxidative capacity (Fratacci et al., 1996a). These are arguments for neonate diaphragm being less fatigue-resistant than the adult diaphragm, which is composed of different proportions of fiber types. An in vivo study in rabbits showed that the neonate diaphragm fatigued more quickly than the adult diaphragm (Le Souef et al., 1988). This paralleled the relative distribution of fiber types as well.
In contrast to these studies, several others have showed the early neonate diaphragm to be more resistant to fatigue than the older diaphragm. Sieck et al. (1991) found that in the cat, resistance to fatigue is highest at birth and decreases with age. This did not parallel the changes in the oxidative capacity of the fiber types, as the relative proportions of type I increased and type II decreased. The authors speculate that the greater fatigue resistance of the neonates could be due to some action of neonatal MHC, since the contractile properties of muscles depend on the ratios of the MHC isoforms expressed in the muscles. Trang et al. (1992) reported a higher resistance to fatigue in the diaphragms of 1 and 3 week old rats in comparison to older rats. More recently, Fratacci et al. (1996b) reported this same result in rats They also propose a higher oxidative potential of the earlier postnatal diaphragm relative to the older diaphragm. It is important to note that all of the studies cited here that showed a greated fatigue resistance of the neonate diaphragm were done in vitro. It is possible that this is affecting the results. Further studies need to be done, possibly using in vivo protocols to ensure against possible systematic error resulting from studies done outside the living body.