Muscle fiber types

Histopathological examination shows the typical corelike lesions in a high proportion of muscle fibers in older patients this may amount to 100%. Most typically the cores are large and centrally-placed, but multiple cores may occur in the same fiber cross section. Most older patients show a striking predominance of type 1 (slow twitch oxidative) fibers and virtually all fibers with cores are type 1. Sometimes younger family members have more normal proportions of type 1 and type 2 fibers but, again, the cores are confined to the type 1 fibers. It is well established that muscle fiber types can interconvert due to altered physiological demands, and it is likely that fibers with cores convert to a basically slow twitch-oxidative metabolism to compensate for the fact that up to 50% of their cross sectional area may be devoid of mitochondria. 

In severe neonatal nemaline myopathy virtually every muscle fiber shows multiple rods and all muscle fiber types are affected. However in juvenile cases, two different patterns of fiber type involvement are seen. In one there is a clear size difference between type 1 fibers, which are abnormally small (hypotrophic or atrophic) and which contain numerous nemaline rods, and type 2 fibers, which are either of normal diameter or hypertrophic and contain few, if any, nemaline rods. Other patients show a gross predominance of type 1 muscle fibers, again with rods virtually confined to this fiber type. These findings may be explicable in terms of the involvement of isoforms of a-actinin specific to slow and fast muscle fiber types. 

Such results suggest a need for further research comparing trained with untrained subjects, along with an examination of muscle fiber type ratios of experimental subjects. Also, the question of subjects habituation to caffeine needs to be explored more fully. For the present, the reported ergogenic effects of caffeine on muscular strength must be viewed with skepticism and perhaps be attributed to the psychostimulant properties of caffeine. 

Table 11.1 Features of Skeletal Muscle Fiber Types...

I realize that this is a side track issue, but relevant all the same. Cycle protocols were an approach intended to facilitate optimal growth of muscle tissue. Remember there are two main muscle fiber types Type I, which is endurance orientated, and Type II which is strength orientated. Type "Ha", "Hb", and Type "He" are responsible for most musculature size and have the greatest potential for growth. Testosterone increases the number of Type II fibers at the expense of the Type I transformation. Growth hormone, Insulin, IGF-1, and thyroid hormones effect growth and hyperplasia of both fiber types. This should be another key relating to protocols that were utilized and why. 

Type of muscle fiber Type I Red muscle Aerobic exercise Type II White muscle Anaerobic exercise... 

Mackle, B. G. and Terjung, R. L. (1983) Blood flow to different skeletal muscle fiber types during contraction. Am. J. 

Martin WH 3rd, Murphree SS, Saffitz JE. [i-Adrenergic receptor distribution among muscle fiber types and resistance arterioles of white, red, and intermediate skeletal muscle. Circ Res 1989 64 1096-1105. 

E7. Engel, W. K., Selective and non-selective susceptibility of muscle fiber types. A new approach to human neuromuscular diseases. Arch. Neurol. (Chicago) 22, 97-117 (1970). 

Skeletal muscle fiber type is determined by innervation. 

The immature muscle fibers appear to be differentiated into fiber types prior to innervation although the final determination of muscle fiber type is the culmination of genetic cues (Brooke et al., 1971), hormonal environment and motor nerve stimulation (Buller et al., 1960 Brown and Lunn,... 

Schiaffino S, Reggiani C (1996) Molecular diversity of myofibrillar proteins gene regulation and functional significance. Physiol Rev 76 371-423 Pette D, Staron RS (1997) Mammalian skeletal muscle fiber type transitions. Into... 

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