See Blaauw et al 4 for an excellent review covering this topic.Ī given specific human muscle displays a typical distribution of the different muscle cell types that is generally reported as the relative number or -proportion of separate muscle fibers of a given type (%). In addition to the three main muscle fiber types, the muscle cells can express several MyHC within the same fiber, typically combinations of the “nearest-neighbor” 3 MyHC according to their shortening velocity and which are termed hybrid fibers (ie 1/2A, 2A/2X). The human type 2A fibers typically show intermediate properties for shortening velocity and fatigability. On the opposite end of the functional scales are the type 2X muscle fibers, which exhibit the largest ATPase activity, highest shortening velocity, and power, while generally having the lowest oxidative capacity and fatigue more easily. Typically, the type 1 muscle fibers have the largest vascular system and apparatus for oxidative metabolism in humans and are thus the most fatigue resistant of the muscle fiber types. The type 1 MyHC contractile protein has the lowest ATPase activity and slowest cross-bridge cycling of the MyHC isoforms and therefore displays the lowest shortening velocity and power of the fiber types. 2 A given muscle fiber type displays a characteristic pattern in functional, biochemical and mechanical properties, although there is some degree of overlap between the different fiber types. 1 In the trunk and extremities the skeletal muscles of consists of three main muscle fiber types, classified as type 1, type 2A and type 2X by myofibrillar ATPase or immunohistochemical staining, dependent on the type of myosin heavy chain (MyHC) protein they express. Human skeletal muscles consist of some hundred to several hundred thousand individual muscle cells or fibers.
The result of this meta-analysis display that the type 1 muscle fiber percentage decrease as a result of reduced muscle activity, although the effect size is relatively small. The CSA of the muscle fiber types decreased after the study period (all P-values < 0.001) with greater reductions in type 2 than type 1 fibers ( P < .001). Conversely, the overall type 2X fiber percentage increased after reduced muscle activity ( P < .001). Meta-regression showed no effect of study duration on type 1 fiber percentage ( P = .98). Overall, the mean type 1 muscle fiber percentage was significantly reduced after interventions (−1.94%-points, 95% CI, P = .008), with no significant differences between intervention models ( P = .86).
Effect sizes were calculated as the mean difference between baseline and follow-up and Generic Inverse Variance tests with random-effects models were used for the weighted summary effect size. Forty-two studies comprising 451 participants were included. The reduced muscle activity models were detraining, leg unloading, and bed rest. We conducted systematic literature searches in eight databases and included studies assessing type 1 fiber percentage visualized by ATPase or immunohistochemical staining before and after a period (≥14 days) of reduced muscle activity. Other objectives were changes in type 2A and 2X percentages and muscle fiber cross-sectional area. The main objective of this systematic review was to examine the effect of reduced muscle activity on the relative number of type 1 muscle fibers (%) in the human vastus lateralis muscle.