Human skeletal muscle fiber type percentage and area after reduced muscle use: A systematic review and meta‐analysis

Abstract

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. Other objectives were changes in type 2A and 2X percentages and muscle fiber cross‐sectional area. 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. The reduced muscle activity models were detraining, leg unloading, and bed rest. Forty‐two studies comprising 451 participants were included. 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. Overall, the mean type 1 muscle fiber percentage was significantly reduced after interventions (−1.94%‐points, 95% CI [−3.37, −0.51], P = .008), with no significant differences between intervention models (P = .86). Meta‐regression showed no effect of study duration on type 1 fiber percentage (P = .98). Conversely, the overall type 2X fiber percentage increased after reduced muscle activity (P < .001). 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). 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.

Muscle Fiber Type Transitions with Exercise Training: Shifting Perspectives

“…It is well characterized that fibers shift toward faster MHC expression during disuse [ 29 ]. However, there appears to be a type IIx overshoot after a period of training and detraining, which corresponds with superior fiber velocity characteristics [ 30 ].…”

THRIFTY: a novel high‐throughput method for rapid fibre type identification of isolated skeletal muscle fibres

“…We would furthermore argue that performing studies on large pools of pure type IIX fibres would be of much less relevance in a healthy and active population. Nevertheless, it is acknowledged that substantial proportions of type IIX fibres have been reported in states of muscle disuse (Andersen et al., 1996 ; Vikne et al., 2020 ), and that validated identification of type IIX fibres using THRIFTY would be beneficial for laboratories with such a research focus. However, as fibre typing with THRIFTY is based on subclass‐specific antibody binding, for laboratories with a particular interest in type IIX fibres and access to antibodies recognizing this subclass, the protocol can easily be modified and validated in‐house.…”

How Postural Muscle Senses Disuse? Early Signs and Signals

“…A recent systematic review by Vikne et al, 2020 provides data from 42 studies involving 451 volunteers who were subject to detraining, unilateral leg suspension (ULLS), and bedrest. The mean type I muscle fiber percentage was significantly reduced after interventions and, conversely, the overall fast fiber percentage increased after reduced muscle activity [ 146 ]. This fiber type transformation is considered to be one of the reasons for increased fatigue and, consequently, reduced performance of postural muscles after functional unloading.…”