Time-course influence of weight-bearing stimulation on muscle fibers with disuse muscle atrophy: based on longitudinal regions in rat soleus muscle

Abstract

Background: Prevention and promotion of recovery from disuse muscle atrophy are important tasks of physical therapy. The soleus muscle has been investigated in many basic studies on these tasks, and data from the middle muscle belly region have been frequently used as an index. However, the functional action of skeletalmuscle differs between the origin and insertion site, and the state of atrophymay not necessarily be homogeneous. If there is a difference among longitudinal regions, it is necessary to devise an intervention method for physical therapy. Purpose: To investigate weight-bearing stimulationinduced recovery from disuse muscle atrophy in longitudinal regions of rat soleus muscle using the frequency of necrotic fibers as an index. Methods: Forty-nine male Wistar rats aged 8 weeks were used. Seven groups were established: animals on the experiment initiation day as a control group (C0), 2week normal maintenance group (C14), group with 2-week hindlimb suspension without weight-bearing (unloading) (H14), and groups with 2-week hindlimb suspension followed by weight-bearing (normal maintenance) for 3, 5, 7, and 14 days (HR3, 5, 7, and 14, respectively). The soleus muscle was excised, and the regions at 25, 50, and 75% of the muscle length from the origin were designated as proximal, middle, and distal regions, respectively. Frozen transverse cross-sections were prepared and stained with HE, and necrotic fibers were counted. Results: The frequency of necrotic fibers was increased by unloading from that in C0 over the time in all proximal, middle, and distal regions (the means in H14 were 1.3, 1.6, and 0.9%, respectively). Reloading further increased the frequency of necrotic fibers, reaching the highest in HR5. The frequency then decreased thereafter, and itwas lower inHR14 than inH14. By regions, the frequencywas high in themiddle region (3.3% in HR5) and low in the distal region (1.9% in HR5). Conclusion(s): It was clarified that the influence of weight-bearing stimulation on muscle fibers of rat soleus muscle with disuse muscle atrophy differed among the longitudinal regions. Based on the results of the transverse cross-sectional areas of the muscle fibers in our previous study, the proximal regionwas strongly influenced by unloading, but the influence of reloading of weight-bearing was smaller than that on the middle region, whereas the distal region was less influenced by unloading and muscle fibers were less injured by weight-bearing. Implications:Considering the overall function of skeletal muscles for clinical physical therapy, combination with an interventionmethod other thanweight-bearingmay be useful for the proximal region, in which weight-bearing has less of an influence and faster recovery can be expected.