Soleus and TA/EDL Myofiber Composition and Cross-Sectional Area Across a Range of Physical Inactivity in Early Postnatal Mice 

It is well known that physical inactivity has a negative effect on health, partcularly that of the muscles. However, less is known about the effects of physical inactivity during early life on muscle development. The Soleus (SOL) and Tibialis Anterior/Extensor Digitorum Longus complex (TA/EDL) are leg muscles relevant for several basic motor functions, such as walking and would be impacted by early life inactivity. Both muscles contrast in size and fiber type composition.The purpose of this study is to examine how early life periods of varying activity and inactivity affect myofiber size and composition after a short, controlled treatment and post-treatment recovery period that encompasses a key period of development where significant growth occurs, in two different leg muscles. 

102 C57BL6 mice were divided into 2 cohorts, and further divided into 4 groups: control (SED), voluntary wheel-running (VWR), small mouse cage (SMC), and hindlimb unloading (HU). In order of increasing physical activity, they go: HU<SMC<SED<VWR. While both cohorts completed the 14 day treatment period, one cohort completed the additional 7 day recovery period. After the experimental period concluded, the mice were euthanized and SOL and TA/EDL muscles  were  collected. 10 micron-thick samples were taken from the muscles and stored onto slides, which were then incubated in antibodies specific to Myosin Heavy Chain I, IIa, IIb, and IIx with appropriate fluorescent secondary antibodies and imaged on a wide-field fluorescence microscope. Sample images were processed with ImageJ and analyzed through Semi-automatic Muscle Analysis using Segmentation of Histology (SMASH) for analysis of myofiber composition, myofiber minimum diameter, and cross-sectional area (CSA). 

Overall, the myofiber size trends seen in HU mice following the 7D period indicate successful recovery from severe physical inactivity, while the myofiber composition results indicate that early life physical inactivity mostly impacts the fastest, less fatigue-resistant types of myofiber. Future studies can investigate these detected trends in the long term, as we ultimately want to determine the impact of early life inactivity on muscle health in adulthood. 

Graphs were generated on GraphPad Prism.