The Effect Of High-intensity Interval Training On Soleus Muscle In Postnatal Growth-restricted Mice

148.1 million children under age five experience growth restriction, which is associated with high morbidity and mortality. Postnatal growth restriction (PNGR) reportedly decreases skeletal muscle cross-sectional area, with an increase in glycolytic skeletal muscle fibers. High-intensity interval training (HIIT) is exercise of alternating intensity known to increase oxidative capacity in the normal population and may improve skeletal muscle function in PNGR. PURPOSE: To investigate whether HIIT affects skeletal muscle cross-sectional area (CSA) and the proportion of glycolytic fibers in PNGR mice. METHODS: 2 weeks before pregnancy, mothers were placed on a normal-protein (20%) diet or a low-protein (8%) diet. Pups born to normal- protein mothers were raised by low- protein mothers (PNGR, n = 7), inducing growth restriction, while other pups born to normal protein mothers were raised by another normal protein mother (CON, n = 9) to induce the same cross-fostering effects. On postnatal (PN) day 21, all mice were weaned to a normal protein diet. At PN 44, mice were assigned to either a HIIT intervention (HIIT: CON, n = 5; PNGR, n = 3) or sedentary group (SED: CON, n = 4; PNGR, n = 4). The HIIT group performed a maximal work test to determine the work intensities for the HIIT intervention. Starting with PN 45, HIIT was performed for 4 weeks, 5 days a week, for 1 hour each day (alternating bouts of 8 minutes at 80% of maximal work and 2 minutes at 50% of maximal work). At PN 73, soleus samples were collected from mice. They were then prepared with OTC, sectioned, stained for myosin isoforms, imaged by confocal microscope, and analyzed in ImageJ to determine fiber type and skeletal muscle CSA. RESULTS: CON have a lower percentage of type IIa fibers (31.40 ± 6.85%) compared to PNGR (40.58 ± 4.95%, p = 0.0166) and a higher percentage of type IIb fibers (31.24 ± 8.68%) compared to PNGR (22.14 ± 4.05%, p = 0.0383). CON have larger type IIa fibers (1900.71 ± 229.04 μm2) than PNGR (1614.61 ± 320.90 μm2, p = 0.0493). After standardizing CSA to body mass, CON HIIT have larger type IIa fibers (93.16 ± 18.64 μm2/g) than CON SED (87.87 ± 12.91 μm2/g, p = 0.0415). There was no difference in PNGR HIIT and PNGR SED. CONCLUSION: Postnatal growth-restricted mice did not have an increase in type IIa fiber size with HIIT and thus, did not respond positively to exercise. Supported by: This project was funded by the Michigan State University College of Education.