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Effect Of Immobilization On Intramuscular Fat Of The Thigh Muscle Groups: 2691 Board #6 May 29, 2

It is well known that muscle atrophy and increase in fat fraction within a given muscle, i.e. intramuscular fat (IMF), is induced as a result of immobilization. It is speculated that increase in IMF content (%IMF) is associated with inactivity, mitochondrial dysfunction and/or adipocytokine actions resulted in insulin resistance. It is not well documented that %IMF increase by immobilization is due to the absolute amount of IMF increases or relative amount of IMF change by atrophy of muscle tissue (MT). PURPOSE: The purpose of this study was to quantify the change of IMF, MT and %IMF during immobilization in the thigh muscle. METHODS: Five patients (65.0 ± 16.7 years, 3 female and 2 male) with fracture of fifth metatarsal bone or fibula participated in this study. Subjects’ involved ankle joint was fixed by plaster cast for 4 to 5 weeks. Axial T1-weighted magnetic resonance images at the mid-thigh were taken just after initiating (pre) and 4 weeks later (post) of immobilization. MIVAP software was used to calculate IMF component, essentially as done previously (Akima et al. 2012). On the pixel number and signal intensity histogram within regions of interest which containing approximately ~50% muscle and 50% fat, we determined signal intensity that divide into fat and MT by Otsu thresholding technique. We also measured cross-sectional area (CSA) of IMF and MT to calculated %IMF [= IMF CSA / (IMF CSA + MT CSA) * 100]. RESULTS: %IMF significantly increased in the quadriceps femoris (QF) compared pre and post, while no changes in hamstrings and adductor. We further investigated individual muscles of QF and found no significant change in IMF among each individual muscle. However, there was significantly decreased in vastus latetaris MT (pre: 14.5 ± 4.0 cm vs. post: 12.6 ± 3.8 cm, p < 0.05), restus femoris MT (pre: 5.9 ± 2.3 cm vs. post: 5.2 ± 1.9 cm, p < 0.05) and vastus intermedius MT (pre: 14.1 ± 3.3 cm vs. post: 12.7 ± 3.8 cm, p < 0.05). Additionally, %IMF significantly increased in vastus latetaris (pre: 10.5 ± 4.5 % vs. post: 17.2 ± 3.9 %, p < 0.05), while no changes in the other muscles. CONCLUSIONS: The pattern of %IMF changes was specific in muscle groups and individual muscles within QF by immobilization. In addition, we should note that this change of %IMF was associated with decrease of skeletal MT, not absolute IMF change.