In the United States, 1.4 million people who rely on wheelchairs for mobility develop serious pressure ulcer (PU) at pressure areas such as ischium and sacrum due to prolonged sitting. Recently, functional electrical stimulation (FES) has been proposed as a means to prevent development of PU through the redistribution and the reduction of the internal stresses within the deep tissue of the buttock, which were achieved by the increase of the muscle thickness, particular in the gluteus maximus. However, rare information about how much the therapeutic effects of the FES application for PU prevention are positively induced through the internal stress relief resulting from the alteration of the muscle thickness achieved by the FES is currently available. The objective of the current study was therefore to evaluate a degree of the internal stress relief through the alteration of the gluteus maximus muscle thickness, by using Finite Element (FE) analysis combined with MRI image analysis. Four FE models were created through 3D reconstruction model made from buttock-thigh MRI images, which were obtained in an actual sitting posture of a specific subject. The internal von Mises stresses within the gluteus maximus were computed and analyzed with alterations of the gluteus maximus muscle thickness considered in the current study. The results showed that the distributions of the high internal von Mises stresses within the gluteus maximus were gradually decreased with the increase of the gluteus maximus muscle thickness. However, the maximum von Mises stresses within the gluteus maximus were irregularly changed. The current study confirms that the internal von Mises stress relief effects of the FES application may have substantial benefits in terms of mechanical responses for PU prevention. It can be concluded that incorporating the FES system into a rehabilitation and treatment program for individuals who have sitting-related PU wounds may promote the healing progress while maintaining their mobility.