In this study, in order to perform standing support that takes into account differences in the muscle strength between individuals, it is necessary to change the support movement according to the muscle strength status of the individual. Therefore, it is necessary to measure the muscle strength of individuals, and most previous studies measured lower limb muscle strength by using electromyographs. However, because the person using the electromyograph needs expertise in muscle strength measurement, it cannot be easily used by ordinary caregivers in the care setting. In addition, measurement is difficult when using the electromyograph because it requires the attachment of many electrodes to the subject's body. At the same time, many previous studies have been conducted on ideal standing movements. The purpose of this study is to propose a stability margin range according to the subject's physical fitness. We proposed a simple method for estimating lower limb muscle strength without using an electromyograph and imposing no burden on the subject and the measurer. Based on the estimation results, we use the body center of gravity and the amount of muscle exertion as indicators and propose a stability margin range according to the subject's physical fitness.