The scale of optimization calculations for estimating muscle tension is reduced. A multiplicity of muscles is categorized into a multiplicity of muscle groups Mi (i = 1, 2, ... n) based on muscle movement directionality or heteronymous muscle facilitation, one or a multiplicity of muscle subgroups is formed from the muscles in each muscle group originating or inserting on the same bone, and the level of muscle activity is treated as the same for muscles affiliated with the same subgroup. One of the aforementioned one or multiplicity of subgroups of some or all of the multiplicity of muscle groups Mi is a first subgroup, which is formed from a single representative muscle, to which an electromyograph has been attached, and the muscles originating or inserting on the same bone as said representative muscle. The number of variables in the optimization calculations is reduced by acquiring the muscle tension of the muscles affiliated with the first subgroup from the myoelectric potential of the aforementioned representative muscle measured during subject activity, or the number of variables in the optimization calculations is reduced by using optimization calculations to estimate the level of muscle activity representing each subgroup.
