Recently, “Nijiku” sports motion theory has been proposed and is being widely disseminated for many types of sports events including association football (soccer). It appears that this theory has some scientific basis for improving performance in sports. However, as the theory did not follow the necessary procedures, to date, its scientific validity has never been examined. This paper was designed to precisely examine the scientific validity of the “Nijiku” sports motion theory in relation to soccer from the viewpoint of science philosophy and biomechanics. Falsifiability is an important concept in science. However, only a few objective data—the ground reaction force of the supporting leg during soccer kicking and the peculiar motion referred to as “unfasten knee” motion—were demonstrated in the related literature with incorrect interpretation, and there were no falsifiable data to prove the existence of “Nijiku” during sports actions. Thus, it has been considered that the “Nijiku” sports motion theory runs contrary to the principle of science. Moreover, there are many misleading descriptions, particularly with regard to the effect of gravity. As a result, it can be considered that the “Nijiku” sports motion theory is classified as a “pseudoscience” labeled inaccurately or misleadingly portrayed as science. Scientific counterarguments to stimulate debate on these matters by the proponent and/or supporters are expected.