[Purpose] Physical therapists frequently perform manual stretching of the ankle joints. Manual stretching procedures are challenging to define because they involve multidirectional joint motions and external forces. Therefore, in this paper, we propose a method for quantitatively and statistically analyzing the manual foot stretching techniques used by physical therapists. [Participants and Methods] The participants were four physical therapists, and three patients who have a spastic foot. We investigated the manual foot stretching techniques employed by the physical therapists using a three-dimensional analysis system and an instrumented brace with force sensors. Principal motion analysis was applied to the obtained data, and principal motions were determined. [Results] The first principal motion was the application of force for the dorsiflexion of the foot; second, the pushing/pulling of the heel; third, the eversion/inversion of the entire foot; and fourth, the eversion/inversion of the forefoot. Furthermore, the manual stretching techniques varied among the physical therapists, even for the same patient, and some techniques occurred only between particular pairs. [Conclusion] This study demonstrated the effectiveness of the principal motion analysis for the statistical assessment of manual stretching techniques and clarifying differences in stretching technique among physical therapists.