Objective The aim was to report the kinetics of masticatory muscles using a 4-dimensional muscle model and kinetically investigate the etiology of square mandible (SQM) in vivo. Study design To ascertain the mandibular movements in patients with SQM, we compared a group of 11 SQM patients with painless limited mouth opening to a matched group of 11 healthy volunteers who acted as control subjects. Mandibular movements were recorded using a 6-degrees-of-freedom jaw-tracking device, with the incisal and kinematic condylar as analytic points. Results For the control group, mean tracking distance for the incisal point was 50.1 ± 6.8 mm, whereas for the SQM group mean tracking distance was 27.1 ± 2.8 mm. With lateral excursions, the control group's mean tracking distance of the incisal point was 9.9 ± 2.01 mm, and the SQM group's was 8.1 ± 1.64 mm. Conclusion Despite a sufficient lateral excursion, motion of the mandible was limited by some factors at opening and suggests that the mode of lateral movements in SQM patients may differ slightly from those in healthy individuals.