Three-dimensional intervertebral range of motion in the cervical spine: Does the method of calculation matter?

•Within-subjects, intervertebral ROM was significantly different among calculation methods.•The method of calculation can significantly affect the group mean ROM.•The differences in group mean ROM due to using different calculation methods decreased from 6.0° to 1.5° when increasing the group size from 5 to 25 subjects.•To analyze motions that occur primarily about the sagittal plane, the first rotation should be calculated about the axis of flexion/extension.•To analyze motions that primarily consist of rotation and/or lateral bending, the first rotation should be calculated about the axis of lateral bending.