The quantification of human muscle contraction is required by many biomechanical studies. Ultrasound imaging has been widely used to study muscle function for decades, and morphological parameters, such as muscle fiber length, muscle thickness, pennation angle and cross sectional area, have been used to reflect muscle contraction. However, few details regarding the motion activity of a muscle group (such as quadriceps muscle, QM) or an individual muscle (such as rectus femoris, RF), during its working for the completion of a movement, were quantitatively investigated. This study introduces a novel approach based on motion feature extraction to provide a new descriptor for local muscle activities during the completion of pre-defined body movements. Specifically speaking, this approach is applied to investigate the QM during isometric contraction in eight subjects using ultrasound imaging. Preliminary results indicate that the proposed approach is capable of providing new perspectives about the activity patterns of both RF and QM.