Although it is known vocal fold adduction is achieved through laryngeal muscle activation, it is still unclear how interaction between individual laryngeal muscle activations affects vocal fold adduction and vocal fold stiffness, both of which are important factors determining vocal fold vibration and the resulting voice quality. In this study, a three-dimensional (3D) finite element model was developed to investigate vocal fold adduction and changes in vocal fold eigenfrequencies due to the interaction between the lateral cricoarytenoid (LCA) and thyroarytenoid (TA) muscles. The results showed that LCA contraction led to a medial and downward rocking motion of the arytenoid cartilage in the coronal plane about the long axis of the cricoid cartilage facet, which adducted the posterior portion of the glottis but had little influence on vocal fold eigenfrequencies. In contrast, TA activation caused a medial rotation of the vocal folds toward the glottal midline, resulting in adduction of the anterior portion of the glottis and significant increase in vocal fold eigenfrequencies. This vocal fold-stiffening effect of TA activation also reduced the posterior adductory effect of LCA activation. The implications of the results for phonation control are discussed.