Cervical muscles are crucial in providing the stability of the cervical spine. Many in vitro studies have investigated the relationship between muscle force and stability directly. However, the effects of different muscle dysfunctions or muscle recruitments on cervical spine stability are not yet clear and therefore, worthy of study. A spine testing apparatus with muscle force replication activated by pneumatic cylinders was developed to find the effect of muscles on spinal stability. Seven porcine cervical spines (C2-T1) were used. Three pairs of cervical muscles, including neck flexors (sternocleidomastoid, SCM) and neck extensors (splenius capitis, SPL; semispinalis capitis, SSC), were simulated. The experimental tests included: 1. no muscle recruitment, 2. full muscle recruitments, 3. SCM dysfunction, 4. SPL dysfunction, and 5. SSC dysfunction. The external pure moment in sagittal plane was applied from 0 Nm to 2 Nm to examine the stability/flexibility of specimens. The spinal stability was evaluated by the neutral zone (NZ), the range of motion (ROM), the reduced NZ (R_NZ), and the reduced ROM (R_ROM). Loading responses of C7-T1 disc were also measured. The results of this study showed: The activation of cervical muscles decreased the NZ and ROM. The degree of decrease among different muscle dysfunctions, however, was not significantly different. The SPL dysfunction induced larger anterior shear force, while the SCM dysfunction exclusively induced extension moment. In conclusion, the muscle forces could stabilize the cervical spine, but significant decrease in spinal stability was not found among dysfunctions of different muscles. The SCM and SPL dysfunction may result in abnormal stress at the C7-T1 disc.