This study aimed to clarify cervical kinematics during daily activities, including level walking and stair ascending, in patients with cervical ossification of the posterior longitudinal ligament (C-OPLL). Eighteen patients with myelopathy caused by C-OPLL and 18 healthy controls were recruited to participate in the study. The sagittal cervical kinematics during level walking and stair ascent were quantitatively assessed using a motion analysis system based on wearable inertial sensors. The Japanese Orthopaedic Association score, Japanese Orthopaedic Association Cervical Myelopathy Evaluation Questionnaire, Neck Disability Index, and deep sensation in the lower extremities were assessed in all participants. Nine of 18 patients with C-OPLL presented with deep sensory disturbances. Patients with C-OPLL with deep sensory disturbances exhibited different sagittal plane cervical motion patterns than healthy controls during level walking and stair ascent. During the first phase of stair ascent, both patients with C-OPLL and healthy controls flexed their necks to the same degree;​ however, during the middle and final phases of stair ascent and all phases of level walking, the mean cervical flexion angle of patients with C-OPLL with deep sensory disturbances was significantly higher than that of patients with C-OPLL without deep sensory disturbance and healthy controls. Our data suggest that patients with C-OPLL presenting with deep sensory disturbances are likely to walk with their necks flexed and gaze downward to observe their steps throughout their daily lives. This habitual neck posture may lead to a vicious cycle of cervical kyphosis and worsening of compressive myelopathy.