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It is well known that sitting posture is associated with discomfort and a number of musculoskeletal disorders. Seat manufacturers have made great strides toward developing seats for equipment which helped in alleviating the vibration transferring to the lower area of the spine; however, increased neck and head motion resulting from these seat designs may have been overlooked. Many cervical spine studies have been developed to estimate the response of the head and neck; however, these current studies do not take head and neck posture into account. The objective of this work was to study and demonstrate the difference in human biomechanical response to WBV when they use different neck postures. Four head and neck postures: up, down, to the side, and normal (straight forward) were investigated.
Ten male subjects with ages ranging from 19 to 28 years were used to test each of the four postures, using the discrete sinusoidal frequencies of 2, 3, 4, 5, 6, 7, and 8 Hz at constant amplitudes of 0.8 m/s^2 RMS and 1.15 m/s^2 RMS in the x-direction (fore-and-aft). Subjects were seated in a rigid seat rigidly mounted to a vibration platform and vibration was generated using a six-degree-of-freedom man-rated shaker table. Subjects were tightly coupled to the seat back, using a neoprene vest and 5 straps, in an effort to reduce any relative motion between the seat and the subject. Subjects reported their head and neck discomfort using the Borg CR-10 scale with each of the postures, and then gave a second discomfort rating for the normal posture for each combination. Motion capture and accelerometer data were used to acquire the motion of the seat, C7 vertebrae, and center-of-head motion. The 3D motion of selected points on the heads and necks of the subjects were acquired using a twelve-camera Vicon motion capture system. Accelerometer data at the head, C7, and seat was used to verify the motion capture data.
For the head-down posture, the magnitude of the discomfort function was higher than the normal posture. The head-to-side and head-up postures have shown less discomfort have shown less discomfort in the critical resonance area; however, these postures show roughly the same discomfort as the normal posture in other frequency ranges. In these postures, the subjects are using major neck-back muscles which create a stiffer system and may explain why there is a shift in the second peak in the head-to-side and head-up postures. Interestingly, the head-to-side and head-up postures show a similar trend as the normal posture, however, the peak transmissibility is attenuated. In addition, the subject's average discomfort was lower in this range compared to the normal posture. The head-down posture had the highest transmissibility and discomfort overall and suggests that workers in vibration environments should reduce any head-down postures to avoid unwanted head accelerations and discomfort. This work has demonstrated the importance of considering the head-neck posture in future seat-design studies.