Biomechanical effects of cranio-cervical positions on cervical musculoskeletal disorders

Abstract

Cervical musculoskeletal disorder become one of the global health problems. It involves the tissue stiffness, muscle weakness, pain syndromes and limited range of movement along the cervical spine. This disease imposes the heavy burden on the economy, society and human well-beings. Proposed mechanism for this disorder is associated with high intrinsic forces along cervical spine, resulting from various types of movements, as well as with malposition of cervical vertebrae at each level. However, it is not well-understood in static situation due to the difficulty of detecting the low-intensity intrinsic forces in the cervical musculoskeletal system. Furthermore, posture as a predictor of internal load, how does cervical musculoskeletal system distribute the loads under various types of spinal postures? Although considerable research has been devoted to normal motion of the cervical spine, such as flexion, extension, lateral bending and rotation, rather less attention has been paid to protraction in term of biomechanical aspect, especially in a static condition. Therefore, the aim of this study is to investigate head-neck posture, cervical spinal posture and load sharing in the cervical musculoskeletal system under three static cranio-cervical positions (neutral, protraction and flexion) during two sitting postures (upright and slump). For investigating the biomechanics of cervical musculoskeletal system and human performance, a platform was established at two levels in this study, including motion capture analysis and musculoskeletal modelling. This study utilized repeated measure design with ten healthy participants performing six experimental conditions. It included cranio-cervical neutral, protraction and flexion positions under upright and slump sitting postures. Three-dimensional posture angles of seated human were measured using the Vicon motion analysis system (Vicon MX, Oxford, UK). Supporting forces under bottom and foot were measured through two force platforms. For predicting inner cervical vertebral angles and load carrying in musculature and cervical joint at each level, the musculoskeletal model with a detailed cervical spine was developed in AnyBody Modelling System. The musculoskeletal model was partially validated through electromyography and previous literature. The result showed that there was a significant interaction between the effects of sitting posture and cranio-cervical position on postural angles (p < 0.05). There were no significant differences in cranial angle between neutral and protraction, approximately 70 degrees both under upright and slump sitting postures. For the cranio-cervical angle, it was significantly greater in protraction than other cranio-cervical positions (p < 0.001), reaching approximately 175 degrees. In musculoskeletal prediction, the cervical tilt angles at the level of C0C1, C2C1, C3C4 and C5C6 varied significantly (p< 0.05) among neutral, protraction and flexion in upright and slump sitting postures respectively. The slumped posture was associated with greater cervical tilt angles compared with the upright posture. The upper vertebral tilt angles in protraction were almost two times as that of the neutral position while the lower vertebral tilt angles in protraction increased by approximately 40% compared with the neutral position during upright sitting. Cervical tilt angles were the greatest for flexion position ranging from 23 to 43 degrees at each vertebral level. However, there was no significant difference between protraction and flexion at the level of C6C7 and C7T1. There was a significant interaction between the effects of sitting posture and craniocervical position on multifidus cervicis, levator-scapular, trapezius-scapular and trapezius-clavicular muscles (p < 0.05). For cervical flexors, it carried approximately 9% of the load in cranio-cervical neutral position during upright sitting. It was also greater than other conditions. As for cervical extensors, load carrying proportions were observed great in cranio-cervical protraction under both upright and slump sitting postures, reaching approximately 95% and 96% respectively. Cervical joint forces in cranio-cervical flexion were significantly greater than neutral or protraction conditions (p < 0.05). The maximum joint reaction forces were 206.3N and 218.6N at the level of C7T1 in cranio-cervical flexion during upright and slump sitting posture, respectively. However, there was no significant difference in joint reaction forces between cranio-cervical neutral and protraction positon. For the validation of the musculoskeletal model, the mean correlation coefficients between the predicted and measured muscle activities over trapezius-clavicular and cervical erector muscles in various types of cranio-cervical positions were 0.313 and 0.471, respectively. There was a fair degree of relationship between the estimated and measured muscle activity (0.25 < r < 0.05, p < 0.05). This study investigated biomechanical characteristics of static cranio-cervical positions under two major sitting conditions through motion analysis and musculoskeletal modelling. These results showed the static behavior of the cervical spine in response to various types of postures. It was concluded that slump posture favored the mobility of cranio-cervical spine and also increased the forward inclination of cervical vertebrae. Cervical extensors including trapezius and multifidus muscles played an important part in maintaining cranio-cervical positions. Cervical flexors produced great force in neutral position. Moreover, superficial muscles were found to be more responsive to the positional changes of head and neck than deep muscles. In this study, a combination of experimental and computational studies provided a versatile platform for interpreting complicated cranio-cervical behaviors under different static postures. Estimation of cervical spinal posture and load carrying along cervical spine was obtained from the validated musculoskeletal model. This work could provide insight into the effects of static loading on musculoskeletal health, and mechanisms underlying cervical musculoskeletal disorders caused by the sedentary occupation, as well as scientific fundamentals for ergonomics design.