Lumbar posture and individual flexibility influence back muscle flexion-relaxation phenomenon while sitting

Abstract

Although numerous studies have documented back muscle flexion-relaxation phenomenon (FRP) in standing postures, few studies have examined the FRP in various seated lumbar postures and individual flexibilities. This study, therefore, recruited 18 male students and assigned to low- and high-flexibility groups (9 in each). Activation of thoracic and lumbar erector spinae (ES) and lumbosacral angles were examined while participants sat in two postures (lordosis and kyphosis) and flexed their trunks at 15°, 30°, 45°, 60°, and maximum flexion. Results showed that kyphotic lumbar posture caused relatively low and unchangeable thoracic and lumbar ES activations, whereas lordotic lumbar posture engendered more contractive and varying thoracic and lumbar ES activations. Flexible participants exhibited higher thoracic ES activation than less flexible participants during lordotic sitting. Thoracic ES seemed to play a compensative role to stabilize the spine in the lordotic sitting posture, especially when the trunk was flexed over 45°. In lordotic lumbar posture, FRP occurred only in the lumbar ES; however, the activation and lumbosacral angles were still higher than those in kyphotic posture. The increased back muscle activation associated with lumbar lordosis may partially share the load on passive interspinous tissues, which are close to the discs during these flexed trunk positions. Relevance to industryThis study suggests that various lumbar postures and individual flexibilities may cause different FRP patterns when sitting. While performing seated tasks, people should exercise caution about the lumbar posture.