Enhancing understanding: Back muscle strength and individual flexibility impact on the flexion-relaxation phenomenon in the lumbar erector spinae

Yi-Lang Chen; Wei-Cheng Lin

doi:10.1016/j.jelekin.2024.102949

Enhancing understanding: Back muscle strength and individual flexibility impact on the flexion-relaxation phenomenon in the lumbar erector spinae

J Electromyogr Kinesiol. 2024 Nov 4:79:102949. doi: 10.1016/j.jelekin.2024.102949. Online ahead of print.

Authors

Yi-Lang Chen¹, Wei-Cheng Lin²

Affiliations

¹ Department of Industrial Engineering and Management, Ming Chi University of Technology, New Taipei 243303, Taiwan. Electronic address: ylchen@mail.mcut.edu.tw.
² Department of Industrial Engineering and Management, Ming Chi University of Technology, New Taipei 243303, Taiwan; Taiwan Research Institute, New Taipei 251401, Taiwan.

PMID: 39520812
DOI: 10.1016/j.jelekin.2024.102949

Abstract

The flexion-relaxation phenomenon (FRP) refers to the deactivation of back muscles during deep forward trunk bending. This study examined the effects of back muscle strength, individual flexibility, and trunk angle on FRP in the back muscles. Forty male participants were classified into four groups according to toe-touch flexibility and back muscle strength. Lumbar erector spinae (LES) activity and the lumbosacral angle (LSA) were measured at incremental trunk flexion angles (0°-90°, with increments of 15°) to analyze FRP. Results indicated significant effects of back muscle strength, flexibility, and trunk angle on LES activity (all p < 0.001). Flexibility (p < 0.05) and trunk angle (p < 0.001) also influenced LSA. Additionally, an interaction between flexibility and trunk angle impacted LES activation (p < 0.001). Flexibility mainly determined FRP onset, while back muscle strength influenced efforts during moderate trunk flexion (30°-60°). These findings indicate that differences in lower back load among individuals with varying back muscle strengths become apparent even at relatively small trunk flexion angles (approximately 30°). When prolonged static trunk flexion in workplace settings places considerable strain on the lower back, we recommend utilizing toe-touch flexibility and back muscle strength assessments as practical screening tools for identifying early and subtle indicators of the FRP in workers.

Keywords: Back muscle strength; Flexibility; Flexion-relaxation phenomenon; Lumbar erector spinae; Trunk flexion.