Difference in selective muscle activity of thoracic erector spinae during prone trunk extension exercise in subjects with slouched thoracic posture.

BACKGROUND: The prone trunk extension (PTE) exercise is often used to strengthen the back extensors. Although altered trunk posture is associated with movement impairment, the influences of a slouched thoracic posture on muscle activity of the thoracic erector spinae and thoracic movement during the PTE exercise were overlooked in previous studies.

OBJECTIVES: To compare the muscle activity of the erector spinae muscles and the relative ratio of the thoracic and lumbar erector spinae muscles during a PTE exercise in subjects with and without slouched thoracic posture.

DESIGN: Cross-sectional.

SETTING: University motion analysis laboratory.

PARTICIPANTS: The study included 22 subjects with slouched thoracic posture (defined as ≥40°) and 22 age- and gender-matched healthy subjects.

METHODS: All participants performed the PTE exercise.

MAIN OUTCOME MEASURES: Bilateral surface electromyographic signals of the longissimus thoracis, iliocostalis lumborum pars thoracis, and pars lumborum muscles were measured during PTE exercises. Thoracic kyphosis (the angle of T1 minus T12) and lumbar lordosis (absolute value of the angle of L5 minus T12) were recorded using inclinometers during the PTE exercise.

RESULTS: The results showed no difference in muscle activity of the erector spinae in subjects with slouched thoracic posture versus those without during the PTE exercise. However, selective recruitment of the erector spinae pars thoracis was decreased significantly, and the thoracic kyphotic angle and lumbar lordotic curve were increased, during the PTE exercise in subjects with a slouched posture.

CONCLUSIONS: Although the PTE exercise has historically been a key component of correction of hyperkyphosis, the increased spinal curvature inhibits muscle activation of the erector spinae pars thoracis in these individuals, thus limiting effective strength gains. Therefore, modified methods to maintain a neutral posture of the spine and facilitate muscle activation of the erector spinae pars thoracis are needed in these individuals.