Associations of erector spinae muscle activity with thoracic kyphosis and thoracic extension

Although an imbalance between the thoracic and lumbar erector spinae muscle activity is known to contribute to spinal dysfunction, the factors influencing this muscle balance, particularly thoracic extension mobility, remain unclear. This study investigated the associations of thoracic kyphosis and thoracic extension mobility with erector spinae muscle activity and muscle balance. Ninety-four healthy participants underwent measurements of thoracic kyphosis angle, thoracic extension angle, and erector spinae muscle activity (longissimus thoracis, iliocostalis lumborum pars thoracis, and iliocostalis lumborum pars lumborum). Muscle balance was assessed using the ratios of longissimus thoracis to iliocostalis lumborum pars lumborum and iliocostalis lumborum pars thoracis to iliocostalis lumborum pars lumborum. Regression analyses were conducted with erector spinae muscle activities and their ratios as independent variables, and thoracic kyphosis and thoracic extension as dependent variables. Regression analyses showed that the ratio of longissimus thoracis to iliocostalis lumborum pars lumborum and the activity of the iliocostalis lumborum pars thoracis significantly predicted thoracic extension mobility (adjusted R 2  = 0.14, p < 0.001), while the activity of the iliocostalis lumborum pars lumborum significantly predicted thoracic kyphosis (adjusted R 2  = 0.04, p = 0.026). These findings suggest that the assessment and intervention of thoracic extension mobility may be significantly associated with the erector spinae muscle balance during trunk extension exercises.