Postural control of the lumbar spine in unstable sitting

OBJECTIVE: To evaluate the neuromuscular strategy adopted during sitting balance on an unstable surface in the frontal plane. DESIGN: Electromyographic evaluation of trunk muscles. SETTING: University spine biomechanics laboratory. PARTICIPANTS: Seventy asymptomatic men (mean age, 34.5 y). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: "Balancers" and "nonbalancers" were identified by principal component analysis of their lumbar spine side flexion angle during sitting balance. Average electromyographic levels were used as a measure of muscle activation. Pearson correlations were used to identify coactivation versus asymmetrical muscle activation of opposite muscle groups. RESULTS: External oblique, internal oblique, and thoracic erector spinae (TES) were most active, and most likely to be used asymmetrically, with other muscles showing low levels of coactivation. Between groups, the average electromyographic levels in the balancers was lower than in the nonbalancers (P<.05), with further differences in the symmetry of external oblique, internal oblique, and TES activation between groups. CONCLUSIONS: Sitting balance in the frontal plane appears to involve a combined feedforward-feedback strategy of muscle activation. Successful balance was characterized by low levels of muscle coactivity, along with higher levels of asymmetric activation in the global trunk muscles, specifically external oblique, internal oblique, and TES.