What is slumped sitting? A kinematic and electromyographical evaluation

Slumped sitting is a commonly used reference posture when comparing effects of upright sitting in both clinical and non-clinical populations alike. The exact nature of slumped sitting has not been clearly defined, including regional differences within the posture, and how the passive nature of slumped sitting compares to an active-flexion posture. Kinematic and electromyographical (EMG) data were collected from 12 males during three repeats of slumped sitting and seated maximum forward flexion. Spine angles were defined in four regions (three thoracic and lumbar) as well as for the pelvis, and EMG was collected from eight muscles bilaterally. Kinematic data were expressed as a range of motion (in degrees), and as a percent of full forward flexion while seated (%SIT-FF) and standing (%STAND-FF). EMG data were normalized to a percent maximum contraction (%MVC). Results showed that slumped sitting is characterized by 10° posterior pelvis rotation, near end-range flexion of the mid- (90%SIT-FF) and lower- (81%SIT-FF) thoracic regions, and mid-range flexion of the upper-thoracic (51%SIT-FF) and lumbar (43%SIT-FF) regions. Comparison of slumped by %STAND-FF showed the upper- and mid-thoracic regions to have high variability and large values (over 100%STAND-FF). Muscle activation showed a significant 3%MVC reduction in the lower-thoracic erector spinae muscle when moving from upright to slumped sitting. These data highlight the postural differences occurring within different spine regions, and interpretations that could be drawn, depending on which normalization (sit or stand) method is used.