This chapter focuses on sitting control and interventions that can be used to improve sitting and sitting balance skills. Careful examination of the patient's overall status in terms of impairments and activity limitations that limit sitting control is necessary. This includes examination of musculoskeletal alignment, range of motion (ROM), and muscle performance (strength, power, and endurance). Examination of motor function (motor control and motor learning) focuses on determining weightbearing status, postural control, and neuromuscular synergies required for static and dynamic control. It also examines use of sensory (somatosensory, visual, and vestibular) cues for sitting balance control and central nervous system (CNS) sensory integration mechanisms. Finally, the patient must be able to safely perform functional movements (activities of daily living [ADL]) in sitting and in varying environments (clinic, home, work [job/school/play], and community).
Biomechanics of Sitting
It is important to understand the foundational requirements of sitting. Sitting is a relatively stable posture with a moderately high center of mass (COM) and a moderate base of support (BOS) that includes contact of the buttocks, thighs, and feet with the support surface. The pelvis is the foundation for sitting and strongly influences postural alignment of the entire axial skeleton. A neutral pelvic position is optimal for sitting. This is characterized by (1) an anterior superior iliac spine (ASIS) that is level or slightly lower than the posterior superior iliac spine (PSIS) (sagittal plane) and (2) a level position of both ASISs (frontal plane). Both ischial tuberosities should be equally weightbearing. The lumbar spine has a natural lumbar lordosis accompanied by extension throughout other areas of spine. The head and trunk are vertical, maintained in midline orientation over the pelvis with a "chin-in" position of the head. During active erect sitting, the line of gravity (LoG) passes close to the axes of rotation of the head and spine. During relaxed erect sitting, the LoG is slightly anterior to these axes of rotation, whereas during slumped or slouched sitting, the LoG is well forward of these axes (Fig. 5.1).1
Normal sagittal plane postural alignment (A) In optimal alignment, the LoG passes close to the axes of rotation of the head, neck, and trunk. (B) During relaxed sitting, the LoG changes very little, remaining close to those axes. (C) During slumped sitting, the LoG is well forward of the spine and hips.
The trunk muscles actively maintain upright postural control and core stability, including co-contractions of the trunk extensors (erector spinae muscles) and flexors (ab-dominals). Activity of the erector spinae muscles is greatest during active erect sitting as opposed to relaxed or slumped sitting.1,2 Lower extremity (LE) muscles are important stabilizers of the trunk and pelvis. Ankle dorsiflexors (anterior tibialis) and hip flexors (iliopsoas) are activated during backward displacements of the trunk, whereas calf muscles ...