Why do we need to stabilize the pelvis when sitting? To answer this, we need to understand hip and pelvis anatomy and the involved kinesiology. First, given the ball and socket nature of the hip joint, significant degrees of motion are available. The hip joint can move from being in a position of entire stability or “closed-packed” positioning to one of mobility, called the “open-packed” position. Standing reflects the closed-packed configuration where the ball and socket of the joint are “locked” into place eliminating almost all motion at the joint. Additional stability is provided through the tightening of the ligaments around the hip as the body moves into standing. Conversely, if the hip moves in the opposite direction, towards flexion and the sitting posture, the open-packed position is approached. As this happens, the hip joint becomes more loose and mobile. The position of the hips in the sitting posture falls at a point mid-way between these two extremes of mobility. This means that the hip joint is in an “unlocked” position in the sitting posture and therefore depends on other anatomical components for stability.
The structural shape of the ischial tuberosities of the pelvis adds an additional consideration to creating pelvic stability. Because the tuberosities are rounded, they can be compared to the rockers of a rocking chair, destabilizing the pelvis in the seated posture. Considering the bodily mechanics of both the hip joint and the pelvis in sitting, the stabilizing factor of muscular activity becomes extremely important to maintain the hip, pelvis and trunk in an erect seated posture.
However, for children and adults with disabilities who struggle with muscle weakness and imbalances, spasticity, and sensory issues, coordinating muscle activity to provide stabilization and control is challenging if not impossible. Slouched posturing, sacral sitting and sliding out of the chair are frequent concerns. In these cases, external pelvic support is needed to facilitate a functional seated posture. External stabilization for the pelvis can also reduce the excessive energy expenditure and muscle fatigue that occurs in the absence of such support.
Ideally, a seated pelvic stabilization system, in addition to supporting upright sitting, will allow the client to shift weight and encourage postural control and functional movement to emerge. Traditionally, supports for the pelvis have included a lap or seat belt over the greater trochanters and rigid stabilizers across the front of the pelvis. Knee blocks have been used as well to provide a posteriorly-directed force through the knees and pommels, or abductors have been placed between the knees or distal thighs to help prevent the pelvis from sliding forward in the seat. These approaches, although moderately effective, can also create poor positioning, discomfort and adverse pressure to the skin or joints if applied incorrectly.
Fortunately, as our understanding of adaptive seating advances, so also do the solutions and positioning options to address such positioning needs. Perhaps a more comfortable and efficient design for pelvic stabilization is Rifton’s pelvic harness, an accessory to the recently redesigned Activity chair. This unique design secures the client’s pelvis comfortably back on the chair seat, maintaining the pelvis in a neutral position with the back well-supported by the backrest.
The harness is first secured to the chair seat and laid out flat with the wide portion towards the back. After the client sits down, the flexible, pliable straps are brought between the client’s thighs, over each leg, and anchored near the greater trochanter. This applies an outward, downward and posteriorly directed force which prevents the pelvis from both thrusting and sliding forward in the seat. Compared to a traditional lap belt, this form of support is notably more proximal, secure and comfortable maintaining a better position of the pelvis even for those with strong extensor patterns, bulky clothing (diapers) or varying girth measurements.
It is no wonder then that this design has become a popular substitute for the regular seatbelt in cases where positioning is a challenge.
With a well-stabilized pelvis, trunk control is improved. This in turn contributes to improved head control and shoulder girdle mobility. The goal of external support is not to make the child or client “look perfect,” but rather to enable freedom of movement in relation to gravity, and to improve function in relation to the environment. Without appropriate support, pelvic thrusting and/or a sacral sitting position with a posterior pelvic tilt can limit or impede head and trunk control, and may contribute to further musculoskeletal problems.
As pelvic stability influences not only shoulder girdle mobility but head control, feeding and communication as well, assuring a stable foundation at the pelvis is more important than many parents and caregivers realize. It is up to the clinician and seating technician to convey how crucial pelvic stabilization is and to present the options available to achieve this important aspect of positioning for adaptive seating.
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