Seating and Positioning: Applying Biomechanical Principles
The knowledge and application of biomechanical principles form the basis for the provision of optimal seating and positioning systems. Biomechanical principles explain body movements. The study of normal healthy body movements is necessary in order to understand how disability and disease cause postural deviations in clients. Optimal seating and positioning systems maintain and improve comfort, health and function. The failure to utilize biomechanical principles is invariably reflected in poorly fitting seating and positioning systems. This failure is realized by reports of discomfort from clients, bodily harm and decreased function. The clients, young or old, may be physically harmed, uncomfortable and unable to function optimally. The understanding and application of biomechanical principles will increase the quality of seating and positioning systems and the overall comfort, function and health of clients. The success of a durable medical equipment (DME) company is dependent upon the knowledge of its employees and the quality of its services.
The word biomechanics originates from the Greek words bíos and méchanikós. Bíos means life and méchanikós means machine, therefore, biomechanics is the study of our life machines or bodies. More specifically, biomechanics is defined as the study of the action of external and internal forces on the living body, especially on the skeletal system. Biomechanical principles offer an understanding of why and how a body is able to move against or with the force of gravity. Anatomy, physiology, physics and geometry must be considered in order to grasp biomechanical principles and their usefulness for seating and positioning.
Anatomy
Anatomy is the study of the shape and structure of a plant or animal. Human bodies, just like human personalities, are unique. However, each body is based on a common skeletal plan. This skeletal plan or system is joined together by joints, ligaments, tendons and muscles. Joints have various designs which dictate their abilities to move certain ways. These movements include flexion and extension, abduction and adduction, as well as internal and external rotation. The hip joint is able to flex and extend, abduct and adduct and internally and externally rotate. The knee joint just flexes and extends. Muscles have an origination point and an insertion point on bones and these points dictate which way they will move a joint. Muscles usually cross one joint and move the joint that they cross. The hips, knees, shoulders, hands, feet, elbows and many other joints each have their own unique range of motion. These ranges are measured in degrees. For example, the popularly known hamstring muscle is actually a group of three muscles on the posterior aspect of the leg. The three hamstring muscles originate on the posterior pelvis and insert on the lower leg beneath the knee joint. Because the hamstring crosses two joints, it moves two joints. The hamstring group extend (straighten) the hip and flex (bend) the knee.
Physiology
Physiology is the study of the function of the parts of a living organism. Whereas anatomy is concerned with structure, physiology is concerned with purpose and function. The brain consists of many parts that work together to control the body. For example, the semicircular canals in the ears are special tilt receptors. These canals are lined with sensors and filled with fluid. The movement of the fluid is registered by receptors to tell us the position of our head and to help us balance our head against gravity. The semicircular canals continually give this tilt input to the cerebellum for processing. If a client is positioned in a way that causes a struggle to balance his or her head against gravity, then he or she will be uncomfortable and may develop pain in neck muscles.
Many parts of the brain work together to balance our muscle tone and to control purposeful movement of our extremities. If a child has cerebral palsy and presents with high muscle tone in his or her arms and legs then there is usually something wrong with the child's motor cortex, a part of the child's brain that controls purposeful movement. Muscles with high tone are constantly working or contracting. These muscles are stiff and tend to stay in one position. When muscles stay in one position for extended periods, they are at risk of sticking in that position permanently. This is called a muscle contracture. Once a muscle has become contracted, it cannot move the joint that it was designed to move. It usually cannot be unstuck without surgery.
Physics
Physics has given us many laws regarding force, torque, equilibrium, and mechanics. Let's consider gravity. Gravity is a constant force that is invisible but undeniable. When standing or sitting upright we have to use our muscles to maintain our position against the force of gravity. Our weight is essentially the gravitational force acting on the mass of our bodies. The greater the mass of a body, the more the force of gravity will act on it and the more a person will weigh. Gravity is a force that pushes our bodies towards the center of the earth. Our center of gravity is the point in our bodies where our bodies are balanced relative to gravity. As we move, our center of gravity moves.
Newton's third law maintains that every action has an equal and opposite reaction. Simply put, everything that you touch touches you back. When you sit on a chair, your weight is supported by the chair surface because the chair surface resists your weight. Newton's law explains that a body that is supported by a seating system is always resisted equally to its force. A client's position in a seating system depends on the properties of the support or seating surface (air, gel, foam, wood, sling) and the placement of the supports. Surfaces equally resist the body's force. For instance, if you sit on a firm medium soft Sunmate foam planar cushion with solid support, your buttocks will remain level (if you have flexible pelvic mobility) as there is a firm surface resisting the ischial tuberosities. On a sling seat the force of the ischial tuberosities is resisted such that the ischial tuberosities sink into the flexible sling surface. By sinking into the surface, the ischial tuberosities are effectively pushed together. This is a position that is not anatomically sound. It is uncomfortable and promotes poor overall posture.
Newton's third law explains why it is necessary to distribute pressure over a large area in order to decrease the risk of developing pressure sores. Force is applied over a surface area. Our weight is the force of gravity coming down on our body. The larger our body the more gravity is able to act upon us and the more we weigh. A force that is applied over a small surface area generates more pressure than the same force applied over a larger surface area (Cook and Hussey, 1995). When a person sits down on a hard surface, the surface resists the weight of the body more in the bony parts of the buttocks, the ischial tuberosities. An air cushion will distribute the force of the body evenly so that not all force is resisted up towards the ischial tuberosities. The resistance from the air cushion distributes the client's weight onto the buttocks and thighs. An action has an equal and opposite reaction but the reaction can be distributed by using cushions with different properties.
Geometry
Simple geometry plays an integral part in the application of biomechanical principles for seating. Geometry is the mathematics of measurements and relationships of points, lines, angles, surfaces, and solids. Linear and angular measurements are necessary to document the movement ability of each body. If the hamstrings are contracted, then they will not allow the hip and knee to move through their normal range of motion. This means that the child with hamstring contractures cannot sit in the classic 90 degrees at hips and 90 degrees at knees posture in which so many seating systems are set up. If the child is placed in a chair that is set up with a 90-degree seat to back angle and 90-degree knee angle then the child's hamstring muscles will be stretched. This will be painful and the child may slide and slouch to try to relieve the stretch on his/her hamstring muscle in order to be more comfortable. By allowing the child to flex his or her knees and extend his or her hips to a range that is available, the child's hamstring muscles will not be stretched.
With the knowledge of biomechanical principles, an RTS or therapist may more easily problem solve to determine the cause of postural deviation. Only by studying how normal skeletal and body systems function can we analyze the postural deviations of our clients. A comprehensive seating and positioning evaluation must include the knowledge of biomechanical principles. Mat examinations and seating simulations are means to accrue biomechanical information and to establish seating and positioning goals. It is important to ask the question: what position are you trying to achieve and what kind of support will achieve this position?
The goals of seating and positioning vary with each unique person. This article merely touches on the varied and complex considerations required for optimal seating and positioning systems. The importance of applying biomechanical principles for optimal seating and positioning systems cannot be overstated. Clients may be harmed, uncomfortable and unable to function well if biomechical principles are not applied during a seating and positioning evaluation. Many other variables must be addressed for the provision of optimal seating and positioning systems. These include, but are not limited to, cognition, compliance, access and motivation. There are exceptions too. Sometimes a client will function the best and be the most comfortable, yet look like they are in a terrible position. But how does an RTS or therapist know what is terrible? And what are the client's baseline anatomical and physiological abilities? The successful provision of quality durable medical equipment can form a foundation for viable customer satisfaction. Success depends on a client's comfort and function. Timeliness, volume and efficiency are not enough positive services if your customer is positioned poorly. Quality work cultivates success. The knowledge of the interplay of human body anatomy, physiology, physics and geometry is paramount for optimal seating and positioning. Knowledge of what happens within a body and how the environment affects a body can empower rehab tech suppliers and therapists to provide better quality seating and positioning systems. Above all, please, primum non nocere, first do no harm.
This article originally appeared in the May 2003 issue of HME Business.