REVIEW OF THE LITERATURE Sitting Biomechanics Part I: Review of the Literature

Objective: To develop a new sitting spinal model and an optimal driver’s seat by using review of the literature of seated positions of the head, spine, pelvis, and lower extremities. Data Selection: Searches included MEDLINE for scientific journals, engineering standards, and textbooks. Key terms included sitting ergonomics, sitting posture, spine model, seat design, sitting lordosis, sitting electromyography, seated vibration, and sitting and biomechanics. Data Synthesis: In part I, papers were selected if (1) they contained a first occurrence of a sitting topic, (2) were reviews of the literature, (3) corrected errors in previous studies, or (4) had improved study designs compared with previous papers. In part II, we separated information pertaining to sitting dynamics and drivers of automobiles from part I. Results: Sitting causes the pelvis to rotate backward and causes reduction in lumbar lordosis, trunk-thigh angle, and knee angle and an increase in muscle effort and disc pressure. Seated posture is affected by seat-back angle, seat-bottom angle and foam density, height above floor, and presence of armrests. Conclusion: The configuration of the spine, postural position, and weight transfer is different in the 3 types of sitting: anterior, middle, and posterior. Lumbar lordosis is affected by the trunk-thigh angle and the knee angle. Subjects in seats with backrest inclinations of 110 to 130 degrees, with concomitant lumbar support, have the lowest disc pressures and lowest electromyography recordings from spinal muscles. A seat-bottom posterior inclination of 5 degrees and armrests can further reduce lumbar disc pressures and electromyography readings while seated. To reduce forward translated head postures, a seat-back inclination of 110 degrees is preferable over higher inclinations. Work objects, such as video monitors, are optimum at eye level. Forward-tilting, seatbottom inclines can increase lordosis, but subjects give high comfort ratings to adjustable chairs, which allow changes in position. (J Manipulative Physiol Ther 1999;22:594-609) Key Indexing Terms: Sitting; Biomechanics; Lordosis; Ergonomics; Spine; Model; Vibration; Posture; Chair Design Journal of Manipulative and Physiological Therapeutics Volume 22 • Number 9 • November/December 1999 Sitting Biomechanics Part I • Harrison et al 595 tromyographic measurements and disc pressure during seated postures, including office chairs, wheel chairs, and driver’s seats. In 1980, the Society of Automobile Engineers17 wrote a manual covering all aspects of car seats, both front and rear. Williams et al18 presented a more recent review of sitting and pain and reported that lordosis is preferred over kyphosis. In a review done especially for automobile seats, Fubini19 wrote a synopsis of safety, comfort, adaptability, practicality, solidity, and suitability. In the late 1800s and early 1900s, several design items were identified. Very early, Parow20 and von Meyer21 concluded that the ischial tuberosities were the chief points of support in the sitting position because of posterior pelvic rotation. Von Meyer21,22 stated that spinal ligaments were not in tension while sitting and that a support is required to give lumbar relaxation. He noted that straight-back chairs did not give support to the spinal column. In 1884, Staffel’s designed chair1 had a lumbar support and a space under this support for the buttocks to slide backward to effect some forward rotation of the pelvis. The debate about what constitutes a normal position of the lumbar spine during sitting (kyphosis or lordosis) occurred as early as 1911. Fick2 thought that the spine should be “ventriflexed,” whereas Staffel1 suggested that the lumbar curve in sitting should be as close as possible to its form in the standing position. Some of the issues (or variables) studied before 1950 included the following: (1) seat-bottom height, (2) seat-bottom incline, (3) seat-bottom contour, (4) seat-bottom width, (5) seat-bottom length, (6) seat-back tilt inclination, (7) seatback lumbar support, (8) seat-back height, (9) table and desk height, (10) the correct sitting posture, (11) muscle activity while seated, (12) thigh angle to trunk, (13) knee angle, and (14) footrest position. Fig 1 illustrates some of these ideas, which Keegan8 suggested for seat design, and these are numbered from most important to least important. Before Snijders et al,23 Fick2 and von Meyer22 discussed sitting with crossed legs. Before Marumoto et al,24 Spitzy25 and Schede4 discussed table height and near sightedness while sitting. Before Lord et al26 and Keegan,8 von Meyer22 and Staffel1 had written about loss of lumbar lordosis from standing to sitting. Before Coleman et al27 and Keegan,8 Staffel1 had incorporated lumbar supports on his designed chairs. Before Hooton28 and Floyd and Roberts,29 Staffel1 discussed table heights for adults and children while sitting. Before Reinecke et al30 presented lumbar support motion as a “new” strategy, Hertzberg31 had designed a pulsating seat cushion and lumbar support for the US Air Force. Let us next examine normal standing posture. Because humans are the only species on earth with an upright stance, all dynamics (after rising from a lying position) are changes from an upright posture. Thus such movements as walking, squatting, climbing, and sitting can be described as changes from the upright position. Therefore normal upright posture must be defined before sitting changes are described. Normal Standing Procedure The details of sitting posture require more than just a definition of upright human posture. Although posture alignment is of vital importance, posture provides few details of the inside spinal alignment. Because the positions of individual vertebrae are desired for a sitting spinal model, a normal upright spinal model is required to discuss spinal changes occurring in the sitting posture. Thus after discussing normal upright posture, a normal spinal model will be reviewed. Postural control is a fundamental but complex motor function, which is involved in nearly every motor task. Different theories have been developed to explain the neural organization required for sitting, standing, breathing, and movement.32 Electromyographic studies have been performed on infants to follow the evolution of sitting to standing posture.33 All authors of postural studies represent the anteroposterior view of upright stance as a true vertical alignment of centers of mass (Fig 2, A). However, in the lateral view, there is debate about which anatomic structures are aligned to a vertical gravity line (Fig 2, B-D). Normal standing posture has been defined as perfect alignment of the ear, shoulder, hip, knee, and ankle (Fig 2, B).34 In 1985, Woodhull et al35 stated that good standing posture is often idealized36 and that studies37-41 reporting the average standing posture indicate that the body center of gravity lies slightly anterior to the talus of the ankle (Fig 2, C). For different ideal posture, Kapandji42 stated that the posterior parts of the head, back, and buttocks Fig 1. Keegan’s7 1953 list of important aspects of seat design: 1, lumbar support; 2, minimum 105-degree tilt angle of backrest; 3, open space for posteriorly projecting sacrum and buttocks; 4, convex thoracic support with height to lower scapulae; 5, shoulder support at 105 degrees; 6, any adjustable tilt of seat back pivoted on a point in line with the hip joints; 7, maximum length of seat bottom (16 in); 8, seat-bottom height above floor (16 in); 9, seat bottom curved down under back of knees; 10, free space for feet under seat bottom; and 11, upward tilt of seat bottom of 5 degrees for maintenance of back against back support.