The Scandinavian Journal of Rehab. Medicine 15:197-203, 1983. “Posture of the Trunk when Sitting on Forward-Inclining Seats,” written by Tom Bendix and Fin Biering-Sorensen from The Laboratory for Back Research, University of Copenhagen, Denmark:
Forward Inclination & Lumbar Lordosis
The above-mentioned study states: “Changes in posture during one hour of sitting were measured by a statometric method on 10 subjects. Four seats were used, one horizontal and three with forward inclinations respectively of 5 degrees, 10 degrees, and 15 degrees. With increasing forward inclination of the seat, the spine moved toward lumbar lordosis.”
Note: Lumbar lordosis is the natural curve of the lumbar spine explained as increased curving of the lumbar spine (which can become flattened when leaning forward while seated on a flat surface, called kyphosis).
Body’s Adaption to Seat Inclination
“A supplementary sample showed that 1/3 of the body’s adaptation to the seat inclination took place in the spine and 2/3 in the hip joints. A tendency to a more vertical position of the trunk as a whole was observed on the 5 degree chair but the posture of the cervical (neck) spine was not influenced by the seat inclination. . .a comfort evaluation showed the 5 degree forward inclination and the horizontal seats to be preferred.”
Previous Studies & Backwards Inclination
The above-mentioned study goes on to discuss the findings of previous studies: “Many investigators (Akerblom, B., 1948; Grandjean, E., 1975; Keegan, J., 1953; Ollefs, H., 1951; Schubert, H., 1962) recommend that the seat surface should be inclined about 5 degrees backwards; one has even suggested 15 degrees backwards, (Rizzi, M., 1969). Others suggest that the seat should be almost horizontal, (Kroemer, K. H. E., 1971; Peters, T., 1969); and in some committees (Comite Europeen de Normalisation, 1979; Engdahl, S., 1971; ISO: Draft International Standard ISO/DIS 5970), a range from 0 degrees to 4 to 5 degrees backwards had been suggested.”
Previous Studies & Forward Inclination
“Another body of opinion focuses on the possible advantages of a forward inclination of either the whole seat (Burandt, U., 1969; Drescher, E.W., 1929; Laurig, W., 1969; Mandal, A.C., 1970 and 1981; Schlegel, K.F., 1940; Staffel, F. 1884), the posterior part (Burandt, U. & Grandjean, E., 1964; Schneider, H.J., et al., 1961), or the anterior half, (Jurgens, H.W. 1969).”
“Mandal suggests a tiltable seat from -5 degrees (backwards) to +15 degrees (forward); and Kroemer (1971) suggests an adjustable seat slope between -6 degrees (backwards) and +6 degrees (forward), to make it possible to changes the position for different tasks.”
Purpose of Study
“The purposes of the above-mentioned study were:
1. “To compare spontaneously chosen posture when sitting for one hour on each of the four seat inclinations — horizontal, 5 degrees, 10 degrees, and 15 degrees forward — and to estimate the adaptability of the trunk and hip joints to different forward inclinations of the seat.
2. “To follow the changes of the spinal curves during one hour of sitting on one seat.
3. “To evaluate comfort in relation to the different seat inclinations.”
Tiltable Office Chair Furnished to Study Subjects
“To accustom the subjects to a forward-inclining seat, their homes were furnished with a tiltable office chair at least two weeks before the experiment. The seat could tilt from 5 degrees backwards to 15 degrees forwards . . . to ensure that the thighs conformed to the seat surface, height adjustment was effectuated by placing the seat at first a little too high, with the legs hanging freely, and later lowering it until the feet rested on the floor with the lower part of the legs vertical. Approximately 2/3 of the thighs were resting on the seat.
Preference for Horizontal & 5 Degree Inclination
“At the end of the period of sitting, all subjects were asked to estimate the degree of comfort on each specific seat inclination, using a scale from 1 (poor) to 5 (excellent.). The same scale was used to rate the tillable chair they had used for two weeks in their home. The comfort evaluation that was done in the above-mentioned study, “shows a preference for the 0 degree and the 5 degree inclinations.”
Study Evaluation of Tiltable Seat
“The corresponding comfort evaluation of the office chair with the tiltable seat, which the subjects had in their homes, was of the median 3.5 (5 execellent to 1 poor) (range 1-5).”
Increasing Inclination = Advantageous for Lordosis
“With increasing seat inclination forward, the spine changed towards lumbar lordosis. Almost all authors claim such a change as an advantage when sitting, (Akerblom, B., 1948; Burandt, U. & Grandjean, E., 1964; Jurgens, H.W., 169; Schlegel, K.F., 1940; Schneider, H.J. & Lippert, H., 1961; and Snorrason, E., 1955; Staffel, F., 1884).”