Bucket Seats: Why They Cause Pain & Discomfort
by Dennis Zacharkow, PT
© 2014
Sitting comfort studies from decades ago found that subjects judged chairs with a slight concavity to the seat surface for the buttocks more comfortable than a flat seat surface (Ridder, 1959; Grandjean et al., 1973).
Some contouring of the seat is also necessary to provide better lateral support for the driver. When making a turn without proper lateral support, flexion and rotation of the lower trunk may occur as the driver slides laterally while his shoulders are stabilized by gripping the steering wheel (Kendall & Underwood, 1968; Bulstrode et al., 1983). Proper seat contouring will help assure that the driver's posture remains as symmetrical as possible.
However, excessive seat contouring (extreme bucket seats) can have deleterious effects on pressure distribution and sitting comfort, often resulting in pain:
- Excessive seat contouring can result in distortion of the gluteus maximus muscles, which are reflected laterally off the ischial tuberosities (sitting bones) when sitting (Bennett, 1928).
- The difference in height between the weight bearing surface of the ischial tuberosities and a potential weight bearing surface of the greater trochanters of the femurs is only approximately 2.5 to 3 cm (Helbig, 1978). Therefore, when the side to side radius of the seat contour is too deep, the greater trochanters of the femurs will bear weight (Figure 1). Based on their structure and function, the trochanters are completely unsuited for supporting the body weight in the sitting position, and this will quickly lead to discomfort and pain (Helbig, 1978; Hertzberg, 1972; Hockenberry, 1977).
- As a result of the distortion from an extreme bucket seat, the femurs will have a tendency to internally rotate (Bennett, 1928; Travell, 1955; Diffrient, 1984). As a result of this internal hip rotation, the greater trochanter will move superiorly. Therefore, the sciatic nerve will be exposed to pressure just lateral to the ischial tuberosity, where it is normally protected within the depth of the ischio-trochanteric gutter between the greater trochanter and ischial tuberosity (Le Floch, 1981). This can be an overlooked cause of sciatica and leg pain.
Suggestions to reduce the excessive concavity of an extreme bucket seat
- By preventing excessive backward tilting of the pelvis, YogaBack's adjustable sacral support will minimize the sinking of the buttocks into the bucket seat, along with preventing the coccyx (tail bone) from weight bearing on the seat.
- Add a piece of medium firm to firm foam over the buttocks region of the bucket seat. The thickness required of the foam will vary based on the degree of concavity of the bucket seat, and also on the driver's sitting anatomy.
References
- Bennett, H.E.: School Posture and Seating. Boston, Ginn and Company, 1928.
- Bulstrode, S., Harrison, R.A., and Clarke, A.K.: Assessment of Back Rests for Use in Car Seats. DHSS Aids Assessment Programme, Health Publications Unit, Lancashire, United Kingdom, 1983.
- Diffrient, N.: The Diffrient Difference. Leading Edge, 5:41-59, June 1984.
- Grandjean, E., Hünting, W., Wotzka, G., and Schärer, R.: An ergonomic investigation of multipurpose chairs. Human Factors, 15:247-255, 1973.
- Helbig, K.: Sitzdruckverteilung beim ungepolsterten sitz. Anthropologischer Anzieger, 36: 194-202, 1978.
- Hertzberg, H.T.E.: The Human Buttocks in Sitting: Pressures, Patterns, and Palliatives. Society of Automotive Engineers, publication no. 720005, 1972.
- Hockenberry, J.: Seating design for worker efficiency. Furniture Design and Manufacturing, 49:42-46, December 1977.
- Kendall, P.H., and Underwood, C.S.: Seats and sitting. Occupational Therapy (London), 31:20-29, 1968.
- Le Floch, P.: Les conditions anatomiques des positions assises. Bulletin de L'Association des Anatomistes, 65:447-457, 1981.
- Ridder, C.A.: Basic Design Measurements for Sitting. Agricultural Experiment Station, University of Arkansas, Fayetteville, Bulletin 616, October 1959.
- Travell, J.: Chairs are a personal thing. House Beautiful, 97: 190-193, 262-266, 269, 1955.