PII: SOOO34870(96)0002~9 Applied Er~onomrc.s zyxwvutsrqponmlkjihgfedcb Vol 27. No. S. pp. 31%3211, IYYh C o p yrig ht @ I IYYC , Else vie r Sc ie nc e I.td Printe d in G re a t Brita in. A ll rig ht> re se we d lNXl.~7llM SIO.(X) + O.lWl ELSEVIER Ergonomic investigation of letter-carrier satchels: Part II. Biomechanical laboratory study C. Joe Liin,* Patrick G. Dempsey,?* James L. Smith,? M. M. Ayoub’ and Tracey M. Bernard” zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF *Chung Yuan Christian University, Department of Industrial Engineering, Chung Li, Taiw an ‘Liberty Mutuar’ Research Center for Safety & Health, Hopkinton, MA 01748, USA ‘M urray State University, Department of Occupational Safety & Health, Murray, KY 42071, USA (Received IO May 1995) A biomechanical investigation of four satchels designed for the purposes of manually carrying and delivering mail was conducted. Twenty United States Postal Service carriers participated in the study. The satchels differed primarily in the presence/absence of a waist belt, the number and design of the shoulder straps, and the number of pouches. The biomechanical analyses were comprised of postural (shoulder and hip) deviation measurements, estimated compressive forces at the L& joint, soft tissue pressure on the shoulder, spinal torsion during mail retrieval, force distribution between the feet, and an anthropometric evaluation. The results indicated that a satchel with fwo shoulder straps and two pouches was more desirable than the single-pouch satchels from a biomechanics standpoint. Copyright @ 1996 Elsevier Science Ltd. Keywords: musculoskeletal disorders, biomechanics, satchel design Introduction Manually carrying and manipulating mail satchels can cause deviations from natural postures, especially affecting the center of gravity of the body. Most carriers utilizing satchels assume postures that involve the shoulders and hips as the weight-bearing structure. Use of the satchels can lead to postural discomfort and pain in the neck-shoulder area and lower back, as well as in other anatomical structures (Murray and Miller, 1984). Letter carriers have been found to have a significantly greater prevalence of joint problems than other postal workers, particularly in the shoulder joint (Wells et al, 1983). Letter carriers have reported rates of shoulder and low-back discomfort as high as 76% and 70% respectively (MIurray and Miller, 1984). There are two major anatomical structures that are of particular concern when considering postural problems associated with mail carriage: (1) the shoulder girdle complex, which is the position the satchel strap is often placed; and (2) the sacroiliac joint (Murray and Miller, 1984). The shoul’der girdle is the site of direct force application from the satchel shoulder strap, and the increased sacroiliac jtoint forces are a result of this force being transmitted through the skeletal system to the supporting structure of the pelvic girdle. $Author to whom correspondence should be addressed 315 The spinal loading resulting from single-strap satchels, which results in the spine being loaded asymmetrically, would also be expected to be larger than the spinal loading resulting from symmetric loading, such as with satchels with two pouches and two straps. Additionally, the asymmetric loading with a single-strap satchel results in greater fatigue compared to either a satchel with two pouches and two straps, or to a satchel with a single pouch and a waist belt (Bloswick et al, 1994; Gerber et al, 1992). The level of discomfort, pain and potential musculo- skeletal disorders can be controlled at the satchel design stage by minimizing postural deviations and asymmetric loading. There is a need to provide carriers with satchels that result in less musculcskeletal stress than satchels currently in use (Gerber et al, 1992; Murray and Miller, 1984). A primary biomechanical goal of any satchel design would be symmetric loading to reduce shoulder, torso and lumbosacral stresses. Symmetric loading is also beneficial to other joints such as the hips, knees and ankles. The study reported here complements a parallel study of field use of satchels by providing detailed biomechanical analyses of the designs. The bio- mechanical analyses selected for this study were comprised of postural (shoulder and hip) deviation measurements, estimated compressive forces at the Ls/SI joint, soft tissue pressure on the shoulder, spinal torsion during mail retrieval, force distribution on the feet and an anthropometric evaluation of the four