Quantification of Wrist Motions during Scanning

A laboratory study was performed to help assess the risk of cumulative trauma disorders (CTDs) associated with the use of scanners in the grocery store environment. In this study experienced and inexperienced cashiers scanned a set of 12 standard grocery items using 19 different combinations of scanners, scanner orientations, and check stands. The motion characteristics of both wrists in three-dimensional space were documented and used as dependent measures of performance. These motions were compared with wrist motion benchmarks of high- and low-risk wrist accelerations. It was found that, in general, scanning motions are of sufficient magnitude to contribute to CTDs of the wrist. It was also found that wrist motion characteristics were greatly influenced by the different combinations of scanners, scanner orientations, and check stand designs. It was concluded that the "front-style" check stand minimizes potentially injurious wrist motions because it permits the checker to split the scanning task between the two hands. The type of scanner and scanner orientation that minimized potentially injurious wrist motions was much more unique to the individual workstation condition. Additionally, it appears that scanners perceived by the checkers as needing fewer wrist deviations, such as those with slanted windows, also minimize wrist motions. The implications of these findings for the ergonomic design of the workplace are discussed. INTRODUCTION The reported incidence of cumulative trauma disorders (CTDs) has grown dramatically over the past several years. The Bureau of Labor Statistics reported that in 1981, 18% of occupational illnesses were attributable to cumulative trauma, whereas in 1991 this figure had grown to 62%. This trend also has surfaced in the grocery retail industry. For example, Buckle, Stubbs, and Baty (1986) reported that 56 percent of supermarket employees complained regularly of musculoskeletal discomfort. Canadian supermarket studies by Wallersteiner (1981), and Stoffman and Sterling (1983) as well as studies in the United States by Margolis and Kraus (1986) and Rosenstock, Barnhart, Longstreth, Mason, and Heyer (1985) have all reported increases in musculoskeletal complaints of checkout personnel. These studies indicated that CTD complaints have increased with the introduction of scanners into the check-out operation. The National Institutes of Occupational Safety and Health (NIOSH) recently performed a health hazard evaluation of workers in supermarkets (Baron, Milliron, Habes, and Fidler, 1991; Orgel, Milliron, and Frederick, 1990) and found an increased risk of developing carpal tunnel syndrome (CTS) if the checker had worked at the job for 10 or more years or had worked more than 25 hours per week. Risk Factors Wrist posture, repetition, tendon force, and wrist acceleration are the four major contributors to CTDs. Wrist posture has frequently been cited as a risk factor for CTS and CTDs (Alexander and Pulat, 1985; Armstrong, 1983, 1986a, 1986b; Armstrong and Chaffin, 1979a, 1979b; Armstrong, Foulke, Joseph, and Goldstein, 1982), tenosynovitis, and De Quervain's disease (Armstrong, 1983). Few researchers have quantified "how much" wrist deviation exposes a worker to CTDs; however, the suggested association between wrist posture and CTDs has been explained biomechanically by Tichauer (1978) and Armstrong and Chaffin (1979b). Repetition also has been cited as a risk factor by Silverstein and colleagues, (Silverstein et al., 1985; Silverstein, Fine, and Armstrong, 1986, 1987), who conducted two epidemiological studies. Unlike static wrist posture, repetition involves the dynamic components of angular velocity and acceleration, which could contribute to the risk for CTS and CTDs. Based on Newton's second law of motion, the extrinsic muscles in the forearm must exert a force proportional to the angular acceleration (rotational inertia) of the hand. …