Shoulder strength of females while sitting and standing as a function of hand location and force direction Amy Y. Chow, Clark R. Dickerson * Department of Kinesiology, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1 article info Article history: Received 29 August 2008 Accepted 14 December 2008 Keywords: Shoulder Strength capability Female workers abstract This study evaluated single-handed isometric push strength capabilities of females working at or above- shoulder level. We examined the influence of force exertion direction (vertical, horizontal and lateral), angle of shoulder flexion from horizontal (0  , 30  , 60  and 90  ) and gross body posture (standing and sitting), on maximal volitional shoulder strength. Force exertion direction had the greatest affect on shoulder strength (p < 0.0001). Strength was greatest in the vertical axis pushing downwards and weakest in the horizontal plane pushing forwards. Angle influenced shoulder strength when considered together with direction (p < 0.0001). However, these effects were dominated by direction results. Marginal differences in strength existed between sitting and standing (p > 0.05). These results can be used to design workspaces that consider individual strength limitations and their dependence on force direction, work orientation, and gross body posture. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Above-shoulder work, coupled with single-handed forceful exertions, often witnessed in automobile assembly, may result in cardiovascular strain and unsafe musculoskeletal loads (Haslegrave et al., 1997a). Depending on the workplace layout, workers stand, sit, kneel or lie, while adopting awkward postures to work above the shoulders. Thus, it is important to determine how posture and the direction of push force exertions limit the force capabilities of workers for many potential situations. These include tasks involving the use of non-powered hand tools or when only one hand is avail- able to install a part as the other hand is holding the part or a tool. Early studies have identified several negative modifiers of shoulder strength. Isometric muscular strength of females is on average two-thirds that of males (Rohmert and Jenik, 1971). A further distance from the body to the point of hand exertion has also been shown to reduce strength capability (Haslegrave et al., 1997a). Strength is further reduced with the use of a single hand or when adopting non-neutral trunk postures to reach the work piece (Coury et al., 1998; Warwick et al., 1980). These findings suggest that strength might be more consistent and useful for biome- chanical models if measured while controlling body configuration. However, small postural constraints, such as foot placement, have a large effect on the ability to exert force (Haslegrave et al., 1997a). For overhead exertions, mean strength values are generally lower when lying supine compared to standing (Haslegrave et al., 1997a). The marginal differences in horizontal force exertions while standing and lying supine indicated that it makes little difference whether forces are reacted through the feet or back, however, standing yielded stronger vertical exertions. The ability to exert maximum force is reduced with closer reaches, except when lying supine, where horizontal maximum push force increases as reach distance is reduced (Haslegrave et al., 1997a). While kneeling, force capability is affected by reach distance and direction of exertion, but not by work height (Haslegrave et al., 1997b). It is also sug- gested that kneeling on two knees will yield lower forces than kneeling on one knee (Haslegrave et al., 1997b). Several studies have observed trends in strength with respect to arm posture and push direction. Lifting and pressing yield the highest forces, lateral pushes yield the lowest (Haslegrave et al., 1997a). In measuring isometric shoulder adduction, strength was greater with the trunk in a neutral position and the handle closer to the body (Coury et al., 1998). Conversely, in the assessment of overhead work at three heights to determine maximum upward push forces, force capability increased as height increased (Sood et al., 2007). However, electromyography measures were unable to show any significant correlation between push force and task height. Nijhof and Gabriel (2006) studied the relationship between an isometric force at the hand and the force direction within the horizontal plane. Maximum forces occurred in a direction approximately along the line connecting the glenohumeral joint and hand, while minimum forces were those directed * Corresponding author. E-mail address: cdickers@uwaterloo.ca (C.R. Dickerson). Contents lists available at ScienceDirect Applied Ergonomics journal homepage: www.elsevier.com/locate/apergo 0003-6870/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apergo.2008.12.003 Applied Ergonomics 40 (2009) 303–308