A 1983 survey of hand tool related injuries in the United States (Meyers & Trent, 1988) showed that of 129 399 cases, 22% were associated with powered hand tools. Recent data indicate that injuries associated with hand tools accounted for 4.5% (74 830) of the total recorded work-related injury cases in the United States (Bureau of Labor Statistics, 2002). Among them, power hand tools accounted for 23.8% (17 852) of the cases, which involved being "struck by" or "struck against" and "overexertion."
Nutrunners are power hand tools that are widely used for securing threaded fasteners in manufacturing and assembly operations, such as in the automotive, mechanical equipment, and electronics industries. Radwin, VanBergeijk, and Armstrong (1989) conducted a study using electromyography (EMG) as an index of muscle effort during pneumatic shut-off nutrunner operation. They found that the EMG activity during threaded fastener torque buildup was affected by tool torque output and torque buildup time. EMG activity during torque buildup was more than three times greater than it was during preparation and shutoff.
Oh and Radwin (1998) observed that the operator initially overcomes the tool reaction force with a concentric muscle exertion. As the force rapidly rises, however, the tool eventually overcomes the operator, causing the motion in opposition to muscle contraction, resulting in an eccentric muscle exertion. This torque reaction effect on muscle length is demonstrated in Figure 1. During an eccentric, or lengthening, contraction the muscle acts like a spring, producing proportionally more force as it lengthens because of passive properties of the muscle.
[FIGURE 1 OMITTED]
Force in eccentric contractions is generally greater than in isometric or concentric contractions (Griffin, 1987; Walmsley, Pearson, & Stymiest, 1986). Furthermore, the physiological cost, as well as perceived exertion, is often less for eccentric contractions as compared with other types at similar intensities (Henriksson, Knuttgen, & Bonde-Peterson, 1972; Pandolf, 1977; Rasch, 1974; Stauber 1989). Repeated eccentric contractions may therefore have negative consequences, including muscle soreness (Dolezal, Potteiger, Jacobsen, & Benedict, 2000; Komi & Buskirk 1972; Talag, 1973) and muscle damage (Boppart et al., 1999; Brown, Child, Day, & Donnelly, 1997; Clarkson & Sayers, 1999; Dolezal et al., 2000). The magnitude of an eccentric exertion is proportional to the magnitude of the tool handle force, velocity, and displacement. Armstrong, Warren, and Lowe (1995) suggested that several mechanical factors corresponding to eccentric contractions, such as high levels of force and velocity, contribute to the initiation and early stages of contraction-induced microinjury in muscles for repetitive skeletal muscle loading.
Psychophysical experiments (Freivalds & Eklund, 1993; Kihlberg, Kjellberg, & Lindbeck, 1993; Kihlberg, Lindbeck, & Kjellberg, 1994) have shown that power hand tool handle displacement response to the torque reaction force is highly correlated with subjective ratings of discomfort. Kihlberg, Kjellberg, and Lindbeck (1995) tested four right-angle nutrunners with target torque levels of 50 and 75 Nm. They concluded that to be accepted by 90% of the operators, the tools should produce handle displacement responses less than 3 cm.
Biomechanical models of the human hand and arm were developed in order to better understand the effects of continuous periodic vibration produced by power hand tools (Fritz, 1991; Louda & Lukas, 1977; Reynolds, 1977; Reynolds & Soedel, 1972; Wood, Suggs, & Abrams, 1978). Those models considered the human operator as combinations of passive springs, masses, and dampers. These empirical models, however, were not applicable to nutrunners of screwdrivers because the mechanical elements were calculated for continuous vibration input in the range of 20 to 2000 Hz, which is much greater than those encountered for impulsive reaction forces in these power hand tools. …