تست کارایی کمک ارگونومیک برای بلند کردن اجسام در کاهش خستگی عضلانی در زنان بیش از یک دوره طولانی مدت از بلند کردن

A personal lift assist device (PLAD) was designed with passive elastic elements that act with a similar line of action to the spine muscles and reduce the extension moment experienced during lifting activities. The purpose of this paper was to evaluate the device's ability to reduce fatigue during a repetitive lifting task. Women (n = 12) lifted a box load representing 20% maximal extensor strength repetitively (12 lift/lowers per minute) for 45 min while electromyography (EMG) was recorded from the lumbar and thoracic erector spinae, and cardiovascular measures were monitored. Subjects were also tested on strength and endurance tests prior to, and after lifting. The increase in EMG RMS amplitude from the start until the end of the lifting session was significantly lower when wearing the PLAD for the TES (91% vs 3%) and the LES (104% vs 16%). The median frequency (MF) drop was also significantly lower when wearing the PLAD for TES and LES. The PLAD delayed the onset of fatigue in women by requiring less muscular effort. Relevance to industry There are numerous industries that still require repetitive manual materials handling tasks to be performed by humans. Repetitive lifting fatigues the musculature involved and may lead to an increased risk of injury. The PLAD reduced fatigue on several measures. This device appears to have potential for industries where women perform repetitive, fatiguing lifts.

The prevalence of musculoskeletal disorders (MSD) and low-back pain (LBP) in industrial workplaces has been recognized as a significant cost to companies, society and the health care system (Andersson, 1997 and Woolf and Pfleger, 2003). In an effort to reduce the number of lost days due to manual materials handling (MMH) related injury (and the associated costs), companies often decide to implement mechanized lift systems as a control strategy meant to alleviate the physical demands present in repetitive tasks. Already of significant cost, mechanized lift systems can be quite limited in terms of versatility as they are typically customized to the workstation, and inherently feature a fixed range of motion (ROM). For these reasons, an assistive device that could be worn on-the-body may serve as a viable option for offloading the erector spinae muscles during MMH tasks; ultimately reducing operator physical demands, regardless of workstation location and/or adopted posture. Throughout the last several years, we have attempted to refine such a technology within our laboratory. As a result, the Personal Lift Assist Device (PLAD) has undergone numerous design iterations but the basic concept has remained unchanged. The PLAD is designed to assist the erector spinae musculature in MMH tasks and static postures by utilizing elastic energy that is stored during trunk/knee flexion and returned during subsequent trunk/knee extension. Two elastic elements are attached to shoulder straps at the upper ends. The lower ends are attached to cables that travel to the feet via an attachment at the knee and pulleys that offset the moment arm at the pelvis by 15 cm. The laboratory prototype (Fig. 1) used in the current paper has recently been modified for use in acceptability trials within the automotive industry.

Fatigue of skeletal muscle has been shown to cause reliable and predictable changes to EMG signal characteristics such as decreases in the MF and increases in RMS amplitude (Basmajian and DeLuca, 1985, DeLuca, 1997, Mannion and Dolan, 1994 and Potvin and Bent, 1997; Ng and Richardson, 1996). When wearing the PLAD, lifters experienced minimal RMS amplitude increases at both erector spinae sites in addition to less severe drops in median frequency throughout the 45 min. Without the PLAD, the onset of muscular fatigue was apparent after 10 min in some cases. While these results are encouraging, the next phase of research should consider whether the PLAD can prevent and/or minimize fatigue over a period of time more representative of an entire workshift. Females demonstrated a differing response to long-term lifting than previous work (Potvin and Norman, 1993; Lotz et al., in press). Females in this study demonstrated more fatigue in the lumbar spine than males did in the Lotz et al., (in press) study as measured by larger relative RMS amplitude increases and greater MF decreases during the No PLAD session. More fatigue in the lumbar spine may indicate that females used a compensatory lifting strategy for the prolonged lifting trial. Kinematic data may be able to elucidate whether they tended to rely on a more stooped posture, which would preferentially activate the LES rather than the TES. Further EMG analysis may reveal that some other muscle group (such as the arms or legs) was less able to tolerate the repetitive lifting and resulted in a reliance on the large spinal column muscles. The TES muscle also experienced greater drops in relative MF for females in the No PLAD condition compared to the response observed in our previous study involving males. Despite the manifestation of localized fatigue, females also benefited from a relatively greater amount of assistance from the PLAD regarding activation of the TES musculature. The PLAD yielded TES RMS increases of 22% for males vs 3% for females, compared to No PLAD increases of 91% and 104%, respectively. Although the box lifting task elicited a greater amount of fatigue within female's lumbar musculature, the data suggest that the PLAD was more effective at reducing the fatigue-inducing demands placed on the upper thoracic muscles. In terms of cardiovascular response, the PLAD was unable to significantly affect female's perceived effort or heart rate response. If perceived effort is related to lumbar muscle fatigue rather than thoracic muscle fatigue, then greater levels of fatigue in the LES combined with a smaller effect of the PLAD on LES would account for higher ratings of perceived effort across the work task. The fact we used a common band stiffness for the PLAD that was not customized to individual body weight may explain why females felt little reduction in perceived effort, as this may have required smaller subjects to excessively utilize their trunk flexors in order to stretch the PLAD elastic elements. If the elastic elements were too stiff, it would result in greater effort required to load the device during the down phase of the lift, thus counteracting any physiological benefit observed in the trunk extensors. The plausibility of this explanation is to be further investigated through subsequent examination of the EMG recorded during the lifting trials. The isometric back extensor strength was found to be significantly higher following the lifting session featuring use of the PLAD compared to the session in which the PLAD was not worn. Furthermore, there was no difference between PLAD and No PLAD sessions in terms of back endurance. These results differ from a previous study completed on males (Lotz et al., in press) whereby the differences between No-PLAD and PLAD sessions were statistically significant for back endurance but not for isometric back strength. The literature has numerous examples of females showing superior back endurance over males (Miller et al., 1993 and Maughan et al., 1986) but they frequently have less back strength (Miller et al., 1993, Mannion et al., 1997 and Mannion et al., 1998). Results from our two studies match the existing literature quite well with women showing endurance times lasting twice as long as males in the unfatigued state (160 s vs 91 s) and similarly, less strength than the males (252 Nm vs 369 Nm). Large coefficients of variation (CV) (62% in the No-PLAD condition and 54% in the PLAD condition) for the endurance test suggest that the fatiguing protocol and relative effect of the PLAD was highly variable depending on the women's individual characteristics. The men demonstrated a more consistent response to the endurance test indicating that the PLAD device did not have the same variable effect on performance (Lotz et al., in press). These results may explain why the effect of the device was not found to be significantly different in the PLAD vs No-PLAD condition for the women as was found in the men's study (Lotz et al., in press). It is unreasonable to think that eliminating all ergonomic risk factors in occupational workplaces is attainable. The PLAD appears to differentially improve noted sex-based limitations to prolonged work by improving back endurance scores in males and back strength scores in females. Thus, a device like the PLAD may be able to manage a worker's exposure to each risk factor and may also be successful at preventing tissues from reaching the injury threshold. The potential of the PLAD in reducing risk factor exposure is contingent on the assumption that additional injury risk scenarios are not created at the expense of minimizing fatigue (e.g. alter lift kinematics and coordination in a deleterious manner). Several limitations concerning the tested population should be addressed. Female subjects recruited to this study had to be a minimum of 165 cm so that the PLAD would fit properly. In addition, they had to be fit enough to lift 20% of their maximum back extensor strength repetitively for 45 min. This population of subjects may not represent the average working female in manual material handling jobs. However, the successful acceptance of the PLAD device in the field will also hinge on a personalized fit and proper choice of tension in the elastic bands. Further research needs to be done to determine the ideal amount of tension in the elastic bands for a variety of working populations. In terms of fit and function, there was a tendency for the hip strap on the PLAD to slip upwards and over the female subject's iliac crests during the forward bend phase, eliminating the tension generated in the elastic bands and negating any possible energy return in the up phase. Many females also complained of substantial levels of pain deep in the upper trapezius muscles. A field-worthy design will need to address these gender differences. Despite being able to identify several areas of the device that required improvement, over half the participants indicated that they would use the PLAD during workplace lifting tasks. The EMG and cardiovascular fatigue indicators suggest that this lifting task was fatiguing for females and that the PLAD was able to significantly reduce the level of fatigue across a 45-min lifting session. Further areas of investigation should address how the PLAD affects movement patterns and should also identify ideal elastic stiffness and personalized fit.