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34 ReferencesThe use of biomechanical measures in the investigation of changes in lifting strategies over extended periods
Abstract
The experiment presented here was conducted for the purpose of investigating changes in lifting patterns that occur due to the effects of learning and due to the effects of lifting over extended periods. It also illustrated the use of biomechanical measures in tracking these changes. Biomechanical measures usually have not been used to make interences about changes in lifting patterns due to such effects as practice, the number of lifts performed in a session, the frequency of lifting, etc. Six inexperienced subjects were asked to lift for an initial 4-hour session, six half-hour practice sessions, and a final 4-hour lifting session. All lifts were conducted at the rate of one lift per minute, from floor to shoulder height, with a weight chosen by the subject. Results indicated that there is a change from a leg lift to a more biomechanically stressful back lift (as inferred from greater joint center moments) due to the effects of training, but once these effects are observed they were maintained through the second 4-hour session. Furthermore, reduction in time per lift over the course of the experiment was accompanied by increases in moments at all of the joint centers, but this was more pronounced in the hip and knee joints for half of the subjects. Conclusions to be drawn from these results are that biomechanical modeling can be used under such circumstances with meaningful results and that there is a natural tendency to lift more quickly and to shift moments to the stronger muscles in the process of learning lifting patterns.
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- The age-related differences in the optimal combinations of the performance measures in the MOO approach may help better understand the distinctions between younger and older adults regarding their motor control mechanisms during lifting. The physical effort is closely associated with the energy expenditure during lifting tasks (Fogleman and Smith, 1995); while minimizing load motion jerk during lifting is related with decreased spinal compressive forces (Hsiang and McGorry, 1997). For the younger adults, the optimal weightings of the physical effort and load motion jerk are 0.9 and 0.1, respectively, which indicates that minimizing energy expenditure are the dominant objective when planning lifting motions.
[Show abstract] [Hide abstract] ABSTRACT: In this study, a novel lifting motion simulation model was developed based on a multi-objective optimization (MOO) approach. Two performance criteria, minimum physical effort and maximum load motion smoothness, were selected to define the multi-objective function in the optimization procedure using a weighted-sum MOO approach. Symmetric lifting motions performed by younger and older adults under varied task conditions were simulated. The results showed that the proposed MOO approach led to up to 18.9% reductions in the prediction errors compared to the single-objective optimization approach. This finding suggests that both minimum physical effort and maximum load motion smoothness play an important role in lifting motion planning. Age-related differences in the mechanisms for planning lifting motions were also investigated. In particular, younger workers tend to rely more on the criterion of minimizing physical effort during lifting motion planning, while maximizing load motion smoothness seems to be the dominant objective for older workers. Relevance to industry: Lifting tasks are closely associated with occupational low back pain (LBP). In this study, a novel lifting motion simulation model was developed to facilitate the analysis of lifting biomechanics and LBP prevention. Age-related differences in lifting motion planning were discussed for better understanding LBP injury mechanisms during lifting.- When muscles became fatigued, the tendency of replacing a leg-lift strategy with a back-lift strategy was observed (Puniello et al., 2001; Sparto, Parnianpour, Reinsel, & Simon, 1997; Trafimow, Schipplein, Novak, & Andersson, 1993; Zhang & Buhr, 2002), and lifting dynamics altered significantly as well (Chen, 2000). Fogleman and Smith (1995) investigated changes in lifting patterns made by the participants practicing during a prolonged period of time, and their investigation indicated a change from a leg lift to a back lift. More recent work, by Marras, Davis, Ferguson, Lucas, and Gupta (2001), revealed that patients with low-back pain tended to limit flexion and motion in an attempt to reduce spine loading, although they failed to do so because of significantly increased muscle coactivity.
[Show abstract] [Hide abstract] ABSTRACT: This study investigated whether relative strength between the back and knees can differentiate and predict lifting strategy and the effects of gender, load magnitude, and knowledge of strength on the strategy. Although muscular strength is thought to play a vital role in the mechanics of lifting, how localized joint strengths and their relations influence lifting strategy remains unclear. Thirty-two participants (16 men and 16 women) underwent isokinetic strength tests and were then divided into two groups: one provided with the knowledge of their strength test results and the other not. They subsequently performed the same set of simulated lifting tasks while their lifting kinematics were being recorded. Postural indices to quantify the lifting strategies were derived from the kinematic data. The ratio of back strength versus total knee strength and gender had significant effects on measures quantifying the lifting strategy. A statistical model incorporating gender, strength, and anthropometry achieved an R2 value of .64 and predicted correctly 76% of lifting strategies used by individual participants. Individuals with back strength greater than their total knee strength tended to use a back-preferred lift strategy, and vice versa, suggesting that muscular strength is a determining factor of lifting strategy. Application: An emphasis on additional knee strengthening in a training program may change the tendency of using and overstressing the back. Application of modeling and simulation technology for ergonomics design can be enhanced by more individually and accurately specified lifting strategies based on anthropometry and strength profiles.- Although squat lifting is generally recommended over stoop lifting (Anderson and Chaffin, 1986), an asymmetric squat lift is not a simple task, and even symmetric squat lifting technique is not always adopted in practice, as it involves higher energy expenditure (Zhu and Zhang, 1990) and subjective rate of discomfort (Straker and Duncan, 2000). Fatigue has also been found to develop more rapidly in squat lifting (Hagen et al., 1993), resulting in a shift from squat to stoop lifting during repetitive lifting tasks (Fogleman and Smith, 1995), and for these reasons only stoop lifting is considered in this study.
[Show abstract] [Hide abstract] ABSTRACT: Sudden changes in load during asymmetric lifting may be associated with a particularly high risk of loss of balance and spinal injury. Centre of pressure (COP) motions and electromyographic responses of trunk and lower limb muscles were studied in 10 normal male volunteers during sudden release of 20, 40, 60 and 80 N stoop lifting loads in symmetric and asymmetric postures. Similar overall COP responses and muscular response strategies to sudden release of load were seen in both postures, although the asymmetric posture showed a larger medio-lateral COP displacements and greater co-contraction asymmetries. While sudden release of load in asymmetric stoop lifting does not seem to involve a greater risk of fall than symmetric lifting, the muscular response results in more complex and asymmetric loading of the trunk, indicating greater localised segmental loading and therefore increased risk of tissue injury. r- In addition, when extending the investigation of muscle function to dynamic tasks, the biomechanics of motion plays a crucial role. Compensatory mechanisms might occur as a product of the progression of fatigue during the task [5], [8], [10], [28], [36], [37]. Such changes in the biomechanics of motion may be interpreted as " strategies, " i.e., ways to facilitate the task when it becomes more challenging to the subject because of the progression of fatigue [5], [27].
[Show abstract] [Hide abstract] ABSTRACT: The analysis of surface electromyographic (EMG) data recorded from the muscles of the back during isometric constant-force contractions has been a useful tool for assessing muscle deficits in patients with lower back pain (LBP). Until recently, extending the technique to dynamic tasks, such as lifting, has not been possible due to the nonstationarity of the EMG signals. Recent developments in time-frequency analysis procedures to compute the instantaneous median frequency (IMDF) were utilized in this study to overcome these limitations. Healthy control subjects with no history of LBP (n = 9; mean age 26.3 +/- 6.7) were instrumented for acquisition of surface EMG data from six electrodes on the thoraco-lumbar region and whole-body kinematic data from a stereo-photogrammetric system. Data were recorded during a standardized repetitive lifting task (load = 15% body mass; 12 lifts/min; 5-min duration). The task resulted in significant decreases in IMDF for six of the nine subjects, with a symmetrical pattern of fatigue among contralateral muscles and greater decrements in the lower lumbar region. For those subjects with a significant decrease in IMDF, a lower limb and/or upper limb biomechanical adaptation to fatigue was observed during the task. Increases in the peak box acceleration were documented. In two subjects, the acceleration doubled its value from the beginning to the end of the exercise, which lead to a significant increase in the torque at L4/L5. This observation suggests an association between muscle fatigue at the lumbar region and the way the subject manipulates the box during the exercise. Fatigue-related biomechanical adaptations are discussed as a possible supplement to functional capacity assessments among patients with LBP.- [Show abstract] [Hide abstract] ABSTRACT: One of the most critical risk factors contributing to injury of the low back is posture, specifically of the torso. Though considerable time and expense is directed towards training workers to use postures which minimize biomechanical stress, there is a growing body of evidence suggesting that this training may not be effective over shifts of extended duration. This may be due to a combination of fatigue and loss of concentration. Ten healthy subjects who had been previously trained in proper lifting technique lifted a load of 120% of their maximum acceptable weight of lift (MAWL) at a frequency of three lifts per minute for two hours or until they were no longer able to continue due to fatigue, whichever came first. Subjects' self-selected postures were quantified in terms of maximum torso flexion and the distance between the hands and the L4/L5 spinal disc. Changes in posture were evaluated as a function of time, heart rate and perception of fatigue.
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