Model for static lifting: relationship of loads on the spine and the knee

Estimation of Lifted Weight and Produced Effort through Perception of Point-Light Display

It has been shown that human observers can estimate the weight of a box from the observation of a point-light display of a lifting motion. We asked observers to report the weight of the box and the effort produced by five lifters ranging in size and strength to determine if observers can perceive lifter size. In experiment 1, five or six weights from each of five lifters were shown to fourteen observers in a random order. Observers showed less error in estimating the amount of effort each lifter produced than in estimating the actual weight of the box. In experiment 2, the lifters were presented individually to forty observers to remove any effect observing a previous lift might have had on estimating the subsequent lift by a different lifter. The results showed an improvement in estimated weight but not in estimated effort. In experiment 3, the actual size of the lifters was given to thirty-four observers, and the estimations of both weight and effort improved. In experiment 4, observers did not improve when observing practice trials and estimating either only weight or only effort. The results from the four experiments suggest that observers are more sensitive to lifter's effort than to the weight lifted, and that observers tend to use changes in the velocity profile of the lift when making their estimates.

The effects of knee angles on subjective discomfort ratings, heart rates, and muscle fatigue of lower extremities in static-sustaining tasks

The purpose of this study was to investigate the effects of knee-flexion angles on subjective discomfort ratings, heart rates, and muscle fatigue using median frequency (MDF) in a static-sustaining task. Thirty healthy participants maintained 13 postures including standing, squatting, sitting, and kneeling postures and then MDFs from the erector spinae, biceps femoris, vastus medialis, gastrocnemius, and tibialis anterior muscles, subjective discomfort, and heart rates were collected every 3 min during a sustained 15 min task. Results showed that the discomfort, heart rate and muscle fatigue were significantly influenced by the body postures. In general, standing and sitting postures showed less discomfort as well as lower heart rates, whereas squatting postures (KF120, KF90, KF60) had higher discomfort and heart rates. Three MDF change trends were reported associated with postures in this study. First, there were less changes of MDFs for standing and sitting postures; second, all patterns of MDFs for KF 150 and KF120 decreased, and lastly some MDFs had increasing trends and others showed decreasing trends for KF30, KF30T, and kneeling postures.

Obesity effect on perceived postural stress during static posture maintenance tasks

Postural stresses related to manual work tasks may be significantly affected by the bodily condition of workers. One such condition is obesity, which is characterised by excess fat mass in the body. This study empirically examined the obesity effect on postural stress during static posture maintenance tasks. In total, 20 obese and 20 non-obese participants performed static box-holding for a set of 84 working postures defined based on the Ovako Working Posture Analysing System. The participants reported postural stresses using the rated perceived exertion scale. Obesity was found to significantly increase postural stress across the 84 working postures and, also, amplify the effects of postural changes on postural stress. The study findings suggest that ergonomic workplace/job design for obese workers would be a challenge requiring a proactive approach and creativity in problem solving. In addition, the use of ergonomic knowledge in design would be more critical when targeting obese than non-obese workers. The study findings are relevant to ergonomic workplace/job design for obese workers.

Evaluation of proper height for squatting stool

Many jobs and activities in people's daily lives have them in squatting postures. Jobs such as housekeeping, farming and welding require various squatting activities. It is speculated that prolonged squatting without any type of supporting stool would gradually and eventually impose musculoskeletal injuries on workers. This study aims to examine the proper height of the stool according to the position of working materials for the squatting worker. A total of 40 male and female college students and 10 female farmers participated in the experiment to find the proper stool height. Student participants were asked to sit and work in three different positions: floor level of 50 mm; ankle level of 200 mm; and knee level of 400 mm. They were then provided with stools of various heights and asked to maintain a squatting work posture. For each working position, they were asked to write down their thoughts on a preferred stool height. A Likert summated rating method as well as pairwise ranking test was applied to evaluate user preference for provided stools under conditions of different working positions. Under a similar experimental procedure, female farmers were asked to indicate their body part discomfort (BPD) on a body chart before and after performing the work. Statistical analysis showed that comparable results were found from both evaluation measures. When working position is below 50 mm, the proper stool height is 100 or should not be higher than 150 mm. When working position is 200 mm, the proper stool height is 150 mm. When working position is 400 mm, the proper stool height is 200 mm. Thus, it is strongly recommended to use proper height of stools with corresponding working position. Moreover, a wearable chair prototype was designed so that workers in a squatting posture do not have to carry and move the stool from one place to another. This stool should ultimately help to relieve physical stress and hence promote the health of squatting workers. This study sought to evaluate and make suggestions on user preference and BPD of proper stool height, at which work can be done comfortably, according to squatting work position. In short, results showed that proper stool height differed according to working position, even with similar squatting tasks.

Lifting strategy and stability in strength-impaired elders

STUDY DESIGN

Ninety-six subjects underwent biomechanical analysis of freestyle box lifting.

OBJECTIVES

To relate lifting strategy to lower extremity muscle strength and postural stability in functionally limited elders.

SUMMARY OF BACKGROUND DATA

Back pain and postural instability in elders is rampant and poorly understood. Much of the literature on lifting relates to young subjects.

METHODS

Lifting strategy for 91 functionally limited elders was classified by timing of peak power in the back and knee joints. Isometric hip and knee extensor strength and postural stability were compared among strategy classifications. Postural stability was analyzed by measuring center of gravity (CG) displacement during lifting.

RESULTS

Three lift strategy groups were established: back-lift, or back dominant strategy (BDS); leg-lift, or leg dominant strategy (LDS); and leg-dominant back-first mixed strategy (LDB). Subjects with relatively strong hip and knee extensors used leg dominant strategy; subjects with relatively strong knee, but weak hip, extensors used leg-dominant back-first mixed strategy; and subjects with weak hip and knee extensors used back dominant strategy. Leg dominant strategy and leg-dominant back-first mixed strategy engendered less center of gravity displacement and thus were posturally more stable than the back dominant strategy.

CONCLUSION

Subjects apparently chose their lift strategy based on their hip and knee extensor strength. Weaker elders using a less stable back dominant strategy could be susceptible to falls and subsequent long bone and vertebral body fractures. Clinicians could identify at-risk elders by muscle testing. Beyond emphasizing strength and endurance exercise in elderly patients, weak elders should be taught to use a leg dominant lifting strategy, or if they are not physically able, to use a combined back/leg strategy.

Gender differences in lifting technique

The importance of different motion patterns in the assessment of work technique is rarely addressed in the literature and even less information can be found regarding gender differences. In this study the possible gender differences in lifting technique from lifting experiments on 12 female and 10 male participants were examined. The participants performed squat and stoop lifts of a box. Movements were measured by means of opto-electronic measurement systems. Kinematic data derived from the measurements revealed some differences between the men and the women, e.g. in trunk motion and knee angle ranges. The hip-knee interjoint coordination was more synchronized for women than for men in terms of the relative phase angle. It is concluded that so far gender differences in motion patterns have not been sufficiently explored and that men and women need to be considered separately in the evaluation of work technique in manual handling tasks. Advantages and disadvantages of different coordination patterns need to be further investigated.

In vitro measurement of the tracking pattern of the human patella

This study sought to determine whether a general pattern describing the three-dimensional tracking characteristics of the human patella could be established, and if not, then to determine the extent and nature of interspecimen variations in the characteristics in a normal population. Using 32 fresh-frozen knees subjected to extensor moment magnitudes similar to those in "static-lifting" and "leg-raising against resistance" maneuvers, patellar displacements were measured in the knee extension range 120 to 0 deg. For static-lifting, a constant foot-floor reaction of 334 N was applied. For leg-raising, a constant net quadriceps tension of 668 N was used throughout the extension range. Measurements were taken with a calibrated six-degree-of-freedom electromechanical goniometer and a displacement coordinate system referenced to the geometry of individual specimens. The three patellar displacements in the plane of knee extension/flexion (extension rotation, and anterior and proximal translations) consistently demonstrated the same pattern in the entire knee extension range with an average coefficient of variation of 13 percent. For knee angles greater than 45 deg, the three other displacements (medial-lateral translation, and rotations about the anterior--posterior and proximal--distal axes) followed a general pattern. However, for knee angles less than 45 deg, these displacements differed considerably between specimens for each loading condition, both in terms of magnitude (average coefficient of variation: 70 percent), and direction.

Determination of the effect of lift characteristics on dynamic performance profiles during manual materials handling tasks

In any quantitative gait or occupational biomechanics investigation, the quantification of the different kinematic, kinetic, and electromyographic parameters is essential towards assessment of functional capacity and development of a biomechanical profile of the task demands. In the current study, the authors presented a methodology for using inferential statistics to evaluate the effect of lift characteristics on phase-dependent and phase-independent variability in performance. Using a database of kinematic and kinetic profiles obtained from a manual lifting study, the phase-dependent effects of lift characteristics: box mass (load), mode (technique of lift), and speed (frequency of lift) were investigated through the use of analysis of variance (ANOVA) techniques, which recognize the vectorial constitution of the profiles. In addition, the Karhunen-Loeve Expansion (KLE) feature extraction method was used for representing the lifting patterns of measured joint angular position, velocity, acceleration, and net muscular torque profiles obtained from a 2-D biomechanical lifting model in order to study the phase-independent effects. In comparison to traditional descriptive statistical analyses currently used in various occupational biomechanics experimental investigations, this method allows the significant information content of the time varying signal to be captured, enhancing the sensitivity of subsequent hypothesis testing procedures. The application of this technique to MMH investigations allows identification of the lift characteristics that dominate the variability of task demands, hence aiding in the design and assessment of ergonomic solutions.

Knee flexion and base of support in asymmetrical handling: effects on the worker's dynamic stability and the moments of the L(5)/S(1) and knee joints

OBJECTIVE: To investigate the effects of knee flexion and of the variations of feet lateral spacing on dynamic stability and on the net joint moments distribution between the back and knees. BACKGROUND: The width of the base of support and knee flexion effects on joint moments in asymmetric handling and especially on the worker's stability have rarely been studied. METHODS: Fourteen healthy male subjects performed an asymmetrical lifting task, using four different techniques: two imposed lateral feet spacings (41 and 57 cm) and two knee flexion amplitudes (slightly and deeply flexed knees). A tridimensional dynamic rigid body model was used to estimate the triaxial net reaction moments at L(5)/S(1) and at the knees, using two force platforms. New developments have been undertaken to characterize workers' stability while handling: the horizontal force required to destabilize the worker was calculated as a measure of dynamic stability. RESULTS: The width of the base of support had little effect on L(5)/S(1) and knee moments; however, the subjects were less stable with the narrow base of support. Using the slightly flexed knees technique, trunk maximal resultant moments were slightly smaller (202 vs. 216 Nm), and maximal resultant knee moments were larger (96 vs. 62 Nm). Furthermore, asymmetric moments at the trunk and the asymmetric position of the knees were reduced with this technique, but subjects were less stable. CONCLUSIONS: The use of a slightly flexed knees technique in asymmetrical handling of low-lying loads appears advantageous because it reduces L(5)/S(1) moments while increasing the knees' flexing moments, although this may compromise workers' stability. RELEVANCE: Handling methods used by workers in asymmetrical handling have rarely been studied. Optimizing the safety of a handling method can involve many parameters, such as reducing joint moments and maximizing stability. The evaluation of the worker's stability while handling in conjunction with joint moments is an interesting alternative to study the safety of handling methods.

Lumbar spinal moments in chronic back pain patients during supported lifting: a dynamic analysis

OBJECTIVES

The main objective of this study was to test the hypothesis that peak L4/L5 moments (torque) placed on the lumbar spine by chronic back pain subjects are reduced using pain-reducing postural adaptations. A secondary objective was to determine the relation between lumbar moments while lifting and self-reported ratings of lower back pain.

STUDY DESIGN

Cohort using seven men with a history of chronic lower back pain. An inverse dynamic model was used to calculate L4/L5 forces and moments while performing five trials each of two lifting styles. Subjective ratings of lumbar back pain were taken before and after the lifts.

RESULTS

Significant (p < .001) differences were found between lifting postures on peak L4/L5 net reaction moments. Two distinct lifting profiles emerged characterized by the amount of lumbar spinal extensor musculature involved. Significant (p < .05) increases in pain were found after a bowed-back lifting style.

CONCLUSIONS

Peak L4/L5 net reaction moments were less (spine extensor loading) for a lifting posture that produced lower levels of self-reported lower back pain. The dynamic model proved reliable and useful for future study of the pathomechanics of lower back pain.

Knee movement strategies differentiate expert from novice workers in asymmetrical manual materials handling

One way to generate hypotheses on appropriate handling principles is to compare the strategies of highly skilled workers (experts) with those of novice workers (novices). This study was conducted to determine whether experts differ from novices in their handling strategies and to determine the effects of these differences in joint motions and net reaction moments at the trunk (L5/S1) and knees. Six experts and five novices were compared transferring two loads (12 and 22 kg) from four different initial low positions to a standardized low position on a trolley. The external forces were obtained from two AMTI force platforms, and two 16 mm Locam cameras coupled with two mirrors were used to obtain the three-dimensional kinematic data. An inverse dynamic analysis was performed on each segment and the net reaction moments were evaluated at L5/S1 and the knees; trunk posture, knee flexion angles and feet spacings were also calculated. Results showed that both groups minimized trunk asymmetries of posture and efforts; however, the experts were characterized by reduced knee flexion (31 degrees vs 53 degrees) and total knee excursion (20 degrees vs 38 degrees), increased flexion moments (72 vs 58 Nm) but reduced extension moments (4 vs 26 Nm) and valgus moments (30 vs 54 Nm). The experts' strategies appear safer overall because they reduce trunk asymmetry, asymmetrical efforts on the knee and stress on the femoropatellar joint. Moreover, these strategies present a potential for reduced mechanical energy expenditure due to lower knee excursions and increased knee stability due to reduced knee flexion.

Calculated stress-shielding in the distal femur after total knee replacement corresponds to the reported location of bone loss

This study sought to determine the similarities between features of calculated stress-shielding and observed bone loss in the distal femur after total knee replacement. Stress-shielding was determined by comparing the magnitudes and distributions of strain energy density, calculated using three-dimensional finite element models of the intact bone, the bone after total knee replacement with bonding assumed at all prosthesis-bone interfaces, and the bone after total knee replacement with bonding assumed only at the distal interface. The loading condition simulated static lifting with the knee flexed at 45 degrees, producing tibiofemoral and patellofemoral joint reactions of 900 and 450 N, respectively. The maximum magnitudes of strain energy density calculated using the total knee replacement models were less than 15% of the corresponding magnitudes from the intact bone model. The greatest difference was found to occur at the anterior distal corner of the femur, suggesting this location as the one most vulnerable to stress-shielding. Clinically observed bone loss after total knee replacement frequently occurs at this location. At the anterior distal corner, the calculated magnitudes for the two total knee replacement models were similar, suggesting that stress-shielding at this location was not reduced by limiting fixation only to the distal interface. Although the study corresponded to one loading condition and one geometry of the total knee replacement femoral component with the inherent limitations of model calculations, the results suggest a possible scenario for stress-shielding.

The validity of visual observation to assess posture in a laboratory-simulated, manual material handling task

A wide range of methods to evaluate posture in work situations relies on simple, unaided visual observation. In the present study the validity of visual observation to assess posture in a dynamic job was evaluated. Postural aspects were observed and recorded during a manual materials handling job simulated in a laboratory. The results from these observations, concerning gross body posture, torso flexion, arms and legs position, and load to be handled, were compared to the results obtained by direct opto-electronic recording. The agreement on a sample-to-sample basis (expressed by Cohen's kappa) was poor for the variables torso flexion (mean value for kappa = 0.38), position of arms (kappa = 0.43) and legs (kappa = 0.46) and load to be handled (kappa = 0.50) and acceptable only for the gross body posture (kappa = 0.79). Moreover, for each variable except gross body posture, the crude distributions of all observations and recordings across categories (irrespective of time) were significantly different. The results demonstrate that the observations are not valid. It is concluded that dynamic work situations require less simple, more time consuming methods (e.g., analyzing film or video records of the job) than a posture registration method based on direct visual observation.

Coordination of the leg muscles in backlift and leglift

Net joint moments are often used to quantify the loading of structures (e.g. the intervertebral disc at L5S1) during lifting. This quantification method is also used to evaluate the loading of the knee, for instance, to determine the effect of backlifting as opposed to leglifting. However, the true loading of the joint as derived from net joint moments can be obscured by a possible co-contraction of antagonists. To unravel the mechanisms that determine the net joint moments in the knee, the leglift was compared to the backlift. Although a completely different net knee moment curve was found when comparing the two lifting techniques, it appeared to be closely related to the ground reaction force vector and its orientation with respect to the joint centre of rotation (R > 0.995). This close relation was established by co-contraction of both flexors and extensors of the knee. Furthermore, a close relation appeared to exist between the joint moment difference between hip and knee and the activity difference between rectus femoris muscle and hamstring (R = 0.72 and 0.83 in leglift and backlift, respectively). The knee-ankle joint moment difference and the activity of the gastrocnemius showed a close relation as well (R = -0.89 and 0.96 in leglift and backlift, respectively). These relations can be interpreted as a mechanism to distribute net moments across joints. It is concluded that during lifting tasks the intermuscular coordination is aimed at coupling of joint moments, such that the ground reaction force points in a direction that provides balance during the movement. The use of net joint moments as direct indicators for joint loading (e.g. knee) seems, therefore, questionable.

Value of biomechanical macromodels as suitable tools for the prevention of work-related low back problems

Biomechanical macromodels are evaluated with respect to their possible usefulness for health professionals and ergonomists, as well as for applied research on the prevention of low back problems. It is concluded that in the context stated geometrically simple models, in particular the model by Schultz and co-workers, are to be favoured over more complex models. However, load predictions in extreme trunk postures should be dealt with carefully. It is recommended that the model load predictions should be used only in the comparison of work situations and not for an assessment of the absolute acceptability of a work situation. Low back problems are related to mechanical (over)load at work. This study shows the pros and cons of various biomechanical macromodels as tools for health professionals and ergonomists, as well as for applied research on the prevention of work-related low back problems.

Relationship between moments at the L5/S1 level, hip and knee joint when lifting

A study was performed to determine the influence of load magnitude on the self selected technique of lifting. Specifically, it was hypothesized that with heavier weights a tendency would occur to lift more with the back and less with the legs. Flexion-extension moments at the L5/S1 level, hip and knee joints were calculated for subjects when lifting boxes weighing from 50 to 250 N. Lifts were performed using a freestyle technique at normal speed. The moment profiles (moment plotted vs time) were analyzed kinematically and as a function of the weight lifted. The kinematics of the lift changed as the weight increased. The moment at the L5/S1 level increased with increasing weight, however, the corresponding knee moment decreased. Thus, an inverse relationship was found between the moment at the L5/S1 level and the knee joint moment. An increase in weight lifted was also associated with an increase in the angular velocity at the knee while lifting. Apparently with heavier weights there is a tendency to extend the knees earlier during the lift than with lighter weights, confirming our hypothesis. This explains the reduced knee moment. Our findings lead to the hypothesis that quadriceps muscle strength limits the subjects' ability to lift with their knees flexed.

Analysis of human torso motion with muscle actuators

A biomechanical model is devised to analyze the motion of the human torso and estimate the load on the lumbar spine, the contraction forces in the trunk muscles, and the sensory signals between the muscle and the nervous system. A state space formulation of three-dimensional (3D) equations of motion of the human torso is presented with muscle forces as input to the system. At least 3 pairs of skeletal muscles are considered to be necessary for 3D motion of human torso. Functional anatomy of these major muscles of the human trunk are discussed. These muscles as well as their feedback and feedforward sensory paths are modeled by linear viscoelastic components and force generators. Stability of the torso with three pairs of muscles is studied both with muscle spindles inactive (open loop) and active (closed loop). Voluntary point-to-point motion of the torso in three-dimensional space is simulated on a digital computer, employing a dynamic controller, where feedback gains are tuned (programmed) by higher centers of nervous system.

Force analysis of the patellar mechanism

The aim of this study is an experimental evaluation of a force analysis of the patellar mechanism based on the assumption that patellofemoral contact is frictionless. At first, the geometric characteristics of contact surfaces, a prior knowledge of which is necessary for quantitative analysis, were measured from radiographs of 42 fresh-frozen knee specimens in the flexion range 0-120 degrees. The results were then used in the analysis to predict the relations between the forces acting on the patella. For the evaluation of the analysis, the ratio of the tension in the ligamentum patellae and the rectus femoris was measured in ten specimens during simulation of two knee functions: (1) "leg raising" against a resistance; and (2) "static lifting". The effect of flexion angle on the ratio is found to be rather complex. With increasing flexion, the ratio increases initially up to 30 degrees, then decreases up to 90 degrees, and finally increases again beyond 90 degrees. The ratio is above unity up to around 45 degrees and below that in the remaining flexion range. The analysis has been found to predict not only the characteristic variation of the ratio but also its magnitude with reasonable accuracy. It has been concluded that for an accurate prediction of the patellofemoral joint reaction, the force analysis needs to be based on the geometry of the contact surfaces. This implies that the mechanical consequences of surgical procedures involving tibial tubercle relocation cannot be inferred simply on the basis of their effect on the patellar mechanism angle, but that they also require consideration of their effect on the contact geometry.

Dynamically and statically determined low back moments during lifting

Assessment of the effects of lifting on the low back has most frequently been done with the aid of static models. Many lifting movements appear to have substantial inertial components. It was of interest, therefore, to determine the size of the difference between statically and dynamically calculated lumbar moments during a demanding but not unusual manual lift observed in a metal fabrication industry. The results of several trials by four young men showed that the dynamic model resulted in peak L4/L5 moments 19% higher on average, with a maximum difference of 52%, than those determined from the static model. The technique adopted in the lift could minimize the difference. When the inertial forces of the load itself and the load weight were incorporated into an otherwise static model (quasi-dynamic) then the resulting L4/L5 moments exceeded those of the fully dynamic model by 25%. In many industrial tasks static analyses may severely underestimate the demands of dynamic lifts. These results show that a reasonably inexpensive approach in lifting task analysis is to measure the dynamic forces of the load on the hands and to use these in an otherwise static model. This results in a conservative assessment of the injury risk of lifts at least of the type reported in this study.

The lifting stress calculator

The Lifting Stress Calculator is a microprocessor-based device, programmed to select from a group of workers the one best suited for a specified lifting task. It also prompts this worker with the ideal position for him to lift the load in question. The concept is based on a theoretical study that deals with lifting as a total body exertion. The goal of this device is to educate the worker for better prevention of lifting injuries by minimizing the loads on the body.