Michiel P. de Looze

Validation of a full body 3-D dynamic linked segment model

Abstract In studies related to human movement, linked segment models (LSMs) are often used to quantify forces and torques, generated in body joints. Some LSMs represent only a few body segments. Others, for instance used in studies on the control of whole body movements, include all body segments. As a consequence of the complexity of 3-dimensional (3-D) analyses, most LSMs are restricted to one plane of motion. However, in asymmetric movements this may result in a loss of relevant information. The aim of the current study was to develop and validate a 3-D LSM including all body segments. Braces with markers, attached to all body segments, were used to record the body movements. The validation of the model was accomplished by comparing the measured with the estimated ground reaction force and by comparing the torques at the lumbo-sacral joint that resulted from a bottom-up and a top-down mechanical analysis. For both comparisons, reasonable to good agreement was found. Sources of error that could not be analysed this way, were subjected to an additional sensitivity analysis. It was concluded that the internal validity of the current model is quite satisfactory.

Effects of precision demands and mental pressure on muscle activation and hand forces in computer mouse tasks

The objective of the present study was to gain insight into the effects of precision demands and mental pressure on the load of the upper extremity. Two computer mouse tasks were used: an aiming and a tracking task. Upper extremity loading was operationalized as the myo-electric activity of the wrist flexor and extensor and of the trapezius descendens muscles and the applied grip- and click-forces on the computer mouse. Performance measures, reflecting the accuracy in both tasks and the clicking rate in the aiming task, indicated that the levels of the independent variables resulted in distinguishable levels of accuracy and work pace. Precision demands had a small effect on upper extremity loading with a significant increase in the EMG-amplitudes (21%) of the wrist flexors during the aiming tasks. Precision had large effects on performance. Mental pressure had substantial effects on EMG-amplitudes with an increase of 22% in the trapezius when tracking and increases of 41% in the trapezius and 45% and 140% in the wrist extensors and flexors, respectively, when aiming. During aiming, grip- and click-forces increased by 51% and 40% respectively. Mental pressure had small effects on accuracy but large effects on tempo during aiming. Precision demands and mental pressure in aiming and tracking tasks with a computer mouse were found to coincide with increased muscle activity in some upper extremity muscles and increased force exertion on the computer mouse. Mental pressure caused significant effects on these parameters more often than precision demands. Precision and mental pressure were found to have effects on performance, with precision effects being significant for all performance measures studied and mental pressure effects for some of them. The results of this study suggest that precision demands and mental pressure increase upper extremity load, with mental pressure effects being larger than precision effects. The possible role of precision demands as an indirect mental stressor in working conditions is discussed.

Segment inertial parameter evaluation in two anthropometric models by application of a dynamic linked segment model

The estimation of segment inertial parameters (SIPs) is an important source of error in inverse dynamic analysis. In most individual cases SIPs are derived from extrapolation of known SIPs of a certain population through regression equations (proportional models). Another well-known method is the use of mathematical approximation of the shape of human body segments combined with estimations of segment densities (geometric models). In the current study five males and five females performed four different lifting movements in the sagittal plane. A full body linked segment model was applied twice to the same data set, once using a proportional and once using a geometric anthropometric model. As a full body linked segment model is an overdetermined system of equations, four equations could be formed to test the summed effect of SIP errors on the inverse dynamic analysis. The overall performance in terms of coefficients of correlation was better for the geometric model as compared to the proportional model. When a back lifting movement was performed, the equations indicated systematic errors in the proportional model. However, when a leg lifting movement was performed, the equations indicated systematic errors in the geometric model. Therefore, analyzing only one kind of movement does not suffice to draw conclusions with respect to the reliability of an anthropometric model.

Flexion relaxation during lifting: Implications for torque production by muscle activity and tissue strain at the lumbo-sacral joint

During the full flexion phase of the back lift movement the lumbar part of the erector spinae muscle exhibits a reduced activity level (flexion relaxation). This study addresses the question how the required extension torque in the lumbo-sacral joint (L5/S1 joint) is balanced during the period in which apparently the lumbar erector spinae ceases to take its share. Six subjects participated in the experiment in which they performed seven lifting tasks. The load, the range of movement, and the phase in which the load was handled (lifting or lowering) were varied. A dynamic linked segment model was applied to determine the momentary torques acting at the L5/S1 joint, while the EMGs of the lumbar and thoracic part of the erector spinae muscle were measured. Furthermore, the lengths between markers on the lumbar and thoracic part of the trunk were determined to reveal changes in length during the movement. The dynamic EMGs were normalized to trunk angle-dependent maximal levels. The L5/S1 joint torques were analysed and combined with the normalized EMG data and the kinematics of the trunk, which are assumed to indicate the elongation of passive tissues. Although in the normalization procedure the change of the length-force relationship of the erector spinae was taken into account, the dynamic lumbar EMG activity decreased to a low-activity level (the phenomenon of flexion relaxation). This coincided with a 25% increase in lumbar length suggesting that passive tissue strain provided part of the required extension torque. In the tasks where a barbell was handled a significant increase in EMG level of the thoracic part of the erector spinae occurred just before the flexion relaxation at the lumbar level. Apparently, the extensor function of the lumbar part is then taken over by the thoracic part of the erector spinae muscle. This suggests that an intricate coordinating mechanism is operative that apportions the load to be balanced over active--(lumbar and thoracic part of the erector spinae) and passive structures (post vertebral ligaments).

Effect of a triathlon wet suit on drag during swimming

The effect of a triathlon wet suit on drag was studied in 12 subjects (eight male, four female) swimming at different velocities (1.10, 1.25 and 1.50 m.s-1). The active drag force was directly measured during front crawl swimming using a system of underwater push off pads instrumented with a force transducer (M.A.D. system: 6). Measurements were made when swimming over the system with and without a wet suit. A 14% reduction in drag (from 48.7 to 41.8 Newtons) is found at a swimming velocity of 1.25 m.s-1, which is a typical swimming speed for triathlon distances. At 1.50 m.s-1 a reduction in drag of 12% was observed, which suggests that the wearing of such a suit might be beneficial in conventional swimming events. The reduction in drag can explain the higher swimming velocities observed in triathletes using a wet suit. The effect of the reduction is probably largely due to an increased buoyancy inducing less frontal resistance. However, since the effect of the suit on the lighter female swimmers was not different from the effect on the heavier male swimmers, a reduction in friction drag and drag coefficient may also be significant.

Low-level activity of the trunk extensor muscles causes electromyographic manifestations of fatigue in absence of decreased oxygenation

This study was designed to determine whether trunk extensor fatigue occurs during low-level activity and whether this is associated with a drop in muscle tissue oxygenation. Electromyography (EMG) feedback was used to impose constant activity in a part of the trunk extensor muscles. We hypothesized that electromyographic manifestations of fatigue and decreased oxygenation would be observed at the feedback site and that EMG activity at other sites would be more variable without fatigue manifestations. Twelve volunteers performed 30-min contractions at 2% and 5% of the maximum EMG amplitude (EMGmax) at the feedback site. EMG was recorded from six sites over the lumbar extensor muscles and near-infrared spectroscopy was used to measure changes in oxygenation at the feedback site (left L3 level, 3 cm paravertebral). In both conditions, mean EMG activity was not significantly different between electrode sites, whereas the coefficient of variation was lower at the feedback site compared to other recording sites. The EMG mean power frequency (MPF) decreased consistently at the feedback site only. At 5% EMGmax, the decrease in MPF was significant at the group level at all sites ipsilateral to the feedback site. These results suggest that the limited variability of muscle activity at the EMG feedback site and at ipsilateral locations enhances fatigue development. No decreases in tissue oxygenation were detected. In conclusion, even at mean activity levels as low as 2% EMGmax, fatigue manifestations were found in the trunk extensors. These occurred in absence of changes in oxygenation of the muscle tissue.

Asymmetric low-back loading in asymmetric lifting movements is not prevented by pelvic twist

Asymmetric lifting is associated with an increased risk of low back disorders. Especially in lifting movements, characterized by a small amount of asymmetry, it is still the question if asymmetric lumbosacral torques occur, or if subjects try to avoid asymmetric back loading by twisting their pelvis with respect to the feet. An increase of the lifting speed or the box weight might amplify the lumbar torques but might also result in an attempt to limit further increase of asymmetric torques by increasing pelvic twist. In the current study, asymmetrical lifting movements were analyzed with the aid of a 3D linked segment model, using cuffs mounted to the body segments. Eight subjects performed lifting movements with five different asymmetry conditions, ranging from 0 to 90 degrees lifting asymmetry with respect to the sagittal plane, using two lifting speeds and two box weights. A significant increase in lateral flexing and twisting low back torque was found for each increase in asymmetry of the lifting movement. Pelvic twist accounted more or less constantly for about 25% of the lifting asymmetry and was hardly influenced by lifting speed or box weight. Even for 10 or 30 degrees of lifting asymmetry, subjects did not twist their pelvis far enough to avoid asymmetric loading of the low back. Assuming that asymmetric loading of the low back is more strenuous to the spine than symmetric loading, the current results indicate that even small deviations of a lifting movement from the sagittal plane can explain an increased risk of low back disorders.

The effects of a passive exoskeleton on muscle activity, discomfort and endurance time in forward bending work

Exoskeletons may form a new strategy to reduce the risk of developing low back pain in stressful jobs. In the present study we examined the potential of a so-called passive exoskeleton on muscle activity, discomfort and endurance time in prolonged forward-bended working postures. Eighteen subjects performed two tasks: a simulated assembly task with the trunk in a forward-bended position and static holding of the same trunk position without further activity. We measured the electromyography for muscles in the back, abdomen and legs. We also measured the perceived local discomfort. In the static holding task we determined the endurance, defined as the time that people could continue without passing a specified discomfort threshold. In the assembly task we found lower muscle activity (by 35-38%) and lower discomfort in the low back when wearing the exoskeleton. Additionally, the hip extensor activity was reduced. The exoskeleton led to more discomfort in the chest region. In the task of static holding, we observed that exoskeleton use led to an increase in endurance time from 3.2 to 9.7 min, on average. The results illustrate the good potential of this passive exoskeleton to reduce the internal muscle forces and (reactive) spinal forces in the lumbar region. However, the adoption of an over-extended knee position might be, among others, one of the concerns when using the exoskeleton.

The influence of task variation on manifestation of fatigue is ambiguous - a literature review

Task variation has been proposed to reduce shoulder fatigue resulting from repetitive hand–arm tasks. This review analyses the effect of task variation, both ‘temporal (i.e. change of work–rest ratio)’ and ‘activity (i.e. job rotation)’ variation, on physiological responses, endurance time (ET) and subjective feelings. Pubmed was searched and complemented with references from selected articles, resulting in 17 articles. Temporal variation had some positive effects on the objective parameters, as blood pressure decreased and ET increased, and on the subjective feelings, as perceived discomfort decreased. The observed findings of activity variation showed both positive and negative effects of increased activity variation, while hardly any effects were found on electromyography manifestations of fatigue. In conclusion, the evidence for positive effects of increasing the level of variation is scarce. The number of studies on variation is limited, while in most studies the findings were not controlled for the amount or intensity of work. Practitioner Summary: Some laboratory-controlled studies showed some positive effects of increasing temporal variation, providing limited support for introducing frequent interruptions in work. Activity variation showed ambiguous effects, meaning there is no evidence for practical implication. In practice, however, other positive effects of activity variation may occur, for example job enrichment and increased motivation.

Position sense acuity of the upper extremity and tracking performance in subjects with non-specific neck and upper extremity pain and healthy controls

OBJECTIVE We aimed to investigate whether position sense acuity of the upper extremity and tracking performance, pen pressure and muscle activity in a tracking task are affected in subjects with neck and upper extremity pain. METHODS Twenty-three subjects with neck and upper extremity pain and 26 healthy controls participated in the study. Position sense acuity of the upper extremity was measured while subjects pointed at targets, without vision of their arm and hand. In the computer tracking task, subjects were instructed to position a cursor within a moving target dot, using a pen on a tablet. RESULTS Position sense acuity of the upper extremity was impaired in subjects with pain. Their variable error was 20-30% larger than in healthy controls. Subjects with pain also showed reduced tracking precision. Both the mean and the standard deviation of the distance to target were significantly larger in subjects with pain than in healthy controls, 10% and 13% respectively. No differences between groups were found in pen pressure and muscle activity during tracking. CONCLUSION The results suggest that subjects with pain are limited in performing precision tasks due to impaired position sense of the upper extremity, but do not compensate with increased effort.