I. Kingma

Flow-related mechanics of the intervertebral disc: the validity of an in vitro model.

Study Design. An in vitro mechanical study on porcine motion segments. Objectives. To test the validity of in vitro studies of the flow-related mechanics of the intervertebral disc and, in particular, to investigate whether fluid flows back into the disc during unloading after a loading cycle. Summary of Background Data. In vivo studies show both the inflow and outflow of fluid in the intervertebral disc. The resistance to flow out of the disc is higher than to inflow. The fluid flow is regulated via unbalance between the external load and the osmotic pressure of the nucleus pulposus. Materials. There were 8 porcine lumbar motion segments (without posterior elements) and 8 isolated discs tested in a physiologic saline bath (39°C). The specimens were preloaded at 0.025 MPa for 15 minutes. Three 15-minute loading periods at 2.0 MPa were applied, each followed by an unloading period of 30 minutes. Loads, axial displacements, and nucleus pressure were recorded online. Results. Over the 3 loading and unloading periods, all specimens showed a net loss of height and mass. The time series of specimen height during the 3 unloading periods showed virtually identical responses. The pressure in the nucleus decreased in the subsequent loading periods and showed no increase during unloading. Conclusion. The data show the limitations of an in vitro model for studying fluid flow-related intervertebral disc mechanics. During loading, outflow of fluid occurred, but inflow appears to be virtually absent during unloading. Poro-elastic behavior cannot be reproduced in an in vitro model.

Fatigue failure in shear loading of porcine lumbar spine segments.

Study Design. An in vitro study on porcine spinal segments. Objectives. To determine the differences in mechanical behavior and fatigue strength in shear loading between intact spinal segments and segments without posterior elements, and between segments in neutral and flexed positions. Summary of Background Data. Limited data are available on shear strength of spinal segments. Literature suggests that shear loading can lead to failure of the posterior elements and failure of the disc, when the posterior elements cannot provide adequate protection. Methods. In 2 experiments, 18 and 20 spines of pigs (80 kg) were used, respectively. Shear strength of the T13–L1 segment was tested, while loaded with 1600-N compression. L2–L3 and L4–L5 segments were loaded with a sinusoidal shear between 20% and 80% of the strength of the corresponding T13–L1 segment and 1600-N compression. In experiment No. 1, the posterior elements were removed in half the segments. In experiment No. 2, half the segments were tested in the neutral position, and half were tested in 10° flexion. Results. The group without posterior elements had failure earlier than the intact group. In the group without posterior element, stiffness increased on failure; in the intact group, it decreased. In experiment No. 2, no differences between groups were found. Conclusions. Repetitive shear loading can induce failure of porcine spinal segments, likely caused by fracture of the posterior elements, and, although repetitive anterior shear forces can also induce disc damage, this appears not to occur in intact segments, not even when flexed close to maximal.

An EMG technique for measuring spinal loading during asymmetric lifting

OBJECTIVES To compare two methods of calibrating the erector spinae electromyographic signal against moment generation in order to predict extensor moments during asymmetric lifting tasks, and to compare the predicted moments with those obtained using a linked-segment model. METHODS Eight men lifted loads of 6.7 and 15.7 kg at two speeds, in varying amounts of trunk rotation. For each lift, the following were recorded at 60 Hz; the rectified and averaged surface electromyographic signal, bilaterally at T10 and L3, lumbar curvature using the 3-Space Isotrak, movement of body segments using a 4-camera Vicon system, and ground reaction forces using a Kistler force-plate. Electromyographic (EMG) and Isotrak data were used to calculate lumbosacral extensor moments using the electromyographic model, whereas movement analysis data and ground reaction forces were used to estimate net moments using the linked-segment model. For the electromyographic technique, predictions of extensor moment were based on two different sets of EMG-extensor moment calibrations: one performed in pure sagittal flexion and the other in flexion combined with 45 degrees of trunk rotation. RESULTS Extensor moments predicted by the electromyographic technique increased significantly with load and speed of lifting but were not influenced by the method of calibration. These moments were 7-40%greater than the net moments obtained with the linked-segment model, the difference increasing with load and speed. CONCLUSIONS The calibration method does not influence extensor moments predicted by the electromyographic technique in asymmetric lifting, suggesting that simple, sagittal-plane calibrations are adequate for this purpose. Differences in predicted moments between the electromyographic technique and linked-segment model may be partly due to different anthropometric assumptions and different amounts of smoothing and filtering in the two models, and partly due to antagonistic muscle forces, the effects of which cannot be measured by linked-segment models. RelevanceAsymmetric lifting is a significant risk factor for occupationally-related low back pain. Improved techniques for measuring spinal loading during such complex lifting tasks may help to identify work practices which place the spine at risk of injury.

Effect of a stiff lifting belt on spine compression during lifting.

Study Design. An in vivo study on weightlifters. Objectives. To determine if and how a stiff back belt affects spinal compression forces in weightlifting. Summary of Background Data. In weightlifting, a back belt has been reported to enhance intraabdominal pressure (IAP) and to reduce back muscle EMG and spinal compression forces. Methods. Nine experienced weightlifters lifted barbells up to 75% body weight while inhaling and wearing a belt, inhaling and not wearing a belt, and exhaling and wearing a belt. IAP, trunk muscle EMG, ground reaction forces, and kinematics were measured. An EMG-assisted trunk model, including IAP effects, was used to calculate spinal compression and shear forces and to reveal the contribution of back muscles, abdominal muscles, and IAP to moment generation. Results. The belt reduced compression forces by about 10%, but only when inhaling before lifting. The moment generated by IAP increased when wearing a belt and inhaling, but this moment was small and the increase was largely negated by the flexing moment generated by abdominal muscles. Conclusions. Wearing a tight and stiff back belt while inhaling before lifting reduces spine loading. This is caused by a moment generated by the belt rather than by the IAP.

The Importance of Antagonistic Cocontraction of Trunk Muscles for Spinal Loads during Lifting and Pulling Tasks: Implications for Modeling Approaches

Tt has been suggested that tasks involving considerable trunk extensor efforts may entail compression forces on the lumbar spine which can reach a high enough level to cause damage to the vertebral endplates and consequent low-hack pain (Dieen, Weinans, & Toussaint, 1999). However, since direct measurements of compression forces are impossible, such conclusions rely on estimates derived from biomechanical analyses. Therefore, the accuracy with which these compression forces can be estimated from a biomechanical analysis deserves attention. The basic problem in this type of analysis is the fact that the many muscles spanning the lumbar joints constitute a mechanically indeterminate system. Consequently, a given net moment about ajoint can be produced by a range of combinations of muscle forces. Therefore, assumptions with respect to the distribution of the net moments across muscles have to be made. Unfortunately, sensitivity analyses have shown that compression force estimates are quite sensitive to such assumptions. Among these, the assumption made regarding the presence and intensity of abdominal cocontraction during extensor efforts has the most prominent effect (Dieen & Looze, 1999). Two common approaches in estimating spinal compression forces treat the issue of cocontraction quite differently. The first approach uses surface electromyography to obtain estimates of forces in individual muscles. The level of cocontraction in abdominal muscles in this case is assumed to be reflected by the electrical activity picked up from a limited number of superficial motorunits in the abdominal muscles. The second approach based on mathematical optimization estimates the distribution of the net moment by minimizing a function of the muscle forces or muscles activities. Since these cost functions are usually based on some form of efficiency criterion, cocontraction is assumed to be absent. The effect of cocontraction is often used as an argument for using the former approach (e.g. Cholewicki, McGill, & Norman, 1995). However, it should be kept in mind that this method is more cumbersome in terms of data acquisition. Furthermore, although it uses more of the available biological information, it still relies on several assumptions, c.g. regarding the activity in deep muscles and regarding the relationship between electrical activity and muscle force. The aim of the present study, therefore, was to compare compression force estimates based on the two approaches in realistic manual materials handling tasks involving substantial extensor efforts (lifting and pulling).

Do arm motions help to recover from a trip

When someone is tripped during gait, an angular momentum is induced in all planes of motion. We measured full-body 3D kinematics in subjects who were tripped during mid-swing. We analyzed the effect of the arm swing on body rotations in all planes of motion during recovery from the trip.

Circumvention of suddenly appearing obstacles in young and older adults

Reduced ability to circumvent an obstacle, which is noticed only shortly before collision, could be a cause of falls and injury, especially in older adults. In this study, we investigated differences in strategies and their characteristics between young and older adults when circumventing a suddenly appearing obstacle. We measured young and older adults while walking over a platform, while in some trials an obstacle suddenly appeared halfway, blocking their passage. Obstacle appearance was timed to provide available response times (ART) of 850, 1000 or 1150 ms. Circumvention strategies could be classified as either side stepping or cross-over stepping, which were observed in both age groups. Strategy choice was affected by ART and age; older adults preferred the side step strategy, especially when ART was shorter. Peak ground reaction forces were higher for the side step strategy. Older adults performed similar circumvention strategies as young adults, with a stronger preference for side stepping. This strategy appears to be more stable, although it is more demanding in terms of force generation.

Effect of a Stiff Lifting Belt on Spine Compression During Lifting

Study Design. An in vivo study on weightlifters. Objectives. To determine if and how a stiff back belt affects spinal compression forces in weightlifting. Summary of Background Data. In weightlifting, a back belt has been reported to enhance intraabdominal pressure (IAP) and to reduce back muscle EMG and spinal compression forces. Methods. Nine experienced weightlifters lifted barbells up to 75% body weight while inhaling and wearing a belt, inhaling and not wearing a belt, and exhaling and wearing a belt. IAP, trunk muscle EMG, ground reaction forces, and kinematics were measured. An EMG-assisted trunk model, including IAP effects, was used to calculate spinal compression and shear forces and to reveal the contribution of back muscles, abdominal muscles, and IAP to moment generation. Results. The belt reduced compression forces by about 10%, but only when inhaling before lifting. The moment generated by IAP increased when wearing a belt and inhaling, but this moment was small and the increase was largely negated by the flexing moment generated by abdominal muscles. Conclusions. Wearing a tight and stiff back belt while inhaling before lifting reduces spine loading. This is caused by a moment generated by the belt rather than by the IAP.

EFFECT OF SHIP MOTION ON LOW BACK MOMENTS DURING MANUAL LIFTING

INTRODUCTION Performing heavy work (such as lifting) on a ship is associated with a high prevalence of musculoskeletal problems [1,2]. Lifting results in high low back loading [3], thereby affecting low back pain risk. Ship motion may have an additional effect on low back loading during lifting [4]. Depending on the timing of a lifting movement, ship motion can either increase (for example when a ship accelerates upwards) or decrease (for example when a ship accelerates downwards) low back loading. The aim of the present study was to find out to what extent ship motion affects the peak low back moment at the L5S1 joint during manual lifting on a ship at sea. Furthermore, it was investigated whether people are able to time their lifting movements in such a way that the effect of ship motion reduces peak low back moments.

2D Analysis of 3D Lifting: How Far can we Go?

Occupational manual material handling (MMH) is generally not limited to the sagittal plane. Yet, for practical reasons, biomechanical modeling of low back loading during occupational MMH is mostly restricted to 2D. In this study, the limitations to such an approach are analyzed through quantification of the errors made during 2D analysis of 3D lifting. In addition, an estimate is given of the improvements that can be obtained using a simple method to resolve one major source of error, i.e. the error due to projection of lumbar markers onto the sagittal plane.