Effects of symmetry and load absorption of a falling load on 3D trunk muscular moments

The effect of a knee support on the biomechanical response of the low back

Stooping and squatting postures are seen in a number of industries (e.g., agriculture, construction) where workers must work near ground level for extended periods of time. The focus of the current research was to evaluate a knee support device designed to reduce the biomechanical loading of these postures. Ten participants performed a series of sudden loading tasks while in a semisquat posture under two conditions of knee support (no support and fully supported) and two conditions of torso flexion (45 and 60 degrees ). A weight was released into the hands of the participants who then came to steady state while maintaining the designated posture. As they performed this task, the EMG responses of the trunk extensors (multifidus and erector spinae) were collected, both during the "sudden loading" phase of the trial as well as the steady weight-holding phase of the trial. As expected, the effects of torso flexion angle showed significant decreases in the activation of the multifidus muscles with greater torso angle (indicating the initiation of the flexion-relaxation response). Interestingly, the results showed that the knee support device had no effect on the activation levels of the sampled muscles, indicating that the loss of the degree of freedom from the ankle joint during the knee support condition had no impact on trunk extensor muscle response. The a priori concern with regard to these supports was that they would tend to focus loading on the low back and therefore would not serve as a potential ergonomic solution for these stooping/semisquatting tasks. Because the results of this study did not support this concern, further development of such an intervention is underway.

Muscular and centre of pressure response to sudden release of load in symmetric and asymmetric stoop lifting tasks

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 80N 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.

Is the trunk movement more perturbed after an asymmetric than after a symmetric perturbation during lifting?

Low back injury is associated with sudden movements and loading. Trunk motion after sudden loading depends on the stability of the spine prior to loading and on the trunk muscle activity in response to the loading. Both factors are not axis-symmetric. Therefore, it was hypothesized that the effects on trunk dynamics would be larger after an asymmetric than after a symmetric perturbation. Ten subjects lifted a crate in which, prior to lifting, a mass was displaced to the front or to the side without the subjects being aware of this. Crate and subject movements, crate reaction forces and muscle activity were recorded. From this, the stability prior to the perturbation was estimated, and the trunk angular kinematics and moments at the lumbo-sacral joint were calculated. Both perturbations only minimally affected the trunk kinematics, although the stability of the spine prior to the lifting movement was higher in the sagittal plane than in the frontal plane. In both conditions the stability appeared to be sufficient to absorb the applied perturbation.

Trunk muscle response to lifting unbalanced loads with and without knowledge of centre of mass

OBJECTIVE

To examine the effects of lifting a bin with a variable centre of mass on muscle activity, with and without knowledge of the centre of mass.

BACKGROUND

Numerous parameters related to lifting have been examined yet the effects of changing the load centre of mass in two dimensions, with or without knowledge, has not been examined.

METHODS

Participants lifted a 6 kg industrial tote bin with a 7 and 11 kg mass randomly placed in each of nine compartments, into which the interior of the bin was partitioned. Participants were not restricted in lifting style other than using the handles, which were equipped with force gauges. Two series (9 lifts per series) were completed using the 7 kg load without knowledge of the load placement and one series with knowledge of the load placement. One series was completed using an 11 kg mass without knowledge of load placement. Electromyographic activity of the upper and lower erector spinae, latissimus dorsi and the external obliques were collected bilaterally.

RESULTS

Left and right muscle pairs demonstrated mirror images for all muscles with lowest activity levels when the load was placed nearer the lifter in the sagittal plane. Peak electromyographic activity of the upper erector spinae and latissimus dorsi increased with the weight in the compartments nearest the body and/or the ipsilateral handle. Conversely, peak electromyographic activity of the lower erector spinae and the obliques increased when the weight was on the contralateral side. Peak upper erector electromyographic activity reached up to 41% of maximum and the lower erectors reached 50% of maximum, while the obliques and latissimus dorsi were below 5% and 7%, respectively. No electromyographic activity differences were found between the known and unknown load placements.

DISCUSSION

A segmental control strategy appears to exist during lifting that works from the upper to lower torso based on peak electromyographic activity activity. When lifting a bin with a varied centre of mass, highest peak electromyographic activity for the upper and lower erector spinae occurred when the load is closest to the body, regardless of load knowledge. Based on our findings with asymmetrical loads, we conclude that the moments acting on the wrist play an important role in spinal loading and must be included in future studies.

RELEVANCE

Asymmetrical loads are often encountered in daily life. Regardless of the lifter's knowledge of the balance of the load, the differential and asymmetrical loading of the muscles of the back play a role in the development of low back pain.

Effects of unexpected lateral mass placement on trunk loading in lifting

STUDY DESIGN

A repeated measurements experiment of spinal loading in healthy subjects.

OBJECTIVES

To test whether unexpected lateral mass placement increases low back loading and trunk movement when subjects are lifting a mass in upright posture.

SUMMARY OF BACKGROUND DATA

Epidemiologic studies suggest that sudden, unexpected loading will lead to low back pain. Also, asymmetric loading is considered to be harmful to the spine. It can be anticipated that unexpected asymmetric loading will increase the risk of injury even more.

METHODS

Ten subjects lifted in an upright posture a crate, in which a mass of 10 kg was placed laterally at the left side either expectedly or unexpectedly. The crate reaction forces, body movements, and trunk muscle activity were measured. From these, the L5-S1 net moments and muscle forces were estimated.

RESULTS

Unexpected lateral placement of the mass caused no clear increase in peak low back loading. The stiffness of the trunk was lower in the unexpected condition, which, in combination with inadequate net moments produced, resulted in movement of the trunk to the side of the displaced mass.

CONCLUSIONS

Unexpected lateral mass placement does not increase the compression force. Perturbed trunk movement and lower muscle forces indicated a decreased stability of the spine, which may imply an injury risk.

Sudden release during a pulling task: the effect of release load on stance perturbation and recovery

Ten subjects performed a horizontal pulling task with sudden release for three randomly allocated loads of 20, 40, and 60 N to examine the effect of sudden release load on centre of pressure (COP) motion and the response strategy with respect to load, and also to determine critical loads at which the response strategy must involve stepping to prevent a fall. Sudden release produces COP deflection directed posteriorly and laterally towards the anteroposterior midline. The magnitude of these deflections increase with release load (P<0.001), as does the time taken to re-establish normal stance following release (P<0.001). Interaction between lateral and anteroposterior shifts in the COP are seen following release, and establishing COP over a favoured limb may be an important factor in the response strategy. This may be so, because if the perturbation is large enough to provoke a protective step, then the COP will have to be located towards one limb before the step can take place. Linear regression of the anteroposterior limit of the COP indicates a wide range of critical loads that will result in a necessary preventive step backwards being taken (80-257 N, mean 153 N), although these values are likely to be overestimated somewhat.