D. Commissaris

Load knowledge affects low-back loading and control of balance in lifting tasks

This study investigated the effect of the presence or absence of load knowledge on the low-back loading and the control of balance in lifting tasks. Low-back loading was quantified by the net sagittal plane torque at the lumbo-sacral joint. The control of balance was studied by the position of the centre of gravity relative to the base of support, the horizontal and vertical momentum of the centre of gravity and the angular momentum of the whole body. In a first experiment, 8 male subjects lifted a rather heavy load (22% of body mass), using a leglift and a backlift, while they were familiar with the load mass. To counteract the threat to balance, imposed by picking up a load in front of the body, the subjects performed specific preparations, based upon the known load mass; prior to load pick-up, profound changes in the horizontal and angular momentum were found. The preparations were technique specific. Preserving balance seemed easier while picking up a load with a backlift than with a leglift. In the second experiment, 25 male subjects lifted a 6 kg box, which they expected to be 16 kg, because, in a series of lifts, the load mass was changed from 16 to 6 kg without their knowledge. Despite the 10 kg difference in actual load mass, the net torque at the lumbo-sacral joint was not different between lifting 6 and 16 kg, until 150 ms after box lift-off. Moreover, lifting of the overestimated load mass caused a disturbance of balance in 92% of the trials. The postural reactions aimed at regaining balance were not accompanied by an increased low-back loading. It was concluded that the absence of load knowledge, and the following overestimation of the load mass to be lifted, lead to an increased mechanical load on the lumbar spine and to an increased risk of losing balance in lifting tasks. Both events may contribute to a higher risk of low-back injury in manual materials handling tasks.

Scaling anticipatory postural adjustments dependent on confidence of load estimation in a bi-manual whole-body lifting task

Abstract Anticipatory control of motor output enables fast and fluent execution of movement. This applies also to motor tasks in which the performance of movement brings about a disturbance to balance that is not completely predictable. For example, in bi-manual lifting the pick-up of a load causes a forward shift of the centre of mass with consequent disturbance of posture. Anticipatory postural adjustments are scaled to the expected magnitude of the perturbation and are initiated well before the availability of sensory information characterising the full nature of the postural disturbance. However, when the postural disturbance unexpectedly changes, the anticipatory adjustment of joint torques is not equilibrated and may result in a disturbance to balance. In a previous study, it was demonstrated that apart from anticipatory postural adjustments, corrective responses after load pick-up are used to further compensate the postural disturbance. In this study it was examined whether the central nervous system (CNS) assembles a strategy that incorporates both anticipatory control and corrective responses, in which the magnitude of the anticipatory postural adjustments depends on the perceived level of predictability of the postural disturbance. Subjects performed series of lifts in which the magnitude of the load was never revealed to the subject. Two boxes equal in size and colour, but different in mass (6 and 16 kg), were used. Differences in expectation were created by several lifts with the 16-kg load before the 6-kg box was presented. It was observed that the number of strong corrective responses (stepping) varied with the number of 16-kg trials that formed the prior experience when the final 6-kg trial was presented. The follow-up question was whether control relied more on anticipation in the stepping trials, compared with trials in which such gross signs of imbalance were absent. In this study it was shown that subjects when stepping (i) exhibited differential anticipatory postural adjustments in comparison with 6-kg trials in which expectation was not shaped by preceding 16-kg trials, and (ii) scaled the anticipatory postural adjustments similar to those preceding lift-off of the 16-kg trial preceding it. These findings emphasise the programmed nature of the anticipatory postural adjustments and the ability of the CNS to selectively tune the anticipatory postural adjustments to stored information gained during the previous lift(s).

Optimizing the determination of the body center of mass.

The position or trajectory of the body center of mass (COM) is often a parameter of interest when studying posture or movement. For instance, in balance control studies the body COM can be related to the ground reaction force or to the base of support. Since small displacements of the body COM are important in balance control studies, it is essential to obtain valid estimates of the body COM. The main source of error in the determination of the body COM is the estimation of the masses and centers of mass of the body segments. Especially the determination of the trunk COM is prone to error. In the current study five subjects maintained three postures, differing in trunk angle, during a few seconds. The relation between the center of pressure of the ground reaction force and the vertical projection of the body COM during the postures was used to optimize the trunk COM position. Additionally the subjects performed two lifting movements. The validity of the body COM trajectory estimation during the lifting movements, both with and without optimized trunk COM, was checked by relating the external moment of the ground reaction force with respect to the body COM to the rate of change of the angular momentum of the whole body. It was shown that the correspondence between the external moment and the rate of change of the angular momentum improved after optimization of the trunk COM. This suggests that the body COM trajectory estimation can be improved by the proposed optimization procedure.

Controlling the Ground Reaction Force during Lifting

The control of the ground reaction force vector relative to the center of gravity (CoG) was examined while subjects performed a back-lifting task. Six male subjects (aged 24.0 +/- 2.5 years) repeatedly lifted a barbell. A biomechanical analysis that used a linked segment model revealed that the summed rotations of body segments during lifting yielded a specific rate of change of the angular momentum of the entire body. This equaled the external moment provided by Fsubg; relative to CoG. This implies that multisegment movements involve control of the angular momentum of the entire body through an appropriately directed Fsubg;. Thus, in dynamic tasks Fsubg; is pointed away from rather than lined up with the CoG, as is the case in static tasks.

Anticipatory postural adjustments before load pickup in a bi-manual whole body lifting task

Balance regulation and movement control were examined in the context of bi-manual lifting. Subjects picked up a load (20% body mass) after several unloaded cycles using the leg-lift technique. The addition of the load to the body caused the system center of mass to shift forward and thus presented the subject with an expected perturbation of balance. To examine whether the disturbances to balance were counteracted by anticipatory postural adjustments, the last cycle, in which the barbell was grasped and lifted, was compared with the preceding unloaded cycle. Using a global mechanical analysis of the movement, we found that anticipatory postural adjustments were present before load pickup in bi-manual lifting. These anticipatory postural adjustments were characterized by a backward directed horizontal momentum, a backward directed horizontal component of the ground reaction force accompanied with a forward shift of the center of pressure, and a backward shift of the center of mass (CoM). These characteristics could all be understood from the mechanical consideration that adding a load in front of the body induces a forward shift of the CoM. However, major compensations of the position of the CoM were also observed after bar grasp. It is therefore proposed that commands giving rise to postural adjustments are closely tied to commands controlling the ongoing movement. On the basis of this insight the strict dichotomy in the control of posture and movement is being questioned.

Anticipatory postural adjustments in the back and leg lift.

This study examined anticipatory postural adjustments in a dynamic multi-joint action in which a relatively fast voluntary movement is being executed while balance is maintained in the field of gravity. In a bi-manual whole body lifting task, the pickup of the load induces a forward shift in the position of the center of mass, challenging the dynamic balance regulation while simultaneously impeding the ongoing extension movement. We investigated whether anticipatory postural adjustments are an addition to a voluntary motor command or an inherent component of this command. Using a global mechanical analysis of the movement, we found that anticipatory postural adjustments are present in bimanual lifting, both in back lifting and leg lifting, and that lifting technique had a significant influence on the pattern of the adjustments. If the mass of the object was reduced unexpectedly, balance was disturbed in 92% of the mass-reduced trials. These findings suggest that the anticipatory postural adjustments to be performed are specified in advance such that the expected changes in the mechanical interaction with the environment are taken into account. The observations lend support to the hypothesis that the control of the observed anticipatory postural adjustments is an integral part of the control of the lifting movement itself. Consequently, the strict dichotomy in the control of posture and movement is being questioned.

Anticipatory postural adjustments in a bimanual, whole body lifting task with an object of known weight

Abstract Anticipatory postural adjustments (APA) were studied in a dynamic multi-joint movement, in which the legs serve both a focal and a postural role. Eight male subjects bimanually lifted a barbell (20% of body mass) after several unloaded movement cycles using two distinct lifting techniques. Picking up a load induces a perturbation to balance, because the centre of mass (CoM) of the combined body and load shifts forward at the moment of load pick-up. Furthermore, the inertia of the load decelerates the backward rotation of the body towards the erect posture. Both perturbations were found to be counteracted by APA in kinematics, kinetics and leg muscle activity patterns. The APA were characterized by an increase in the backward directed horizontal CoM momentum, an increase in the backward directed whole body angular momentum and a forward shift of the centre of foot pressure (CoP) for both techniques. Anticipatory adjustments in activity of muscles crossing the ankle joint were shown to control the CoP position, which was important to accomplish the required combination of anticipatory changes in horizontal and angular momenta. The APA in kinematics and kinetics were modulated according to the dynamic requirements of each lifting technique, although it could have been expected that picking up and lifting the same load at equal speed would have yielded a similar perturbation in each condition.

Adaptation of center of mass control under microgravity in a whole-body lifting task

Abstract Human balance in stance is usually defined as the preservation of the vertical projection of the center of mass (COM) on thesupport area formed by the feet. Under microgravity conditions, the control of equilibrium seems to be no longer required. However, several reports indicate preservation of COM control in tasks such as arm or leg raising, tiptoe standing, or trunk bending. It is still unclear whether COM control is also maintained in complex multijoint movements during short term exposure to microgravity. In the current study, the dynamics of equilibrium control were studied in four subjects performing two series of seven whole-body lifting movements under microgravity during parabolic flights. The aims of the study were to examine whether the trajectory of horizontal COM motion during lifting movements changes in short-term exposure to microgravity and whether there is any sign of recovery after several lifting movements. It was found that, compared with control movements under normal gravity, the horizontal position of the COM was shifted backward during the entire lifting movement in all subjects. In the second series of lifting movements under microgravity, a partial recovery of the COM trajectory toward the normal gravity situation was found. Under microgravity, angles of the ankle, knee, hip, and lumbar joints differed significantly from the angles found under normal gravity. Recovery of joint angular trajectories in the second series of lifting movementsmainly occurred for those angles that could contribute to a reduction of the backward COM shift. It is to be pointed out that COM control under microgravity is not redundant but functional. Persisting COM control under microgravity may be required for pure mechanical reasons, since rotational movements of the body are dependent on adequate control of the COM position with respect to external forces. It is shown that, from a mechanical perspective, subjects can benefit from a backward displacement of the COM in the downward as well as the upward phase of the lifting movement under microgravity.

Anticipatory postural adjustments in a bimanual, whole-body lifting task seem not only aimed at minimising anterior–posterior centre of mass displacements

Anticipatory postural adjustments (APAs) were studied in a bimanual whole-body lifting task, using a mechanical analysis of the downward movement phase preceding loaded versus unloaded lifts. APAs in the backward ground reaction force were found to lead the perturbing forward box reaction with approximately 400 ms, thus inducing a backward centre of mass momentum. Both the APA onset and magnitude were scaled as a function of the load to be lifted. We conclude that, in this lifting task, the APAs served the generation of an appropriate extending moment of the ground reaction force after box pick-up, rather than the traditionally defined goal of minimising anterior-posterior centre of mass displacements.

Comparison of the postural and physiological effects of two dynamic workstations to conventional sitting and standing workstations.

Abstract Increasing evidence is being found for the association of health risk factors with work-related physical inactivity. An increasing number of people are being exposed to this form of inactivity, and as a result, various interventions aimed at increasing physical activity during working hours are being developed. This study aims to investigate the differences in postural, muscular and physical activities resulting from two dynamic workstations, namely an elliptical trainer and a treadmill workstation, compared with a conventional sitting and standing workstation. Twelve participants completed five standardised office tasks in a laboratory setting at all workstations. No significant effect was found regarding changes in posture and the muscular activity was only significantly higher for the trapezius muscle (50th percentile: 8.1 %MVC) at the dynamic workstations. For the dynamic workstations, physical activity ranged from 4.0 to 14.9 × 10−2 g, heart rate from 14.3 to 27.5 %HRR and energy expenditure from 1.8 to 3.1 METs. Practitioner Summary: Work-related physical inactivity is associated with health risk factors. In this study, physiological and postural effects of dynamic workstations were assessed in comparison to conventional workstations. No significant effects were found regarding changes in posture and muscular activity. Physical activity, heart rate and energy expenditure increased for the dynamic workstations.