Ulf P. Arborelius

Effects of changes in sitting work posture on static neck and shoulder muscle activity

Abstract In order to analyse the effect of changing the sitting posture on the level of neck and shoulder muscular activity, an electromyographic (EMG) study of ten healthy experienced female workers from an electronics plant was undertaken. A standardized, simulated task was performed in eight different sitting work postures. Using surface electrodes, the level of muscular activity was recorded as normalized, full-wave rectified low-pass filtered EMG. The results showed that the whole spine flexed sitting posture gave higher levels of static activity in several neck and shoulder muscles than the posture with a straight and vertical spine, which in turn gave higher levels than the posture with slightly backward-inclined thoraco-lumbar spine.

Mechanisms of blast induced brain injuries, experimental studies in rats

Traumatic brain injuries (TBI) potentially induced by blast waves from detonations result in significant diagnostic problems. It may be assumed that several mechanisms contribute to the injury. This study is an attempt to characterize the presumed components of the blast induced TBI. Our experimental models include a blast tube in which an anesthetized rat can be exposed to controlled detonations of explosives that result in a pressure wave with a magnitude between 130 and 260 kPa. In this model, the animal is fixed with a metal net to avoid head acceleration forces. The second model is a controlled penetration of a 2mm thick needle. In the third model the animal is subjected to a high-speed sagittal rotation angular acceleration. Immunohistochemical labeling for amyloid precursor protein revealed signs of diffuse axonal injury (DAI) in the penetration and rotation models. Signs of punctuate inflammation were observed after focal and rotation injury. Exposure in the blast tube did not induce DAI or detectable cell death, but functional changes. Affymetrix Gene arrays showed changes in the expression in a large number of gene families including cell death, inflammation and neurotransmitters in the hippocampus after both acceleration and penetration injuries. Exposure to the primary blast wave induced limited shifts in gene expression in the hippocampus. The most interesting findings were a downregulation of genes involved in neurogenesis and synaptic transmission. These experiments indicate that rotational acceleration may be a critical factor for DAI and other acute changes after blast TBI. The further exploration of the mechanisms of blast TBI will have to include a search for long-term effects.

Load moments and myoelectric activity when the cervical spine is held in full flexion and extension

Abstract Sustained joint load in extreme positions (namely maximally flexed or extended positions) has been described as causing pain. The aim of the present study is to analyse eight different sitting work postures with respect to extreme positions, and to assess the mechanical load and the levels of muscular activity arising in defined extreme positions of the cervical spine. Ten healthy female workers from an electronics plant took part in laboratory experiments. For seven of these, levels of neck and shoulder muscular activity in sitting postures with the cervical spine in different manually-adjusted extreme positions were recorded using surface electrodes. Loading moments of force about the bilateral motion axis of the atlanto-occipital joint (Occ-C1) and the spinal cervico-thoractc motion segments (C7-T1) were calculated. Extreme or almost extreme positions occurred in sitting postures with the thoracolumbar back inclined slightly backwards or with the whole spine flexed. Electromyographic (EMG) act...

Power output and work in different muscle groups during ergometer cycling.

SummaryThe aim of this study was to calculate the magnitude of the instantaneous muscular power output at the hip, knee and ankle joints during ergometer cycling. Six healthy subjects pedalled a weight-braked bicycle ergometer at 120 watts (W) and 60 revolutions per minute (rpm). The subjects were filmed with a cine camera, and pedal reaction forces were recorded from a force transducer mounted in the pedal. The muscular work at the hip, knee and ankle joint was calculated using a model based upon dynamic mechanics described elsewhere. The mean peak concentric power output was, for the hip extensors, 74.4 W, hip flexors, 18.0 W, knee extensors, 110.1 W, knee flexors, 30.0 W and ankle plantar flexors, 59.4 W. At the ankle joint, energy absorption through eccentric plantar flexor action was observed, with a mean peak power of 11.4 W and negative work of 3.4 J for each limb and complete pedal revolution. The energy production relationships between the different major muscle groups were computed and the contributions to the total positive work were: hip extensors, 27%; hip flexors, 4%; knee extensors, 39%; knee flexors, 10%; and ankle plantar flexors 20%.

Power and work produced in different leg muscle groups when rising from a chair

The aim of this study was to determine the power output and work done by different muscle groups at the hip and knee joints during a rising movement, to be able to tell the degree of activation of the muscle groups and the relationship between concentric and eccentric work. Nine healthy male subjects rose from a chair with the seat at knee level. The moments of force about the hip and knee joints were calculated semidynamically. The power output (P) and work in the different muscle groups surrounding the joints was calculated as moment of force times joint angular velocity. Work was calculated as: work = f Pdt. The mean peak concentric power output was for the hip extensors 49.9 W, hip flexors 7.9 W and knee extensor 89.5 W. This power output corresponded to a net concentric work of 20.7 J, 1.0 J and 55.6 J, respectively. There was no concentric power output from the knee flexor muscles. Energy absorption through eccentric muscle action was produced by the hip extensors and hip flexors with a mean peak power output of 4.8 W and 7.4 W, respectively. It was concluded that during rising, the hip and knee muscles mainly worked concentrically and that the greatest power output and work were produced during concentric contraction of the knee and hip extensor muscles. There was however also a demand for eccentric work by the hip extensors as well as both concentric and eccentric work by the hip flexors. The knee flexor muscles were unloaded.

Influence of sitting postures on neck and shoulder e.m.g. during arm-hand work movements

The aim of the study was to analyze the effect of different sitting postures on the level of activity in some neck and shoulder muscles in a standardized work cycle involving movement of the arm/hand. Ten experienced female workers volunteered in a laboratory study. Full-wave rectified, low-pass filtered, time-averaged and normalized e.m.g. was used. Surface electrodes were applied unilaterally at six locations. The course of the level of muscle activity during the standardized work cycle is presented. There was a tendency to higher level of activity when the arm/hand was moving along the high part of the work object compared to along the low part. The level of activity during the standardized work movement with the arm/hand was significantly influenced by the sitting postures chosen. The highest activity levels were found in the posture with the whole spine flexed. A marked reduction of the level of activity was obtained when a posture with the thoraco-lumbar spine slightly inclined backward was used.

Effects of arm support on shoulder and arm muscle activity during sedentary work.

The aim of this study was to evaluate different arm supports by comparing the activity of shoulder and arm muscles during various work tasks, with and without the lower arm supported. Twelve female subjects, aged between 23 and 37 years, were asked to perform three types of tasks: typing, simulated assembly work (in two different positions), and pipetting. The supports used were: fixed arm support (FIX), horizontal movable arm support (HOR), and spring-loaded arm support (SLA). During the experiments, the electromyograms (EMG) of four muscles were simultaneously recorded: m. deltoideus anterior and lateralis, m. trapezius pars descendens and m. extensor carpi radialis brevis. Normalization was made against maximum isometric contraction. The mean values of the normalized EMG levels showed a reduced EMG level of the shoulder muscles when using arm supports in all the tasks, and for all muscles but the wrist extensor, compared to the EMG levels without arm supports. The horizontal movable support was more effective in reducing the EMG levels of the shoulder muscles than other arm supports, in tasks at table height. Thus, it is possible to reduce muscle activity of the shoulder region by using arm supports. Further research is needed to make biomechanical calculations to compare the EMG level of these muscles using suspension and the effects of inclination of work task.

Load on knee joint and knee muscular activity during machine milking

Abstract Ten professional milkers simulated 20 different standardized machine-milking work postures. Various differences in level between milkers and cow, and various postures at each level, were studied. Knee muscular activity was recorded electromyographically. In the milking procedure when attaching the teat cups to the udder the body position was photographed and the loading moment of force about the bilateral knee axis was calculated by means of a biomechanical model. Working with straight knees resulted in comparatively high moments tending to extend the knee (with the highest mean of 56 N m). This moment was for some subjects close to their maximum strength capacity. A 65° knee angle in the standing postures with small level-difference between cow and milker gave the least knee loading moment (i.e. around zero) independently of body weight. The horizontal distance between operator and cow showed high importance for the amplitude of the knee load. The model for determining knee load can generally be...

Shoulder load during machine milking An electromyographic and biomechanical study

Abstract Ten professional milkers simulated 20 different machine-milking postures related to different vertical distances between milker and cow. The loading moment with respect to the shoulder joint bilateral axes was calculated, and EMGs from four shoulder muscles were recorded. The loading moments were related to isometric strength and EMG was standardized against an isometric maximum contraction. Muscle activity was low to moderate. Load moment increased with increasing level difference between milker and cow, but the simultaneous increase in strength utilization and muscular activation was lower than expected. About 25% of maximum shoulder flexor strength was required. The study illustrates some effects of the interaction between shoulder loading moment, trunk inclination and shoulder flexion. The method used gives load both in absolute form (loading moment) and in relation to individual capacity (strength utilization and normalized EMG).

The load on the low back and hips and muscular activity during machine milking

Abstract We investigated the load on the lumbo-sacral and hip joints and muscle activity in the erector spinae, oblique abdominal, rectus femoris, biceps femoris and gluteus maximus muscles in ten professional milkers simulating 20 different standardized machine-milking work postures in our laboratory. A computerized biomechanical sagittal plane model was used for calculations of load moments and compressive forces. Muscle activity was recorded as time-averaged, full-wave-rectified EMGs, and normalized, ie., expressed as a percentage of an isometric test-contraction. Muscle activity was low in the erector spinae and very low in the oblique abdominal muscles. Activity in hip muscles were generally low compared to that in other activities such as lifting a 12 kg burden, descending stairs and walking. The load on the low back ranged between 64 Nm (corresponding to 1.9 times body-weight compressive force at the L5-S1 disc) and 170 Nm (3.8 times body weight). The lowest induced lumbar load occurred with a vertical level difference (pit depth) of 0.85 m or 1.0 m between milker and cow. The peak load moment about the hip was 96 Nm; the lowest, 31Nm, occuring in the posture with a difference in level of 1.0 m between milker and cow. In this posture only 15 percent of the maximum muscle strength was used to counteract the load moment. Because the load during machine milking is relatively static, the recorded muscle activity is considered to be sufficiently high to partially occlude circulation to the muscle and therefore may result in muscle fatigue and pain.