Neutrophil chemotaxis in moving gradients of fMLP.

In this study the fluid gradient chamber, a modified version of the Boyden chamber that enables mobile gradients, was used to study the migration of human granulocytes in gradients of fMLP. Temporal chemotactic gradients were created by moving density-stabilized spatial gradients at different velocities in relation to migrating cells. Random and directed cell migration was quantified by applying a theoretical population distribution model to experimental cell distributions obtained from cell counts at different depths in the filters. Rates of random and directed migration generally increased with gradient velocity. At negative gradient velocities, i.e., when the gradients were moved in a direction opposite to that of cell migration to decrease fMLP concentration over time, random and directed migration was inhibited. At positive gradient velocities, migration rates were not significantly different from those seen in immobile gradients. The fact that the rate of directed migration was smaller at negative gradient velocities indicates that negative temporal gradients reduced the average speed and/or orientation of the chemotactically migrating cells. In immobile gradients, the cells generated a small concentration increase over time when they migrated in the up-gradient direction. Consequently, a positive temporal gradient as perceived by the cells may act as a positive feedback signal to maintain chemotactic migration.

Total shoulder and relative muscle strength in the scapular plane.

Strength profiles of the shoulder joint are measured experimentally for two arm positions in "the scapular plane" in order to present quantitative data on the shoulder strength. Apart from yielding the actual force a subject can exert in various directions, these measurements also exhibit e.g. the strongest and weakest directions, in fact the relative strength in all directions. The inter-individual variation of the direction of maximal force was at most 14 degrees (sd). The experimental profiles are compared with the corresponding theoretical profiles, obtained by using a shoulder model. The calculations were made both with default muscle parameters and individually adapted parameters. The results show that the employed shoulder model, which is based on data from an elderly population, may be adapted to other populations and that the necessary changes in relative muscle strength are those expected on biomechanical grounds. Without model changes the difference between measured (in the mean) and predicted maximal force directions was at most 50 degrees. Muscle parameter adjustment reduced this difference to 23 degrees. The strength profiles clearly indicate in what direction a person can produce larger forces and which muscles that contribute.

A subpopulation analysis of f-MLP stimulated granulocytes migrating in filters.

Leukocyte migration in vitro has been studied extensively during many years without providing satisfactory theoretical models for the different migratory behaviors (chemotaxis and chemokinesis) of leukocyte populations. The present study utilized the fluid gradient chamber, which is a new method to study leukocyte migration in filters. Human neutrophils were applied between two stacked filters and migrated in all directions under the influence of constant concentrations or chemotactic gradients of f-MLP, maintained in fluid phase density gradients. The distributions of the granulocytes over filter depth were fitted to theoretical functions composed by 1-3 Gaussian distributions, representing subpopulations. The results showed that the neutrophils migrated as two discrete subpopulations during chemokinetic stimulation (a constant concentration of f-MLP). One of the subpopulations showed less active and passive (slow sedimentation under the influence of gravity) translocation. The most mobile subpopulation was divided into two new subpopulations when exposed to chemotactic stimulation (concentration gradient of f-MLP), one of which responded chemotactically and one of which migrated in random directions. The properties of the different subpopulations where characterized in terms of diffusion coefficient (random migration), convection velocity (chemotactic migration) and sedimentation coefficient (passive translocation).

Structure and internal consistency of a shoulder model.

A three-dimensional biomechanical model of the shoulder is developed for force predictions in 46 shoulder structures. The model is directed towards the analysis of static working situations where the load is low or moderate. Arbitrary static arm postures in the natural shoulder range may be considered, as well as different kinds of external loads including different force and moment directions. The model can predict internal forces for the shoulder muscles, for the glenohumeral, the acromioclavicular and the sternoclavicular joint as well as for the coracohumeral ligament. A solution to the statistically indeterminate force system is obtained by minimising an objective function. The default function chosen for this is the sum of the squared muscle stresses, but other objective functions may be used as well. The structure of the model is described and its ingredients discussed. The internal consistency of the model, its structural stability and the compatibility of the elements that go into it, is investigated.

Voluntary redistribution of muscle activity in human shoulder muscles.

Four shoulder muscles (the supraspinatus, the infraspinatus, the anterior and middle portion of the deltoid, and the descending part of the trapezius) were examined with electromyography in abducted arm positions. By using feedback techniques, we found that the subjects could reduce the EMG activity voluntarily by 22-47% in the trapezius muscle while keeping different static postures. This was not true for any other muscle investigated. When the trapezius activity was reduced there was a tendency towards an increase of EMG activity in some other shoulder muscles, particularly the infraspinatus. The findings may be related to relaxation from an initial overstabilization of the shoulder, or redistribution of load among synergists. It is suggested that the possibility of reducing trapezius activity may be of ergonomic significance. It is also noted that EMG trapezius activity may not serve as a universal descriptor of total muscular load in the shoulder.

Biomechanical model of the human shoulder joint--II. The shoulder rhythm.

A method to investigate the rhythm of the human shoulder, i.e. the interplay between the motion of constituent parts of the shoulder, has been devised and tested. The method is based upon numerical evaluation of low dose roentgenstereophotogrammetric motion pictures of subjects equipped with radiation dense implantations in the bones. Evaluation of the method shows that it may be used in determining motion patterns and that the employed interpolation techniques can be used to simulate motions not actually performed in the laboratory. The shoulder rhythm has been previously poorly investigated and quantified results published pertain to one plane only. Our results on motion patterns correlate with previous investigations. With this method, we show that the absolute position of the bones varies significantly between individuals while the relative displacement of the bones during motion exhibit similarities. In particular the results show that, under normal conditions, the individual rhythm is very stable and insensitive to small hand-loads.

Clinical assessment of rheumatic diseases using viscoelastic parameters for synovial fluid.

For the first time it is clearly exhibited that synovial fluid (SF) is thixotropic. Although no hysteresis loops were observed for SF, not even at high shear rates, thixotropy may be exhibited by measuring the rate of recovery after extensive shearing. The rebuilding of the structure in a small-amplitude oscillatory state following the high-shear-rate state reveals the thixotropic behaviour. Five different viscoelastic parameters for various synovial fluids (SF) were obtained using oscillatory rheometry. It was also shown that for SF in the low frequency range, corresponding to a knee joint almost at rest, the shear loss modulus G" is greater than the shear storage modulus G', since the system is allowed to dissipate energy at rest. However, with movement, G' increases and eventually becomes greater than G" at a characteristic frequency above which the system has insufficient time to dissipate energy and hence responds as an elastic body. This functional behaviour, characteristic for normal SF, broke down in the SF of rheumatoid arthritis. It was also absent in the SF of knee joints with meniscus lesions and ligament defects.

Intramuscular pressure and electromyography in the supraspinatus muscle at shoulder abduction.

In ergonomic and biomechanic research, estimations of shoulder muscle load and evaluation of different measuring techniques are important. Intramuscular pressure (IMP), using the microcapillary infusion technique and bipolar intramuscular electromyography (EMG), was recorded from the same part of the supraspinatus muscle. In 12 subjects, IMP and EMG were recorded at shoulder abduction angles of 0 degrees, 30 degrees, 60 degrees, 90 degrees, and 135 degrees with no or a 1- or 2-kg hand load in each position. The shoulder torque was calculated for each test position. A correlation was found for both mean IMP and mean EMG versus shoulder torque if the position with extreme muscle shortening (135 degrees abduction) was excluded. IMP was high (greater than 50 mmHg) in almost all test situations. In seven other subjects, isometric force in abduction was correlated with IMP and EMG. Both methods showed an equally good correlation with external shoulder force, IMP gives as good an estimation of relative muscle force as EMG, but any comparison between EMG and IMP must be done at the same muscle length.

Biomechanical model of the human shoulder--I. Elements.

In a first step of a biomechanical modelling of the human shoulder the points of application of muscle forces to the bones were determined in a dissection study on four human shoulder specimens. A kinematic description of the shoulder was presented employing bone fixed coordinate systems. Actions of the shoulder muscles were modelled as a system of forces acting along ideal strings. The shoulder complex was treated as a three rigid body twelve degrees of freedom system.

Shoulder pain and heavy manual labor.

The problem of the increasingly common shoulder pain syndromes in industry was investigated by biomechanic and epidemiologic methods. Rotator cuff tendinitis constitutes a major problem in people with arduous occupations, i.e., shipyard welders (prevalence ratio, 18.3%), and steel plateworkers (16.2%). Static work seems to induce symptoms at an earlier age. The clinical diagnosis was supraspinatus tendinitis. The biomechanic studies confirmed the view that the shoulder muscles are heavily loaded when the arm is elevated. Excessive hand tool mass increases the strain markedly in some muscles, particularly the supraspinatus and the infraspinatus. The hypovascularity of the supraspinatus tendon is then likely to be accentuated by high intramuscular pressure that reduces the blood flow through the muscle. The strain on the supraspinatus muscle in overhead work is an important factor contributing to prolonged shoulder disability. An analysis of the distribution of loads imposed on the shoulder in practical working situations can be based on observations of working postures and external forces.