In order to plan and control movements the central nervous system (CNS) needs to continuously keep track of the state of the musculoskeletal system. Therefore the CNS constantly uses sensory input from mechanoreceptors in muscles, joints and skin to update information about body configuration on different levels of the CNS. On the conscious level, such representations constitute proprioception. Different tests for assessment of proprioceptive acuity have been described. However, it is unclear if the proprioceptive acuity measurements in these tests correlate within subjects. By using both uni- and multivariate analysis we compared proprioceptive acuity in different variants of ipsilateral active and passive limb position-matching and ipsilateral passive limb movement velocity-discrimination in a group of healthy subjects. The analysis of the position-matching data revealed a higher acuity of matching for active movements in comparison to passive ones. The acuity of matching was negatively correlated to movement extent. There was a lack of correlation between proprioceptive acuity measurements in position-matching and velocity-discrimination.

Clinical research has presented evidence that chronic neck-shoulder pain is associated with impairments ofproprioception (perception oflimb position and movement without vision and touch) and motor control. Thus, assessment ofproprioceptive and motor function of the upper limb may be powerful tools both for research and clinical practice. However, insufficient knowledge of certain features of human sensorimotor control haropers both development and interpretation of results of clinically relevant tests. For example, evidence is lacking which proprioception submodalities (position and movement sense) are reflected in common tests of shoulder proprioception. With respect to testing of upper limb motor function, a better understanding of the control of goal directed arm movements would be needed.

The overall purpose of the thesis was to gain further insights into the sensorimotor control of the upper limb in healthy subjects, with implications for clinical testing and ergonomics. The main aims were: ( 1) to study relationships of outcomes of different psychophysical tests for assessment ofproprioceptive acuity in the shoulder joint and (2) to study control strategies in repetitive bimanual pointing tasks by analysis of the structure ofjoint angle variability.

Proprioceptive acuity was assessed in several variants of ipsilateral position- matching and velocity-discrimination by testing subjects' ability to repeat a memorized arm location and to discriminate between two different velocities of arm movement, respectively. Sensory discrimination thresholds were represented by Yariable Errors (YEs) for position-matching and by Just Noticeable Differences (JNDs) for velocity-discrimination. The pattem of correlations of the YEs and JNDs was analyzed by Principal Component Analysis. The main finding was that two uncorrelated mechanisms based either on perception of position or movement might underlie perception of limb location in ipsilateral position-matching. This depended on the extent of arm movement and on association of the memorized arm location with an active location-focused searching task. The results provided important information for interpretation of common tests of shoulder proprioception with implications for design of novel tests allowing for specific proprioception submodalities to be addressed.

Control strategies in bimanual pointing in 2D and 3D space were studied within the Uncontrolled Manifold (U CM) hypothesis. The structure of joint angle variance was computed wjth respect to the vectorial distance between the endpoints of the arms and with respect to the endpoint coordinates of each arm separately (selected task variables). Joint angle variability was decomposed in variance affecting (YUN) and not affecting (Y COMP) a task variable. The U CM hypothesis predicts that the central nervous system stabilizes a task variable by minimizing Y UN, while allowing Y COMP to be high. Thus, the ratio of these variance components quantifies the degree of control of the task variable. The results showed that the variance in joint space was structured according to the predictions of the U CM hypothesis. It was also shown that the arms were united into one synergy to significantly larger degree than joints within each arm were united into single-arm synergies. It was concluded that the U CM method might quantify components ofmotor variability during repetitive motor tasks, which are not detectable by conventional performance measures.

The central nervous system processes sensory input from proprioceptors to form a state estimate of body configuration, which represents proprioception on theconscious level. Proprioception can be divided in two submodalities: sense of movement and sense of position. It is unclear whether tests, involving psychophysical methods and assessing proprioceptive acuity, can separate between perception of movement and position and thus, measure specific proprioception submodalities. In order to investigate if position matching and velocity discrimination tests can separate perception of position and perception of movement, correlations between outcomes of these tests were analyzed. Sixteen young healthy right-handed and gender balanced subjects performed movements in the right glenohumeral joint in all tests. In the ipsilateral position matching test subjects replicated an arm location, achieved in a criterion movement. Movements were performed from positions Q, 4Q and 80- degrees to target positions 16, 32, 48 and 64 degrees with respect to the body sagittal plane. Four conditions, different in terms of movement mode to the target position during criterion and replication movement, were used: passive-active, passive-passive, active-active, semipassive-semipassive. In the latter condition subjects slightly resisted the passive movement. In the ipsilateral velocity discrimination test subjects assessed if a latter comparison arm movement was faster or slower than a previous criterion movement. Criterion movements were performed at velocities of 3Q and 5Q deg/s. T wo conditions were used: passive-passive and semipassive-semipassive. Variable errors (VE) andjust noticeable differences (JND) were measures of proprioceptive acuity in the position matching and velocity discrimination, respectively. Hierarchical cluster analysis (H CA) and principal component analysis (PCA) were applied to study correlations between outcomes in conditions of both tests. In the H CA solution, the position matchi ng and velocity discrimination conditions were located in clusters on opposite poles of the cluster structure. The PCA results were interpreted for six significant components. The position matching and velocity discrimination conditions loaded mainly on components one to five and on component six, respectively. Both the H CA and the PCA separated all test conditions in t wo groups: one contained the position matching and another the velocity discrimination conditions. There were no clear differences between conditions within each group. The study showed that the outcomes VE and JND of the position matching and velocity discrimination, respectively, are mainly uncorrelated. This implies that these tests likely measure different proprioceptive perceptual components or their different combinations. The position matching and velocity discrimination tests could in general separate between perception of position and movement, although no direct evidence was obtained for ex act match of each test with specific proprioception submodality. Much clearer separation of the measured outcomes could be achieved in subjects, heterogeneous with respect to proprioceptive acuity, i.e. with larger variability of performance.

Previous studies have shown an association of visual demands during near work and increased activity of the trapezius muscle. Those studies were conducted under stationary postural conditions with fixed gaze and artificial visual load. The present study investigated the relationship between ciliary muscle contraction force and trapezius muscle activity across individuals during performance of a natural dynamic motor task under free gaze conditions. Participants (N = 11) tracked a moving visual target with a digital pen on a computer screen. Tracking performance, eye refraction and trapezius muscle activity were continuously measured. Ciliary muscle contraction force was computed from eye accommodative response. There was a significant Pearson correlation between ciliary muscle contraction force and trapezius muscle activity on the tracking side (0.78, p < 0.01) and passive side (0.64, p < 0.05). The study supports the hypothesis that high visual demands, leading to an increased ciliary muscle contraction during continuous eye–hand coordination, may increase trapezius muscle tension and thus contribute to the development of musculoskeletal complaints in the neck–shoulder area. Further experimental studies are required to clarify whether the relationship is valid within each individual or may represent a general personal trait, when individuals with higher eye accommodative response tend to have higher trapezius muscle activity.

The present study aimed to identify whether or not an increase in ciliary-muscle contraction force, when the eye-lens is adjusted for viewing at a near distance, result in an increase in trapezius muscle activity, while performing a natural work task. Twelve participants, ranging in age from 21 to 32 years, performed a computer-mouse work task during free gaze conditions. A moving visual target was tracked with a computer mouse on a screen placed at two different distances from the eyes, 25 cm and 50 cm. Tracking performance, eye accommodation, and bilateral trapezius muscle activity were measured continuously. Ciliary-muscle contraction force was computed according to a formula which takes into account the age-dependent, non-linear relationship between contraction force of the ciliary muscle and the produced level of eye accommodation. Generalized Estimating Equations analyses were performed. On the dominant hand side and for the nearest screen distance, there was a significant effect of ciliary-muscle contraction force on the trapezius muscle activity (p<0.001). No other effects were significant (p>0.05). The results support the hypothesis that high visual demands, during computer mouse work, increase ciliary muscle contraction force and contribute to a raise of the sustained level of trapezius muscle activity. The current study specifically clarifies the validity of the relationship between ciliary-muscle contraction force and trapezius muscle activity and demonstrates that this relationship is not due to a general personal trait. We conclude that a high level of ciliary muscle contraction force can contribute to a development of musculoskeletal complaints in the neck-shoulder area.

The structure of joint angle variability and its changes with practice were investigated using the uncontrolled manifold (UCM) computational approach. Subjects performed fast and accurate bimanual pointing movements in 3D space, trying to match the tip of a pointer, held in the right hand, with the tip of one of three different targets, held in the left hand during a pre-test, several practice sessions and a post-test. The prediction of the UCM approach about the structuring of joint angle variance for selective stabilization of important task variables was tested with respect to selective stabilization of time series of the vectorial distance between the pointer and aimed target tips (bimanual control hypothesis) and with respect to selective stabilization of the endpoint trajectory of each arm (unimanual control hypothesis). The components of the total joint angle variance not affecting (V(COMP)) and affecting (V(UN)) the value of a selected task variable were computed for each 10% of the normalized movement time. The ratio of these two components R(V)=V(COMP)/V(UN) served as a quantitative index of selective stabilization. Both the bimanual and unimanual control hypotheses were supported, however the R(V) values for the bimanual hypothesis were significantly higher than those for the unimanual hypothesis applied to the left and right arm both prior to and after practice. This suggests that the CNS stabilizes the relative trajectory of one endpoint with respect to the other more than it stabilizes the trajectories of each of the endpoints in the external space. Practice-associated improvement in both movement speed and accuracy was accompanied by counter-intuitive lack of changes in R(V). Both V(COMP) and V(UN) variance components decreased such that their ratio remained constant prior to and after practice. We conclude that the UCM approach offers a unique and under-explored opportunity to track changes in the organization of multi-effector systems with practice and allows quantitative assessment of the degree of stabilization of selected performance variables.

Changes in the structure of motor variability during practicing a bimanual pointing task were investigated using the framework of the uncontrolled manifold (UCM) hypothesis. The subjects performed fast and accurate planar movements with both arms, one moving the pointer and the other moving the target. The UCM hypothesis predicts that joint kinematic variability will be structured to selectively stabilize important task variables. This prediction was tested with respect to selective stabilization of the trajectory of the endpoint of each arm (unimanual control hypotheses) and with respect to selective stabilization of the timecourse of the vectorial distance between the target and the pointer tip (bimanual control hypothesis). Components of joint position variance not affecting and affecting a mean value of a selected variable were computed at each 10% of normalized movement time. The ratio of these two components (RV) served as a quantitative index of selective stabilization. Both unimanual control hypotheses and the bimanual control hypothesis were supported both prior to and after practice. However, the RV values for the bimanual control hypothesis were significantly higher than for either of the unimanual control hypothesis, suggesting that the bimanual synergy was not simply a simultaneous execution of two unimanual synergies. After practice, an improvement in both movement speed and accuracy was accompanied by counterintuitive changes in the structure of kinematic variability. Components of joint position variance affecting and not affecting a mean value of a selected variable decreased, but there was a significantly larger drop in the latter when applied on each of the three selected task variables corresponding to the three control hypotheses. We conclude that the UCM hypothesis allows quantitative assessment of the degree of stabilization of selected performance variables and provides information on changes in the structure of a multijoint synergy that may not be reflected in its overall performance. Keywords. Coordination - Variability - Voluntary movement - Bimanual - Human

Purpose

We intended to assess consequences of reduced visual acuity for performance in a natural simple motor task (tracing) using objective kinematic performance measures. Specifically, we intended to elucidate the kind of relationship between the task performance and best corrected binocular visual acuity and to determine the threshold of visual acuity when task performance starts to deteriorate.

Methods

Ninety-five individuals with different best corrected visual acuity participated in the study (age 49 ± 12 years, mean ± SD, 27 men and 68 women). The participants manually traced maze-like visual patterns of different spatial complexity presented on the screen of a portable notebook computer using Clinical Kinematic Assessment Tool software. Tracing error was computed as performance measure in each trial with a spatial pattern matching technique – rigid point set registration method.

Results

The segmented linear regression analysis showed that the relation between visual acuity and tracing errors was best described with a regression function having a break point between two data segments. Tracing performance was unaffected by values of visual acuity below 0.2 on logMAR scale, but when logMAR values increased above this critical limit (i.e. when visual acuity is further reduced), tracing errors linearly increased. The rate of the increase of the tracing error correlated with the complexity of visual stimulus shape.

Conclusion

Testing of fine motor functions with objective kinematic measures during visuomotor tasks may help differentiating between actual effects of reduced visual acuity on eye–hand coordination in individuals with similar levels of impairment of visual acuity.

The authors present a study of the relationship between individual variation in working memory capacity (WMC) and visually guided hand control in the face of visual distraction. WMC was assessed with the automated operation span task. Hand control was measured by requesting participants to track a visual target with a hand-held touch screen pen. Tracking error increased when nontarget visual objects (distractors) appeared, especially in individuals with low WMC. High-WMC individuals are less impaired by distractors than their low-WMC counterpart, because they resume target tracking more quickly after distractor onset. The results suggest that visual distractors cause a momentary interruption to tracking movements and that high WMC attenuates this interruption by facilitating visual search.

Human voluntary movements face a problem of kinematic redundancy: The number of degrees of freedom for the peripheral mechanical apparatus (e.g., a Iimb) is higher than the number of variables necessary to describe movement execution. Thus, there is an infinity of different ways to execute a given motor task. The recently developed Uncontrolled Manifold (UCM) hypothesis suggests that the central nervous system (CNS) generates solutions such that important task related variables are selectively stabilized. Each motor task is associated with stabilizing a .time series of a task variable. At each instant, the CNS selects, in the state space af elements participating in the task, a manifold (UCM) corresponding to a fixed value of the selected task variable. We study a planar bìmanual task, when one hand moves a target and the other hand moves a pointer that must reach the target. We hypothesized that the stabilized task variable was the vectorial difference of the pointertip and the target. The 6 dimensional state space was defined bys 'joint configuration vectors" whose elements were intersegmental joint angles (shoulder, elbow and wrisst in both arms). The subjects repeated the movements IS times, and the movements were recorded by a movement analysis system. Then, the subjects practiced the movements (300 trials). After practice IS trials were recorded again. We computed the variance of the joint configurations before and after practice. Six joint rotations affected the 2 dimensional task variable. The UCM corresponding to this variable is 4- dimensional, while the subspace of the state space that is orthogonal (ORT) to the UCM is 2-dimensional. The variance within the UCM was larger than in the ORT conforming to the UCM hyphothesis. After practice the joint variance decreased and the drop in the component of variance that did not affect the task vaeriable was larger thnn the drop 'of the other component. Thus, practice lead to more stable time courses of the task variable and of the corresponding joint configuration.

Musculotendinous tenderness was evaluated in ambulance personnel, and their working load was studied by means of biomechanical analysis of the standard loading and unloading the `stretcher+patient' in and from the vehicle. Swedish and Japanese subjects were tested. The medical examination demonstrated increased tenderness of selected muscles active in work related movement tasks, as well as in spinous processes in both populations. However, a number ofparticular tests suggested higher Ievel of tenderness in the Swedish, as compared with the Japanese personnel. Biomechanical analysis of working tasks demonstrated similar forces, but higher LS-SI, hip and shoulder torques acting in Swedish personnel. This difference appears to be caused by the differences in stretcher construction. Specifically, the Swedish stretcher requires separate lifting of each stretcher side starting from a squat body position, while the Japanese system requires only pushing the stretcher in and pulling it out from the vehicle. We conclude that differences in working technique imposed by available working equipment could contribute to the observed differences in muscle tenderness especially around the shoulders between the two tested populations. However, further studies are needed to elaborate working load and muscle activity while performing standard tasks of ambulance personnel, as well as to explore other possible causes of increased musculotendinous tenderness. Keywords: Ambulance, Muculoskeletal disorders, Muscle tenderness, Spinous process, Biomechanical analyses

Background: Unlike the usual skeletal muscles, ciliary muscles responsible for focusing the crystalline eye lens and extraocular muscles responsible for convergence eye movements appear resistant to fatigue. Purpose: The dual goals of this article are to briefly outline the current evidence that suggests that probing into blood flow and hemodynamic prefrontal brain activity with Near-Infrared Spectroscopy (fNIRS) could advance progress in visual ergonomic research, and to provide pilot data exemplifying the proposed approach. Methods: The vision task consisted of sustained focusing on a contrast-varying black and white Gabor grating. Four participants with a median age of 46 (IQR 44 – 50) fixated the grating from a distance of 65 cm. Three counterbalanced 10-min tasks required central fixation and accommodation/convergence on the grating target through: (i) 0.0 diopter (D) lenses, (ii) −1.5 D lenses, and (iii) −3.5yD lenses while maintaining maximal focus. Non-invasive measurements of local oxyhemoglobin (HbO₂) were quantified with a one-channel Near Infrared Spectrometer, NIRS. The NIRS probe was placed on the prefrontal cortex in the vicinity of the right dorsolateral prefrontal cortex or Brodmann area 46 (DLPFC, BA 46). Accommodation response and contrast threshold was measured in parallel. Results: General estimating equation analyses showed that baseline subtracted DLPFC blood flow (ΔHbO2) increased significantly over time in all three lens conditions. The effect of time may be caused by a continuous increase in mental effort to compensate for progressively more mental fatigue induced by increased visual attention. The increase of DLPFC ΔHbO2 was also larger in magnitude in participants with larger amplitudes accommodation response (i.e., in participants who minimized deterioration in visual performance). Conclusion: The results from this study indicate that oxyhemoglobin changes recorded over DLPFC with NIRS can be used to assay the degree to which the visual system is strained during demanding near work.

This study is concerned with a new method for partly automating forestry harvesting work. Work-related injuries and constant demands for a higher productivity are two of the many arguments for why forestry work must be improved. Forestry work places great mental demands on the driver because they must continuously evaluate and act on relevant parts in a heavy visual information flow. Against this background the purpose of the present study was to extend the knowledge of functional linkages between visual and mental fatigue, performance, and prefrontal cortex activity, during semi-automated and conventional forestry harvesting work. Eleven healthy participants, range 21–51 years old, with a minimum of 1-year work experience, carried out the task of loading logs along a standardized path in a machine simulator during two counterbalanced 45-min periods: (i) conventional forest harvesting, and; (ii) semi-automated forest harvesting. Equal emphasizes was put on accuracy and speed. During manual forest harvesting the driver controlled the crane arm, used to load logs into the load space of the forest vehicle (“forwarder”), by manually operating the joysticks and so guide the crane to the location of the log and then back to the load space. During semi-automatic forest harvesting the driver moved the crane with the press of a button to a pre-programmed location near the log and then, after another button press, to a pre-programmed location within the load space. The following joystick usage parameters were considered for the statistical analysis: Sequential work cycle number, work phase (1-loading in basket, 2-movement to log, 3-picking up log, 4-movement to load space), number of simultaneously used controls across samples of one phase, number of direction changes of joystick movements per phase. Mental load was assessed by quantification of oxygenated hemoglobin (HbO₂) concentration changes over the right dorsolateral prefrontal cortex (dlPFC) via non-invasive functional near infrared spectrometry (fFNIRS: PortaLite mini, Artinis Medical Systems, Zetten, the Netherlands). The frequency and duration of horizontal amplitudes of eye/head/neck angles was assessed continuously with 8 SmartEye cameras and used as a measure of visual load. NASA-TLX and Borg CRS was used to assess perceived mental and physical fatigue. Linear Mixed Model will be used to test and to analyze the effect of the duration of work, joystick usage, work type (manual or semi-automated) and perceived mental and physical effort on the outcome of oxygenated hemoglobin concentration. This study contributes with new knowledge of the consequences of the current increase in automation. The 4^th industrial revolution can have tremendous implications on how we perceive and organize work in the future, but little is still known about the impact on human body and brain.

The purpose of the present study was to extend the knowledge offunctional linkages between visual and mental load, performance, and prefrontalcortex (PFC) activity, during forestry forwarding work. Eleven healthy participants,range 21–51 years old, with a minimum of 1-year work experience,carried out the task of loading logs along a standardized path in a machinesimulator during two counterbalanced test conditions: (i) conventional cranecontrol, and; (ii) semi-automated crane control. Mental load was assessed byquantification of oxygenated hemoglobin (HbO2) concentration changes overthe right dorsolateral prefrontal cortex (dlPFC) via non-invasive functional nearinfrared spectrometry (fNIRS). Visual, autonomic, and motoric control variableswere measured and analyzed in parallel along with the individual level of performance.Linear Mixed Models (LMM) analysis indicated more mental loadduring conventional crane work. Collectively, our data suggest that fNIRS is aviable tool which can be used in neuroergonomic research to evaluate physiologicalactivity levels in PFC.

This study is concerned with a new method for partly automating forestry harvesting work. Work-related injuries and constant demands for a higher productivity are two of the many arguments for why forestry work must be improved. Forestry work places great mental demands on the driver because they must continuously evaluate and act on relevant parts in a heavy visual information flow. Against this background the purpose of the present study was to extend the knowledge of functional linkages between visual and mental fatigue, performance, and prefrontal cortex activity, during semi-automated and conventional forestry harvesting work. Eleven healthy participants, range 21–51 years old, with a minimum of 1-year work experience, carried out the task of loading logs along a standardized path in a machine simulator during two counterbalanced 45-min periods: (i) conventional forest harvesting, and; (ii) semi-automated forest harvesting. Equal emphasizes was put on accuracy and speed. During manual forest harvesting the driver controlled the crane arm, used to load logs into the load space of the forest vehicle (“forwarder”), by manually operating the joysticks and so guide the crane to the location of the log and then back to the load space. During semi-automatic forest harvesting the driver moved the crane with the press of a button to a pre-programmed location near the log and then, after another button press, to a pre-programmed location within the load space. The following joystick usage parameters were considered for the statistical analysis: Sequential work cycle number, work phase (1-loading in basket, 2-movement to log, 3-picking up log, 4-movement to load space), number of simultaneously used controls across samples of one phase, number of direction changes of joystick movements per phase. Mental load was assessed by quantification of oxygenated hemoglobin (HbO₂) concentration changes over the right dorsolateral prefrontal cortex (dlPFC) via non-invasive functional near infrared spectrometry (fFNIRS: PortaLite mini, Artinis Medical Systems, Zetten, the Netherlands). The frequency and duration of horizontal amplitudes of eye/head/neck angles was assessed continuously with 8 SmartEye cameras and used as a measure of visual load. NASA-TLX and Borg CRS was used to assess perceived mental and physical fatigue. Linear Mixed Model will be used to test and to analyze the effect of the duration of work, joystick usage, work type (manual or semi-automated) and perceived mental and physical effort on the outcome of oxygenated hemoglobin concentration. This study contributes with new knowledge of the consequences of the current increase in automation. The 4^th industrial revolution can have tremendous implications on how we perceive and organize work in the future, but little is still known about the impact on human body and brain.

Introduction:Several studies have reported impaired postural control in people with neck pain. Many of these studies have analysed centre of pressure data from a force plate in quiet standing but to our knowledge none has investigated the kinematics of postural sway in people with neck pain. From studies on healthy controls there are two well established strategies of maintaining upright stance: hip and ankle strategies. Recent work has shown that these co-exist simultaneously. The purpose of this study is to investigate these kinematic strategies in people with neck pain. This could give a greater understanding of the mechanisms behind the postural control impairments and give implications for specific rehabilitation interventions.Methods:Seventy subjects (neck pain n=44, controls n=26) stood with their feet together, arms crossed and their eyes closed for 180 s. An electromagnetic tracker was used to record the kinematics for a two segment model (leg, trunk). An in-phase pattern between these segments corresponds to ankle strategy and an anti-phase pattern to hip-strategy. The strength of the in-phase pattern was quantified using coherence analysis.Results:Preliminary results indicate that people with neck pain rely less on ankle strategy in quiet standing than healthy controls. There were no differences in variability of the segment angles between groups. However people with neck pain tended to stand with a greater degree of extension in the hip.Conclusion:It’s generally considered that hip-strategy is used more when the difficulty of the task is higher. One speculative interpretation to the weaker ankle strategy in the neck pain group is that they perceived the task as more demanding. The method of coherence analysis used in this study can be a valuable tool in future studies for understanding postural sway in people with spinal pain.