Aging and Bimanual Effects on Finger Center of Pressure during Precision Grip: Different Strategies for Spatial Stability

The purpose of this study was to examine aging and bimanual effects on finger spatial stability during precision grip. Twenty-one older and 21 younger adults performed precision grip tasks consisting of a single task (grip and lift an object with the thumb and index finger) and a dual task (the grip-lifting task with one hand and a peg board task with the other hand). The center of pressure (COP) trajectory and the grip force were evaluated using a pressure sensor with a high spatial resolution. In the COP trajectory, the main effects of age for the thumb (F1,140 = 46.17, p < 0.01) and index finger (F1,140 = 22.14, p < 0.01) and task difficulty for the thumb (F1,140 = 6.47, p = 0.01) were significant based on ANCOVA. The COP trajectory was statistically decreased in the older adults. The COP trajectory was also decreased in the dual task, regardless of age. The results suggest the existence of a safety strategy to prioritize the spatial stability in the elderly group and in the dual task. This study provides new insights into the interpretation of the COP trajectory.

The effect of visually manipulating back size and morphology on back perception, body ownership, and attitudes towards self-capacity during a lifting task

Body re-sizing illusions can profoundly alter perception of our own body. We investigated whether creating the illusion of a muscled and fit-looking back (Strong) influenced perceived back size, body ownership, and attitudes towards self-capacity during a lifting task. Twenty-four healthy male volunteers performed a standardised lifting task while viewing real-time (delay < 20 ms) video of their own back through a head-mounted display under four different conditions (Normal size, Strong, Reshaped, Large; order randomised). The MIRAGE-mediated reality system was used to modify the shape, size, and morphology of the back. Participants were poor at recognizing the correct appearance of their back, for both implicit (perceived width of shoulders and hips) and explicit (questionnaire) measures of back size. Visual distortions of body shape (Reshaped condition) altered implicit back size measures. However, viewing a muscled back (Strong condition) did not result in a sense of agency or ownership and did not update implicit perception of the back. No conditions improved perceptions/attitudes of self-capacity (perceived back strength, perceived lifting confidence, and perceived back fitness). The results lend support for the importance of the embodiment of bodily changes to induce changes in perception. Further work is warranted to determine whether increased exposure to illusory changes would alter perceptions and attitudes towards self-capacity or whether different mechanisms are involved.

Spatial distribution of erector spinae activity is related to task-specific pain-related fear during a repetitive object lifting task

ABSTRACTFear-avoidance beliefs, particularly the fear of lifting an object with a flexed spine, were shown to be associated with reduced spinal motion during object lifting in both individuals with and without low back pain (LBP). LBP patients thereby also showed potentially clinically relevant changes in the spatial distribution of back muscle activity, but it remains unknown whether such associations are also present in pain-free individuals. The aim of this study was therefore to investigate the relationship between fear-avoidance beliefs and the change in spatial distribution of lumbar paraspinal muscle activity in pain-free individuals during a repetitive object lifting task. Thirty participants completed two pain-related fear questionnaires and performed 25 repetitions of lifting a 5kg-box from a lower to an upper shelf and back, while multi-channel electromyographic signals were recorded bilaterally from the lumbar erector spinae muscles. Changes in spatial distribution were determined by calculating the differences in vertical position of the weighted centroids of muscle activity (centroid shift) between the first and last few repetitions. Multiple linear regression analyses were performed to examine the relationship between the centroid shift and fear-avoidance belief scores. The analyses showed that the fear of lifting an object with a flexed spine was negatively associated with erector spinae activity centroid shift (R2 adj. = 0.1832; p = 0.045), which might be an expression of behavioral alterations in order to prevent the back from possible harm.

Personalizing the control law of an upper-limb exoskeleton using EMG signal

Implementing an intuitive control law for an upperlimb exoskeleton dedicated to force augmentation is a challenging issue in the field of human-robot collaboration. The goal of this study is to adapt an EMG-based control system to a user based on individual characteristics. To this aim, a method has been designed to tune the parameters of control using objective criteria, improving user's feedback. The user's response time is used as an objective value to adapt the gain of the controller. The proposed approach was tested on 10 participants during a lifting task. Two different conditions have been used to control the exoskeleton: with a generic gain and with a personalized gain. EMG signals was captured on five muscles to evaluate the efficiency of the conditions and the user's adaptation. Results showed a statistically significant reduction of mean muscle activity of the deltoid between the beginning and the end of each situation (28.6%, standard deviation (SD) 13.5% to 17.2%, SD 7.3%, of Relative Maximal Contraction for the generic gain and from 24.9%, SD 8.5%, to 18%, SD 6.8%, of Relative Maximal Contraction for the personalized gain). When focusing on the first assisted movements, the personalized gain induced a mean activity of the deltoid significantly lower (29%, SD 8.0%, of Relative Maximal Contraction and 37.4%, SD 9.5%, of Relative Maximal Contraction, respectively). Subjective evaluation showed that the system with a personalized gain was perceived as more intuitive, and required less concentration when compared to the system with a generic gain.

Kinematic analysis of movement impaired by generalization of fear of movement-related pain in workers with low back pain

Purpose To identify impaired trunk movement during work-related activity in individuals with low back pain (LBP) and investigate whether abnormalities were caused by generalized fear of movement-related pain. Methods This cross-sectional study was conducted at a hospital in Japan. We recruited 35 participants with LBP (LBP group; 26 males, 9 females) and 20 healthy controls (HC group) via posters at our hospital. The task required lifting an object. We used a 3D motion capture system to calculate the peak angular velocity of trunk flexion and extension during a lifting task. Pain-related factors for the LBP group were assessed using the visual analogue scale (VAS) for pain intensity over the past 4 weeks and during the task, the Tampa Scale for Kinesiophobia (TSK), the Pain Catastrophizing Scale (PCS), and the Pain Anxiety Symptoms Scale-20 (PASS-20). We compared kinematic variables between groups with a generalized linear mixed model and investigated the relationship between kinematic variables, VAS scores, and psychological factors by performing a mediation analysis. Results The peak angular velocity of trunk extension showed significant main effects on the group factors (LBP group vs. HC group) and their interactions; the value of the kinematic variable was lower at Trial 1 in the LBP group. No LBP participant reported pain during the experiment. The mediation analysis revealed that the relationship between the VAS score for pain intensity over the past 4 weeks and the peak angular velocity of trunk extension in the first trial was completely mediated by the TSK (complete mediation model, 95% bootstrapped CI: 0.07–0.56). Conclusion Individuals with LBP had reduced trunk extension during a lifting task. Generalized fear of movement-related pain may contribute to such impaired trunk movement. Our findings suggest that intervention to ameliorate fear of movement may be needed to improve LBP-associated disability.

Inter-individual variability in a repetitive lifting task

Trunk kinematics directly impact the biomechanical loading of the tissues of the low back. Quantifying the variability in trunk kinematics may provide deeper insights into biomechanical loading and low back injury risk. Inter-lifter variability in trunk kinematics was assessed as twenty participants performed a repetitive lifting task at three levels of the NIOSH Lifting Index. Trunk kinematics were captured and Levene’s test of homogeneity of variance was used to test the hypothesis that variance in kinematic parameters increased as a function of level of lifting index. Results showed considerable levels of variability in all kinematics parameters, and for sagittal range of motion, mean sagittal velocity, transverse range of motion, and mean transverse velocity the variance was significantly affected (p<0.05) by level of lifting index. The results of this study demonstrate that variability (both inter- and intra-lifter) should be considered as one considers the relative risk of a lifting task.

Design Human-Robot Collaborative Lifting Task Using Optimization

In this paper, an optimization-based dynamic modeling method is used for human-robot lifting motion prediction. The three-dimensional (3D) human arm model has 13 degrees of freedom (DOFs) and the 3D robotic arm (Sawyer robotic arm) has 10 DOFs. The human arm and robotic arm are built in Denavit-Hartenberg (DH) representation. In addition, the 3D box is modeled as a floating-base rigid body with 6 global DOFs. The interactions between human arm and box, and robot and box are modeled as a set of grasping forces which are treated as unknowns (design variables) in the optimization formulation. The inverse dynamic optimization is used to simulate the lifting motion where the summation of joint torque squares of human arm is minimized subjected to physical and task constraints. The design variables are control points of cubic B-splines of joint angle profiles of the human arm, robotic arm, and box, and the box grasping forces at each time point. A numerical example is simulated for huma-robot lifting with a 10 Kg box. The human and robotic arms’ joint angle, joint torque, and grasping force profiles are reported. These optimal outputs can be used as references to control the human-robot collaborative lifting task.