Joint angle variability and co-variation in a reaching with a rod task

The problem at the heart of motor control is how the myriad units of the neuromotor system are coordinated to perform goal-directed movements. Although for long these numerous degrees of freedom (DOFs) were considered redundant, recent views emphasize more that the DOFs should be considered abundant, allowing flexible performance. We studied how variability in arm joints was employed to stabilize the displaced end-effector in tool use to examine how the neuromotor system flexibly exploits DOFs in the upper extremity. Participants made pointing movements with the index finger and with the index finger extended by rods of 10, 20, and 30 cm. Using the uncontrolled manifold (UCM) method, the total joint angle variance was decomposed into two parts, the joint angle variance that did not affect the position of the end-effector (VUCM) and the variance that results in a deviation of the position of the end-effector from its mean (VORT). Analyses showed that some angles depended on length of the rod in use. For all rod lengths, VUCM was larger than VORT, and this did not differ over rod lengths, demonstrating that the arm was organized into a synergy. Finally, the variation in the joint angles in the arm as well as the degree of co-variation between these angles did not differ for the rod’s tip and the hand. We concluded that synergies are formed in the arm during reaching with an extended end-effector and those synergies stabilize different parts of the arm+rod system equally

Are movement disorders and sensorimotor injuries pathologic synergies? When normal multi-joint movement synergies become pathologic

The intact nervous system has an exquisite ability to modulate the activity of multiple muscles acting at one or more joints to produce an enormous range of actions. Seemingly simple tasks, such as reaching for an object or walking, in fact rely on very complex spatial and temporal patterns of muscle activations. Neurological disorders such as stroke and focal dystonia affect the ability to coordinate multi-joint movements. This article reviews the state of the art of research of muscle synergies in the intact and damaged nervous system, their implications for recovery and rehabilitation, and proposes avenues for research aimed at restoring the nervous system’s ability to control movement

Analyse biomécanique du mouvement de préhension contraint et altéré : indices quantitatifs de la gestion de la redondance motrice

Through the assessment of quantitative indexes, this work focused on the biomechanical analysis of the upper limb redundancy and motor coordination during a reach-to-grasp movement. Based on the Uncontrolled Manifold approach, we considered, on one hand, a particular aspect of the synergies, i.e. the covaration of elemental variables of a system that lead to a stabilization of a particular performance variable. In this framework, it was hypothesized that degrees of freedom were not frozen as suggested by Bernstein but combined in order to ensure performance and stability. On the other hand, we used a robotic index, the manipulability, allowing the assessment of wrist displacement capacities during the grasping task. The aim of the present work was to study the motor coordination during constrained and altered upper limb movements through the evaluation of these two. In this framework, the first analysis consisted in evaluating the effect of a spatial constraint materialized by an obstacle during the transport phase of the reach-to-grasp movement. The second part consisted in identifying motor coordination of the upper limb for spinal cord injury patients, especially quadriplegic patients, using both kinematics and electromyographic (EGM) analyses. Two experiments were conducted to record kinematics and EMG activity of upper limb muscles with adapted apparatus. In the presence of an obstacle, results showed that wrist Cartesian trajectory was stabilized through flexible combination of joint synergy. Also, the results showed an increase of goal equivalent configurations to stabilize the elbow trajectory when an obstacle was present. This result suggests that the increase of the joint angle flexibility may be a mechanism by which the central nervous system takes into account a spatial constraint during obstacle avoidance. In the same time, the increase of the manipulability of the wrist joint center was found. For quadriplegic patients, EMG analysis showed a compensation of the weakness or paralysis of particular muscles, particularly the triceps brachii, by an increase of the relative activation of the shoulder muscles. In addition, these subjects presented a variance decomposition similar to the one of the control group suggesting that, despite the presence of a motor deficiency, goal equivalent joint configuration during the execution of the movement are still present or have been recovered. Also, for manipulability, patients have strong similarities with the control group. Considering these results, we think that the exploration of synergies through the proposed indices could be an interesting tool to study the impact of external constraint and impairment on the biomechanical parameters of movement. These works open the way for interesting applications in the field of movement simulation by considering the variability of joint trajectories in relation to a performance variable. Also, they may be applied to the assessment of the coordination of impaired subjects in order to evaluate and improve rehabilitation programs by providing quantitative information on the contribution of all the degrees of freedom.Au travers de l'évaluation d'indices quantitatifs, ce travail se focalise sur l'analyse biomécanique de la redondance et des coordinations motrices du membre supérieur lors de la réalisation du mouvement de préhension. En nous basant sur la théorie de « l'uncontrolled manifold », nous abordons, d'une part, un aspect particulier de la notion de synergie qui est relatif à la covariation des paramètres d'état du système afin de stabiliser une variable de performance. En effet, la thématique de la redondance motrice qui est une des caractéristiques principale du système musculo-squelettique a été récemment reformulée. Dans ce cadre, il est postulé que les degrés de liberté ne sont jamais éliminés comme le suggère la théorie proposée par Bernstein mais combinés afin d'assurer performance et flexibilité. D'autre part, nous utilisons un indice issu de la robotique, la manipulabilité permettant d'évaluer les capacités de déplacement du poignet au cours de ce mouvement de saisie. L'objectif de ce travail consiste à appliquer ces deux analyses à des mouvements contraints ou altérés afin d'identifier les modifications de la coordination motrice par rapport à des conditions contrôles. Dans cette optique, la première analyse consiste à évaluer l'effet d'une contrainte spatiale matérialisée par un obstacle lors de la phase d'approche d'un mouvement de préhension par une mesure des paramètres cinématiques du membre supérieur. La seconde étude vise à identifier la coordination motrice du membre supérieur de sujets atteints d'une lésion médullaire et souffrants de tétraplégie par une analyse combinée des paramètres électromyographiques (EMG) de certains muscles impliqués et des paramètres cinématiques. Pour cela nous avons mis en place deux protocoles expérimentaux au cours desquels les angles articulaires du membre supérieur ainsi que le signal EMG ont été évalués par des moyens adaptés. En présence d'un obstacle, nos résultats montrent de manière classique que la position du poignet est stabilisée au travers d'une synergie articulaire. De plus, il semble que la présence d'une contrainte spatiale renforce les synergies pour stabiliser la trajectoire du coude dans la seconde partie du mouvement correspondant au franchissement de l'obstacle. Ce renforcement se caractérise par une utilisation accrue de configurations articulaires équivalentes. Ce résultat suggère que l'augmentation de la flexibilité des configurations articulaires serait un mécanisme par lequel le système nerveux central pourrait prendre en compte la présence d'une contrainte spatiale. Parallèlement, la présence de l'obstacle entraine des modifications de la manipulabilité du poignet. Pour les sujets souffrants de tétraplégie, l'analyse EMG a montré une compensation de la faiblesse ou la paralysie de certains muscles, le triceps brachial notamment, par une augmentation de l'activité relative des muscles de l'épaule. De plus, ces sujets présentent une décomposition de la variance des angles articulaires similaire à celle du groupe contrôle suggérant que, malgré la présence d'une déficience motrice, la flexibilité des configurations articulaire au cours de l'exécution du mouvement est toujours présente ou a été récupérée. De même pour la manipulabilité, les patients présentent de fortes similitudes avec les sujets valides avec, dans certains cas, des valeurs de manipulabilité du poignet supérieures. Ces résultats nous amènent à penser que l'étude des synergies au travers des indices proposés pourrait constituer un outil intéressant afin d'étudier l'impact d'une contrainte et d'une déficience motrice sur les paramètres biomécaniques du mouvement. Les présents travaux ouvrent des voies intéressantes pour des applications dans le domaine de la simulation du mouvement et de l'évaluation fonctionnelle de la coordination de sujets déficients moteurs en vue d'évaluer et améliorer la rééducation par l'apport d'informations quantitatives aux cliniciens

Postural prevalence, time of day and spent time activities during smartphone weekday use among students: A survey to prevent musculoskeletal disorders

The long time spent on smartphones in awkward postures exposes young users to the risk of developing musculoskeletal disorders (MSDs). This study aimed to investigate 1) how the duration of smartphone use varies by the time of day and activities and 2) the risks of MSDs based on an analysis of the postures used when interacting with smartphones. A cross-sectional survey was conducted among 263 university students. The duration of smartphone use during a typical weekday was investigated over four times of the day and seven activities. After checking for normality, a nonparametric Friedman test was used to study the differences in the time spent using a smartphone according to the time of day and activity. Postural prevalence during weekdays was analyzed using a taxonomy called SmarTaxo, consisting of 41 postures. The Rapid Upper Limb Assessment (RULA) ergonomic score was chosen to assess the MSD risks associated with each posture.Smartphone use was the highest in the evening (301.1 min; 95 % confidence interval [CI]: 277.4–324.8 min, p < 0.05). Texting (170.8 min; 95 % CI: 152.0–189.6 min) and watching videos (163.6 min; 95 % CI: 146.3–180.9 min) were the most common activities. Three sitting and two walking postures were primarily used in the morning (29.3–36.9 %), afternoon (27.0–44.4 %), and evening (28.9%–38.9 %). Standing postures were preferred in the morning and afternoon (36.9 % and 42.2 %, respectively), while one lying posture was widely reported in the evening (39.2 %). The RULA scores for these postures ranged from 3 to 4. However, four lying postures, often observed during the evening (frequency between 20.5 % and 37.6 % of the time), had RULA scores of 6. In conclusion, the study identified an existing MSD risk among smartphone users, especially with long durations of daily use. Special emphasis should be placed on addressing the reclining postures adopted during evening smartphone use, as they subject students to a significantly elevated risk of MSDs

Effect of day time on smartphone use posture and related musculoskeletal disorders risk: a survey among university students

Abstract Background Musculoskeletal disorders (MSDs) are one of the most important problems among young smartphone users worldwide. Portability leads to a wide variety of postures during the different activities of the day. The objective evaluation of these postures coupled with ergonomic tools allows evaluating the level of MSD risk to which users are exposed. Methods The purpose was to investigate the effect of the time of day on the posture adopted during smartphone use among university students. The study was conducted through a cross-sectional survey of 263 university sports students. Four time of day, i.e. morning, afternoon, evening and night, and a taxonomy of 41 postures called SmarTaxo were considered. SmarTaxo included 18 sitting, 11 standing, 10 lying and 2 walking postures and their ergonomic score. After checking the normality of the data, a non-parametric Kruskal–Wallis test was used to study the effect of the time of day on the use duration of the different postures. Results The total mean duration use per typical weekday was 5.39 ± 2.19 h for males and 5.15 ± 1.60 h for females with maximal duration during evening. The average smartphone use durations were statistically longer in afternoon and evening for all sitting (9.44 and 9.22 min respectively, p  5 h per day), young people will remain highly exposed to MSDs

Joint intra-individual variability simulation based on UCM hypothesis

International audienceno abstrac

Effect of the presence or absence of upper limb support on posture when a smartphone user is in a seated position under ambient light conditions

International audienceBackground: Home, public transport, work, the number of environmental positions constraining the use of smartphones is significant.Research question: How can the presence of an upper limb support influence the sitting posture of smartphone users under fixed illumination and brightness?Methods: Twelve subjects (21,6 ± 5,5 years old) performed web browsing under two environmental positions (POSITION): sitting with and without support (table). The users' postures were evaluated through relative kinematics joint angles analysis. A repeated measure analysis of variance and Tukey post-hoc tests were performed to test the effect of POSITION on posture.Results: In sitting position in front of a table, neck is less stressed (flexion < 10°) and trunk and shoulder are supported which suggests less constrains for the joints.Relevance to industry: To prevent injury or pain, the use of an upper limb support (to lean on) should be considered for people/workers who use the smartphone frequently