Is equilibrium point control feasible for fast goal-directed single-joint movements?

Several types of equilibrium point (EP) controllers have been proposed for the control of posture and movement. EP controllers are appealing from a computational perspective because they do not require solving the "inverse dynamic problem" (i.e., computation of the torques required to move a system along a desired trajectory). It has been argued that EP controllers are not capable of controlling fast single-joint movements. To refute this statement, several extensions have been proposed, although these have been tested using models in which only the tendon compliance, force-length-velocity relation, and mechanical interaction between tendon and contractile element were not adequately represented. In the present study, fast elbow-joint movements were measured and an attempt was made to reproduce these using a realistic musculoskeletal model of the human arm. Three types of EP controllers were evaluated: an open-loop α-controller, a closed-loop λ-controller, and a hybrid open- and closed-loop controller. For each controller we considered a continuous version and a version in which the control signals were sent out intermittently. Only the intermittent hybrid EP controller was capable of generating movements that were as fast as those of the subjects. As a result of the nonlinear muscle properties, the hybrid EP controller requires a more detailed representation of static muscle properties than generally assumed in the context of EP control. In sum, this study shows that fast single-joint movements can be realized without explicitly solving the inverse dynamics problem, but in a less straightforward manner than implied by proponents of conventional EP controllers. Copyright © 2006 The American Physiological Society

Can proprioceptive training improve motor learning?

Recent work has investigated the link between motor learning and sensory function in arm movement control. A number of findings are consistent with the idea that motor learning is associated with systematic changes to proprioception (Haith A, Jackson C, Mial R, Vijayakumar S. Adv Neural Inf Process Syst 21: 593-600, 2008; Ostry DJ, Darainy M, Mattar AA, Wong J, Gribble PL. J Neurosci 30: 5384-5393, 2010; Vahdat S, Darainy M, Milner TE, Ostry DJ. J Neurosci 31: 16907- 16915, 2011). Here, we tested whether motor learning could be improved by providing subjects with proprioceptive training on a desired hand trajectory. Subjects were instructed to reproduce both the time-varying position and velocity of novel, complex hand trajectories. Subjects underwent 3 days of training with 90 movement trials per day. Active movement trials were interleaved with demonstration trials. For control subjects, these interleaved demonstration trials consisted of visual demonstration alone. A second group of subjects received visual and proprioceptive demonstration simultaneously; this group was presented with the same visual stimulus, but, in addition, their limb was moved through the target trajectory by a robot using servo control. Subjects who experienced the additional proprioceptive demonstration of the desired trajectory showed greater improvements during training movements than control subjects who only received visual information. This benefit of adding proprioceptive training was seen in both movement speed and position error. Interestingly, additional control subjects who received proprioceptive guidance while actively moving their arm during demonstration trials did not show the same improvement in positional accuracy. These findings support the idea that the addition of proprioceptive training can augment motor learning, and that this benefit is greatest when the subject passively experiences the goal movement. © 2012 the American Physiological Society

Multi-scale adaptations and vulnerability transfer in an artificial society: From sugarscape to sugarscale

In a context of global changes, coastal social-ecological systems are submitted to growing and combining pressures. To reduce their vulnerability, agents respond through adaptations that occurs at multiple scales (from individual to collective), leading to nested feedback loops with consequences that can be expected or not. Artificial societies are of great help to explore these complex and uncertain adaptation dynamics. Taking inspiration from Anderies et al. robustness framework, we modified Epstein and Axtell' Sugarscape model to integrate a multi-scale perspective and explore vulnerability transfers following adaptation processes. Harvesters (users) are evolving in a cell-environment composed of spice and sugar (resources) which is divided in 6 States (infrastructure providers). This environment is unequally submitted to two types of perturbations: a slowly increasing demographic pressure (exogenous drivers on users) and shocks on resources availability that simulate climatic extreme events (exogenous drivers on resource). Harvesters and States are responding by the implementation of adaptation strategies, respectively migration and opening or closing of the State borders (infrastructures). Vulnerability indicators were developed at several levels (individual, State and environment) to analyse consequences of adaptation, and more particularly vulnerability transfer across levels. We demonstrate a cumulating effect of perturbations, as well as mitigated effects of adaptation strategies. We discuss the outcome of developing multi-scales agentbased models to explore adaptation to global changes, moving from Sugarscape to Sugarscale. At this point, agents decision making are based on abstract Epstein and Axtell's Sugarscape model (based on welfare function defined in economics). The resulting model is to be used as a conceptual thought experiment that include main issues of coastal systems

Conclusions on motor control depend on the type of model used to represent the periphery

Within the field of motor control, there is no consensus on which kinematic and kinetic aspects of movements are planned or controlled. Perturbing goal-directed movements is a frequently used tool to answer this question. To be able to draw conclusions about motor control from kinematic responses to perturbations, a model of the periphery (i.e., the skeleton, muscle-tendon complexes, and spinal reflex circuitry) is required. The purpose of the present study was to determine to what extent such conclusions depend on the level of simplification with which the dynamical properties of the periphery are modeled. For this purpose, we simulated fast goal-directed single-joint movement with four existing types of models. We tested how three types of perturbations affected movement trajectory if motor commands remained unchanged. We found that the four types of models of the periphery showed different robustness to the perturbations, leading to different predictions on how accurate motor commands need to be, i.e., how accurate the knowledge of external conditions needs to be. This means that when interpreting kinematic responses obtained in perturbation experiments the level of error correction attributed to adaptation of motor commands depends on the type of model used to describe the periphery

Determinants of frequent and infrequent STI testing and STI diagnosis related to test frequency among men who have sex with men in the eastern part of the Netherlands: a 6-year retrospective study.

Men who have sex with men (MSM) remain vulnerable to sexually transmitted infections (STIs) and are advised to be tested at least twice a year. The aim of this study was to assess the determinants of test frequency and their associations with an STI diagnosis

Pedagogía social : revista interuniversitaria

Monográfico con el título: "Redes territoriales de acción socioeducativa: una apuesta por la innovación social colaborativa"Título, resumen y palabras clave también en portuguésResumen basado en el de la publicaciónEstudio cualitativo, localizado en Estados Unidos, presenta un análisis en profundidad sobre el liderazgo en escuelas y organizaciones comunitarias que ayudan a conectar a los alumnos y a las familias con recursos educativos fundamentales. Los datos han sido recopilados a partir de 132 entrevistas con personas que han experimentado las complejidades sociales de la indigencia. Los resultados sugieren que el liderazgo participativo fomenta el aprendizaje, el simbolismo, el desarrollo de la identidad y la responsabilidad. El tipo de liderazgo promete brindar oportunidades para los desfavorecidos, que no tienen acceso a recursos o relaciones de importancia.ES