The goal of the research is to reconstruct the unknown cost (objective) function(s) presumably used by the neural controller for sharing the total force among individual fingers in multifinger prehension. The cost function was determined from experimental data by applying the recently developed Analytical Inverse Optimization (ANIO) method (Terekhov et al. 2010). The core of the ANIO method is the Theorem of Uniqueness that specifies conditions for unique (with some restrictions) estimation of the objective functions. In the experiment, subjects (n = 8) grasped an instrumented handle and maintained it at rest in the air with various external torques, loads, and target grasping forces applied to the object. The experimental data recorded from 80 trials showed a tendency to lie on a 2-dimensional hyperplane in the 4-dimensional finger-force space. Because the constraints in each trial were different, such a propensity is a manifestation of a neural mechanism (not the task mechanics). In agreement with the Lagrange principle for the inverse optimization, the plane of experimental observations was close to the plane resulting from the direct optimization. The latter plane was determined using the ANIO method. The unknown cost function was reconstructed successfully for each performer, as well as for the group data. The cost functions were found to be quadratic with nonzero linear terms. The cost functions obtained with the ANIO method yielded more accurate results than other optimization methods. The ANIO method has an evident potential for addressing the problem of optimization in motor control.
Xun Niu, Alexander V. Terekhov, Mark L. Latash and Vladimir M. Zatsiorsky
Jaebum Park, Brian S. Baum, You-Sin Kim, Yoon Hyuk Kim and Jae Kun Shim
The aim of this study was to test the mechanical advantage (MA) hypothesis in multifinger torque production tasks in humans: fingers with longer moment arms produce greater force magnitudes during torque production tasks. There were eight experimental conditions: two prehension types determined by different mechanical constraints (i.e., fixed- and free-object prehension) with two torque directions (supination and pronation) and two torque magnitudes (0.24 and 0.48 N·m). The subjects were asked to produce prescribed torques during the fixed-object prehension or to maintain constant position of the free hand-held object against external torques. The index of MA was calculated for agonist and antagonist fingers, which produce torques in the same and opposite directions to the target torques, respectively. Within agonist fingers, the fingers with longer moment arms produced greater grasping forces while within antagonist fingers, the fingers with shorter moment arms produced greater forces. The MA index was greater in the fixed-object condition as compared with the free-object condition. The MA index was greater in the pronation condition than in the supination condition. This study supports the idea that the CNS utilizes the MA of agonist fingers, but not of antagonist fingers, during torque production in both fixed- and free-object conditions.
Casey M. Breslin and Mark Fischman
We tested the end-state comfort effect (ESCE) under varying reach extents (Experiment 1) and a balancing task (Experiment 2). We hypothesized that as reach extent increased, or when participants had to perform a simultaneous balancing task, sensitivity to end-state comfort would decrease. Participants were divided by height (taller or shorter than 160 cm) to understand the impact of reach extent. In Experiment 1, 86 participants grasped an overturned glass from a shelf, turned the glass upright, placed it on a counter, and then filled the glass with water. Shelf heights were 95, 145, and 168 cm above the floor. In Experiment 2, 82 participants climbed a stepstool, which posed a modest balance challenge, before grasping an overturned glass from a shelf 213.4 cm high. They then turned the glass upright and filled it with water. Three trials were performed. In each experiment, for individuals taller than 160 cm, a Cochran’s Q test revealed a majority used an awkward thumb-down grip to initially grasp the glass in all conditions. However, for participants shorter than 160 cm, sensitivity to end-state comfort decreased. This interaction suggests that the ESCE is influenced by both participant height and tasks requiring extreme reaches. Results are interpreted in context of a constraint hierarchy within a model of posture-based motion planning.
Anastasiya Khomutova and Alex Channon
This paper explores the representation of female athletes within the Legends Football League (LFL; formerly ‘Lingerie Football League’) in 2013. Specifically, the study seeks to understand the vision of women’s sport, and female athletes, which the LFL propagates. Drawing on a combined content and semiotic analysis of the commentary and visuals featured in 26 LFL matches broadcast online in 2013, our data suggests that the LFL presents apparently conflicting images of sexualized, but ‘powerful’ female athletes. We discuss these data relative to competing interpretive approaches to female sexuality in sports media, and join other sports media scholars in advocating audience reception research as a necessary next step to grasping this emergent sport’s significance. Cet article examine la représentation d’athlètes féminines au sein de la Legends Football League (« Ligue de football des légendes » ou LFL, anciennement la « Lingerie Football League » ou « Ligue de football en lingerie ») en 2013. L’étude cherche spécifiquement à comprendre la vision du sport féminin et des athlètes féminines que la LFL propage. En se basant sur une analyse de contenu et sémiotique des commentaires et images trouvés dans les retransmissions en ligne de 26 matchs de la LFL en 2013, les données suggèrent que la LFL présente des images apparemment contradictoires d’athlètes sexualisées mais « puissantes ». Nous discutons ces données par rapport à d’autres approches interprétatives pour étudier la sexualité féminine dans les médias sportifs. Nous nous joignons aussi à d’autres chercheurs sur les médias sportifs en prônant la recherche sur la réception de l’audience comme prochaine étape afin de mieux saisir la signification de ce sport émergent.
Julien Jacquier-Bret, Nasser Rezzoug and Philippe Gorce
In the presence of motor redundancy, recent studies have shown that goal equivalent configurations of the body segments might be used by the central nervous system (CNS) instead of stereotypical movement patterns. In particular, some authors have shown that the CNS might choose a subset of joint configurations (termed the uncontrolled manifold or UCM) such that variability (goal equivalent variance or GEV) in this subset does not affect the value of a particular performance variable while variability in the orthogonal subset ORT (non-goal equivalent variance or NGEV) does. This hypothesis has been used successfully to test whether specific performance variables such as endpoint trajectory or segment global orientation are stabilized by the CNS or to study the influence of constraints on the organization of the movement. Few studies have examined the redundancy problem when considering obstacle avoidance during a grasping task. Indeed, the majority of the works on this topic considers non redundant arm models or do not take into account the movement variability. In the present work, we sought to study the coordination of the trunk and the arm during a reaching task involving an obstacle and to test whether such a spatial constraint in extrinsic space may induce particular adaptations in term of joint flexibility when considering the shoulder, elbow, and wrist joint center positions. In this framework, the upper limb three-dimensional kinematics was recorded. From the calculated joint angles, the variability in joint space related to the three joint center positions was computed and decomposed into GEV and NGEV. In agreement with the UCM hypothesis, results showed higher values of GEV than NGEV for all the experimental conditions. The main finding of the study is that joints’ synergy is strengthened for the stabilization of the elbow joint center position during the late phases of the movement. This strengthening seems to be due mainly to an increase of GEV. Therefore, our results suggest that an increase of joint flexibility may be a mechanism by which the CNS takes into account a spatial constraint in extrinsic space represented by an obstacle.
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