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Katherine L. Hsieh, Yaejin Moon, Vignesh Ramkrishnan, Rama Ratnam and Jacob J. Sosnoff

measure postural stability, such as the functional reach task, 6 trunk sway, 7 and center of pressure (COP) measures (ie, velocity, area). 8 One method of measuring postural stability is determining virtual time to contact (VTC). VTC provides an estimate of how long it would take an individual to lose

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Anat V. Lubetzky, Bryan D. Hujsak, Gene Fu and Ken Perlin

are limited to the research laboratory setting and cannot be utilized in the clinic. Recent advances in virtual reality (VR) technology such as the Oculus Rift (Oculus VR, LLC; Menlo Park, CA) and the HTC Vive (HTC Corporation, New Taipei City, Taiwan) could potentially help identify movement patterns

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Kevin R. Ford, Anh-Dung Nguyen, Eric J. Hegedus and Jeffrey B. Taylor

Virtual environments with real-time feedback can simulate extrinsic goals that mimic real life conditions. The purpose was to compare jump performance and biomechanics with a physical overhead goal (POG) and with a virtual overhead goal (VOG). Fourteen female subjects participated (age: 18.8 ± 1.1 years, height: 163.2 ± 8.1 cm, weight 63.0 ± 7.9 kg). Sagittal plane trunk, hip, and knee biomechanics were calculated during the landing and take-off phases of drop vertical jump with different goal conditions. Repeated-measures ANOVAs determined differences between goal conditions. Vertical jump height displacement was not different during VOG compared with POG. Greater hip extensor moment (P < .001*) and hip angular impulse (P < .004*) were found during VOG compared with POG. Subjects landed more erect with less magnitude of trunk flexion (P = .002*) during POG compared with VOG. A virtual target can optimize jump height and promote increased hip moments and trunk flexion. This may be a useful alternative to physical targets to improve performance during certain biomechanical testing, screening, and training conditions.

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Jurjen Bosga and Ruud G. J. Meulenbroek

In this study we investigated redundancy control in joint action. Ten participantpairs (dyads) performed a virtual lifting task in which isometric forces needed to be generated with two or four hands. The participants were not allowed to communicate but received continuous visual feedback of their performance. When the task had to be performed with four hands, participants were confronted with a redundant situation and between-hand force synergies could, in principle, be formed. Performance timing, success rates, cross-correlations, and relative phase analyses of the force-time functions were scrutinized to analyze such task-dependent synergies. The results show that even though the dyads performed the task slower and less synchronized in the joint than in the solo conditions, the success rates in these conditions were identical. Moreover, correlation and relative phase analyses demonstrated that, as expected, the dyads formed between-participant synergies that were indicative of force sharing in redundant task conditions.

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Manuel E. Hernandez, Erin O’Donnell, Gioella Chaparro, Roee Holtzer, Meltem Izzetoglu, Brian M. Sandroff and Robert W. Motl

in this aging population. Standard tests of gait function in persons with MS include the timed 25-foot walk, yet increased sensitivity may be achieved through the use of more balance-demanding walking tasks ( Stellmann et al., 2014 ). In this study, we examine virtual beam walking (VBW) tasks on a

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Anat V. Lubetzky, Daphna Harel, Helene Darmanin and Ken Perlin

normal and abnormal responses to changing visual cues with eyes open in situations relevant to daily living ( Jeka et al., 2006 ). In the past few years, there have been substantial advancements in virtual reality (VR) technology. The ability to carefully manipulate visual environments has become simpler

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Paolo Taboga, Alena M. Grabowski, Pietro Enrico di Prampero and Rodger Kram

In the 2012 Paralympic 100 m and 200 m finals, 86% of athletes with a unilateral amputation placed their unaffected leg on the front starting block. Can this preference be explained biomechanically? We measured the biomechanical effects of starting block configuration for seven nonamputee sprinters and nine athletes with a unilateral amputation. Each subject performed six starts, alternating between their usual and unusual starting block configurations. When sprinters with an amputation placed their unaffected leg on the front block, they developed 6% greater mean resultant combined force compared with the opposite configuration (1.38 ± 0.06 vs 1.30 ± 0.11 BW, P = .015). However, because of a more vertical push angle, horizontal acceleration performance was equivalent between starting block configurations. We then used force data from each sprinter with an amputation to calculate the hypothetical starting mechanics for a virtual nonamputee (two unaffected legs) and a virtual bilateral amputee (two affected legs). Accelerations of virtual bilateral amputees were 15% slower compared with athletes with a unilateral amputation, which in turn were 11% slower than virtual nonamputees. Our biomechanical data do not explain the starting block configuration preference but they do explain the starting performance differences observed between nonamputee athletes and those with leg amputations.

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Bob W. Kooi and Max Kuipers

The maneuvers of a competition diver on a springboard before takeoff may serve to maximize the height of the flight phase. To simplify analysis, it is often assumed that the diver performs motions at the top of a single degree-of-freedom (DOF) system, usually consisting of one mass and one linear spring. This system is expected to simulate the behavior of the board sufficiently. In this paper we propose a new single DOF system approximating the effects of a board with passable accuracy. This model is applied to three types of springboards to obtain numerical values for their virtual masses at the tip.

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Robert Tibold, Gabor Fazekas and Jozsef Laczko

A three-dimensional (3-D) arm movement model is presented to simulate kinematic properties and muscle forces in reaching arm movements. Healthy subjects performed reaching movements repetitively either with or without a load in the hand. Joint coordinates were measured. Muscle moment arms, 3-D angular acceleration, and moment of inertias of arm segments were calculated to determine 3-D joint torques. Variances of hand position, arm configuration, and muscle activities were calculated. Ratios of movement variances observed in the two conditions (load versus without load) showed no differences for hand position and arm configuration variances. Virtual muscle force variances for all muscles except deltoid posterior and EMG variances for four muscles increased significantly by moving with the load. The greatly increased variances in muscle activity did not imply equally high increments in kinematic variances. We conclude that enhanced muscle cooperation through synergies helps to stabilize movement at the kinematic level when a load is added.

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Wei Zhang, Halla B. Olafsdottir, Vladimir M. Zatsiorsky and Mark L. Latash

We studied the mechanical variables (the grip force and the total moment of force) and multidigit synergies at two levels (the virtual finger-thumb level, VF-TH, and the individual finger level, IMRL) of a hypothetical control hierarchy during accurate rotation of a hand-held instrumented handle. Synergies were defined as covaried changes in elemental variables (forces and moments of force) that stabilize the output at a particular level. Indices of multidigit synergies showed higher values at the hierarchically higher level (VF-TH) for both normal and tangential forces. The moment of force was stabilized at both hierarchical levels during the steady-state phases but not during the movement. The results support the principles of superposition and of mechanical advantage. They also support an earlier hypothesis on an inherent tradeoff between synergies at the two hierarchical levels, although the controller showed more subtle and versatile synergic control than the one hypothesized earlier.