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Matthew T.G. Pain and John H. Challis

Wobbling mass models have been used to gain insight into joint loading during impacts. This study investigated the sensitivity of a wobbling mass model of landing from a drop to the model's parameters. A 2-D wobbling mass model was developed. Three rigid linked segments designed to represent the skeleton each had a second mass attached to them, via two translational nonlinear spring dampers, representing the soft tissue. Model parameters were systematically varied one at a time and the effect this had on the peak vertical ground reaction force and segment kinematics was examined. Model output showed low sensitivity to most model parameters but was sensitive to the timing of joint torque initiation. Varying the heel pad stiffness in the range of stiffness values reported in the literature had the largest influence on peak vertical ground reaction force. The analysis indicated that the more proximal body segments had a lower influence on peak vertical ground reaction force per unit mass than the segments nearer the contact point. Model simulations were relatively insensitive to variations in the properties of the connection between wobbling masses and the skeleton. If the goal is to examine the effects of wobbling mass on the system, this insensitivity is an advantage, with the proviso that estimates for the other model parameters and joint torque activation timings lie in a realistic range. If precise knowledge about the motion of the wobbling mass is of interest, however, this calls for more experimental work to precisely determine these model parameters.

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Mark A. King, Cassie Wilson, and Maurice R. Yeadon

This study used an optimization procedure to evaluate an 8-segment torque-driven subject-specific computer simulation model of the takeoff phase in running jumps for height. Kinetic and kinematic data were obtained on a running jump performed by an elite male high jumper. Torque generator activation timings were varied to minimize the difference between simulation and performance in terms of kinematic and kinetic variables subject to constraints on the joint angles at takeoff to ensure that joints remained within their anatomical ranges of motion. A percentage difference of 6.6% between simulation and recorded performance was obtained. Maximizing the height reached by the mass center during the flight phase by varying torque generator activation timings resulted in a credible height increase of 90 mm compared with the matching simulation. These two results imply that the model is sufficiently complex and has appropriate strength parameters to give realistic simulations of running jumps for height.

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Ângela Fernandes, Andreia S. P. Sousa, Nuno Rocha, and João Manuel R. S. Tavares

The aim of this study was to compare postural control strategies during gait initiation in single- and dual-task conditions in individuals in early stages of Parkinson’s Disease (PD). The activation timing of tibialis anterior occurred significantly later in the individuals with PD than in the controls (p = .05), and a significant interaction between the groups, conditions and limbs was found (p = .027). Differences between the single- and dual-task conditions were observed for the activation timing of the tibialis anterior (p = .042) and for the magnitude of soleus (p = .007), with lower values for the dual-task condition. Furthermore, not all the individuals followed the previously reported pattern of soleus inhibition followed by tibialis anterior activation. The duration of the mediolateral displacement of the center of pressure was longer in the individuals with PD than in the controls (p = .019). The anticipatory postural adjustments during gait initiation are impaired in PD and are expressed by an activation failure of tibialis anterior in both single- and dual-task conditions. Hence, it is important that during rehabilitation, intervention should concentrate on the tibialis anterior TA.

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Carlo J. De Luca

This lecture explores the various uses of surface electromyography in the field of biomechanics. Three groups of applications are considered: those involving the activation timing of muscles, the force/EMG signal relationship, and the use of the EMG signal as a fatigue index. Technical considerations for recording the EMG signal with maximal fidelity are reviewed, and a compendium of all known factors that affect the information contained in the EMG signal is presented. Questions are posed to guide the practitioner in the proper use of surface electromyography. Sixteen recommendations are made regarding the proper detection, analysis, and interpretation of the EMG signal and measured force. Sixteen outstanding problems that present the greatest challenges to the advancement of surface electromyography are put forward for consideration. Finally, a plea is made for arriving at an international agreement on procedures commonly used in electromyography and biomechanics.

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Maurice R. Yeadon and Mark A. King

The use of computer simulation models in studies of human movement is now widespread. Most of these models, however, have not been evaluated in a quantitative manner in order to establish the level of accuracy that may be expected. Without such an evaluation, little credence should be given to the published results and conclusions. This paper presents a simulation model of tumbling takeoffs which is evaluated by comparing the simulation output with an actual performance of an elite gymnast. A five-segment planar model was developed to simulate tumbling takeoffs. The model comprised rigid foot, leg, thigh, trunk + head, and arm segments with two damped linear springs to represent the elasticity of the tumbling track/ gymnast interface. Torque generators were included at the ankle, knee, hip, and shoulder joints in order to allow each joint to open actively during the takeoff. The model was customized to the elite gymnast by determining subject-specific inertia and torque parameters. Good agreement was found between actual and simulated tumbling performances of a double layout somersault with 1% difference in the linear and angular momenta at takeoff. Allowing the activation timings of the four torque generators to vary resulted in an optimized simulation that was some 0.32 m higher than the evaluation simulation. These simulations suggest the model is a realistic representation of the elite gymnast, since otherwise the model would either fail to reproduce the double layout somersault or would produce a very different optimized solution.

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Manuel J. Escalona, Daniel Bourbonnais, Michel Goyette, Damien Le Flem, Cyril Duclos, and Dany H. Gagnon

(ST and BF) (Figure  2 ). Figure 2 —Group average ( n  = 20) for each of the four muscle synergies found in healthy participants at SLOW (red), NAT (green), and FAST (yellow) walking speeds. (a) Activation timing profiles for each synergy over the gait cycle. (b) Muscle synergies average and SD

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Nicola Marotta, Andrea Demeco, Gerardo de Scorpio, Angelo Indino, Teresa Iona, and Antonio Ammendolia

anterior shear force, which directly loads the ACL. Our study involving male and female professional soccer players intended to quantify muscular activation timing before ground contact, to compare the times of initial contraction of the quadriceps and hamstrings in soccer players to a range of normality

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Matthew K. Seeley, Hyunwook Lee, S. Jun Son, Mattie Timmerman, Mariah Lindsay, and J. Ty Hopkins

alters quadriceps activation timing during stair ascent and descent in a way similar to PFP, delaying vastus medialis oblique onset relative to vastus lateralis onset. 8 In 2 separate running studies, EKP inhibited gluteus medius, vastus medialis, vastus lateralis, and gastrocnemius activation. 7 , 10

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Desiree Camara Miraldo, Renato Naville Watanabe, and Marcos Duarte

, because various speeds are present in daily life activities. Data on the timing of TA muscle activation in healthy subjects during walking could also be useful for gait event estimation and the development of a functional electrical stimulation device for people with foot drop. This activation timing can

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Jordyn Vienneau, Sandro Nigg, and Benno M. Nigg

to quantify differences in activation timing and frequency. The hypothesis, based on changes in the cognitive loads between tasks, was that muscle activation during the COG would differ significantly from the WALK with respect to both timing (H1) and frequency (H2). Results of this study will further