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Rahman Davoodi and Gerald E. Loeb

Computer models of the neuromusculoskeletal systems can be used to study different aspects of movement and its control in humans and animals. SIMM with Dynamics Pipeline (Musculographics Inc., Chicago) and SD-Fast (Symbolic Dynamics Inc., Mountain View, CA) are software packages commonly used for graphic and dynamic simulation of movement in musculoskeletal systems. Building dynamic models with SIMM requires substantial C programming, however, which limits its use. We have developed Musculoskeletal Modeling in Simulink (MMS) software to convert the SIMM musculoskeletal and kinetics models to Simulink (Mathworks Inc., Natick, MA) blocks. In addition, MMS removes SIMM’s run-time constraints so that the resulting blocks can be used in simulations of closed-loop sensorimotor control systems.

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Gertjan J.C. Ettema, Emma Taylor, J. David North and Vaughan Kippers

This study’s aim was to identify the effect of oscillation of torques in isometric tasks under identical mechanical conditions on the muscle synergies used. It was hypothesized that bi-functional muscles would play a lesser role in torque oscillation, because they would also generate an undesired oscillation. Thus, changes in muscle synergies were expected as a consequence of oscillation in torque generation. The effect of the trajectory of torque generation was investigated in dual-degrees-of-freedom submaximal isometric oscillation torque tasks at the elbow. The torques were flexion-extension and supination-pronation. Oscillation torques were compared with static torque generations at four torque positions during oscillation. Muscle activity was determined with surface electromyography. Compared with the static torque tasks, the oscillation tasks showed an overall increased muscle activity. The oscillation tasks, however, showed similar activity patterns and muscle synergies compared to the static composite tasks. It was found that the motor system is well able to control different orthogonal combinations of slow torque oscillations and constant torques by employing a single oscillating muscle synergy.

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Gertjan J.C. Ettema, Steinar Bråten and Maarten F. Bobbert

A ski jumper tries to maintain an aerodynamic position in the in-run during changing environmental forces. The purpose of this study was to analyze the mechanical demands on a ski jumper taking the in-run in a static position. We simulated the in-run in ski jumping with a 4-segment forward dynamic model (foot, leg, thigh, and upper body). The curved path of the in-run was used as kinematic constraint, and drag, lift, and snow friction were incorporated. Drag and snow friction created a forward rotating moment that had to be counteracted by a plantar flexion moment and caused the line of action of the normal force to pass anteriorly to the center of mass continuously. The normal force increased from 0.88G on the first straight to 1.65G in the curve. The required knee joint moment increased more because of an altered center of pressure. During the transition from the straight to the curve there was a rapid forward shift of the center of pressure under the foot, reflecting a short but high angular acceleration. Because unrealistically high rates of change of moment are required, an athlete cannot do this without changing body configuration which reduces the required rate of moment changes.

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Michael S. Guss, John P. Begly, Austin J. Ramme, David P. Taormina, Michael E. Rettig and John T. Capo

Context: Major League Baseball (MLB) players are at risk of hook of hamate fractures. There is a paucity of data assessing the effect of a hook of hamate fracture on MLB players’ future athletic performance. Objective: To determine if MLB players who sustain hook of hamate fractures demonstrate decreased performance upon return to competition when compared with their performance before injury and that of their control-matched peers. Design: Retrospective case-control design. Setting: Retrospective database study. Participants: 18 MLB players who sustained hook of hamate fractures. Methods: Data for 18 MLB players with hook of hamate fractures incurred over 26 seasons (1989–2014) were obtained from injury reports, press releases, and player profiles ( and Player age, position, number of years in the league, mechanism of injury, and treatment were recorded. Individual season statistics for the 2 seasons immediately prior to injury and the 2 seasons after injury for the main performance variable—Wins Above Replacement—were obtained. Eighteen controls matched by player position, age, and performance statistics were identified. A performance comparison of the cohorts was performed. Main Outcome Measures: Postinjury performance compared with preinjury performance and matched-controls. Results: Mean age at the time of injury was 25.1 years with a mean of 4.4 seasons of MLB experience prior to injury. All injuries were sustained to their nondominant batting hand. All players underwent operative intervention. There was no significant change in Wins Above Replacement or isolated power when preinjury and postinjury performance were compared. When compared with matched-controls, no significant decline in performance in Wins Above Replacement the first season and second season after injury was found. Conclusion: MLB players sustaining hook of hamate fractures can reasonably expect to return to their preinjury performance levels following operative treatment.

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Logan A. Lucas, Benjamin S. England, Travis W. Mason, Christopher R. Lanning, Taylor M. Miller, Alexander M. Morgan and Thomas Gus Almonroeder

3 times an athlete’s body weight. 7 These impact forces generate a transient spike in acceleration which is transmitted throughout the musculoskeletal system from the foot to the head. 8 Although the lower-extremity musculature can help to attenuate these impact accelerations, passive tissues also

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Nancy D. Groh and Greggory M. Hundt

Self-efficacy shares a causal relationship with performance. Few studies have examined self-efficacy in relationship to athletic training and instead limit themselves to areas focused on the musculoskeletal system. The purpose of this study was to develop and validate a scale for measuring athletic trainer self-efficacy in the assessment and diagnosis of medical conditions and illness. A principal component analysis yielded 19 items that loaded on three factors. The analysis resulted in acceptable levels of correlation (KMO = .93; Bartlett x2 = 2152.58, df = 171, p < .001) and internal consistency (α = .943). This scale demonstrates both validity and reliability.

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Walter Herzog, Timothy Koh, Evelyne Hasler and Tim Leonard

We hypothesize that the neuromuscular system is designed to function effectively in accomplishing everyday movement tasks. Since everyday movement tasks may vary substantially in terms of speed and resistance, we speculate that agonistic muscles contribute differently to varying movement tasks such that the mechanical, structural, and physiological properties of the system are optimized at all times. We further hypothesize that a mechanical perturbation to the musculoskeletal system, such as the loss of an important joint ligament or the change of a muscle’s line of action, causes an adaptation of the system aimed at reestablishing effective function. Here. we demonstrate how the specificity of the cat ankle extensors is used to accommodate different locomotor tasks. We then illustrate how the loss of an important ligament in the cat knee leads to neuromuscular adaptation. Finally, we discuss the adaptability of skeletal muscle following an intervention that changes a muscle’s line of action, moment arm, and excursion.

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Maarten F. Bobbert and A.J. “Knoek” van Soest

Prilutsky's paper is mainly concerned with the coordination of one- and two-joint muscles. This commentary on the paper addresses the question why we have two-joint muscles in the first place. From an evolutionary point of view, two-joint muscles must have contributed to fitness by presenting a solution to problems that could not be solved with musculoskeletal systems comprising only one-joint muscles. One such problem, not mentioned by Prilutsky, is the following. In a system equipped with only one-joint muscles, satisfying directional constraints would demand, in certain phases of movements, deactivation of muscles that are shortening. Consequently, the work output of these muscles would be limited. The incorporation of two-joint muscles helps to overcome this problem. The reason is that it offers the possibility to redistribute energy across joints, thereby making it possible to accomplish more successfully the difficult task of producing work while steering the movement.

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Damien McKay, Carolyn Broderick and Katharine Steinbeck

With the advent of long-term athlete development programs and early sport specialization, the training of elite athletes now spans the period of adolescence. Adolescence represents a period of physical, psychosocial and cognitive development, but also a time of physical and psychological vulnerability. Changes in skeletal and physiological attributes coincide with an increased risk of sport related injury. A window of vulnerability is shaped by the properties of the musculoskeletal system, the influence of pubertal hormones and the lag time between physical and cognitive development. This article aims to challenge the assumption of adolescence as a time of increased vigor alone, by highlighting the presence of specific vulnerabilities, and proposing that the hormonal, musculoskeletal, and neurocognitive changes of adolescence may represent intrinsic risk factors for sport related injury.

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Zong-Ming Li

Mechanical degrees of freedom (DOF) are defined as the minimum number of independent coordinates needed to describe a system’s position. The human musculoskeletal system has many mechanical DOF through which countless movements are accomplished. In the motor control field, one of the aspirations is to understand how the many DOF are organized for movement execution—the so-called DOF problem. Natural movements are characterized by the coordination of the DOF such that few vary independently. The concept of functional degrees of freedom (fDOF) is introduced to describe the very limited DOF of purposeful, coordinated movements. Deterministic (i.e., constraint satisfaction) and statistical (i.e., principal component analysis) approaches are used to determine fDOF. In contrast to DOF as a mechanical descriptor, fDOF emphasizes the mechanisms of human movements and corroborates our search for the solution to the DOF problem.