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Michael J. Grey, Charles W. Pierce, Theodore E. Milner and Thomas Sinkjaer

The modulation and strength of the human soleus short latency stretch reflex was investigated by mechanically perturbing the ankle during an unconstrained pedaling task. Eight subjects pedaled at 60 rpm against a preload of 10 Nm. A torque pulse was applied to the crank at various positions during the crank cycle, producing ankle dorsiflexion perturbations of similar trajectory. The stretch reflex was greatest during the power phase of the crank cycle and was decreased to the level of background EMG during recovery. Matched perturbations were induced under static conditions at the same crank angle and background soleus EMG as recorded during the power phase of active pedaling. The magnitude of the stretch reflex during the dynamic condition was not statistically different from that during the static condition throughout the power phase of the movement. The results of this study indicate that the stretch reflex is not depressed during active cycling as has been shown with the H-reflex. This lack of depression may reflect a decreased susceptibility of the stretch reflex to inhibition, possibly originating from presynaptic mechanisms.

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Nicole C. George, Charles Kahelin, Timothy A. Burkhart and David M. Andrews

Traditional rigid-link segment biomechanical models are unable to accurately represent the impact response of the musculoskeletal system of living humans because they lack separate wobbling mass (fat mass and lean mass) components, which have been shown to influence the magnitude of forces

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Shyh-Chour Huang

This paper presents a description of a general -purpose nonlinear model of the human body. The model is developed to simulate human response to high force and high acceleration as typically experienced in vehicle accidents. The model is composed of connected bodies of segments representing the torso and limbs of the human frame. Nonlinear springs and dampers are used at the connection joints to represent human anatomical characteristics and limits imposed by muscles, ligaments, and soft tissue. The governing dynamical equations are developed using Kane's equations (Kane <&: Levin-son, 1985) and multibody dynamics analysis procedures developed by Huston et al. (1974, 1975, 1978). These equations and procedures form the basis for the algorithms of a computer code. The equations are solved numerically using a fourth-order Runge-Kutta integrator. The results of several accident simulations are also presented.

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Richard Bailey, Charles Hillman, Shawn Arent and Albert Petitpas

Despite the fact that physical activity is universally acknowledged to be an important part of healthy functioning and well-being, the full scope of its value is rarely appreciated. This article introduces a novel framework for understanding the relationships between physical activity (and specifically sport-related forms of physical activity) and different aspects of human development. It proposes that the outcomes of physical activity can be framed as differential ‘capitals’ that represent investments in domain-specific assets: Emotional, Financial, Individual, Intellectual, Physical, and Social. These investments, especially when made early in the life course, can yield significant rewards, both at that time and for years to come. The paper presents a new model—the Human Capital Model—that makes sense of these effects, outlines the different capitals, and briefly articulates the conditions necessary for the realization of Human Capital growth through physical activity.

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Brad D. Carlson and D. Todd Donavan

By integrating social identity theory with brand personality, the authors test a model of how perceptions of human brands affect consumer’s level of cognitive identification. The findings suggest that consumers view athletes as human brands with unique personalities. Additional findings demonstrate that athlete prestige and distinctiveness leads to the evaluation of athlete identification. Once consumers identified with the athlete, they were more likely to feel an emotional attachment to the athlete, identify with the athlete’s team, purchase team-related paraphernalia and increase their team-related viewership habits. The findings extend previous research on human brands and brand personalities in sports. Marketers can use the information gleaned from this study to better promote products that are closely associated with well-recognized and attractive athletes, thereby increasing consumer retail spending. In addition, the findings offer new insights to sports marketers seeking to increase team-related spectatorship by promoting the image of easily recognizable athletes.

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Yujiang Xiang

Human carrying is simulated in this work by using a skeletal digital human model with 55 degrees of freedom. An optimization-based approach is used to predict the carrying motion with symmetric and asymmetric loads. In this process, the model predicts joint dynamics using optimization schemes and task-based physical constraints. The results indicate that the model can predict different carrying strategies during symmetric and asymmetric load-carrying tasks. The model can also indicate the risk factors for extreme loading situations. With such robust prediction capability, the model could be used for biomedical and ergonomic studies.

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Senshi Fukashiro, Dean C. Hay and Akinori Nagano

This paper reviews the research findings regarding the force and length changes of the muscle-tendon complex during dynamic human movements, especially those using ultrasonography and computer simulation. The use of ultrasonography demonstrated that the tendinous structures of the muscle-tendon complex are compliant enough to influence the biomechanical behavior (length change, shortening velocity, and so on) of fascicles substantially. It was discussed that the fascicles are a force generator rather than a work generator; the tendinous structures function not only as an energy re-distributor but also as a power amplifier, and the interaction between fascicles and tendinous structures is essential for generating higher joint power outputs during the late pushoff phase in human vertical jumping. This phenomenon could be explained based on the force-length/velocity relationships of each element (contractile and series elastic elements) in the muscle-tendon complex during movements. Through computer simulation using a Hill-type muscle-tendon complex model, the benefit of making a countermovement was examined in relation to the compliance of the muscle-tendon complex and the length ratio between the contractile and series elastic elements. Also, the integral roles of the series elastic element were simulated in a cyclic human heel-raise exercise. It was suggested that the storage and reutilization of elastic energy by the tendinous structures play an important role in enhancing work output and movement efficiency in many sorts of human movements.

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Akinori Nagano, Senshi Fukashiro and Taku Komura

Contribution of series elasticity of the human mm. triceps surae in cyclic heel-raise exercise (similar to hopping but the feet do not leave the floor) was examined via computer modeling and simulation. A two-dimensional skeletal model of the human body was constructed. Upright posture was maintained throughout the simulation to prevent the model from falling. A mathematical representation of the mm. triceps surae was implemented in the skeletal model. The muscle was activated by the neural activation input signal with a time resolution of 0.050 sec. Cyclic heel-raise exercises of cycle duration ranging from 0.300 sec to 0.900 sec, corresponding to the motion frequency of 200 to 66.7 cycles/min, were generated using an optimization approach. The goal of the numerical optimization was to generate cyclic motions with as much range of motion as possible. As a result, realistic heel-raise motions were generated with the range of motion between 0.0023 m (cycle duration = 0.300 sec) and 0.0414 m (cycle duration = 0.900 sec). It was found that contribution of the series elasticity in positive mechanical work output of the muscle-tendon complex during the pushoff phase (from the lowest position to the termination of a cycle) increased as motion frequency increased (3% at 66.7 cycles/min to 47% at 200 cycles/min). Relatively higher muscle activation was found during the downward moving phase when the motion frequency was higher. These tendencies are consistent with the findings reported in preceding studies involving experimental animals as well as human participants. It is suggested that series elasticity plays an integral role in the generation of cyclic human motions.

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Ildus I. Ahmetov, Olga L. Vinogradova and Alun G. Williams

The ability to perform aerobic or anaerobic exercise varies widely among individuals, partially depending on their muscle-fiber composition. Variability in the proportion of skeletal-muscle fiber types may also explain marked differences in aspects of certain chronic disease states including obesity, insulin resistance, and hypertension. In untrained individuals, the proportion of slow-twitch (Type I) fibers in the vastus lateralis muscle is typically around 50% (range 5–90%), and it is unusual for them to undergo conversion to fast-twitch fibers. It has been suggested that the genetic component for the observed variability in the proportion of Type I fibers in human muscles is on the order of 40–50%, indicating that muscle fiber-type composition is determined by both genotype and environment. This article briefly reviews current progress in the understanding of genetic determinism of fiber-type proportion in human skeletal muscle. Several polymorphisms of genes involved in the calcineurin–NFAT pathway, mitochondrial biogenesis, glucose and lipid metabolism, cytoskeletal function, hypoxia and angiogenesis, and circulatory homeostasis have been associated with fiber-type composition. As muscle is a major contributor to metabolism and physical strength and can readily adapt, it is not surprising that many of these gene variants have been associated with physical performance and athlete status, as well as metabolic and cardiovascular diseases. Genetic variants associated with fiber-type proportions have important implications for our understanding of muscle function in both health and disease.

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Yasuo Kawakami, Yoshiho Ichinose, Keitaro Kubo, Masamitsu Ito, Morihiro Imai and Tetsuo Fukunaga

This paper reviews three of our recent studies on human muscle architecture in vivo. 1. Hypertrophic changes: From B-mode ultrasonograms, pennation angles and thickness of triceps brachii were determined for normal subjects and highly-trained bodybuilders. There was a significant correlation between muscle thickness and pennation angles. It was confirmed that hypertrophy was accompanied by an increase in pennation angles. 2. Variation of fascicle architecture: Fascicle lengths and pennation angles were obtained from different positions in the gastrocnemius muscle while the subjects relaxed and performed isometric plantar flexion. The fascicle length was uniform throughout the muscle and shortened by contraction (30-34% at 50% of the maximal force). On the other hand, pennation angles differed among positions and increased by contraction. The muscle thickness did not change by contraction. Pen-nation angles were significantly correlated with muscle thickness within muscle. 3. Joint position-fascicle length relationships: Ultrasonic images of the gastrocnemius and soleus muscles were obtained while the subject performed maximal isometric plantarflexion at various joint positions, from which fascicle lengths and angles were determined. The length-force relationship of each muscle was estimated. It was suggested that human muscle architecture has an ability to make substantial changes to adapt to environmental conditions.