Effect of support foot placement on football instep kick performance.

Although the support foot plays an important role in kicking a football, there has been a paucity of research exploring the effect of the placement of the support foot on kicking performance. To investigate the kick performance under different support foot positions, ten male footballers were recruited to participate in two experiments: one determining the maximum ball speed and the second determining accuracy. The participants were instructed to plant their support foot on one of nine different spots marked in the form of a 3 × 3 shape on a piece of artificial grass and asked to kick the ball. In the first (maximum speed) test, the participants tried their best to kick the ball at the maximum ball speed from nine different support foot positions. In the second (accuracy) test, the participants kicked the ball toward the target area without restricting the support foot position. The ball speed, as well as the success rate, were recorded from each support foot position. Significantly higher ball speed and accuracy were obtained at medial positions than was the case at lateral positions from the nine spots. It was concluded that although footballers may choose different positions for support foot placement, the maximum ball speed and better accuracy could be expected when the support foot was next to or slightly in front of the ball centre without too much side-by-side separation (27-37 cm).

Biomechanical fidelity of athletic training using virtual reality head-mounted display: the case of preplanned and unplanned sidestepping.

Virtual reality has recently been recognised as an effective tool for investigating visual-perceptual tasks. To develop a sport-specific virtual environment with realistic locomotion, it is crucial to examine the effect of using virtual reality devices on athletes performing intense and complex movements. Twelve collegiate football players were instructed to perform pre-planned and unplanned sidestepping in both environments with the same dimension and experimental setup in the virtual environment as in the real one. Analysis of the performance and knee biomechanical parameters showed that movements performed in the two environments were generally comparable. Consistent changes in approach velocity and knee angle/moment under unplanned conditions (compared with preplanned conditions) were also found in the virtual environment as in the real one, except for the significantly larger peak flexion angle (p < .05) observed in the virtual environment. Interestingly, half of the participants changed from producing abduction to adduction moment at the weight acceptance phase in the preplanned condition (p < .05). These findings suggested that while it is generally feasible to use virtual reality head-mounted displays for designated experiments and training, the effect of wearing virtual reality devices could be somewhat subject-specific.

Control of a virtual vehicle influences postural activity and motion sickness in pre-adolescent children.

Among adults, persons in control of a vehicle (i.e., drivers) are less likely to experience motion sickness compared to persons in the same vehicle who do not control it (i.e., passengers). This "driver-passenger effect" is well-known in adults, but has not been evaluated in children. Using a yoked-control design with seated pre-adolescent children, we exposed dyads to a driving video game. In each dyad, one child (the driver) drove the virtual vehicle. Their performance was recorded, and later shown to the other child (the passenger). Thus, visual motion stimuli were identical for the members of each dyad. During exposure to the video game, we monitored the quantitative kinematics of head and torso movements. Participants were instructed to discontinue participation immediately if they experienced any symptoms of motion sickness, however mild. Accordingly, the movements that we recorded preceded the onset of motion sickness. Results revealed that Passengers (73.08%) were more likely than Drivers (42.31%) to state that they were motion sick. Drivers tended to move more than passengers, and with a greater degree of multifractality. The magnitude of movement was greater among participants who later reported motion sickness than among those who did not. In addition, for the multifractality of movement a statistically significant interaction revealed that postural precursors of motion sickness differed qualitatively between Drivers and Passengers. Overall, the results reveal that control of a virtual vehicle reduces the risk of motion sickness among pre-adolescent children.

Differences in Baseball Batting Movement Patterns Between Facing a Pitcher and a Pitching Machine.

Purpose: Pitching machines are widely used for baseball batting practice. However, these machines cannot precisely simulate a live pitcher's pitching motion. To understand if a batter's movement strategy would be altered due to disparate visual information provided by a pitching machine as opposed to a live pitcher, the present study aimed to compare differences in baseball batting movement patterns under the two delivery conditions. Methods: To examine movement variations and different strategies of each high-level athlete rather than obtaining averaged group performance, single-subject analysis was adopted. Four professional baseball players were recruited to bat around 50 pitches under each delivery condition. Vertical ground reaction forces of the participants' batting movements were recorded. Relative timings of key events and values of several kinetic parameters during batting were examined. Results: When batting against the pitching machine, batters initiated forward stepping earlier (relative to ball release), had smaller loading rate at landing the step, and altered the duration from forward bat swing to ball impact. These results might be attributed to lacking visual cues of a live pitcher's whole body kinematics prior to ball release. Without sufficient visual information, greater uncertainty and incomplete weight transfer affected the batter's decision making and movement execution. Conclusion: The batters in this study did adjust their movement strategy in batting against a pitching machine. With dissimilar batting movement patterns under the two delivery conditions, extensive reliance on training with pitching machines is not recommended.

Effects of physical driving experience on body movement and motion sickness among passengers in a virtual vehicle.

Virtual vehicles (e.g., driving video games) can give rise to visually induced motion sickness. Typically, people drive virtual vehicles. In the present study, we investigated motion sickness among participants who were exposed to virtual vehicles as passengers; that is, they observed vehicle motion, but did not control it. We also asked how motion sickness and the postural precursors of motion sickness might be influenced by participants' previous experience of driving physical vehicles. Participants viewed a recording of a virtual automobile in a driving video game. Drivers were young adults with several years of experience driving physical automobiles, while non-drivers were individuals in the same age group who did not have a driver's license and had never driven an automobile. During exposure to the virtual vehicle, we monitored movement of the head and torso. The independent measures included the incidence and severity of motion sickness. After exposure to the virtual vehicle, the incidence and severity of motion sickness did not differ between Drivers and Non-Drivers. By contrast, postural movement differed between participants who later became motion sick and those who did not. In addition, during exposure to the virtual vehicle, physical driving experience was related to patterns of postural activity that preceded motion sickness. The results are consistent with the postural instability theory of motion sickness, and illuminate relations between the control of physical and virtual vehicles.

Role of heel lifting in standing balance recovery: A simulation study.

Although lifting the heels has frequently been observed during balance recovery, the function of this movement has generally been overlooked. The present study aimed to investigate the functional role of heel lifting during regaining balance from a perturbed state. Computer simulation was employed to objectively examine the effect of allowing/constraining heel lifting on balance performance. The human model consisted of 3 rigid body segments connected by frictionless joints. Movements were driven by joint torques depending on current joint angle, angular velocity, and activation level. Starting from forward-inclined and static straight-body postures, the optimization goal was to recover balance effectively (so that ground projection of the mass center returned to the inside of the base of support) and efficiently by adjusting ankle and hip joint activation levels. Allowing/constraining heel lifting resulted in virtually identical movements when balance was mildly perturbed at the smallest lean angle (8°). At larger lean angles (8.5° and 9°), heel lifting assisted balance recovery more evidently with larger joint movements. Partial and altered timings of ankle/hip torque activation due to constraining heel lifting reduced linear and angular momentum generation for avoiding forward falling, and resulted in hindered balancing performance.

Does knee motion contribute to feet-in-place balance recovery?

Although knee motions have been observed at loss of balance, the ankle and hip strategies have remained the focus of past research. The present study aimed to investigate whether knee motions contribute to feet-in-place balance recovery. This was achieved by experimentally monitoring knee motions during recovery from forward falling, and by simulating balance recovery movements with and without knee joint as the main focus of the study. Twelve participants initially held a straight body configuration and were released from different forward leaning positions. Considerable knee motions were observed especially at greater leaning angles. Simulations were performed using 3-segment (feet, shanks+thighs, and head+arms+trunk) and 4-segment (with separate shanks and thighs segments) planar models. Movements were driven by joint torque generators depending on joint angle, angular velocity, and activation level. Optimal joint motions moved the mass center projection to be within the base of support without excessive joint motion. The 3-segment model (without knee motions) generated greater backward linear momentum and had better balance performance, which confirmed the advantage of having only ankle/hip strategies. Knee motions were accompanied with less body angular momentum and a lower body posture, which could be beneficial for posture control and reducing falling impact, respectively.

A unified approach for revealing multiple balance recovery strategies.

In human balance recovery, different strategies have been proposed with generally overlooked knee motions but extensive focus on the ankle, hip, and step strategies. It is not well understood whether maintaining balance is regulated at the lower "muscular-articular" level of coordinating segment joints or at a higher level of controlling whole body dynamics. Whether balance control is to minimize joint degrees of freedom (DOF) or utilize all the available DOF also remains unclear. This study aimed to use a realistic musculoskeletal human model to identify multiple balance recovery strategies with a single optimization criterion. Movements were driven by neural excitations (which activated muscle force generation) and were assumed to be symmetric. Balance recoveries were simulated with forward-inclined straight body postures as the initial conditions. When the position of the toes was fixed, balance was regained with virtually straight knees and mixed ankle/hip strategies. Under a severely perturbed condition, use of the forward hop strategy after releasing the fixed-toes constraint indicated spontaneous recruitment or suppression of DOF, which mimicked functions of optimally computed CNS commands in humans. The results also indicated that increase/decrease in the number of DOF depends on the imposed perturbation intensity and movement constraints.

Role of arm motion in feet-in-place balance recovery.

Although considerable arm movements have been observed at loss of balance, research on standing balance focused primarily on the ankle and hip strategies. This study aimed to investigate the effect of arm motion on feet-in-place balance recovery. Participants stood on a single force plate and leaned forward with a straight body posture. They were then released from three forward-lean angles and regained balance without moving their forefeet under arm-swing (AS) and arm-constrained (AC) conditions. Higher success rates and shorter recovery times were found with arm motion under moderate balance perturbations. Recovery time was significantly correlated with peak linear momentum of the arms. Circumduction arm motion caused initial shoulder extension (backward arm movement) to generate reaction forces to pull the body forward, but later forward linear momentum of the arms helped move the whole body backward to avoid forward falling. However, greater lean angles increased difficulty in balance recovery, making the influences of the arms less significant. Since arm motions were observed in all participants with significantly enhanced performance under moderate balance perturbation, it was concluded that moving the arms should also be considered (together with the ankles and hips) as an effective strategy for balance recovery.

Perform kicking with or without jumping: joint coordination and kinetic differences between Taekwondo back kicks and jumping back kicks.

We investigated joint coordination differences between Taekwondo back kicks and jumping back kicks, and how jumping (in performing the latter) would alter engaging ground reaction forces (GRF) in executing kicking. Ten skilful athletes volunteered to perform both kinds of kicking within the shortest time for three successful trials. Three high-speed cameras and two force platforms were used for data collection, and the trial with the shortest execution time was selected for analysis. Movements were divided into the rotation and attack phases. With comparable execution time and maximum joint linear/angular speeds, back kicks and jumping back kicks differ mainly in larger GRF in the latter, and in greater target acceleration in the former probably because the support leg prevented athletes' rebounding after impact. In addition, more prominent antiphase and in-phase coordination between the shoulder segment and knee joint, and elongated rotation phase were found in jumping back kicks. Larger GRF values in jumping back kicks were generated for jump take-off rather than for a more powerful attack. In back kicks although the support leg remained ground contact, greatly decreased GRF in the attack phase suggested that the support leg mainly served as a rotation axis.

Effect of arm swing on single-step balance recovery.

Balance recovery techniques are useful not only in preventing falls but also in many sports activities. The step strategy plays an important role especially under intense perturbations. However, relatively little is known about the effect of arm swing on stepping balance recovery although considerable arm motions have been observed. The purpose of this study was to examine how the arms influence kinematic and kinetic characteristics in single-step balance recovery. Twelve young male adults were released from three forward-lean angles and asked to regain balance by taking a single step under arm swing (AS) and arm constrained (AC) conditions. It was found that unconstrained arms had initial forward motion and later upward motion causing increased moment of inertia of the body, which decreased falling angular velocity and allowed more time for stepping. The lengthened total balance time included weight transfer and stepping time, although duration increase in the latter was significant only at the largest lean angle. In contrast, step length, step velocity, and vertical ground reaction forces on the stepping foot were unaffected by arm swing. Future studies are required to investigate optimal movement strategies for the arms to coordinate with other body segments in balance recovery and injury reduction.

The three-dimensional kinematics of a barbell during the snatch of Taiwanese weightlifters.

The purpose of this study is to characterize the trajectory of a barbell and clarify whether there is a standard pattern in the barbell trajectory for each lifter. Two high-speed cameras (mega-speed MS1000, sampling rate=120 Hz) were used to film the barbell trajectories of male Taiwanese weightlifters under competitive conditions. Twenty-four successful lifts were filmed and classified into 3 groups (n=8 per group) by relative barbell-mass (RBM): the better-performance group (RBM>1.63), the middle group (1.28

Real-time physics-based 3D biped character animation using an inverted pendulum model.

We present a physics-based approach to generate 3D biped character animation that can react to dynamical environments in real time. Our approach utilizes an inverted pendulum model to online adjust the desired motion trajectory from the input motion capture data. This online adjustment produces a physically plausible motion trajectory adapted to dynamic environments, which is then used as the desired motion for the motion controllers to track in dynamics simulation. Rather than using Proportional-Derivative controllers whose parameters usually cannot be easily set, our motion tracking adopts a velocity-driven method which computes joint torques based on the desired joint angular velocities. Physically correct full-body motion of the 3D character is computed in dynamics simulation using the computed torques and dynamical model of the character. Our experiments demonstrate that tracking motion capture data with real-time response animation can be achieved easily. In addition, physically plausible motion style editing, automatic motion transition, and motion adaptation to different limb sizes can also be generated without difficulty.

The relationship between joint strength and standing vertical jump performance.

The effect of joint strengthening on standing vertical jump height is investigated by computer simulation. The human model consists of five rigid segments representing the feet, shanks, thighs, HT (head and trunk), and arms. Segments are connected by frictionless revolute joints and model movement is driven by joint torque actuators. Each joint torque is the product of maximum isometric torque and three variable functions of instantaneous joint angle, angular velocity, and activation level, respectively. Jumping movements starting from a balanced initial posture and ending at takeoff are simulated. A matching simulation reproducing the actual jumping movement is generated by optimizing joint activation level. Simulations with the goal of maximizing jump height are repeated for varying maximum isometric torque of one joint by up to +/-20% while keeping other joint strength values unchanged. Similar to previous studies, reoptimization of activation after joint strengthening is necessary for increasing jump height. The knee and ankle are the most effective joints in changing jump height (by as much as 2.4%, or 3 cm). For the same amount of percentage increase/decrease in strength, the shoulder is the least effective joint (which changes height by as much as 0.6%), but its influence should not be overlooked.

The mechanisms that enable arm motion to enhance vertical jump performance-a simulation study.

The reasons why using the arms can increase standing vertical jump height are investigated by computer simulations. The human models consist of four/five segments connected by frictionless joints. The head-trunk-arms act as a fourth segment in the first model while the arms become a fifth segment in the second model. Planar model movement is actuated by joint torque generators. Each joint torque is the product of three variable functions of activation level, angular velocity dependence, and maximum isometric torque varying with joint angle. Simulations start from a balanced initial posture and end at jump takeoff. Jump height is maximized by finding the optimal combination of joint activation timings. Arm motion enhances jumping performance by increasing mass center height and vertical takeoff velocity. The former and latter contribute about 1/3 and 2/3 to the increased height, respectively. Durations in hip torque generation and ground contact period are lengthened by swinging the arms. Theories explaining the performance enhancement caused by arms are examined. The force transmission theory is questionable because shoulder joint force due to arm motion does not precisely reflect the change in vertical ground reaction force. The joint torque/work augmentation theory is acceptable only at the hips but not at the knees and ankles because only hip joint work is considerably increased. The pull/impart energy theory is also acceptable because shoulder joint work is responsible for about half of the additional energy created by arm swings.

Role of arms in somersaulting from compliant surfaces: a simulation study of springboard standing dives.

The role of arms in compliant-surface jumping for maximizing backward somersault rotations is studied using multi-segment models and is applied to springboard diving. The surface (springboard) is modeled by a rigid bar with a rotational spring with a hinged end and point mass at the tip. Planar four- and five-segment human models are used (with the fifth segment representing the arms) and are driven by torque actuators at the ankle, knee, hip, and shoulder. Each joint torque is the product of maximum isometric torque and three variable functions depending on instantaneous joint angle, angular velocity, and activation level, respectively. Movement simulation starts from a balanced initial posture and ends at jump takeoff. The objective is to find joint torque activation patterns during board contact so that the number of backward rotations in flight is maximized. Kinematic differences in jumps with and without arms are mainly in smaller takeoff vertical velocity and more flexed knee and hip in the former. In both jumps, joint torque/activations are similar in their minor flexion-full extension patterns. Maximum hip torque is larger with arms but maximum knee torque is larger without arms. Except at the knee, more joint work can be done with arm swing. Total angular momentum is increased considerably by arm motion because of its remote contribution. Consequently segment remote contributions to total angular momentum are much larger in jumping with arms. Shoulder strength helps generate angular momentum only to a certain limit. If more work is used to generate horizontal velocity away from the board, the amount of total angular momentum is reduced.

Optimal discus trajectories.

A general 3-D dynamic model for men's and women's discus flight is presented including precession of spin angular momentum induced by aerodynamic pitching moment. Dependence of pitching moment coefficient on angle of attack alpha is estimated from experiment. Numerical integration of 11 equations of motion for nominal release speed v(0)=25 m/s and axial spin p(0)=42 rad/s also requires 3 other release conditions; initial discus flight path angle beta(0), pitch attitude theta(0), and roll angle phi(0). Optimal values for these release conditions are calculated iteratively to maximize range and are similar for both men and women. The optimal men's trajectory and range R=69.39 m is produced by the strategy beta(0)=38.4 degrees, theta(0)=30.7 degrees, and phi(0)=54.4 degrees. Initial angular velocities except spin are chosen to minimize wobble but an optimal initial spin rate p(0)=25.2 rad/s exists that also maximizes range. Optimal 3-D range exceeds that predicted by 2-D models because, although angle of attack and lift are negative initially, 3-D motion allows advantageous orientation of lift later in flight, with tilt of the axis of symmetry from vertical becoming much smaller at landing. Optimal strategies are discontinuous with wind speed, resulting in slicing and kiting strategies in large head and tail winds, respectively. Sensitivity of optimal range is largest to initial beta(0) and least to phi(0). Present calculations do not account for dependence of initial release angle or spin on release velocity or among other release conditions.

Optimal compliant-surface jumping: a multi-segment model of springboard standing jumps.

A multi-segment model is used to investigate optimal compliant-surface jumping strategies and is applied to springboard standing jumps. The human model has four segments representing the feet, shanks, thighs, and trunk-head-arms. A rigid bar with a rotational spring on one end and a point mass on the other end (the tip) models the springboard. Board tip mass, length, and stiffness are functions of the fulcrum setting. Body segments and board tip are connected by frictionless hinge joints and are driven by joint torque actuators at the ankle, knee, and hip. One constant (maximum isometric torque) and three variable functions (of instantaneous joint angle, angular velocity, and activation level) determine each joint torque. Movement from a nearly straight motionless initial posture to jump takeoff is simulated. The objective is to find joint torque activation patterns during board contact so that jump height can be maximized. Minimum and maximum joint angles, rates of change of normalized activation levels, and contact duration are constrained. Optimal springboard jumping simulations can reasonably predict jumper vertical velocity and jump height. Qualitatively similar joint torque activation patterns are found over different fulcrum settings. Different from rigid-surface jumping where maximal activation is maintained until takeoff, joint activation decreases near takeoff in compliant-surface jumping. The fulcrum-height relations in experimental data were predicted by the models. However, lack of practice at non-preferred fulcrum settings might have caused less jump height than the models' prediction. Larger fulcrum numbers are beneficial for taller/heavier jumpers because they need more time to extend joints.

Optimal jumping strategies from compliant surfaces: a simple model of springboard standing jumps.

A simple model of standing dives is used to investigate optimal jumping strategies from compliant surfaces and applied to springboard diving. The human model consists of a massless leg actuated by knee torque, and a lumped torso mass centered above the leg. The springboard is modeled as a mass-spring system. Maximum jump height for a male and a female is calculated by controlling knee-torque activation level as a function of time. The optimization includes constraints on minimum and maximum knee angle, rate of change of normalized activation level, and contact duration. Simulation results for maximal springboard depression and diver takeoff velocity agree reasonably with experimental data, even though larger board tip velocities are necessarily predicted earlier during the contact period. Qualitatively similar multiple pulse knee-torque activation patterns are found over various conditions and are different from those in rigid-surface jumping. The model is less able to predict accurately jump height at high fulcrum number since jumpers may have difficulty behaving optimally at non-preferred fulcrum settings. If strength is proportional to the product of mass and leg length, increasing leg length is more effective in increasing jump height than is increasing mass.