Biomechanics of the ski cross start indoors on a customised training ramp and outdoors on snow

An effective start enhances an athlete's chances of success in ski cross competitions. Accordingly, this study was designed to investigate the biomechanics of start techniques used by elite athletes and assess the influence of different start environments. Seven elite ski cross athletes performed starts indoors on a custom-built ramp; six of these also performed starts on an outdoor slope. Horizontal and vertical forces were measured by force transducers located in the handles of the start gate and a 12-camera motion capture system allowed monitoring of the sagittal knee, hip, shoulder, and elbow kinematics. The starting movement involved Pre, Pull, and Push phases. Significant differences between body sides were observed for peak vertical and resultant forces, resultant impulse, and peak angular velocity of the shoulder joint. Significantly lower peak vertical forces (44 N), higher resultant impulse (0.114 Ns/kg), and knee joint range of motion (12°) were observed indoors. Although movement in the ski cross start is generally symmetrical, asymmetric patterns of force were observed among the athletes. Two different movement strategies, i.e. pronounced hip extension or more accentuated elbow flexion, were utilised in the Pull phase. The patterns of force and movement during the indoor and outdoor starts were similar.     

多点摄像与解析方法在跳台滑雪技术分析中的应用

在运动影像解析技术领域里,为了准确地获取运动目标的三维空间坐标位置,一般使用定点、定焦摄像方式。对于大范围运动的动作解析,需要多台摄像机在多点位置进行定点、定焦动作捕捉。跳台滑雪运动是一个比较典型的、在较大范围内动作复杂的运动项目。对于优秀跳台滑雪运动员的三维动作解析问题是我国冰雪体育科研领域的前沿研究项目,其中如何捕捉和获取大范围运动信息是分析过程的关键环节之一。通过研究多点摄像解析方法在跳台滑雪运动中的应用,为处理大范围运动的动作捕捉与解析研究提供理论参考。     

Validation of an integrated experimental set-up for kinetic and kinematic three-dimensional analyses in a training environment

Biomechanical analyses using synchronized tools [electromyography (EMG), motion capture, force sensors, force platform, and digital camera] are classically performed in a laboratory environment that could influence the performance. We present a system for studying the running sprint start that synchronizes motion capture, EMG, and ground reaction force data. To maximize motion capture (Vicon 612 with six cameras), a special dim environment was created in the stadium. "Classical" tools were combined with "purpose-built" tools intended to analyse the different aspects of movement. For example, a synchronization system was built to create a common time-base for all data recordings and a portable EMG system was synchronized by a cable that was "disconnected" by the athlete's movement out of the blocks. This disconnection represented an independent event recorded by different tools. A "gap" was measured for some sprint start events between kinetic and kinematic (motion capture) data. Calibration results, measurements of time "gap", and duration of the independent event were used to validate the accuracy of motion capture and the synchronization system. The results validate the entire experimental set-up and suggest adjustment values for motion capture data. This environment can be used to study other movements and can easily be applied to several sports.     

Aerodynamic force data for a V-style ski jumping flight

To determine the flight of a ski jumper it is essential to know what aerodynamic forces are acting on the ski jumper. However, few data on this are available, especially for a V-style ski jumping flight. We have measured the aerodynamic forces during the free flight phase for a V-style, as well as a parallel-style, ski jump by employing a full-size model in a wind tunnel. The aerodynamic force data, (drag, lift and pitching moment) were obtained to create an aerodynamic database. These forces are given in polynomial form as functions of the angle of attack, the body-ski (forward leaning) angle and the ski-opening (V-style) angle. Using the polynomial form database is a convenient way of obtaining the aerodynamic forces. Moreover, the wind tunnel was equipped with a ground effect plate to measure the aerodynamic forces during the landing phase. It was found that the difference between the lift with and without the ground effect plate increases with the ski-opening angle. The longitudinal stability in the pitching motion of a body-ski combination is also discussed on the basis of the pitching moment data. This indicates that a stable pitching oscillation of the body-ski combination may arise around an equilibrium point in the angle of attack, the trim angle of attack, during flight.     

Influence of foot-stretcher height on rowing technique and performance

Strength, technique, and coordination are crucial to rowing performance, but external interventions such as foot-stretcher set-up can fine-tune technique and optimise power output. For the same resultant force, raising the height of foot-stretchers on a rowing ergometer theoretically alters the orientation of the resultant force vector in favour of the horizontal component. This study modified foot-stretcher heights and examined their instantaneous effect on foot forces and rowing technique. Ten male participants rowed at four foot-stretcher heights on an ergometer that measured handle force, stroke length, and vertical and horizontal foot forces. Rowers were instrumented with motion sensors to measure ankle, knee, hip, and lumbar–pelvic kinematics. Key resultant effects of increased foot-stretcher heights included progressive reductions in horizontal foot force, stroke length, and pelvis range of motion. Raising foot-stretcher height did not increase the horizontal component of foot force as previously speculated. The reduced ability to anteriorly rotate the pelvis at the front of the stroke may be a key obstacle in gaining benefits from raised foot-stretcher heights. This study shows that small changes in athlete set-up can influence ergometer rowing technique, and rowers must individually fine-tune their foot-stretcher height to optimise power transfer through the rowing stroke on an ergometer.     

Forward acceleration of the centre of mass during ski skating calculated from force and motion capture data

The purpose of this paper was to present and evaluate a methodology to determine the contribution of bilateral leg and pole thrusts to forward acceleration of the centre of mass (COM) of cross-country skiers from multi-dimensional ground reaction forces and motion capture data. Nine highly skilled cross-country (XC) skiers performed leg skating and V2-alternate skating (V2A) under constant environmental conditions on snow, while ground reaction forces measured from ski bindings and poles and 3D motion with high-speed cameras were captured. COM acceleration determined from 3D motion analyses served as a reference and was compared to the results of the proposed methodology. The obtained values did not differ during the leg skating push-off, and force–time curves showed high similarity, with similarity coefficients (SC) >0.90 in the push-off and gliding phases. In V2A, leg and pole thrusts were shown to contribute 35.1 and 65.9% to the acceleration of the body, respectively. COM acceleration derived from ground reaction forces alone without considering the COM position overestimated the acceleration compared to data from motion analyses, with a mean difference of 17% (P < 0.05) during leg push-off, although the shapes of force–time curves were similar (SC = 0.93). The proposed methodology was shown to be appropriate for determining the acceleration of XC skiers during leg skating push-off from multi-dimensional ground reaction forces and the COM position. It was demonstrated that both the COM position and ground reaction forces are needed to find the source of acceleration.     

A ski robot system for qualitative modelling of the carved turn

A robot that simulates a number of human leg joint motions during carved turns has been developed. Each leg had six degrees of freedom like those of human athletes. An on-board computer controlled the sequence of joint angles in an open-loop mode during skiing on an artificial grass slope. The relations among joint motions, reacting forces and turn trajectory were investigated by programming various motions of the robot. At first, the effect of basic joint motions, such as abduction–adduction and flexion–extension of the hip, knee and ankle joints were investigated. Then the sequence of a top athlete’s joint motions, measured in a separate study, was applied to investigate its effect on the ski turn. The human-inspired programme produced a more even force balance between the skis and also a higher-quality turn. The requirements for a successful physical model of a human skier are discussed.     

Kinetic function of the lower limbs during baseball tee-batting motion at different hitting-point heights

ABSTRACT

The aim of this study was to investigate the kinetic functions of the lower limbs at different hitting-point heights to provide key information for improving batting technique in baseball players. Three-dimensional coordinate data were acquired using a motion capture system (250 Hz) and ground reaction forces were measured using three force platforms (1000 Hz) in 22 male collegiate baseball players during tee-batting set at three different hitting-point heights (high, middle, and low). Kinetic data were used to calculate joint torque and mechanical work in the lower limbs by the inverse dynamics approach. The peak angular velocity of the lower trunk about the vertical axis was smaller under the low condition. The joint torques and mechanical works done by both hip adduction/abduction axes were different among the three conditions. These results indicate that hip adduction/abduction torques mainly contribute to a change in the rotational movement of the lower body about the vertical axis when adjusting for different hitting-point heights. In order to adjust for the low hitting-point height which would be difficult compared with other hitting-point heights, batters should focus on rotating the lower trunk slowly by increasing both hip abduction torques.     

Pose tracking with rate gyroscopes in alpine skiing

The motion capture of alpine skiing is technically challenging. Motion capture systems using inertial sensors such as gyroscopes and accelerometers have begun to be used in sport. The capture volume of the system can be large enough to cover the whole area of a particular sporting event, because sensors are affixed to the human body itself. The current study developed a method of tracking the pose of an alpine skier using gyroscopes for 3 min with a tracking error of 2° (RMSE). The technical significance of the presented method, which uses one common simple algorithm to improve measurement of pose, applies to many experimental conditions. The adoption of a high resolution (24 bit) analog-to-digital converter and high sampling rate (1 kHz) also contribute to the improvement of measurement time.     

Imitation jumps in ski jumping: Technical execution and relationship to performance level

ABSTRACT

Imitation jumps are frequently used in training for ski jumping. Yet, the dynamics of these jumps differ considerably. Thus, the relevance of imitation jumps for ski jumping performance is not elucidated. The aim of this study was to investigate the relationship between the technical execution of imitation jumps and ski jumping performance level. We compared the imitation jumps of 11 ski jumpers of different performance levels using a Spearman correlation transform of time traces of the kinetics (measured using force cells and motion capture) of imitation jumps. The kinetic aspects that were related to performance centred on the moment arm of ground reaction force to the centre of mass before the onset of the push-off, angular momentum early in push-off, thigh angle during the main period of push-off and vertical velocity towards the end of push-off. We propose that the thigh angle may be a key element allowing high development of linear momentum while preparing for appropriate aerodynamic position. Furthermore, the findings suggest that the kinetic development prior to (and during) push-off is more important than the kinematic end state at take-off.     

Effect of ski boot rear stiffness (SBRS) on maximal ACL force during injury prone landing movements in alpine ski racing: A study with a musculoskeletal simulation model

A common anterior cruciate ligament (ACL) injury situation in alpine ski racing is landing back-weighted after a jump. Simulated back-weighted landing situations showed higher ACL-injury risk for increasing ski boot rear stiffness (SBRS) without considering muscles. It is well known that muscle forces affect ACL tensile forces during landing. The purpose of this study is to investigate the effect of different SBRS on the maximal ACL tensile forces during injury prone landings considering muscle forces by a two-dimensional musculoskeletal simulation model. Injury prone situations for ACL-injuries were generated by the musculoskeletal simulation model using measured kinematics of a non-injury situation and the method of Monte Carlo simulation. Subsequently, the SBRS was varied for injury prone landings. The maximal ACL tensile forces and contributing factors to the ACL forces were compared for the different SBRS. In the injury prone landings the maximal ACL tensile forces increased with increasing SBRS. It was found that the higher maximal ACL force was caused by higher forces acting on the tibia by the boot and by higher quadriceps muscle forces both due to the higher SBRS. Practical experience suggested that the reduction of SBRS is not accepted by ski racers due to performance reasons. Thus, preventive measures may concentrate on the reduction of the quadriceps muscle force during impact.     

Automatic measurement of key ski jumping phases and temporal events with a wearable system

We propose a new method, based on inertial sensors, to automatically measure at high frequency the durations of the main phases of ski jumping (i.e. take-off release, take-off, and early flight). The kinematics of the ski jumping movement were recorded by four inertial sensors, attached to the thigh and shank of junior athletes, for 40 jumps performed during indoor conditions and 36 jumps in field conditions. An algorithm was designed to detect temporal events from the recorded signals and to estimate the duration of each phase. These durations were evaluated against a reference camera-based motion capture system and by trainers conducting video observations. The precision for the take-off release and take-off durations (indoor < 39 ms, outdoor = 27 ms) can be considered technically valid for performance assessment. The errors for early flight duration (indoor = 22 ms, outdoor = 119 ms) were comparable to the trainers' variability and should be interpreted with caution. No significant changes in the error were noted between indoor and outdoor conditions, and individual jumping technique did not influence the error of take-off release and take-off. Therefore, the proposed system can provide valuable information for performance evaluation of ski jumpers during training sessions.     

The suitability of Sanders’ model for calculation of the propulsive force generated by the hands during sculling motion

This study examined whether Sanders’ model is suitable for estimating accurately the propulsive force generated by the hands’ motion in swimming comparing the calculated force obtained using the model and the measured force during an actual propulsive action. The measured and calculated forces were obtained from 13 swimmers who, while tethered, performed a sculling motion in a prone position for the purpose of displacing the body by moving it forward. Kinematic analyses were conducted to obtain the calculated force, while the measured force was obtained via the use of a load cell. The calculated force was lower than the measured force and accounted for only a small part of the variation in the measured force. The forces could not be used interchangeably, and there were fixed and proportional differences between them. Consequently, this study indicates that Sanders’ model is not suitable for estimating accurately the propulsive force generated by the swimmer’s hands during sculling motion. However, research that integrates analyses from different approaches could result in improvements to the model that would render it applicable for estimating the propulsive forces during movements that are characterised by directional changes of the hands.     

Friction reduction using self-waxing alpine skis

A continuously waxed ski has been developed that releases a thin film of lubricant under the base of a ski. This replicates the melt water layer observed in snow skis which is caused by frictional and solar heating. The system is particularly effective on artificial (dry) slopes where skiers slide on plastic bristles rather than snow. Speeds comparable to those achieved on snow are achieved using this system and this improves the experience for the skier. Speed enhancements on plastic slopes of up to 50?% have been achieved using solutions of polyethylene glycol in water. There is speed enhancement of approximately 9?% on artificial snow and 2?% on fresh alpine snow. The latter value is highly significant as it can be the difference between winning a medal in ski competitions and finishing outside the top ten. In addition to the quantitative data, qualitative athlete perceptions were also recorded and show that a feel like snow can be achieved on artificial surfaces. Because the lubrication system is attached to the ski, it allows personal performance enhancement irrespective of a water misting system being in operation or not. The design complies with the equipment regulations of the skiing??s international governing body so it can be used in competition.     

Musculoskeletal loading during the round-off in female gymnastics: the effect of hand position

Chronic elbow injuries from tumbling in female gymnastics present a serious problem for performers. This research examined how the biomechanical characteristics of impact loading and elbow kinematics and kinetics change as a function of technique selection. Seven international-level female gymnasts performed 10 trials of the round-off from a hurdle step to flic-flac with ‘parallel’ and ‘T-shape’ hand positions. Synchronized kinematic (3D-automated motion analysis system; 247 Hz) and kinetic (two force plates; 1,235 Hz) data were collected for each trial. Wilcoxon non-parametric test and effect-size statistics determined differences between the hand positions examined in this study. Significant differences (p < 0.05) and large effect sizes (ES>0.8) were observed for peak vertical ground reaction force (GRF), anterior–posterior GRF, resultant GRF, loading rates of these forces and elbow joint angles, and internal moments of force in sagittal, transverse, and frontal planes. In conclusion, the T-shape hand position reduces vertical, anterior–posterior, and resultant contact forces and has a decreased loading rate indicating a safer technique for the round-off. Significant differences observed in joint elbow moments highlighted that the T-shape position may prevent overloading of the joint complex and consequently reduce the potential for elbow injury.     

Kinetic analysis of the lower limbs in baseball tee batting

The purpose of this study was to investigate effects of the ground reaction forces on the rotation of the body as a whole and on the joint torques of the lower limbs associated with trunk and pelvic rotation in baseball tee batting. A total of 22 male collegiate baseball players participated in this study. Three-dimensional coordinate data were acquired by a motion capture system (250 Hz), and ground reaction forces of both legs were measured with three force platforms (1,000 Hz). Kinetic data were used to calculate the moment about the vertical axis through the body’s centre of mass resulting from ground reaction forces, as well as to calculate the torque and mechanical work in the lower limb joints. The lateral/medial ground reaction force generated by both legs resulted in the large whole body moment about its vertical axis. The joint torques of flexion/extension of both hips, adduction of the stride hip and extension of the stride knee produced significantly larger mechanical work than did the other joint torques. To obtain high bat-head speed, the batter should push both legs in the lateral/medial direction by utilising both hips and stride knee torques so as to increase the whole body rotation.     

How do elite ski jumpers handle the dynamic conditions in imitation jumps?

We examined the effect of boundary conditions in imitation ski jumping on movement dynamics and coordination. We compared imitation ski jumps with – and without – the possibility to generate shear propulsion forces. Six elite ski jumpers performed imitation jumps by jumping from a fixed surface and from a rolling platform. The ground reaction force vector, kinematics of body segments, and EMG of eight lower limb muscles were recorded. Net joint dynamics were calculated using inverse dynamics. The two imitation jumps differed considerably from each other with regard to the dynamics (moments, forces), whereas the kinematics were very similar. Knee power was higher and hip power was lower on the rolling platform than on the fixed surface. Mean EMG levels were very similar for both conditions, but differences in the development of muscle activity were indicated for seven of eight muscles. These differences are reflected in a subtle difference of the alignment of ground reaction force with centre of mass: the ground reaction force runs continuously close to but behind the centre of mass on the rolling platform and fluctuates around it on the fixed surface. This likely reflects a different strategy for controlling angular momentum.     

Ground reaction forces and knee kinetics during single and repeated badminton lunges

Repeated movement (RM) lunge that frequently executed in badminton might be used for footwear evaluation. This study examined the influence of single movement (SM) and RM lunges on the ground reaction forces (GRFs) and knee kinetics during the braking phase of a badminton lunge step. Thirteen male university badminton players performed left-forward lunges in both SM and RM sessions. Force platform and motion capturing system were used to measure GRFs and knee kinetics variables. Paired t-test was performed to determine any significant differences between SM and RM lunges regarding mean and coefficient of variation (CV) in each variable. The kinetics results indicated that compared to SM lunges, the RM lunges had shorter contact time and generated smaller maximum loading rate of impact force, peak knee anterior-posterior force, and peak knee sagittal moment but generated larger peak horizontal resultant forces (Ps < 0.05). Additionally, the RM lunges had lower CV for peak knee medial-lateral and vertical forces (Ps < 0.05). These results suggested that the RM testing protocols had a distinct loading response and adaptation pattern during lunge and that the RM protocol showed higher within-trial reliability, which may be beneficial for the knee joint loading evaluation under different interventions.     

Optimization-based motor control of a Paralympic wheelchair athlete

The following research approximated how the central nervous system of Paralympic wheelchair athletes resolve kinematic redundancies during upper-limb movements. A multibody biomechanical model of a tetraplegic Paralympic athlete was developed using subject-specific body segment parameters. The angular joint kinematics throughout a specified Paralympic sport movement (i.e., wheelchair curling) were experimentally measured using inertial measurement units. The motor control system of the Paralympian was mathematically modelled and simulated using forward dynamics optimization. The predicted kinematics from different optimization objective functions (i.e., minimizing resultant joint moments, mechanical joint power, and angular joint velocities and accelerations) were compared with those experimentally measured throughout the wheelchair curling movement. Of the optimization objective functions under consideration, minimizing angular joint accelerations produced the most accurate predictions of the kinematic trajectories (i.e., characterized with the lowest overall root mean square deviations) and the shortest optimization computation time. The implications of these control findings are discussed with regards to optimal wheelchair design through predictive dynamic simulations.