Removing swing from a handstand on rings using a properly timed backward giant circle: a simulation solution.

In the rings event in men's gymnastics, marks are deducted if the rings and gymnast are swinging during a held handstand position. The unwanted swing can be reduced in the next handstand position if the gymnast is able to properly time the start of the connecting giant circle. The purpose of this study was to search for the optimal time to commence a backward giant circle in order to attenuate swing in the succeeding handstand. Computer simulations, using a four-segment and a three-segment model which employed two-pulse muscular control strategies, were used to search for the optimal timing solution. Qualitative validation tests between the performance of a world class gymnast and the simulation models indicated that a three-segment model comprising a cables-rings segment, an arms segment with a shoulder torque generator, and a head-torso-legs segment, produced similar results to that of a four-segment model which separated the legs segment from the torso and employed an additional torque generator at the hip joint. The results from the simulation indicated that a gymnast should be advised to initiate a backward giant circle when his swinging handstand has reached the bottom of its swing-arc. For a handstand with an original swing-amplitude of 10 degrees, the simulation results indicate that a properly timed backward giant circle can reduce this amplitude to a negligible 1.5 degrees of swing.

In vitro attenuation of impact shock in equine digits.

This study was designed to test the impact characteristics of the equine digit in vitro with the objective of providing a better understanding of the role of the digital structures in the attenuation of impact shock. Uni-axial accelerometers were mounted on cadaver digits on the distolateral hoof wall, the proximolateral hoof wall, the dorsal surface of the second phalanx, and the mid-lateral first phalanx. The hoof-mounted accelerometers were aligned with the hoof tubules while the bone-mounted accelerometers were oriented along the longitudinal axis of the bone. Each digit was mounted in a test apparatus designed to simulate impact of the hoof with the ground during locomotion. The digits were subjected to 3 impact trials against a barrier at each of 3 vertical impact velocities that simulated a forward trotting velocity in the range of 2.67 to 4.46 m/s. The impact deceleration tended to increase with impact velocity. Attenuation of the impact shock by the digital tissues resulted in a reduction in impact decleration in the more proximal measuring locations. The interphalangeal joints appeared to play a larger role in amplitude attenuation than the hoof wall or the soft tissue structures within the hoof wall. The signal frequency data showed that the soft tissues within the hoof acted as a 'lowpass' filter, attenuating the higher deceleration frequencies. The hoof wall and the interphalangeal joints showed little frequency attenuation.

Biomechanical modelling of the human sacroiliac joint.

From a mechanical point of view, the human pelvis can be considered as a stable, complex three link structure. This three-link closed-chain system explains why there is so little motion in the sacroiliac joint. Based on the minimum total potential energy principle, a quasi-static model of the human pelvis with its three joints is developed. In the model, the articular cartilage linings of the joint surfaces are considered as thin layers with a geometric non-linear behaviour. They lie between two rigid curved surfaces that are represented by small three-node elements. Accessory ligaments and capsules are represented by a number of non-linear springs. A primary model is developed based on a female cadaver. According to the primary model, the translation of the sacroiliac joint in the direction of force is about 0.5 mm in the lateral direction, about 1.8 mm in the antero-posterior direction, and about 1.5 mm in the superior or inferior direction, when a load of 1000 N is applied to the sacrum. When a load of 50 N m-1 is applied to the sacrum, the rotation in the load direction is about 1.6 degrees in axial rotation, about 1.0 degree in flexion or extension and about 1.1 degrees in lateral bending.

Radiographic calculation of anteversion in acetabular prostheses.

The position of the acetabular prosthesis is critical for preventing dislocation following total hip arthroplasty. The reliability of a mathematical model for radiographically calculated acetabular cup version was examined. A porous-coated anatomic acetabular prosthesis was mounted in a mold. Anteroposterior radiographs were taken with the cup in five different positions of anteversion. These were reviewed by five orthopaedic surgeons, and measurements were taken from each radiograph. From these measurements, the mathematically derived degree of version was calculated. The results were examined for accuracy and intraobserver reliability. It was concluded that intraobserver reliability was very good and that the accuracy was within a clinically acceptable range. This technique could be useful in studying the "safe zone" for acetabular prostheses.