The influence of speed of approach and accuracy constraint on the maximal speed of the ball in soccer kicking.

The kicking skill of the individual player is of great importance in soccer, and two parameters can be identified as being important in soccer kicking--achieving high speed of the ball and accuracy of performance. The purpose of the current study was to examine the influence of different speeds of approach on the maximal speed of the ball when kicking a stationary ball, and to examine the influence of accuracy of performance on the maximal speed of the ball relevant to a penalty kick. Seven male soccer players kicked for maximal speed of the ball (I) with a self-selected angle and speed of approach, (II) with accuracy demand placed on the subjects, (III) with a straight-line approach of different velocities ranging from 0 m/s to maximal running speed. The maximal speed of the ball was between 28.60 and 34.48 m/s. An accuracy constraint caused the speed of the ball to decline to 85%. With a straight-line approach, the maximal speed of the ball was between 25.64 and 32.26 m/s. When the subjects approached the ball at speeds other than the self-selected speed of approach, the speed of the ball declined, indicating a subject-specific optimal speed of approach.

Biomechanical differences in soccer kicking with the preferred and the non-preferred leg.

The aims of this study were to examine the release speed of the ball in maximal instep kicking with the preferred and the non-preferred leg and to relate ball speed to biomechanical differences observed during the kicking action. Seven skilled soccer players performed maximal speed place kicks with the preferred and the non-preferred leg; their movements were filmed at 400 Hz. The inter-segmental kinematics and kinetics were derived. A coefficient of restitution between the foot and the ball was calculated and rate of force development in the hip flexors and the knee extensors was measured using a Kin-Com dynamometer. Higher ball speeds were achieved with the preferred leg as a result of the higher foot speed and coefficient of restitution at the time of impact compared with the non-preferred leg. These higher foot speeds were caused by a greater amount of work on the shank originating from the angular velocity of the thigh. No differences were found in muscle moments or rate of force development. We conclude that the difference in maximal ball speed between the preferred and the non-preferred leg is caused by a better inter-segmental motion pattern and a transfer of velocity from the foot to the ball when kicking with the preferred leg.

EMG activity of the iliopsoas muscle and leg kinetics during the soccer place kick.

The purpose of the study was to develop a method to record intramuscular electromyogram (EMG) from the iliopsoas muscle and to relate this activity to the kinetics during the soccer place kick. Seven skilled soccer players performed 3 maximal velocity place kicks. The kicks were filmed with a high-speed camera (400 Hz) and EMG recordings were obtained from 5 muscles of the kicking leg, including wire electrodes inserted into the m. iliopsoas. The EMG signals were compared to the kinetics of the kicking leg, which were calculated from the digitised film. The results showed hardly any torque reversal about the hip joint before impact. Angular deceleration of the thigh segment did not increase the angular velocity of the shank (work -3.57 to 0.0%). M. iliopsoas was active during the entire kicking motion (average EMG 65.1-100.9%), even in the period when the thigh was decelerating. Wire electrodes can successfully be applied to EMG recordings of fast unloaded movements.

Prophylactic reoperation for strut fractures of the Björk-Shiley convexo-concave heart valve.

Outlet strut fracture of the Björk-Shiley Convexo-Concave (C/C) heart valve is the current paradigm of structural failure of mechanical heart valve prostheses. The estimated risk of fracture for C/C valves varies from 0.02% to 2.5% per patient-year, depending on valve size, opening angle, date of manufacture and position as a mitral or aortic replacement. This report examines our experience with 396 implants, comprising 346 C/C 60 degrees valves and 50 C/C 70 degrees valves. The 30-day mortality was 1.8%. Through April 1993, mean follow up was 8.9 years, incorporating 2971 patient-years and knowledge of the current status of 91.5% (356/389) of operative survivors. Late mortality was 3.7% per patient-year, including one 60 degrees and one 70 degrees outlet strut fracture. To evaluate the risk of semi-elective valve re-replacement at our institution, we reviewed the results of isolated re-replacements of failing or incipiently failing bioprostheses in 65 patients who had no serious co-morbidity and who were NYHA Class III or lower immediately before the procedure. The 30-day mortality was 4.6% and the overall serious morbidity rate was 10.8%, necessitating six repeat reoperations. When these factors were balanced against the life expectancy and general status of five patients with large-size, higher-fracture-risk 70 degrees C/C valves, it was concluded that only two patients remained possible reoperation candidates. Consideration for re-replacement of a normally functioning C/C heart valve should take into account institution-specific experience because structural failure remains a low risk compared to valve-related complications engendered by reoperation.