Eric J. Sprigings

A three-dimensional kinematic method for determining the effectiveness of arm segment rotations in producing racquet-head speed

The contribution that a segments anatomical rotations make to racquet-head speed depends on both the segments angular velocity and the instantaneous position of the head of the racquet with respect to the segments axes of rotation. Any analysis of racquet swing technique that does not consider both of these factors simultaneously is, at best, incomplete. With this in mind, a three-dimensional kinematic method was developed to determine the effectiveness of the anatomical rotations of the upper arm, forearm, and hand in producing racquet-head speed. The method entailed developing a system of vector equations for three-dimensional upper limb rotations that used displacement histories of 10 selected landmarks as input. The required three-dimensional displacement histories were obtained using three cine cameras and the DLT approach. To test the diagnostic capabilities of the method, a tennis serve was selected for analysis. For the player and serve analyzed, the greatest contribution to racquet-head speed at impact was produced by internal rotation of the upper arm (8 m s-1). Forearm pronation, although exhibiting the fastest rotation at impact (24 rad s-1), ranked only fourth in terms of its contribution (4 m s-1) to racquet-head speed. To test the performance of the method, a comparison was made between the racquet-head speed measured directly from film and the racquet-head speed computed by summing all of the individual segment contributions to speed commencing at the start of forward swing and ending at ball contact. The results indicate that the method can successfully determine the individual contributions that the different anatomical rotational velocities of the arm segments make to the measured instantaneous racquet-head speed.

Fluid forces on kayak paddle blades of different design

Three kayak paddle blades of different design (Conventional, Norwegian, Turbo) were tested in a low-speed wind tunnel at a maximum chord Reynolds number of Re = 2.2–2.7 × 105 (corresponding to speed through water of ≈1 m/s). The mean drag force and side force acting on each blade were measured, as the yaw and pitch angles were varied. The results were compared with those recorded for a finite rectangular flat plate of similar area and aspect ratio. For zero pitch angle of the blades, the results indicate that the drag coefficient was mostly independent of the blade design as the yaw angle was varied between ± 20°, with only the Norwegian blade design displaying a marginally higher drag coefficient than either of the other two blades or the flat plate. Increasing the pitch angle to 30°, while maintaining the yaw angle at zero, resulted in a 23% reduction of the drag coefficient for the flat plate, but only a 15% reduction of the drag coefficients for the three blades. For all designs, the drag coefficient reduction followed a simple cosine relationship as the pitch angle or yaw angle was increased. The wind tunnel experiments revealed that the side force coefficients for all three paddle blade designs were entirely independent of the blade design and were indistinguishable from those recorded for a flat plate. In summary, the study showed that the nondimensional force coefficients are largely independent of the paddle blade design.

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

In the rings event in mens 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.

Pre-flight characteristics of Hecht vaults

This study reports the techniques used by gymnasts to perform the Hecht vault and compares them with techniques used for the handspring somersault vault (Takei and Kim, 1990). Our main aim was to establish how the pre-flight characteristics of the Hecht vault influence post-flight performance. Data were obtained on 27 elite gymnasts performing the Hecht vault at the 1993 Canadian National Championships using two-dimensional video analysis with the direct linear transformation (DLT) technique. The maximum height reached by the mass centre during post-flight was significantly correlated (P < 0.001) with the vertical velocity of the mass centre and the body angle at horse contact. The backwards rotation of the body was significantly correlated (P = 0.015) with the shoulder angle at horse contact. The competition score was significantly correlated (P = 0.043) with the body angle at horse contact and was also related to the maximum height of the mass centre during post-flight. For the Hecht vault, the gymnasts had longer, lower and faster pre-flights with slower rotation at horse contact compared with the handspring somersault vaults.

An insight into the reversal of rotation in the Hecht vault

In a Hecht vault the direction of somersault rotation is reversed during horse contact. Gymnasts contribute actively to this reversal using muscle-generated shoulder torques during this contact phase. There is also the possibility that the way in which the gymnast contacts the horse may contribute to the reversal of somersault rotation by creating naturally occurring total body rotations at horse impact. To investigate this, a two-segment model was used to simulate an instantaneous inelastic impact during which internally generated shoulder torque was constrained to zero. The simplicity of the model used in the simulation provided valuable insight into the role that the preflight trajectory plays in the reversal of total body rotation at horse impact. It was found using realistic takeoff conditions from the board, that over half of the reversal of rotation could be produced by a suitable preflight trajectory.

Simulation of the force enhancement phenomenon in muscle

This paper demonstrates that the mathematical modeling of the mechanical force enhancement phenomenon in muscle can be achieved using the simple rheological model proposed by Houk. The simulation of force enhancement for this model requires that the pre-stretch of the muscle be entered as an input function to the rheological models equations. The results of the mathematical simulation suggest that the main characteristics of the force enhancement phenomenon could quite conceivably be explained in terms of a greater induced compliance of the series elastic component as a result of a stretch placed on the muscle by an external load.

Evaluation of the plumb-bob method for reading greens in putting.

This study evaluated the validity of the plumb-bob method as used to determine the break of a putt. Two separate experiments were conducted to examine the consequence of violating inherent assumptions in the method. In the first experiment, a controlled putting environment was constructed to assess the plumb-bob method in determining the break of a putt, if the slope of the green was not constant from the position of the golfer behind the ball through to the hole. It was determined that if the slope of the green beneath the golfer was different from the slope between the ball and the hole, then the plumb-bob method would provide an incorrect indication of break. The second experiment examined the ability of a golfer to stand perpendicular to a slope. Half of the participants in the study deviated by ± 1.5° or greater from standing perpendicular to a slope. A + 1.5° error on a 1.4 m (∼ 4.5 ft) putt translates into reading an extra 0.08 m of break and a missed putt. The plumb-bob method was found to be an invalid system for determining the break of a putt.

A method for personalising the blade size for competitors in flatwater kayaking

The intent of this project was to explore the feasibility of personalising the paddle blade size for individual flatwater kayakers based on their power output profiles. Twelve elite male kayakers performed on a kayak ergometer at the same intensity and resistance that they would normally experience while paddling at race pace for 500 m on the water. The kayak ergometer was instrumented so that power profiles could be determined from the instantaneous force and velocity of the representative centre point of the paddle blade. From the power profile information, the researchers calculated a personalised blade size that was expected to improve performance for those kayakers differing more than 5% from the calculated ‘ideal’ size. For the elite kayakers studied, it was recommended that seven of the paddlers should increase their blade size by approximately 5–10%. For the remaining five paddlers, the results indicated that their current blade sizes were within the expected measurement error of their predicted ideal value and should be retained. It is anticipated that this research will provide the theoretical rationale for elite kayakers to see the need to personalise their blade size based on their own muscle power profiles.