Influence of preflight on the horse impact phase of a simulated yurchenko layout vault

Introduction

Recently, key mechanical variables associated with an optimal performance of the Yurchenko layout vault for an elite female gymnast were identified [4]. The optimal vault was one that satisfied the postflight height and distance requirements stipulated by the 1997 FIG Code of Points, while maintaining a fully extended posture throughout postflight. The optimised impact technique was a combination of several factors: a greater "body angle of attack"; an increase in the vertical horse takeoff velocity of the CM; and, a larger preflight angular momentum that increased during horse impact, relative to the recorded performance. However, the individual influence of the body angle of attack or the level of preflight angular momentum on vertical horse takeoff velocity and horse impact duration is not known. The purpose of the study was therefore to investigate how selected preflight variables affects the vault performance.

Methods

A combined optimal control and parameter selection approach with the MISER3 optimal control software [1] was used in this study. A five-segment model (symmetry assumed), comprising the hands, upper limbs, upper trunk, lower trunk and lower limbs, was personalised with respect to the physical attributes of an elite level gymnast using the technique of [2]. Model evaluation was reported in [3]. The model was driven by joint torque histories of the wrist, shoulder, mid-trunk and hip from the optimised vault of [4]. Two separate simulations (Sim1 and Sim 2) were conducted to investigate how the initial body angle of attack or the angular momentum at impact affected the optimal vault identified by [4]. Data from the best trial reported in [4] was used as input variables. In Sim1, only the segment angular velocities were allowed to vary. In Sim2, only the initial impact configuration was allowed to vary. The simulations commenced at impact and finished at the end of postflight. Input comprised the inertial parameters, the initial segment angular coordinates and angular velocities, and the initial CM position and velocity. Model output comprised the time histories of the optimised joint torques, the segment angular orientations and velocities and the CM trajectory of the system.

Results

Table 1. Impact time, body angle at impact, and whole body CM velocities and angular momentum at start and end of impact

			Vertical CM velocity (ms-1)
Source	Impact Time (s)	Body Angle (°)	At impact	Horse Takeoff
Data (9.2 score vault, best trial) a	0.175	31.9	2.40	2.27
Koh et al. (2003) [5]	0.172	41.4		2.61
Simulation 1 (ωi, allowed to vary)	0.175	31.9		2.63
Simulation 2 (Oi, allowed to vary)	0.163	42.6		2.68
Oi,ωi-angular coordinates and velocities of hand, arm, upper and lower trunk and leg segments; a Best trial CM velocity data used as input into simulation studies 1, 2 and Koh et al. (2003)

Discussion / Conclusions

Compared to the data, Sim1 (see Table 1) produced an 11% increase in the whole body angular momentum at initial impact, which increased to 50% at the end of impact. Sim2 increased angular momentum by 35% at the end of impact. The implication is that the recorded vault does not have sufficient angular momentum to produce the optimal vault. In Sim2, body angle at initial impact increased by 34% relative to the data. A reduction in impact duration by 0.012 s was also observed. It may be concluded that impact duration is a function of the body angle at impact. Main modifications in CM horse takeoff velocities were in the vertical component. Both simulations reported gains, Sim1 (10 %) and Sim2 (12%), from the initial level contrary to the data and [5]. The discrepancies may be attributed to the layout somersaults in the actual performances not being fully extended. The increase in the vertical CM horse takeoff velocity for both simulations strongly indicates that vertical CM takeoff velocity is a critical variable in postflight amplitude and thus to the successful performance of a high scoring vault [5]. There exists an optimal combination of pre-flight angular momentum and body angle for each gymnast as identified by [4]. A gymnast making initial contact with the horse at too steep an angle may not be able to use this phase of the vault to influence the postflight trajectory, in particular the vertical CM takeoff velocity; or to alter the magnitude of the angular momentum to higher levels that can produce the optimal vault.

References

1. Jennings. L.S., et al. (2000). The University of Western Australia, Crawley. http://www.cado.uwa.edu.au/miser.
   
2. Jensen, R.K. (1978). J. Biomech., 11:349-358.
   
3. Koh, M. & Jennings, L. (2003). J. Biomech.,36:1177-1183.
   
4. Koh, M., et al. (2003). J. Appl.Biomech. (in press).
   
5. Kwon, Y. H., et al. (1990). Int. J. Sport Biomech., 6:157-176.