Practical Use of Airborne Simulation in a Release-Regrasp Skill on the High Bar

Transcription

1 332 JOURNAL OF APPLIED BIOMECHANICS, 2002, 18, by Human Kinetics Publishers, Inc. Practical Use of Airborne Simulation in a Release-Regrasp Skill on the High Bar Patrice Holvoet Sports Science University, Lille Patrick Lacouture and Jacques Duboy Sciences University, Poitiers The aim of this study was to objectively predict individual improvements in a release-regrasp tkatchev skill. The prediction was based on a kinematic analysis of failed and successful trials. The modification of release conditions, and the correction of hip and shoulder joint motions during the aerial phase of failed trials, were determined by considering the successful trials as target executions. Computer simulations were used to confirm the effect of the corrected parameters on the flight trajectory and angular motion of the body over the bar. The results indicated that when time of release is initiated earlier, this presents a major problem the gymnast must overcome in order to grasp the bar. Moreover, the moment when the body s center of gravity is vertically above the bar represents a critical instant for the gymnast in initiating the hip and shoulder movements. The rotation motion analysis of the segments indicated that the stabilization motion of the upper limbs could be a good strategy for improving the failed tkatchev. This study showed that simple computer simulation using hypothetical data based upon real data could be an effective tool for improving acrobatic skills. Key Words: kinematics, predictive model, acrobatic skill, gymnastics Introduction Many sports require athletes to generate and control airborne rotation. Particularly in gymnastics events, following giant circles, gymnasts must perform flight elements on the horizontal bar (International Gymnastics Federation, 1997, Code of Points). The tkatchev is one of the most frequently performed release-regrasp skills P. Holvoet, Laboratory of Human Movement Studies, Sports Sciences University, Lille II, 9 rue de l Université, Ronchin, France; P. Lacouture and J. Duboy, Laboratory of Physics Metallurgy and Solids Mechanics, Sciences University, BP 179, Poitiers Futuroscope, France. 332

2 Airborne Simulation and the High Bar 333 that may be executed during the airborne phase, either with straddled or adducted legs or with piked or stretched body form. From a biomechanical point of view many factors contribute to the success or failure of the tkatchev skill. Its successful execution depends on the release of the body s center-of-gravity (GC) position and linear velocity that determine the projectile characteristics of the gymnast. Most important in this regard are the shape of the parabolic curve and the time available for the flight which guarantee a backward vault over the bar and a safe regrasp of the bar. Yet, due to the body s backward arched position relative to the bar at release, the gymnast, while airborne, must make accurate adjustments to segmental rotations in order to execute the required half forward somersault according to the conservation-of-angular-momentum principle. Studies traditionally have focused on elite gymnasts and the formulation of the profile of the kinematic requirements of dismounts and flight motions with regrasp of the bar during high level competitions (Borms, Moers, & Hubbelinck, 1976; Brüggemann, Cheetham, Alp, & Arampatiz, 1994; Gervais & Tally, 1993; Newton, Greenwood, & Turner, 1993; Smith, 1982; Takai, Nohara, & Kamimura, 1992; Witten, Brown, Witten, & Wells, 1996). These approaches have established that during the preparatory giant swing, the gymnast s ability to generate a swing motion with a large range of hip flexion and substantial shoulder movement of flexion, in order to achieve sufficient linear and angular momentum at release, was decisive for a successful execution of the tkatchev skill. Although those studies have clearly described the release requirements of many flight elements and the mechanics of the associated giant swing, only a few studies have examined how segmental movements executed by less skilled gymnasts could be modified during flight for successful performance of a release-regrasp exercise. Nissinen, Preiss, and Brüggemann (1985) simulated the dismount to modify the gymnast s moment of inertia concerning the transverse axis; they found that, using the same release parameters as measured for a given athlete doing the double layout somersault, the triple tucked somersault was possible. Kerwin, Yeadon, and Lee (1990) modified the layout somersault dismount to change the characteristic backward arch to a stretched body position: the additional angular momentum needed to complete the simulated movement was calculated and found to be no more than 13%. Yeadon (1990d; 1997) found that the tilt angle in dismount twisting techniques relates to body configuration and number of twists. Canal, Coffignal, Brochot, Bon, and Thomas (1990), Yeadon, Hiley, and Kerwin (1996), and Arampatzis and Brüggemann (1998) used the net muscle moments as model input for optimizing the swing preceding the flight but did not examine the effect on flight movement itself. To the best of our knowledge, only Nissinen et al. (1985) investigated whether an original forward somersault pike with release and regrasp of the bar would be possible using a straight-body form during the airborne phase. The aim of this paper was to objectively predict individual improvements of a release-regrasp tkatchev skill performed unsuccessfully or with poor technique. The first step involved a kinematic analysis of successful and unsuccessful trials in order to characterize the features of different skill executions. The release requirements, which determine the flight trajectory of the body s center of gravity, were examined. The segmental rotations that were analyzed relative to the CG reference frame in light of the fact that angular motion is governed during flight by the principle of conservation of angular momentum calculated to the CG.

3 334 The next step was to correct the errors in failed trials. By considering the successful trials as target executions, the differences of position and linear velocity of the CG between the failed trials and the bar-regrasped ones were considered errors that had to be corrected in order to improve the body s airborne translational motion. Correction of the hip and shoulder joint motions was based on segmental movements of the bar-regrasped trials and on the results of neurophysiological studies (Bardy & Laurent, 1998; Berthoz & Pozzo, 1994; Fowler & Turvey, 1978; Newell & McDonald, 1992; Pozzo, Berthoz, & Lefort, 1992) which demonstrated that upper segment stabilization motions that permit visual and vestibular control are good strategies for performing acrobatic skills. The final step was to confirm, by computer simulation, the effect of the corrected release parameters and corrected airborne segmental rotations on the body s flight trajectory and angular motion over the bar in order to validate the possibilities for improved movements. Methods The gymnast was considered as a planar 8-link kinematic chain. The feet, shanks, thighs, trunk, head, arms, forearms, and hands were assumed to be rigid segments linked by 7 rotational joints and were delimited by body landmarks. The trunk was considered as a one-part segment. The anthropometric estimates of positions of the segmental centers and CG, segmental masses, and moments of inertia were calculated using the tables of Dempster and Gaughran (1967). The multisegmental model related to the sagittal plane was connected to an elastic resistance modeling the high bar as a tension-compression spring with negligible inertia. The spring constant was determined experimentally by static measured flexions of the bar; it permitted the use of the linear relation between the tension and deflection of the bar from its neutral position in order to determine the exact point of application of the bar s reaction force. A 2-D video analysis was carried out on a French national level male gymnast (20 yrs, 1.65 m, 58 kg) during training while he performed a series of giant swings culminating in a release-regrasp tkatchev skill. The coach and four judges, in order to characterize the features of failed and successful executions and also to achieve realistic parameters, selected five trials executed with adducted legs and representative of different levels of performance. Trials 1 and 2 were failed trials without regrasp of the bar. Trial 3 was a successful tkatchev as performed with a piked body, followed by regrasp of the bar and continuation to the next routine. Trial 4 was an error in which, although the bar was regrasped, the upper limbs were bent and very near to the bar. Trial 5 was performed with a regrasp of the bar but then the gymnast fell under the bar. The two non-regrasped tkatchev skills (Trials 1 & 2) were accepted to be improved so that the bar could be regrasped. The successful piked tkatchev (Trial 3) was accepted to be improved so that the backward vault would be executed with a stretched body form. The coordinates of the body landmarks were collected at 50 fps using a CCD camera (Sony XC-75CE) set up at right angles to the bar s sagittal plane. The film data were obtained by digitizing each film frame with an image-capture board (Matrox Meteor, Dorval, PQ, Canada) interfaced with image analysis software (Noesis, Les Ulis, France). The position data were filtered using a 4th-order Butterworth filter (Allard, Blanchi, Gauthier, & Aissaoui, 1990) whose

4 Airborne Simulation and the High Bar 335 best cutoff frequency was found to be 6 Hz. Hence the root mean square (RMS) errors relative to the segmental lengths were 8% lower for the hand, foot, and head, and 5% lower for the other segments. The velocity of the joints and segmental centers were determined by differentiation with time and smoothed with a method based on least-squares quartic polynomial fitting across a moving window within the data (Stavitsky & Golay, 1964). A first diagnosis of the gymnast s translational movement over the bar was established by considering the release and regrasp values of Trials 4 and 5 as limit values and those of Trial 3 as target data. The difference in position and velocity of the CG at release and regrasp between failed and bar-regrasped trials was considered as the error that led to the imperfect flight curve in the non-regrasped Trials 1 and 2. To determine the sufficient projectile qualities of an airborne straight body execution, we added the length of the trunk to that of the lower limb to determine the vertical peak (h) of the parabolic trajectory of the CG. In this case a sufficient vertical component of release velocity (Vvr) could be calculated from the following equation: Vvr = 2gh where g was equal to the acceleration of gravity. By considering the release and regrasp position of the CG during the regrasped-bar trials as target values, using the classical law of falling bodies, we could determine sufficient time of flight and sufficient horizontal component of the release velocity. We conducted a second diagnosis of the gymnast s airborne angular motion by analyzing, during the flight phase, the segmental rotations according to the principle of conservation of angular momentum. The original shoulder joint motion of the successful trials, which were in agreement with experimental observations made by neurophysiologists, formed the basis of the segmental rotational error-detection process. In order to validate the corrections provided by the airborne translational and rotational movement analysis of the unsuccessful trials, we developed a 2-D computer simulation model of aerial movement based on the study of Yeadon (1990a, b, c). The airborne simulation process that comprised the inertial parameters of the segments, release values of the CG location and linear velocity, release angular momentum about the transverse axis through the CG, and 7 joint-angles time history as input yielded differential equations which we solved in order to determine the motion of the last segment. This movement together with the known joint movements defined the motion of the entire multisegmental model. To determine the effect of corrected release and regrasp requirements and segmental stabilization motions, we carried out three simulations by modifying the time of release and time of flight of failed trials and by modifying shoulder or hip motions to maintain the upper or lower limb in constant orientation during the gymnast s preparation to regrasp the bar. Results The release values of this study with regard to the release characteristics means and standard deviations presented by Gervais and Tally (1993) and Brüggemann et al. (1994), and included for comparison, did not appear to differ significantly

5 336 Table 1 Release and Regrasp Characteristics of the Body s Center of Gravity Horiz. Vertical Horiz. Vertical Angular position position velocity velocity momentum (m) (m) (m/s) (m/s) (kg m 2 /s) Failed 1 tkatchev Release Regrasp 22 Failed 2 tkatchev Release Regrasp 28 Successful piked tkatchev Release Regrasp Fall-under-the-bar tkatchev Release Regrasp Bent upper limb tkatchev Release Regrasp Study by Gervais & Tally (n = 7) (SD) Release (0.08) (0.38) Regrasp 25 (8) Study by Brüggemann et al. (n = 23) (SD) Release (0.12) (0.35) Regrasp 29 (4) between the regrasped-bar trials and the failed ones (Table 1). The failed trials were performed with a greater vertical position and horizontal velocity of the CG at release, and a lower horizontal position and vertical velocity at release, than the bar-regrasped trials. The aerial segmental contributions to the total angular momentum indicated for all trials that lower limb rotation was very important (>50%) just after release and after the gymnast had vaulted away from the vertical position over the bar (Figure 1). The contribution of the torso and head was great from release to vertical passing over the bar, and decreased rapidly during the regrasp phase. Much lower percentage contributions of the upper limb were calculated (<10%). The lower limbs and the trunk and head values of the two failed tkatchevs represented the greatest deviation from the data of the bar-regrasp trials.

6 Airborne Simulation and the High Bar 337 LOWER LIMBS body position angle contribution to total angular momentum (%) TRUNK AND HEAD body position angle UPPER LIMBS Figure 1 Segmental contributions to total angular momentum. successful piked tkatchev fall-under-the-bar tkatchev bent upper limb tkatchev failed 2 tkatchev failed 1 tkatchev body position angle

7 338 SUCCESSFUL PIKED TKATCHEV vertical position (m) horizontal position (m) Figure 2 Rotation movements of the two supra-segments relative to the body s CG reference frame. The rotation movement during the bar-regrasp phase of the successful piked tkatchev of two supra-segments linked at the hip one being the lower limb delimited by the foot and hip, the other delimited by the hip and hand and including the torso, head, and upper limb was precisely analyzed in relation to the body s CG reference frame in each second video-field (Figure 2). The orientation of the upper segment appeared to be nearly constant while the lower segment rotated with a large hip extension. The corrected release positions and velocities of the CG which permitted us to obtain adequate flight paths for improving the failed Trial 1, the failed Trial 2, and the piked tkatchev were obtained using earlier release conditions (0.06, 0.08, and 0.02 s, respectively) in order to achieve lower horizontal velocity and vertical position of the CG, and greater vertical velocity and horizontal position of the CG (Figure 3). The constant orientation which permitted a stabilization motion of the upper supra-segment during the bar-regrasp phase was retained as a corrected joint angle to modify the hip or shoulder angle flexion after the release (Figure 4). These corrected release conditions and airborne hip and shoulder joint movements were used to carry out three simulations (Figure 5). In the case of the failed Trial 1 tkatchev, the simulation process indicated that the translation motion of the body s CG and the time of initiation of hip extension and shoulder flexion during the regrasp phase were the parameters that could be corrected in order to execute a successful regrasp of the bar. These corrections suggested that the release must be

8 Airborne Simulation and the High Bar 339 Figure 3 Experimental and corrected flight curves. experimental; corrected.

9 340 hip angle ( ) hip angle ( ) hip angle ( ) shoulder angle ( ) body position angle ( ) body position angle ( ) shoulder angle ( ) shoulder angle ( ) body position angle ( ) body position angle ( ) body position angle ( ) body position angle ( ) Figure 4 Experimental and simulated motions of the hip and shoulder joints. (A, B) failed Trial 1 tkatchev; (C, D) failed Trial 2 tkatchev; (E, F) piked tkatchev.

10 Airborne Simulation and the High Bar 341 backward vault backward vault release release regrasp fall backward vault backward vault fall release regrasp release piked backward vault stretched body backward vault regrasp release regrasp release Figure 5 Computer-generated sequences (1 frame over 3) of (A) the failed Trial 1 tkatchev and (B) the simulated movement; (C) the failed Trial 2 tkatchev and (D) the simulated movement; (E) the piked tkatchev and (F ) the simulated movement.

11 342 initiated 0.06 s earlier, and that hip extension and shoulder flexion must begin immediately after the gymnast completes a vertical position over the bar. The improvement of the failed Trial 2 and the piked tkatchev was obtained not only with a 0.08-s earlier release but also with modified joint motions. In the failed Trial 2 tkatchev, the simulation process indicated that the stabilization motion of the upper limbs, which was initiated when the gymnast was vertically over the bar and maintained during the regrasp phase, was a good strategy for ensuring regrasp of the bar. Due to the conservation of angular momentum during the airborne phase, upper limb stabilization was possible because hip extension began earlier and with larger amplitude. The results also indicated that the execution of the stretched body tkatchev required an earlier release by 0.02 s and a maintained hip extension of 180 when the gymnast moved away 70 relative to the horizontal axis. The consecutive movement of the upper limbs must lead to a reduction of shoulder flexion. Discussion Most biomechanical studies in gymnastics focus on successful skills executed by top level gymnasts (Brüggemann, 1994). The present study showed that an aerial acrobatic skill could be improved based on individual mechanical corrections of failed executions. Using real data of several failed and successful trials, we found that simple computer simulation could be an effective tool for correcting and improving the execution of release-regrasp skills. Analysis of the translation motion of the CG was a preliminary means for determining flight trajectories which guaranteed the vault over the bar and its regrasp. Modifying the release kinematic parameters in order to obtain lower horizontal velocity and vertical position of the CG, and greater vertical velocity and horizontal position of the CG, we defined better shapes of the CG parabolic curve. The simulations supported the fact that an earlier initiation of release is a major problem the gymnast must overcome in order to grasp the bar. Analysis of the segmental rotations suggested that the stabilization motion of the upper limbs could be a good strategy for improving the tkatchev. This motion is in agreement with observations made by neurophysiologists (Berthoz & Pozzo, 1988, 1994) who have shown that upper segmental rotations are minimized in the sagittal plane during aerial movement in order to keep the head stable relative to the environment: this upper segment stabilization would permit the vestibular system to be used as a stable platform for the navigational inertial system (Pozzo et al., 1992). The present study showed that when executing the tkatchev release-regrasp, the gymnast must recognize the moment when the CG moves vertically over the bar as the critical moment for initiating hip and shoulder movements. The computer simulations validated larger hip extension and reduction of shoulder flexion or upper limb stabilization motion as movements which permit regrasp of the bar and continuation to the next routine. Although the joints forces and moments responsible for producing the segmental motions involved in the experimental and simulated tkatchev movements were not determined, the findings in this study suggest that, in regard to the mean values and standard deviations of the release conditions presented by Gervais and Tally (1993) and Brüggemann et al. (1994), the proposed modifications are fea-

12 Airborne Simulation and the High Bar 343 sible. However, it would be appropriate to use net muscle moments as model input to produce the required modifications for performing the release-regrasp skill. The sensitivity of the adjustment of the release conditions indicated that improvement in release-regrasp skill requires biomechanical models and methods in order to provide the error corrections which could not easily be determined by visual observation. It would seem appropriate that technical instruction and feedback to the gymnast be based on an individual biomechanical analysis. Given that the advice coaches give to their gymnasts on the adjustment of these airborne movements tends to be based more on experience and intuition than on a predictive model, intraindividual studies with predictive simulation could provide useful information toward the improvement of acrobatic skills. References Allard, P., Blanchi, J.P., Gautier, G., & Aissaoui, R. (1990). Technique de lissage et de filtrage de données biomécaniques [Biomechanical data and smoothing and filtering techniques]. Science et Sports, 5, Arampatzis, A., & Brüggemann, G.P. (1998). A mathematical high bar-human body model for analysing and interpreting mechanical-energetic processes on the high bar. Journal of Biomechanics, 31, Bardy, B.G., & Laurent, M. (1998). How is body orientation controlled during somersaulting? Journal of Experimental Psychology: Human Perception and Performance, 24, Berthoz, A., & Pozzo, T. (1988). Intermittent head stabilisation during postural and locomotory tasks in humans. In B. Amblard, A. Berthoz, & F. Clarac (Eds.), Posture and gait: Development, adaptation and modulation (pp ). Amsterdam: Elsevier Science Publ. B.V. Berthoz, A., & Pozzo, T. (1994). Head and body coordination during locomotion and complex movements. In S.P. Swinnen, H. Hauer, J. Massion, & J. Casaer (Eds.), Interlimb coordination: Neural, dynamical & cognitive constraints (pp ). New York: Academic Press. Borms, J., Moers, R., & Hebbelinck, M. (1976). Biomechanical study of forward and backward swings. In P.V. Komi (Ed.), Biomechanics V-B (pp ). Baltimore: University Park Press. Brüggemann, G.P. (1994). Biomechanics of gymnastic techniques. Sport Science Review, 3(2), Brüggemann, G.P., Cheetham, P.J., Alp, Y., & Arampatzis, D. (1994). Approach to a biomechanical profile of dismounts and release-regrasp skills of the high bar. Journal of Applied Biomechanics, 10, Canal, M., Coffignal, G., Brochot, J.J., Bon, M., & Thomas Y. (1990). Modélisation du geste à la barre fixe [Modeling of gymnast skills on the horizontal bar]. In V. Nougier & J.P. Blanchi (Eds.), Pratiques sportives et modélisation du geste (pp ). Grenoble: Université Joseph Fourier Grenoble I. Dempster, W.T., & Gaughran, G.R.L. (1967). Properties of body segments based on size and weight. American Journal of Anatomy, 120, Fowler, C.A., & Turvey, M.T. (1978). Skill acquisition: An event approach with special reference to searching for the optimum of a function of several variables. In G.E. Stelmach (Ed.), Information processing and motor control (pp. 1-40). New York: Academic Press.

13 344 Gervais, P., & Tally, F. (1993). The beat swing and mechanical descriptors of three horizontal bar release-regrasp skills. Journal of Applied Biomechanics, 9, Kerwin, D.G., Yeadon, M.R., & Lee, S.G. (1990). Body configuration in multiple somersault high bar dismounts. International Journal of Sport Biomechanics, 6, Newell, K.M., & McDonald, P.V. (1992). Searching for solutions to the coordination function: Learning as exploratory behavior. In G.E. Stelmach & J. Requin (Eds.), Tutorials in motor behavior II (pp ). Amsterdam: Elsevier Science. Newton, J., Greenwood, M., & Turner, R. (1993). Biomechanical analysis of the preparation swing for the double layout somersault dismount from high bar. In F. Goubel & S. Metral (Eds.), XIV International Congress on Biomechanics Paris (pp ). International Society of Biomechanics. Nissinen, M.A., Preiss R., & Brüggemann, G.P. (1985). Simulation of human airborne movements of the horizontal bar. In D.A. Winter, R.W. Norman, R.P. Wells, K.C. Hayes, & A.E. Patla (Eds.), Biomechanics IX-B (pp ). Champaign, IL: Human Kinetics. Pozzo, T., Berthoz, A., & Lefort, L. (1992). Head kinematics during complex movements. In A. Berthoz, P.P. Vidal, & W. Graf (Eds.), The head-neck sensory-motor system (pp ). Oxford: Oxford University Press. Smith, T. (1982). Gymnastics: A mechanical understanding. Toronto: Hodder & Stoughton. Stavitsky, A., & Golay, M.G.E. (1964). Smoothing and differentiation of data by simplified least squares procedures. Analytic Chemistry, 36, Takei, Y., Nohara, H., & Kamimura, S. (1992). Techniques used by elite gymnasts in the 1992 Olympic compulsory dismount from the horizontal bar. International Journal of Sport Biomechanics, 8, Witten, W.A., Brown, E.W., Witten, C.X., & Wells, R. (1996). Kinematic and kinetic analysis of the overgrip giant swing on the uneven parallel bars. Journal of Applied Biomechanics, 12, Yeadon, M.R. (1990a). The simulation of aerial movement I. The determination of orientation angles from film data. Journal of Biomechanics, 23, Yeadon, M.R. (1990b). The simulation of aerial movement II. A mathematical inertia model of the human body. Journal of Biomechanics, 23, Yeadon, M.R. (1990c). The simulation of aerial movement III. The determination of the angular momentum of the human body. Journal of Biomechanics, 23, Yeadon, M.R. (1990d). The simulation of aerial movement IV. A computer simulation model. Journal of Biomechanics, 23, Yeadon, M.R. (1997). Twisting double somersault high bar dismounts. Journal of Applied Biomechanics, 13, Yeadon, M.R., Hiley, M.J., & Kerwin, D.J. (1996). Optimisation of the accelerated backward giant circle. In A. Hoffer (Ed.), Proceedings, Ninth Biennial Conference, Canadian Society for Biomechanics (pp ). Vancouver: Canadian Society of Biomechanics. Acknowledgments We would like to acknowledge the French national training camp of La Madeleine gymnastics club for its cooperation.