EFFECT OF THE STARTING BLOCK POSTURE ON THE 3D JOINT ANGULAR VELOCITY IN SPRINTERS. Jean Slawinski1,2, Guy Ontanon2, Raphaël Dumas3, Laurence Chèze3, Christian Miller2, Alice Mazure-Bonnefoy1,2. 1 Laboratoire d’analyse du mouvement, CIC-IT 805, AP-HP, CHU Raymond Poincaré, Garches, France 2 Team Lagardère, centre d’expertise, 26 avenue du général Sarrail 75016 PARIS. 3 Université de Lyon, F-69622, Lyon, France ; Université Lyon 1, F-69622, Villeurbanne, France ; INRETS, UMR_T9406 Laboratoire de Biomécanique et Mécanique des Chocs, F-69675, Bron, France. SUMMARY The aim of this study was to measure the effect of the modification of the posture during a sprint start on 3D joint angular velocity. This was performed using a 3D kinematic analysis of the whole body. Ten trained sprinters started using three different starting positions in the starting blocks (bunched, medium and elongated). They were equipped with 63 passive reflective markers, and an opto-electronic Motion Analysis® system (12 digital cameras 250 Hz) was used to collect the 3D marker trajectories. During the pushing phase on the blocks and the first step, the 3D angular velocity (EJAV) and the norm of the joint angular velocity (NJAV) were calculated. These results demonstrated that the modification of the initial posture of the sprinter in the starting block significantly influenced the NJAV of the rear elbow, hip and knee. The 3D kinematic analysis of the whole body demonstrated that joints such as shoulders, thoracic or hips did not reach their maximal angular velocity with a movement of flexion-extension, but with a combination of flexion-extension, abduction-adduction and internal-external rotation. To understand postural adaptations in skilled movement the NJAV approach could be a useful tool. and the first step. A segment coordinate system (SCS) was defined on each body segment based on the markers. The orientation of their axes was carefully carried out using the ISB recommendations (Wu et al. 2002; 2005). From the reconstructed spatial trajectories of the markers, the segment mass, position of the centre of mass and inertia tensor were estimated from scaling equations [1]. The rotation sequence proposed by the ISB to describe the lower and the upper limbs joint movements was used. For this study, 16 rigid segments were used in order to model the body. The joint angular velocity and its norm were computed from proximal and distal segment angular velocities: Ω i / i −1 = Ω i / 0 − Ω i −1 / 0 . The contribution of each degree of freedom was also computed. For this, the joint angular velocity was projected on each axis of the joint coordinate system (JCS) in order to ( MATERIAL AND METHODS Ten trained sprinters started using three different starting positions in the starting blocks (bunched, medium and elongated). They were equipped with 63 passive reflective markers, and an opto-electronic Motion Analysis® system (12 digital cameras 250 Hz) was used to collect the 3D marker trajectories during the pushing phase on the blocks ) retrieve the Euler angles derivatives α& , β& , γ& : Ω i / i−1 = (e × e )• Ω e + (e × e )• Ω e + (e × e )• Ω e (e × e )• e 1(e44×2e 4 )• 4 (e × e )• e e 1442443 3 1442443 2 i / i −1 3 1 where 3 i / i −1 1 1 2 α& INTRODUCTION Biomechanical studies shown that the velocity of the centre of mass (VCM) at block clearing is higher when the interblock spacing is increased as a result of a more effective force-impulse [2, 3]. From Kisler or Henry the biomechanical tools and the morphology of the sprinter have been considerably modified and these modifications could influence these old results. A Recent study demonstrated that complex movement such as sprint start must be studied using a 3D kinematical analysis because joints such as shoulders, thoracic or hips did not reach their maximal angular velocity with a movement of flexion-extension, but with a combination of flexion-extension, abductionadduction and internal-external rotation [5]. The aim of this study was to measure the effect of the modification of the posture during a sprint start on 3D joint angular velocity. Ω i / i −1 (NJAV), 3 1 i / i −1 2 2 1 2 3 β& 3 1 2 3 γ& e1 is a selected axis from the matrix Ti −1 / 0 , e3 is a selected axis from the matrix Ti / 0 and e2 = e" × e1 . These values, called Euler Angular Velocity (EAV), could be used, in addition to the numerical value of NJAV, to know which degree of freedom (flexion/extension, adduction/abduction and internal/external rotation) is more important during the pushing phase of the sprint start. From the NJAV, the maximal values, NJAVmax were calculated to analyse the data. Rear and front joints were respectively associated with the side of the rear and the front legs in the starting blocks. RESULTS These results demonstrated that the modification of the initial posture of the sprinter in the starting block significantly influenced the NJAVmax of the rear elbow, hip and knee. Indeed, NJAVmax of the rear shoulder tends to be greater while NJAVmax of the rear hip and knee tends to be lower in the bunched condition (fig. 1). DISCUSSION AND CONCLUSION The 3D kinematic analysis of the whole body demonstrated that joints such as shoulders, thoracic or hips did not reach their maximal angular velocity with a movement of flexion- REFERENCES extension, but with a combination of flexion-extension, abduction-adduction and internal-external rotation. To understand postural adaptations in skilled movement the NJAV approach could be a useful tool. 1- Dumas, R., Cheze, L.,Verriest, J. P.,. Adjustments to McConville et al. and Young et al. body segment inertial parameters. J Biomech 2007 ; 40, 543-553. 2- Henry MF. Force time characteristics of the sprint start. Res Q 1952; 23(3): 301-318. 3- Kisler JW. Study of the distribution of the force exerted upon the blocks in starting the 4- Schot PK, Knutzen KM. A biomechanical analysis of four sprint start positions. Res Q Exerc Sport 1992; 63: 137-47. 5- Slawinski J, Bonnefoy A, Ontanon G, Leveque JM, Miller C, Riquet A, Cheze L, Dumas R. 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