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Changes of motor-control strategies in precision jump according to distance constraints

Juillet 2015, ISPGR, Séville Support oral de la conférence de La Société Internationale pour la Posture et la Marche

Changes of motor-control strategies in precision jump according to distance constraints

David PAGNON – GIPSA-lab, LFREP, Grenoble (France), contact@david-pagnon.com
Olivier MARTIN – GIPSA-lab , Grenoble (France), olivier.martin@gipsa-lab.fr

INTRODUCTION
• Precision jump (PJ): Standing jump toward a precise reception area. Requires an adaptive motor control to reach accurately the landing zone and then stabilize the posture without any additional step.
• Aim: Analyzing the distance-dependent motor control strategies in PJ, using different jump amplitudes. No study has yet questioned the effects of distance to jump on the preparatory and on-line motor processes controlling PJ.

METHODS
• Subjects: 7 male experts in Parkour (6.7±3.2 years of practice) having especially high level PJ skills.
• Task: Precision jumping from the impulse force plate, toward the 44mm wide bar fastened on the reception force plate.
– Reception area at 40%, 60% and 80% of the maximal jump distance (Dmax) performed during pre-tests.
– Each PJ condition repeated 10 times. Resting period and jump start at the subjects’ decision.
• Analysis:
– Jump distance comparison (t-tests)
– 60% vs. 40% (D1 effect), and 80% vs. 60% (D2 effect)
– For each phase of successful trials, from subject‘s means of variables
– 6 phases composing a PJ:
– PR: Preparation,
– CM: Counter-movement,
– PO: Push-off,
– FL: Flight,
– RE: Reception,
– ST: Stabilization. ST defined as going from 1 to 2 seconds after RE.
– Dependent variables
– Duration T
– Dynamics of the ground forces: antero-posterior forces Fap, angle Fang
– Integrated surface electromyography (iEMG) of tibialis anterior TA, soleus SO, quadriceps QC, hamstring HA, tensor fascia latea TFL, erector spinae ES, deltoidus DE

RESULTS

CONCLUSION
• Preparation: Getting from 60% to 80% of Dmax perceived as a major difficulty by the neuro-motor control system, so it switches to a more predictive strategy expressed by longer preparation times helping to calibrate the PJ.
• Counter-movement to reception: Getting from 40% to 60% of Dmax involves an increase of iEMGs during push-off, as well as a stronger impulse and a more grazing trajectory. Conversely, getting from 60% to 80% of Dmax shows that an increases of iEMGs no longer leads to significant changes in impulse forces. Push-off resources have reached their limits so another mode of energy generation takes over, expressed in the stronger counter-movement generating pliometric force, and in the different onsets and actuations of motor.
• Stabilization: Getting from 60% to 80% of Dmax makes the flight long enough for on-line computations. Moreover, the time of flight is not as increased as in the condition result since the subject extends himself in order to damp more and to get an anticipated proprioceptive feedback. As a consequence, there is no further decrease of success rate.

Increasing the distance of a precision jump modifies both the predictive and on-line control strategies, and there seems to be a turning point in motor control strategies of PJ around 60% of Dmax

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