INTRODUCTION: In alpine skiing the loading constrains make the legs to resist to high level of peak forces and vibrations (Mueller 2001, Pozzo 2006). This loading conditions are not easy to be reproduced by means of Standard strength exercise used in training protocols. The aim of this study is to verify the dynamic and neuromuscular responses of knee flexion-extension exercises executed with different techniques: vertically (VE) and with leaning the body sideways like to the normal turn movements on the ski (SSE), performed on a new ski Simulator under Vibration conditions. The Simulator used has two breech footboards, with normal ski boots bindings, which can rotate around their longitudinal axis and can vibrate with a frequency between 0 to 50 Hz. The athlete is tied with a waist belt from which two lateral cables and one, directed to middle line of the feet, are connected to a traction lever which is controlled by an external operator METHODS 2D-Kinematics: movement sequences were acquired via CCD-camera (50 Hz) and dedicated SIMI-software; joint variations by electrogoniometer on the knee (100Hz). Dynamics: reaction force on the feet were recorded by insoles with pressure cell sensors (PEDAR 100 Hz). Vertical and lateral forces acting on the subject's belt were recorded (100Hz) by load cells. Muscle activity: EMG: bipolar electrodes BIOVISION (1000 Hz) for following muscles gastro medialis and lateralis, tibialis anterior, rectus femoris, vastus medialis and lateralis. Six jung competitive skiers (16 ±1,7year) participated to the investigation. They performed VE and SSE in series of 6 to 10 repetitions with a skiing like cadence and maximal recovery in between. RESULTS: Maximal knee flexion obtained in the VE and SSE (110° and 85° external and internal leg) are consistent with corresponding parameter during normal skiing (96° and 60°) respectively. Peak reaction force in VE reaches the highest value during 30 Hz vibrations (1460 N total force) while, during SSE the maxima were 900 N and 780 N for external and internal leg respectively. Average external peak forces exerted on the body by the traction cables were 450 N for VE and 530 N (external leg) and 400 N (internal leg) for SSE. The muscular activity show different patterns according to diverse loading constrains and Vibration conditions (VC). During VE exercises, the peak RMS value of total muscles activity was 80-84% (with respect to MVC) up to 30 Hz VC with a decrease to 70% for higher VC. During SSE exercises, values were about 60% when the leg was internal, and 80-86% when the was external. CONCLUSIONS: The different response observed in the kinetics and neuromuscular Parameter permits to differentiate some essential aspects of adaptation in the neuromuscular coordination processes according to the specificity of the loading constrains. On the other hand, due to age of the subjects, it is possible to clearly identify some thresholds of the acting Stimulus (loading force and Vibration frequency) which should be taken into account for optimizing training programs. According to these results, new assumption can be made in order to improve the constrains of the Simulator and make it more useful in training or in the rehabilitation.
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