INTRODUCTION: Double poling (DP) in cross country (XC) skiing is an increasingly important technique especially for sprint skiing. Earlier DP studies have mainly focused on kinematics and kinetics (Hoffman et al. 1995; Millet et al. 1998; Nilsson et al. 2004) but only few have investigated EMG activity (e.g. Holmberg et al. 2005). Little is known about neuromuscular control patterns during DP, which may also have an important positive effect on athletic performance. In running and jumping studies, it could be shown that Stretch shortening cycle contraction (SSC) enhances motor performance (Gollhofer et al. 1992, Komi 2000, Kyröläinen et al. 2005). Therefore, the goal of this study was to evaluate neuromuscular activation patterns during DP across increasing speeds. METHOD: 12 elite XC skiers (Swedish National Team) performed DP roller skiing at treadmill velocities of 9, 15, 21, 27km-h~1 and individual speed maximum. Elbow angle (goniometer: Megatron, Germany) and axial pole forces (strain gauge) were measured (2000 Hz). Neuromuscular behaviour of triceps brachii (TRI), pectoralis major (PEC) lattisimus dorsi (LAT) and biceps brachii (BfC) was analyzed by surface EMG. The band-pass filtered (10-SOOHz) and rectified Signals of at least 15 cycles were averaged to pole ground contact and were normalized to muscle specific MVC conditions. Mean EMG amplitudes for the elbow flexion (FLEX) and extension (EXT) phases, and for a 90 ms window before pole plant (PRE activation) were calculated. Data collection and processing was performed using the Biovision-system (Werheim, Germany) and Noraxon-Software (USA). Repeated ANOVA were calculated to analyze changes in all variables across speeds (statistical level P < 0.05). RESULTS: Directly after pole plant an elbow flexion-extension occurred, resulting in a SSC movement of the TRI. Angle minimum decreased and peak pole force plus other force characteristics increased (P<0,05). Both took place almost simultaneously, Poling time shortened (0.67±0.11 to 0.26±0.03 s) and angular velocity increased (104±81 to 442±128°-s" 1) across speeds (P<0.05). For PRE (Fig 1) and FLEX an increase of EMG activity could be shown for all muscles (P<0.05). During FLEX phase EMG amplitudes over 100% MVC were found in LAT and PEC at higher speed levels. DISCUSSION and CONCLUSION: MVC normalization underestimates the full activation Potential of DP movements at higher speeds. This corresponds to results from Kyröläinen et al. (2005) for leg muscles during running. The increase especially of the PRE and FLEX phase may increase the stiffness of the tendo-muscular system and therefore enhance the rote of stretch reflexes for muscular performance at high speeds. From the results, we conclude that SSC contraction may play an important role for DP performance in XC skiing sprint. Therefore principles known from SSC training should be integrated into strength and technique training.
© Copyright 2007 4th International Congress on Science and Skiing. Published by University of Salzburg. All rights reserved.