DEVELOPMENT OF CROSS COUNTRY SKI SKATING ABILITIES

Skiing techniques together with the development of equipment and the ski tracks has undergone major changes during the last decade. In the beginning of the decade ski skaters in Finland were considered weak. A project designed to develop the skating techniques for the 0lympic level Finnish racers was started in1992. Athletes were followed for three years, 1992-1994. During the follow up period five different 3-D film analyses were performed; 1) in a training camp, November 1993, 2) January,1993, 3) at the Falun World Ski Championships, 4) November, 1993and 5)in another training camp, November, 1994. Skiers were filmed performing the V2 technique on a steep uphill section of the track (8 degree slope). The data was digitized using the APAS-motion analysis system. From the digitized data the contact phases of poles and skis, speed and height of the centre of mass, and the directions of the velocity vectors were calculated. At the Falun World Championships the change of technique during the 50km race was analyzed at three different phases of competition. major change in techniques during the race were observed. During the three year follow-up period the timing of the pole work and kick phases changed remarkably, leading from very inefficient speed production to a smooth centre of mass velocity curve and velocity vector directions. The height of the centre of mass of good skiers became lower. We concluded that individual skating techniques can be changed over the long term

The repeatability of motion analysis and the reproducibility of athletes in sprint hurdles

Grainger et al. (1983, Biomechanics VIII-B, pp. 1239- 1247) concluded that biomechanical variables from a single trial based on film data were unstable even in a well-learned pattern such as walking. This leads to the question: how reliable is the analysis of more complex tasks, which are common in most applied sports research? The aim of this study was to investigate the repeatability of motion analysis system and the reproducibility of athletes in the event of sprint hurdles. Eight athletes (4 females and 4 males) performed 2 sets of 4 trials over 4 hurdles in the training situation. The clearances over the third hurdle were filmed with the two genlocked video camera recorders (50Hz). The cameras were located 29 m away from the hurdle in front and symmetrically on both sides of the lanes creating a 90 ' angle from the midpoint of hurdle. The filmed view was 6.5 m in the direction of running ensuring as large an image size as possible. All the eight trials per subject were digitised using the "Kine analysis" software. Additionally two trials (one female and one male) were randomly selected and re-digitised eight times. Twentyeight kinematic variables were studied and coefficient's of variation (CV) were calculated for each subject and for both re-digitid data sets independently. Individual CV's varied from 0.4% to 138.0% for the female athletes and from 0.8% to 181.2% for the male athletes. The ranges for the female and male redigitised trial were 0.1-151.2% and 0.2-198.7%, respectively. There were 8 variables in both female and male groups, in which the CV of the re-digitised trial were greater than the lowest CV of individual athletes in the same gender. This indicates that the digitising and analysis process is sensitive enough to potentially separate technical aspects of performance. However, it also indicates that in these eight variables (which were not the same variables for both genders) the variation of results may be due to the digitising and analyses process. Although, some of the CV's were high, there were only 8 variables were one or more of the females produced over 10% CV (9 variables for males). For the re-digitised trial there were 16 and 13 variables for the female and male, respectively, in which the CV did not exceed the 2% level. Thus, it can be concluded that one operator can perform the digitising process repeatably and that in this context the analyses is reliable for most of the variables. This study, however, did not clarify the accuracy and reliability of motion analysis in absolute terms. Furthermore, the athletes were able to reproduce their performance within certain limits despite such a complex movement. However, there were differences in reproducibility between females and males due to different clearance technique. In conclusion, it is clear that for some variables more digitised trials are needed to clarify individual technical aspects of performance