Date: 9/17/02
Author: J. H. M. Bergmann, M. G. Feltham, M. Kortsmit, F. J. Oosterwerff, Faculty of Human Movement Sciences - Vrije University

Results

ANOVA showed a significant main effect between the two groups (table 4) and between the different conditions (table 5). The wakeboard group showed a significant lower value of variation of sway then the non-sportsmen. A significant interaction between group and condition on the cumulative magnitudes of variation of postural sway was found in anterior-posterior direction in all time periods. For the lateral sway a significant difference between the two groups was only found after twenty seconds (table 6).

The post hoc test used to explore the relationship between visual vs. non-visual revealed no significant difference when the wakeboarders and non-sportsmen group performed the bipedal task. No change was observed over time, as the level of significance was constant at 1.000. A similar result was found for all directions of sway for each group (graph 1). A significant difference was found between the visual and non-visual conditions during the unipedal and balance bag task in both the wakeboarding and non-sportsmen group in the anterior-posterior and lateral direction over every ten-second period.

A Spearman's correlation between the level of performance of each wakeboarder and the cumulative magnitude of variation of postural sway showed a significant correlation in two conditions during six specified time periods. In the anterior-posterior direction, significance was found in condition 3 (Unipedal, eyes open), for the ten second (r = -.610, p = .004), twenty second (r = -.505, p = 0.023) and thirty second (r = -.536, p = .015) time period. The lateral direction showed a significance in condition 2 (Bipedal, eyes closed) for the ten second ( r = -.524, p = .018) and twenty second (r = 0.487, p = 0.029) time period and in condition 3 for the twenty second ( r = -.396, p= .084).

Discussion and Conclusion

Regarding our first hypothesis the study demonstrates that wakeboarders have a significant better general sense of balance than non-sportsmen in both directions when only the groups are compared. When conditions were taken into account a significant difference between the groups could constantly be found in anterior posterior direction but not in lateral direction. According to these results, a transfer of motor ability in the performance of balance by wakeboarders compared to sedentary subjects during all conditions only took place in the anterior-posterior direction.

There are two leading theories in the context for the transfer of motor abilities. Adam's (1987) general motor ability hypothesis suggests that any human skill should remain observable among various tests. This theory is supported by research of Kioumourtzoglou et al. (1997) who found that elite groups of gymnastics performed better in static balance tasks than control groups. Contrary to Adam's general motor ability hypothesis is Henry's theory that predicted that transfer among skills should be quite low because motor ability is specific to a particular task. The study of Hugel et al. (1999) supports this hypothesis, the authors concluded that there was no automatic transfer of balance skill in classically trained ballet dancers. Transfer can only occur if the tasks being performed are made sufficiently difficult as reported by Lin et al. (2000) and Vuillerme et al. (2001a). The current study took this into account and the increased difficulty was demonstrated by the high number of falls, especially in the control group. The current study showed a transfer of motor skill in the anterior-posterior balance but no transfer in the lateral balance, indicating that the transfer of balance in a certain direction of the sway may be more specific to a particular task or to the amount of difficulty of the task. The performance of the wakeboarders on each of the separate conditions did not differ significant from that of the non-sportsmen, the main exception was the condition involving the balance bag with eyes closed where a significant difference between the groups was noted. This condition could be seen as task specific, because standing on a balance bag may be comparable to the dynamic conditions involved in wakeboarding. A possible "ceiling effect" could be present for the easy tasks, a possible transfer could then not be seen. Results of this current study could be used to support either Henry's or Adams' theory. A weak transfer of balance skills seems present.

For the second hypothesis the results imply that for both groups visual information became more important for maintaining balance as the tasks became increasingly difficult. In both the wakeboard and the control group a significant difference was found between visual and non-visual conditions, leading to the conclusion that visual information is an important factor for maintaining balance. A similar result was found for the bipedal tasks in the study of Hugel et al. (1999). They found no significant effect on the postural balance between dancers and non-dancers in the absence of vision.

A significant effect between visual and non-visual conditions was found on maintaining balance during the unipedal task and the balance bag task. These results support the findings of Day et al.(1993), who found that the dependence on visual information for maintaining balance increased during increasing stance difficulty. Wakeboarders were significantly better in maintaining balance on the balance bag without visual input than the non-sportsmen.

The performance of the wakeboarders on each of the separate non-visual conditions did not significantly differ from the non-sportsmen except under one condition. A significant difference between both groups was observed in the performance on the balance bag with eyes closed. A possible explanation is provided by the findings of Inglis et al. (1995), whose results indicated that although the vestibular system may be important for maintaining balance during fast dynamic movements, it plays a lesser role in static postures. The balance bag clearly required a better sense of dynamic balance. Wakeboarders move over a constant changing surface in combination with high speed and varying environmental conditions such as wind and rain. It is therefore possible that wakeboarders train another system that is less involved during static balance. This leads to the conclusion that wakeboarders maintenance of dynamic balance might depend more on vestibular and somatosensory sources, than that of the control group.

A correlation between wakeboarder performance level and the amount of variation of postural sway in different conditions during increasing task difficulty, was only found in six of the 36 possible combinations formed by condition, direction of sway and time period. This was in contradiction to the third hypothesis. In general no relation between the level of performance of the wakeboarder and the dependence on visual information for maintaining static balance during increasing task difficulty was found. Thus the required skill for maintaining balance during static balance tasks did not depend on the skills required to be a good wakeboarder.

The results showed no direct evidence for an automatic transfer of the balance skill of wakeboarding to different static balance tasks. Nevertheless, the wakeboarders performed better in the anterior-posterior direction of the sway. Visual information is a major input for balance control and the importance of visual information for maintaining balance is increased as the tasks became more difficult. However, wakeboarders seem to rely less on the visual system than non-sportsmen. This indicates they might have learned to rely more on other systems than the visual system to control their balance.

When the experiment was conducted subjects were physically not at their best. One participant of the expert group reported pain in the right ankle under condition 6 and three participants used narcotic substances the night before. The subjects were not excluded from this study because they declared that the narcotic substances or the pain did not diminish their wakeboarding skills on the day of measurement. The after effect of these substances could have had an influence on the current results. Another limitation of the study was that a number of subjects lost their balance during the experiment and fell off the force platform, before they had reached the 30 second time limit. In order to include their data in the statistical analysis the current study chose to use a penalty system, in which a percentage was added to the worst cumulative variation of postural sway in the direction of movement. This means that results are artificially manipulated. However, the number of times a subject fell off is a ranking of performance. The current study also did not divide the wakeboarding group into categories (motorboat, cable and kite). Future research could measure if there is any noticeable difference between motorboat/cable wakeboarding and kiting.

In conclusion, the present findings suggest that balance skills required for becoming a better wakeboarder are task specific and that these are not strongly transferred to other tasks. Such a finding implies that to become an expert in wakeboarding participants must train and develop task specific skills. Visual information is a major input for balance control and the importance of visual information for maintaining balance is increased as the tasks become more difficult.

Special thanks to "Cable waterskicenter Lido Almere" and "Cable waterskicenter Twente" for their hospitality and cooperation.

REFERENCES

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Hugel, F., Cadopi, M., Kohler, F., & Perrin, Ph. (1999). Postural Control of Ballet Dancers: A Specific Use of Visual Input for Artistic Purposes. International Journal Sports Medicine, 20, 86-92.

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ACKNOWLEDGEMENTS

Dr. A. Ledebt

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