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

ABSTRACT

The importance of visual information for maintaining static balance in wakeboarders was investigated in this study. Forty-two subjects, matched for age, height, weight and sex formed a wakeboard and a control group. All subjects were measured at ten, twenty and thirty seconds for the displacement of the centre of pressure (postural sway) during static balance tasks. Three increasingly difficult balance tasks were used with and without removal of vision. The results showed that in general wakeboarders performed better on the balance tasks in the anterior-posterior direction than non-sportsmen. Visual information became more important when the task was made more difficult. No relation was found between the level of performance of the wakeboarder and the importance of visual information for maintaining static balance. Visual information is very important for balance control for both wakeboarders and non-sportsmen. It was concluded that the importance of visual information for maintaining balance is increased as the tasks become more difficult. Finally the findings also demonstrated that wakeboarders develop specific modalities of balance, which are only partly transferable to static posture control tasks.

Introduction

Wakeboarding is a novel sport where an athlete rides a specially designed board skimming over the surface of the water. If a motorboat tows the wakeboarder then the wake of the boat modulates the surface. Alternatively the board can be propelled using a cable and winch. The objective is to perform various acrobatic tricks. Performing the complex movements, similar to those performed by gymnasts and dancers (Vuillerme et al., 2001a), wakeboarders require a great sense of balance, Kiting is similar to wakeboarding only in this case the board is being propelled by a kite.

Postural stability is defined as the ability to maintain or control the centre of mass in relation to the base of support to prevent falls and complete desired movements. Balancing is the process by which postural stability is maintained (Westcott et al., 1997). The control of posture is known to be critical for both the acquisition and control of motor abilities and is an essential requirement for physical activities in daily life (Vuillerme et al., 2000). In wakeboarding and other motor activities the dynamic stability of the posture during movement is dependent on the subject's capacity to maintain balance in a constant altering environment (Hugel et al., 1999). Balancing depends on feedback of sensory information from visual, vestibular and somatosensory sources. The central nervous system processes the information by comparing them to a 'postural' body scheme built by the subject's anterior experiences, and on reflex motor activities (Hugel et al., 1999).

It has been proved in one study (Kioumourtzoglou et al., 1997) that athletes can demonstrate a better sense of balance than sedentary subjects. This issue is under debate in the context of the two leading theories proposed for the transfer of motor abilities. General motor ability hypothesis suggests that any human skill should remain observable among various tests. However, the transfer of motor ability might not be such a simplistic mechanism. In fact, Henry's hypothesis predicted that transfer among skills should be quite low because motor ability is specific to a particular task (Vuillerme et al., 2001a).

The current study attempts to determine the importance of visual information on the maintenance of balance in wakeboarders by:

1. Investigating the hypothesis that wakeboarders demonstrate a better sense of balance than sedentary subjects under different static conditions.

2. Investigating the hypothesis that wakeboarders and non-sportsmen depended more on visual information to maintain static balance when task difficulty is increased.

3. Investigating the hypothesis that a negative relation exists between the level of performance of the wakeboarder and the amount of variation of postural sway in different conditions.

METHODS

Participants

Two groups of subjects were formed, one group existed of wakeboarders and the other existed of people who do not participate in regular physical activities or who participate in sport activities that do not require high balance skills. The groups were constructed so that there was no significant difference in age, weight and height, because body-properties are known to be determinant, for postural tasks (Berger et al., 1992) (see table 1.).

The wakeboarding group consisted of 15 males ranging from 16 to 48 years (mean: 23.6 years) and 6 females ranging from 14 to 31 (mean: 20.5 years). It was limited to participants with experience of wakeboarding and/or kiting.

The average time spent wakeboarding was 13.75 hours a week (range: 4-48 hours per week) and the time spent kiting by the three participants was 2, 5 and 8 hours a week.

Eight of the wakeboarders had ongoing injuries (see table 2.). However all of them were able to wakeboard unencumbered. A six month injury free criteria was used for injuries involving an ankle, following the results of Holme et al. (1999), who reported that four months after injury, reduced ankle strength and postural control were no longer noticeable. Thirteen of the twenty-one wakeboarders participated in competitions at national level or higher and six wakeboarders were involved in a sporting activity other then wakeboarding.

The control group consisted of twenty-one healthy subjects of which fifteen did not participate in any regular physical activity and had no long-term experience in any other sporting activity requiring balance. Six subjects performed regular sport activities but not at a particularly high level and without need of great balance skills. Fifteen males ranging from 15 to 51 years, (mean: 24.3 years) and six females between 13 and 25 years (mean: 19.5 years) formed the control group. One woman reported a history of low back pain (table 2.).

Participants

A strain-gauge force platform made of four vertical pressure gauges and a statokinesimeter were used to measure postural behaviour in terms of cumulative magnitude of the variation of postural sway in the anterior-posterior (a-p) direction and in the lateral direction.

Experimental Setup

The force platform was levelled and positioned 1.5 meter from a wall in an environment free from as many perturbations as possible. The display of the statokinesimeter was placed on a spacer behind the participants such that it was not possible for them to monitor their own progress during the experiment. A Soehnle digital scale was used (maximum weight 130 kg) to measure the participants weight.

Design and Test Procedure

After signing an informed consent, the participants were asked to complete a group specific questionnaire. The subjects were briefed about the purpose of the study and given an opportunity to ask questions before commencing the experiment. The participants were asked to perform three different static postural tasks whereby the visual source of information was manipulated per condition (see table 3). Tasks were made increasingly difficult by using a unipedal stand and a bag filled with air (Tilia balance bag) which amplifies any loss of balance. The subjects were barefoot and the placement of their feet was predetermined. Before testing each condition the participants were given a verbal explanation of the procedure and asked to confirm their understanding. All experiments involved standing as still as possible in an upright position on the force platform for a 30-second period. The participants were told to place their hands behind their back, keep their head upright, fixate on a position in front of them and stand as still as possible. To initiate the different conditions the researcher used a count of five. On the count of one the participant took up position on the force platform. At the count of four the participant closed his eyes (for condition 2, 4 and 6) and lifted one foot (for condition 3 and 4). The measurement started on the count of five.

Placement of the feet on the bipedal task (the base task) was determined by fixed positioned blocks. For the unipedal task the participant had to place his foot of preference on a dotted line, which indicated the middle of the force platform. A Tilia© balance bag with a radius of 33 centimeters was used for the last task (condition 5 and 6). The amount of contact of the feet with the surface of the force platform had to be kept at a minimum while standing on the Tilia balance bag. The position of the feet were the same as used in task 1. After performing each condition the participants took a short rest (<1 min).

Statistical Analysis

Statistical analyses techniques were applied to the data to evaluate the three postulated hypotheses.

Before analysing the data, a student t-test was performed to check whether the height, weight and age differed between the two groups. As mentioned earlier no significant (p<0.05) difference was found between the two groups.

Subjects who failed to stay on the platform for the required thirty seconds were given penalty scores. The following percentage was added to the worst cumulative variation of postural sway of all subjects to acquire a suitable penalty score:

• Falling once: five percent was added.
• Falling twice: ten percent.
• Falling three times: fifteen percent.

The first hypothesis was tested using a two-way analysis of variance (ANOVA). The amount of variation of postural sway was set as a dependent variable and the group and task as independent variables. Main effect of the independent variables was carried out for each ten-second period and direction (posterior-anterior and lateral sway). Analyses of variance (two groups x six conditions) was applied on the data for each time period and direction to find any significant interaction between group and condition. This allowed the current study to see if wakeboarders had a better maintenance of balance than non-sportsmen.

In order to test the second hypothesis a post hoc Scheffé-test was used to highlight significant difference. The sway for one task was compared under two different conditions (visual vs. non-visual) for each group separately. This was done to determine if the subjects performed differently with or without visual information.

The third hypothesis was tested using a Spearman's correlation between the level of wakeboarding and the cumulative magnitude of the variation of postural sway in the anterior-posterior and lateral direction. The objective was to see if a wakeboarder classified in the high level group showed lesser amount of variation of postural sway under different conditions during increasing task difficulty. A certified competition adjudicator determined the level of performance the wakeboarder using a scale from one to ten with ten being the highest level (mean: 5.0; range: 1.5-9).

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