Effects of 8-Week Complex Balance Training in Young Alpine Skiers: A Pilot Study

Objectives The purpose of this study was to determine the effects of an 8-week complex balance training program on dynamic balance in skiers according to the new balance training protocol. Design Intervention study, comprising 8 weeks of core stability, plyometric, balance, and stretching exercises. Participants Ten young skiers volunteered to take part in this study (average age, height, and body mass were 16,44 +/- 1,07 years, 172,76+/-8,84 cm, and 67,4 11,44 kg, respectively (mean +/- SD)). Main Outcome Measures Subjects' dynamic balance performance was assessed and retested after training completion with the use of dynamic balance measurement within the Optojump Next System. Results The results of 8-week complex balance training showed significant improvements for jump height (H) and flight time (FT) for the left leg and jumping area in both legs. Conclusion The complex balance training program improved parameters of dynamic stability in young skiers and led to decreased asymmetry between lower extremities.


Introduction
Regardless of the discipline, sports training proved to have a positive influence on body balance. The high level of balance ability allows achieving many benefits including performance improvement, injury prevention [1][2][3], and faster rehabilitation [4]. These potential profits make a specific balance training essential for every sport discipline. Recent years show a growing number of implemented balance training protocols in different sports disciplines such as team sports [5][6][7][8][9][10][11], downhill skiing [12,13], figure skating [14], capoeira [15], tennis [16], running [17], field hockey [18], and gymnastics [19]. Due to this diversity and different specificity of sports the gold standards of balance training and exercises, common for most situations, seem not to exist [2]. The protocols mentioned above were not always defined and described. The terms used in the descriptions were sometimes misleading and confusing. The following terms are most commonly used: balance and core stability exercises [20][21][22][23], neuromuscular and proprioceptive training with mixed strength, balance and plyometric exercises [4,6,13,23], and sensorimotor training [24,25]. The differences in training volume are also substantial. Some authors used full training units [22,26,27]; others used only a warm-up [19,28,29]. One can also find in the literature specific suggestions for balance training application [3,30]. These review studies define training modalities such as exercises time and frequency, number of sessions, duration of a single training session, number of exercises and sets per training session, and effective training dose.
Skiing is considered an extreme sport of high-risk injury incidence, where the most prevalent are the knee injuries [31]. The contemporary style of skiing emphasizes, above all, the speed and movement technique of the skier [32]. Apart from energy expenditure requirements, skiing demands high level of motor abilities (e.g., speed, balance) [33,34]. The neuromuscular system must be adequately trained for balance to enable efficient movement down the ski slope and to prevent falls [35]. Falls during ski training and competition can lead to the occurrence of multiple injuries of different etiology 2 BioMed Research International [32]. Just like any other sports with high speed and strength demands skiing motivates high level of motor control over the narrow base of support (skis). Specifically, the postural control should be emphasized during the training which in consequence leads to better balance during performance. One can speculate that this will decrease falls occurrence and injuries. Since skiing is very dynamic in nature essentially the dynamic balance control is needed. To address this issue one should incorporate dynamic exercises, which are focused on core stability and plyometrics, into the training [36]. This is important both in young and adult skiers who spend most of their training time on the ski slope working on the skiing technique. The area of motor preparation in sports is in constant development and takes advantage from the current achievement in physiology and biomechanics. Unfortunately, the specificity of alpine skiing forces athletes to devote much time of the training to the on-snow training and less time remains for an effective basic motor preparation [37]. Incorporation of specific balance training in their routine could be another factor improving overall performance [38]. Therefore, the aim of this study was to examine the influence of specific balance training on dynamic balance in elite polish young skiers. We hypothesize that the proposed balance training will elicit changes in skiers' dynamic balance performance.

Materials and Methods
. . Experimental Approach to the Problem. The study was designed to evaluate the effects of a complex balance training program on dynamic balance in skiers. The program comprised exercises focused on core stability, plyometrics, balance with devices, and stretching. The training was executed twice a week, along with the regular practice. All the subjects were tested with the use of Drift protocol within the Optojump Next System (Microgate, Italy), which is designed to evaluate dynamic balance. The measurements were taken before and after the 8-week training program.
. . Subjects. Ten top young polish skiers with FIS points under 100 and competing on the national and international level volunteered to take part in this study. The skiers were under a special athletic program in the school being the center of alpine skiing preparation for youth.
Their average age, height, and body mass were 16,44 +/-1,07 years, 172,76+/-8,84 cm, and 67,4 11,44 kg, respectively (mean +/-SD). Athletes, parents, and coaches were informed about the aims of the study, and written informed consent was acquired from the subjects prior to their participation in the investigation. The study was approved by the Institutional Ethic Board.
. . Testing Procedures. Subjects' balance performance was assessed prior to 8-week balance training program and retested after its completion with the use of Drift protocol included within testing procedures in the Optojump Next System, which has been proved to be valid and reliable [39]. The test was designed to evaluate athletes' jumping . All parameters were established for both legs separately, denominated by R (right) and L (left) in the variable names. Additionally the differences between the right and left leg in performance were calculated, being the measure of functional asymmetry. Prior to testing, subjects performed a warm-up that consisted of running and dynamic stretching exercises. The Drift test protocol consists of five, consecutive, unilateral, and vertical jumps for the right and left leg. The series of jump were repeated two times (foot parallel and orthogonal to the Optojump bars) in order to achieve the two dimensional picture of the dynamic stability of the jumping performance. To standardize the test execution each subject had to elevate their foot at the knee height of the contralateral leg and keep it at this level during the jumps. Subject's hands were in the akimbo position to eliminate the influence the technique on the jumping performance. The instruction for the subjects was to jump 5 times as fast and as high as they can. The optimal score on Drift test is performed in the smallest area, the highest height of jumps, and the shortest contact time with the ground. It requires a high level of coordination (dynamic balance, core stability) and strength from the subjects.
. . Training Protocol. The training lasted 8 weeks (twice a week on non-consecutive days) and comprised core stability, plyometric, balance, and stretching exercises. The training was supervised at the beginning by trained physiotherapists and then continued by the instructed coach. Appropriate instructions were given to the athletes throughout the training program. A standardized warm-up routine comprising running and dynamic stretching exercises was used before the training which did not differ from their standard warmup routine ( Table 1). The outline of the training program is presented in Table 2.
. . Statistical Analysis. The basic descriptive statistics were calculated. The Shapiro-Wilk test was used to verify the normality of the distribution of data. The differences between the analyzed variables pre-and posttraining were estimated with the use of Student's t-test for dependent variables or Wilcoxon signed-rank test where applicable. All calculations were done with the use of the Statistica software package (ver. 12). The level of significance was p<0,05.

Results
The effects of 8-week complex balance training showed significant differences between pre-to posttests for the dependent   We did not find significant differences in average drift in the mediolateral direction after the training for left (t(9)=1.66,   Single leg stance on sensory pad The same was observed for the standard deviation of the average drift in ML and in AP directions. The Wilcoxon signed rank test did not show significant differences in ML direction for the left (Z=0.87, p<0.39) and right leg (t(9)=-1.66, p=0.132). In AP direction after the training the results were the following for left leg (t(9)=-1,35 p = 0,210) and right leg (t(9)=-1,86 p = 0,096).
Finally the data indicated a decrease in the asymmetry between the lower extremities in the Sd-DR-ML variable after the training, which was present and statistically significant before the training (

Discussion
We aimed to examine the influence of a specific balance training on dynamic balance and lower extremities power and strength performance in young alpine skiers. Our hypothesis was that the proposed balance training will elicit significant changes in skiers' dynamic performance and balance. In general our main findings were the significant improvements in the jump height, flight time, and area for the left leg and area for right leg induced by the balance training. Positive changes in jump height and flight time are very closely related. What is interesting is the increase in the occupied area during jumps in both legs. This means that the increase in strength capabilities did not involve improvement in the control processes; the higher the jump the harder to control the behavior of the body. Therefore, we advocate for the importance of the core stability training along with the plyometrics which may lead to improvement in the dynamic control of performance. This aspect can be potentially improved in the examined group.
We have also observed a significant decrease in an average drift in the mediolateral direction after the proposed balance training. The new approach to the balance training proposed by Brachman et al. [30] was verified and its effectiveness for improvement in dynamic stability proved to be positive in the examined group. Our findings emphasize the significant impact of balance training on sport performance (its improvement).
Several studies are in accordance with our results, showing that the introduction of a balance training component to the practice schedule allow for improvement in a wide range of movements, starting from the simple movement such as a vertical jump [40], through the complex ones executed during downhill slalom skiing [13]. Other research presented in the review study by Markovic [41] shows significant improvement in jump height after plyometric training. Similarly, like in our study, there are researches where improvement of balance leads to a decrease in functional asymmetry in performance [40]. Not all balance training programs (e.g., resistance and vibration training) reported in the literature lead to the balance improvement [12]. Comparison of balance and resistance training was done in other studies and the results showed that the resistance training was more beneficial to jump height than the balance training [42]. In our research, we used core stability exercises as a kind of resistance training leading to improvement in sport performance. The effect of training protocol, which we used, should be compared with effects of specific resistance training protocols in the future studies. We are also aware that few days of ski training can induce positive changes in some parameters of balance [43].
Our results seem to support the utility of balance exercises incorporated into the training schedule in the decrease of the lower extremity performance asymmetry being the difference in mediolateral drift between the left and right leg. This constitutes one of the injury prevention goals. We consider this as one of the most significant effect of the conducted 8-week balance training. This was also present in other studies [2][3][4]44]. These findings confirm the usefulness of the balance training protocol used in these studies. It is necessary to examine other athletes from different disciplines with different competition level to create the gold standard of balance training procedures. In conclusion, we believe that complex balance training might lead to increase in the dynamic balance and overall performance. This is an important part of sports conditioning that contribute to improved specific performance in skiers.
. . Study Limitations. Although the study has reached it aims there were some unavoidable limitations. First, the study was conducted on a small number of selected population, which limits our ability to draw more meaningful conclusions. On the other hand it allows for improved design in the future based on sample size and power of the test statistics. Another limitation is the results concerning the performance only of the lower part of the body. It would be very useful to use measurements of the upper body kinematics to better estimate dynamic balance performance. The low number of subjects and the lack of a control group are detrimental to the final results which one can only speculate to be based on the conducted balance training, therefore one should treat this study as a promising pilot. Another limitation of this paper is a difficulty to create an aimed functional training program for a specific group such as skiers. Skiers spend most of the season time on outdoor training in the mountains, so close cooperation with the coaches is essential. We did not control the effects of applied training one week and four weeks after the end of the training program, which would be essential to estimate the retention of these effects. Another limitation is to adequately incorporate this training into the training macrocycle in order to boost the potential effects. The reliability of the standalone Drift test was not established yet; however, there were several studies reporting reliability and validity of the Optojump Next system [39]. We are aware that this is not a trivial question and the high reliability of the measurement system may not convey to the separate tests. This is still to be done in the future.
. . Practical Applications. The complex balance training program improved some parameters of dynamic stability in young skiers. Moreover, it leads to decreased asymmetry between lower extremities. Balance training protocol of this study should be an essential tool for all sports trainers and coaches.

Data Availability
The data can be reached through Open ICPSR data repository service under http://doi.org/10.3886/E102440V1

Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.