Different Gymnastic Balls Affect Postural Balance Rather Than Core-Muscle Activation: A Preliminary Study

Round 1

Reviewer 1 Report

I believe that this paper follows a logical sequence and the statistical analyzes are adequate. Also, the various parts of this article are well written. But I'm afraid there are some points in the paper which could deserve further explanations. My major comments are as follows:

Title.

• The title should say something to impress readers. Please perform a new title, which gives clinical relevance of the paper and shows more interest to the reader.

Materials and Methods.

• During which months/years was the study conducted?
• where do the participants come from?
• The sample size is too small. How was the sample size calculated?
• Please clearly define the primary and secondary variables of the study.
• Provide more information on the platforms and tests used to measure the variables.
• The statistics being used, and the underlying purpose for the various analyses is confusing and needs to be made clear

Results

• I think the results are confusing for the reader. The authors should explain it more clearly.

Discussion

• I will ask authors to provide to summarize the main findings in bulleted format and put in a new section as clinical perspectives.
• I recommend adding a section with the limitations of the study.

Conclusion

• As a result of some of the previous comments, specially the scope of this paper, the conclusions do not seem to be very relevant. What does this article contribute to scientific knowledge? What are the news? Why are the results of this article useful?In the discussion or the conclusions section, these questions should be answered.

Author Response

Reviewer 1

• I believe that this paper follows a logical sequence and the statistical analyzes are adequate. Also, the various parts of this article are well written. But I'm afraid there are some points in the paper which could deserve further explanations.

We thank the reviewer for the positive comment. We tried to do our best to improve the quality of the manuscript following his/her detailed observations.

My major comments are as follows:

 Title

• The title should say something to impress readers. Please perform a new title, which gives clinical relevance of the paper and shows more interest to the reader.

We thank the reviewer for the pertinent observation. The title has been changed accordingly. Now it reads: “Different gymnastic balls affect postural balance rather than core-muscle activation: a preliminary study”.

Materials and Methods.

• During which months/years was the study conducted?

Data collection started in January but, due to the COVID-19 emergency, it was suspended until May 2020. Data collection ended in July 2020.

• where do the participants come from?

Participants were from Veneto, a north-east region of Italy along the Adriatic Sea.

• The sample size is too small. How was the sample size calculated?

We thank the reviewer for the pertinent observation. We are aware that the sample size is small. However, according to Cohen's interpretation, the partial eta squared values referred to our primary variables (i.e., the COP-related parameters) are very large. Further, we performed a post-hoc power analysis with the G*Power software, and power (1-β err prob) was higher than 0.95 for all the COP-related variables, highlighting a low percentage of running into a Type II error. However, the small sample size of the study has been acknowledged as a limitation of the study, and we clearly stated in the manuscript that a larger sample size with a more heterogeneous sample is needed to strengthen our preliminary results.

• Please clearly define the primary and secondary variables of the study.

We thank the reviewer for the comment. In the last part of the introduction, now we specified the primary and the secondary aim of the study.

• Provide more information on the platforms and tests used to measure the variables.

The suggestion of the reviewer has been acknowledged. In the new version of the manuscript, we added the technical specifications of the force platform and we reported the dimensions of the wooden board screwed on it.

• The statistics being used, and the underlying purpose for the various analyses is confusing and needs to be made clear

We thank the reviewer for the suggestion. The statistical analysis section has been reorganized to be less confusing to the reader.

 Results

• I think the results are confusing for the reader. The authors should explain it more clearly.

The section has been rewritten. In the new version of the manuscript, results have been grouped into two separate subparagraphs, including the COP-related parameters and the EMG core-muscle activation.

Discussion

• I will ask authors to provide to summarize the main findings in bulleted format and put in a new section as clinical perspectives.

We thank the reviewer for the suggestion. Indeed, we think that the main findings of the study are summarized in the conclusion paragraph. Therefore, the addition of bullet points will result in a repetition of the conclusions.

• I recommend adding a section with the limitations of the study.

Following the reviewer's suggestion, we reported a paragraph with the limitations of the study at the end of the discussion.

 Conclusion

• As a result of some of the previous comments, specially the scope of this paper, the conclusions do not seem to be very relevant. What does this article contribute to scientific knowledge? What are the news? Why are the results of this article useful? In the discussion or the conclusions section, these questions should be answered.

We thank the reviewer for the observation. Our study is the first to investigate the effect of different destabilizing devices on balance control and core-muscle activation. Previous works analyzed exclusively different exercises (Vera-Garcia et al. Phys. Ther. 2000, 80, 564–9) or compared stable versus unstable conditions (Chulvi-Medrano et al. J. strength Cond. Res. 2010, 24, 2723–30). Therefore, the results of our study could sensibilize trainers and therapists going beyond the only choice of the exercise and integrating it with the proper device. These concepts have been expressed more clearly in the new version of the discussion and conclusion sections.

Reviewer 2 Report

The presented research procedure is correct, I consider the following positive:

- determination of three balls in terms of size and shape as well as normative pressure;

- precise description of the subjects' positions (distances related to somatic parameters - length of the lower limb measured according to the principles of anthropometry - anterior superior iliac spine and medial malleolus);

range of motion in the knee joint of the non-dominant limb;

- lifting the leg and arms according to the frequency measured by the metronome 50x / 1 min.

- during the eye exercise, directing the straight line on a white wall at a distance of 80 cm;

- each time performing three tests and averaging the measurement results;

- analyzing the EMG signal and averaging it from three muscles (straight abdomen, oblique abdominal and spine extensor);

1. The weakness of the presented work is the small size of the research group. (It would be worth considering using a program that calculates the minimum number of patients necessary for the study. (Warto byłoby rozważyć wykorzystanie programu obliczającego minimalną liczbę pacjentów koniecznych do badania.)
2. The justification of the statistics used is not very clear. I understand that the obtained results have the character of a normal distribution, which was not clearly stated in the text, but due to the application of the D'Agostino Pearson test based on moments from the samples, and then the use of analysis of variance (ANOVA), which usually leads to testing the significance of differences between the means, gave rise to the conclusion that the distribution was normal. It seems that based on the results in order to "normalize" the distribution of kurtosis, the authors used such a transformation due to the small size of the research group. They did so because by combining two tests (skewness and kurtosis) a test is sensitive to deviations from normality.
3. There was no significant correlation between the size of the ball and the level of activation of core muscles.
4. Significant differences were found in the level of stability due to the type of ball used, which did not affect the level of muscle activation (in the results of the study).

Summing up, it should be said that the research idea is interesting. The research procedure was planned correctly. However, too small a study group was planned. It causes that in spite of the applied statistical tests, the inference is very difficult and may be incorrect. The key is to understand what effect sample size has on the results of statistical inference. With a small sample, it is difficult to prove e.g. research hypotheses (especially in the case of high variability of the analyzed variable and extremely small sample size - e.g. 11), while with a very large sample, it is possible to demonstrate statistical significance of any small difference. At this stage, the material presented and the results obtained are more an observation than a study.

Author Response

The presented research procedure is correct, I consider the following positive:

- determination of three balls in terms of size and shape as well as normative pressure;

- precise description of the subjects' positions (distances related to somatic parameters - length of the lower limb measured according to the principles of anthropometry - anterior superior iliac spine and medial malleolus);

- range of motion in the knee joint of the non-dominant limb;

- lifting the leg and arms according to the frequency measured by the metronome 50x / 1 min.

- during the eye exercise, directing the straight line on a white wall at a distance of 80 cm;

- each time performing three tests and averaging the measurement results;

- analyzing the EMG signal and averaging it from three muscles (straight abdomen, oblique abdominal and spine extensor);

We thank the reviewer for the positive comments. We are glad he/she appreciated the research procedures we adopted in the study design, data collection, and data analysis.

The weakness of the presented work is the small size of the research group. (It would be worth considering using a program that calculates the minimum number of patients necessary for the study.

We thank the reviewer for the pertinent observation. We are aware that the sample size is small. However, the partial eta squared values referred to our primary variables (i.e., the COP-related parameters) are very large according to Cohen’s interpretation. Further, we performed a post-hoc power analysis with the G*Power software and power (1-β err prob) was higher than 0.95 for all the COP-related variables, highlighting a low percentage of running into a Type II error.

The justification of the statistics used is not very clear. I understand that the obtained results have the character of a normal distribution, which was not clearly stated in the text, but due to the application of the D'Agostino Pearson test based on moments from the samples, and then the use of analysis of variance (ANOVA), which usually leads to testing the significance of differences between the means, gave rise to the conclusion that the distribution was normal. It seems that based on the results in order to "normalize" the distribution of kurtosis, the authors used such a transformation due to the small size of the research group. They did so because by combining two tests (skewness and kurtosis) a test is sensitive to deviations from normality.

We thank the reviewer for the observations. The statistical analysis section has been reorganized to be less confusing to the reader.  

There was no significant correlation between the size of the ball and the level of activation of core muscles.

Significant differences were found in the level of stability due to the type of ball used, which did not affect the level of muscle activation (in the results of the study).

Summing up, it should be said that the research idea is interesting. The research procedure was planned correctly. However, too small a study group was planned. It causes that in spite of the applied statistical tests, the inference is very difficult and may be incorrect. The key is to understand what effect sample size has on the results of statistical inference. With a small sample, it is difficult to prove e.g. research hypotheses (especially in the case of high variability of the analyzed variable and extremely small sample size - e.g. 11), while with a very large sample, it is possible to demonstrate statistical significance of any small difference. At this stage, the material presented and the results obtained are more an observation than a study.

Again, we are glad the reviewer appreciated our research idea and the overall study design. The small sample size of the study has been acknowledged as a limitation of the study. Following the reviewer’s consideration, we clearly stated in the manuscript that a larger sample size with a more heterogeneous sample is needed to strengthen our preliminary results. The title of the paper has been changed accordingly.

Round 2

Reviewer 1 Report

Accept in present form

Author Response

We thank the reviewer. We are glad he aprreciated our efforts to improve the quality of the paper following his/her suggestions.

Reviewer 2 Report

Thank you for all the answers to the research. I accept all the changes you have made. A very positive aspect of your response is the presentation of the method of calculating the number of people needed to perform the test. I think it would be worth adding a section on these calculations. This will work in favor of the presented work. The clarity of the text on statistics has definitely improved. The proposed new title is also adequate to the presented research. This encourages us to follow the results of the authors' research. I appreciate the contribution made to the changes and improvement of work.

Author Response

We thank the reviewer for the comments. We added the post-hoc power analysis in the manuscript, accordingly.