Professional dancers subject their feet to high forces through frequent jumping, repeated heel rises, insufficient shoe cushioning, and a high degree of foot mobility. To prevent injuries and improve jumping performance, dancers are recommended to strengthen the muscles of the lower extremities.

The foot muscles, especially the toe flexors, play an important role as shock absorbers and motors for the foot by absorbing and generating mechanical energy, which is essential for human locomotion. Furthermore, they serve as a link between the powerful leg extensor muscles and the ground, transmitting the energy generated by the larger muscle groups. This makes them essential components of overall athletic performance. However, the impact of foot muscle strength on dancers´ jumping performance remains uncertain. Therefore, the aim of this study is to investigate the effectiveness of two training programs designed to strengthen the toe flexor muscles in dancers. For this project, a 6.5-week intervention with ballet students from a vocational ballet school is planned. Before and after the intervention, all study participants will perform tests to assess foot muscle strength and jumping performance.

This study aims to develop two different toe flexor (TF) exercise programs for vocational dancers to enhance TF strength and jumping performance. It is hypothesized that i) both training programs increase TF strength and jumping performance, ii) horizontal jumps are more affected than vertical jumps, and iii) the TF exercise program with the TF strengthening device has a greater impact on TF strength.

Participants

Participants will be recruited from a vocational ballet school via an email detailing the study outline. They and their legal guardians (for participants under 18 years old) will be informed of the potential risks and benefits of the study. They will provide written informed consent prior to the measurements. Participants will then be assigned to either a control group, a toe flexor device experimental group, or a heel rise experimental group. The schedule includes three and a half weeks of training, a 12-day break due to the school schedule, followed by another three weeks of training. This will result in a total of 26 training sessions. An experienced trainer will supervise all training sessions. The control group will continue with the ballet school´s normal training schedule.

Data collection and management

An intervention protocol will be kept to collect information on compliance and progression of the training, such as the number of repetitions and the resistance applied. Data analysis will be performed according to the intention-to-treat (ITT) principle, in which participants are analyzed according to their original treatment group assignment. An additional researcher will manage reporting for adverse effects (such as delayed onset muscle soreness) and adverse events. All data collected in this study will be anonymized, personal identifiers removed, and data coded to protect the identities of the participants.

Sample size

Due to the study´s novel character, no sample size calculations have been conducted. To the researchers' best knowledge, no comparable protocols to improve TF strength have been investigated, so a sample size calculation was not warranted. The study is, therefore, exploratory.

Statistical analysis

Variables will be presented as the mean ± standard deviation. The Shapiro-Wilk test will be performed to assess the normality of the measured variables. Repeated-measures Analysis of Variance (ANOVA) with Bonferroni correction will be used to analyze variables across different times and groups. Non-normally distributed variables will be analyzed using Friedman ANOVA and Kruskal-Wallis ANOVA, with multiple comparisons of mean ranks for individual comparisons. Statistical analyses will be performed using IBM SPSS Statistics for Windows (Version 29.0), and the statistical significance level will be set to .05. If the primary and/or secondary outcomes yield null findings, equivalence testing will be conducted to determine whether the intervention is equivalent to the usual training (control group). The two one-sided tests (TOST) procedure will be applied, using either standardized differences (e.g., Cohen's d) or raw differences.

Inclusion Criteria

• Being a ballet student at the Elmhurst Ballet School.
• Aged 14 to 19.
• Written informed consent (by the participant and/or their legal guardian).

Exclusion Criteria:

• Any recent lower leg or foot injuries.
• Chronic pain in the lower extremities.

Sensitive data of the students and the school.

After the participants have completed the pre-measurements, they will be assigned to their group allocation. The intervention´s nature precludes blinding the participants and the trainer; however, a supervisor will monitor data collection.