Comparative effectiveness of neutral and plantarflexion position training in male adolescent soccer players with lateral ankle sprain

2.1 Participants

The study sample size was determined using G*power version 3.1.9.4 (G*power, University of Düsseldorf, Düsseldorf, NRW, Germany), and the analysis incorporated a repeated measures design with a within-between interaction, setting the α error probability at 0.05, the power (1 − β error probability) at 0.95, and the effect size (f) at 0.25. Based on these parameters, the required sample size was calculated to be 56.

Participants were recruited through convenience sampling from new soccer clubs until the desired sample size was reached. Researchers visited 18 first-division soccer clubs, recruiting eligible and consenting players from 15 of these clubs. The study’s purpose, the intervention program, and its potential benefits were explained to both coaches and players. Players’ injury histories were reviewed, and those who had experienced a LAS within the previous month and had been diagnosed and treated for LAS more than twice in the same period were selected. Furthermore, these players had recently been excluded from training and matches due to LAS. An orthopedic specialist diagnosed and examined the LAS, evaluating the severity and determining participant eligibility for inclusion in the study. Exclusion criteria included a history of lower-extremity surgery or significant pain and inflammation that would impede the examination and training program implementation. Of the eligible players, 28 were assigned to the plantarflexion position training (PPT) group, while the remaining 28 were assigned to the neutral position training (NPT) group. The study ultimately included 56 athletes with a mean age of 16.3 ± 1.1 years and a mean body weight of 65.3 ± 8.1 kg.

Given that the participants were minors, written informed consent was obtained from both the participants and their legal guardians. The researcher provided a detailed explanation of the potential benefits of participating in the study, including access to support for training programs and coverage of testing expenses. It was also emphasized that there would be no penalties for choosing not to participate. Additionally, participants were assured that they could withdraw from the study at any time without facing any negative consequences.

2.2 Study procedures

A total of 56 participants were included in the study, with 28 players assigned to the PPT group and 28 to the NPT group with recurrent LAS. To minimize researcher bias, the participants were assigned to groups using a registration number system: odd numbers were allocated to the NPT group, and even numbers to the PPT group. This study employed a double-blind procedure; investigators were not involved in intervention allocation or the order and method of testing, which were managed by an assistant blinded to the study design and participant characteristics. The professional tester conducting the evaluation was also blinded to the study design and participants.

Training sessions were conducted by five research assistants who provided educational materials and feedback to the participants. The intervention program was designed to be accessible either at home or at a training facility and was delivered through mobile device-compatible programs. Participants also had access to mobile question-and-answer support. The eight-week intervention required participants to engage in training sessions twice daily, in the morning and evening. They were instructed to maintain self-training within a tolerable pain threshold on the 10-point visual analog scale (VAS), targeting a range of 3–4. If discomfort exceeded a VAS of 5 or if LAS recurred, participants were advised to reduce training intensity and inform the researcher via mobile phone. Before and after the intervention, participants visited a rehabilitation clinic for a series of assessments, including the Foot and Ankle Outcome Score (FAOS), ankle isokinetic inversion and eversion tests, and the Y-balance test.

2.3 Self-assessment questionnaire

The participants were advised to rate the severity of their symptoms using FAOS across five domains: pain, symptoms, daily activities, sports and recreational physical activities, and quality of life [22]. This questionnaire has been validated for reliability, with an intraclass correlation coefficient (ICC) of 0.85, and it demonstrated a correlation with the pain scale of r = 0.675 (p < 0.001) [23]. The FAOS consists of 42 questions designed for participants to subjectively assess the severity of their symptoms in the aforementioned domains. Each item within the domains is rated on a scale from 0 to 4 points. These ratings are then converted to a scale of 0–100 points for each domain, where higher scores represent better conditions and lower scores indicate greater discomfort.

2.4 Ankle strength test with isokinetic device

Ankle strength was assessed using a CSMi isokinetic dynamometer, a clinical tool designed for measuring isokinetic muscle strength. This device quantifies muscle strength within a specified range of motion by employing computer-controlled equipment to regulate speed, with results reported in Nm [24]. This device was confirmed to have reliability (ICC: 0.71–0.95%), specificity (50.7–86.8%), and sensitivity (60.4–81.1%) for measuring muscle strength [25]. In this study, ankle inversion and eversion dynamics were evaluated using the concentric contraction mode at angular velocities of 30°/s and 120°/s. The testing protocol involved a range from 40° of inversion to 30° of eversion relative to the neutral position of the ankle. Before testing, participants underwent preparation, practice and demonstration sessions with the assessor to become familiar with the equipment and procedures. Each test session included four repetitions with appropriate rest intervals between angular speeds. Testing was performed bilaterally, starting with the healthy ankle to assess stability, followed by the evaluation of the injured ankle, with the maximum value recorded. To account for differences in body weight, muscle strength and power measurements were normalized by dividing the values by the participant’s body weight.

2.5 Dynamic balance and hop tests

Dynamic balance was assessed using the Y-balance test (YBT) device (FMS Inc., Chatham, VA, USA), which evaluates balance in three directions: anterior, posteromedial and posterolateral. These directions were selected due to their high reliability in assessing dynamic ankle balance and proprioception, with ICC ranging from 0.80 to 0.91 [26]. Prior to testing, each participant received guidelines from the examiner and underwent three practice sessions, each lasting 10 minutes [27]. Participants removed their shoes and socks and positioned the test foot on the central test plate. From this position, the opposite foot was extended as far forward as possible in each of the three directions, maintaining the posture for over 2 seconds. While movement of the arms and lifting of the heels were permitted to help maintain balance, any part of the foot touching the ground was considered a test error. The maximum distance achieved was recorded, and the test was repeated three times in each direction. Testing began with the healthy ankle, followed by the injured ankle, with a 2-minute rest between sets. The highest value from the two tests was used for analysis [28].

Hop tests, used to evaluate leg muscle power, demonstrated ICC values ranging from 0.84 to 0.98 [29]. The participants were allowed a 10-minute practice period before the actual test, with a 2-minute rest interval between test sets [30]. Each test was performed twice, with the highest value used for analysis. In the single hop test, participants started with one foot behind the starting line, ensuring that the heel did not cross the line. At the signal, they jumped forward as far as possible. For the crossover hop test, a line was drawn perpendicular to the starting line. Participants hopped alternately to the left and right of the line three times consecutively from the starting line, aiming to cover the maximum distance. The distance from the starting line to the heel at landing was recorded in centimeters [31].

2.6 Intervention training

The NPT and PPT were designed based on electromyography, ultrasonography experiments, and intervention studies with similar designs [18, 19, 20]. Both interventions were conducted offline and face-to-face, focusing primarily on strength training with elastic exercise tubing (Theraband®; Hygenic Corp., Akron, OH, USA) and balance training with BOSU (BOSU®; HEDSTROM Fitness, Ashland, OH, USA). The BOSU device has been validated against traditional unstable exercise equipment, such as the wobble board, with an ICC ranging from 0.88 to 0.96 [32, 33]. Participants engaged in training twice daily for 8 weeks, with each session lasting approximately 20–30 minutes. During each session, participants performed concentric contractions for approximately 5 seconds while the ankle everted, followed by eccentric contractions to return to the starting position. Each set consisted of 20 repetitions, and participants completed five sets. The intensity of strength training was increased by changing the color of the elastic tube band from red to green. For balance training, participants maintained their center of gravity on the BOSU, standing for 2 minutes across a total of five sets. The intensity of balance training was increased by progressing to more challenging green and blue BOSU tools. The training intensity was maintained for the first four weeks and subsequently increased during the second four weeks. Athletes opting for home-based training were supervised through real-time video app monitoring at least once a week. Alternatively, participants were asked to film their training sessions and send the footage to researchers via a messaging app for feedback, ensuring adherence and accuracy of movement.

NPT: In this position, strength training involved ankle eversion using the tube band. Balance training required maintaining the ankle in a neutral position on an unstable tool inclined at 30°. Participants grounded the entire sole of the foot on the tool and performed balance exercises to maintain the center of gravity in this position (Fig. 1) [18, 19]. The athlete was asked to perform tube exercises while lying face down on a 50 cm high bed. The hip and knee joints were bent at approximately 45°. A tube band was installed on the forefoot, and the ankle was kept in a neutral position (0°) based on the anatomical position. One repetition was performed after performing the eversion movement, and the return was performed at a training speed of 1 time/2 s were repeated. Balance training was performed on the hard surface of the BOSU, and the center of gravity was maintained in a slightly inclined state so that an approximately 30° incline was generated.

Fig. 1.Ankle training at a neutral position.

PPT: The participants maintained plantarflexion of the ankle while performing eversion exercises using the tube band. Balance training involved lifting the heel, sustaining plantarflexion, and maintaining posture by counteracting the loss of the center of gravity (Fig. 2) [18, 19]. The athlete was instructed to sit with his knees extended and a tube band was placed around his forefoot. The exercise speed was 1 time/2 s. The ankle was instructed to maintain 40–50° of plantarflexion and then eversion and return. Balance training was performed by lifting the heel in the center of the BOSU and maintaining plantar flexion to maintain maximum balance.

Fig. 2.Ankle training at a plantarflexion position.

2.7 Statistical analysis

Data analyses were conducted using SPSS version 25.0 (IBM Corp., Armonk, NY, USA). Paired t-tests were used for intra-group comparisons of pre- and post-intervention data, while independent t-tests were applied for inter-group comparisons. Categorical variables, including playing position, sprain grade, and injury frequency, were analyzed using the chi-square test. Repeated two-way analysis of variance (ANOVA) was utilized to examine main effects and interactions by time and group. Effect sizes were determined using Cohen’s d (d) and phi (φ), with effect sizes classified as small (<0.3), medium (0.3–0.5), and large (>0.5). Statistical significance was set at p < 0.05.

3.1 Comparison of information profiling of participants

Table 1 presents a comparison of the general characteristics between the NPT and PPT groups. The average ages of the NPT and PPT groups were 16.0 ± 1.1 years and 16.5 ± 1.0 years, respectively, with no significant difference between the groups. No significant differences were found in height, weight, body mass index, or athletic experience between the groups (p > 0.05). Additionally, there were no significant differences in the VAS scores, which reflect self-assessed subjective pain related to the injury, injury side, position or injury grade.

3.2 Changes in self-assessment score

Over the 8-week intervention period, both groups demonstrated significant improvements in their self-assessment scores. There were no significant differences between the groups before or after the intervention (Table 2). Pain improved by 19.9% in the NPT group and 15.1% in the PPT group. Sports and recreation scores improved by 8.8% in the NPT group and 14.2% in the PPT group (p < 0.05). No significant interaction effects between time and group were observed for any domain (p > 0.05).

3.3 Changes in ankle strength

Post-intervention isokinetic strength testing indicated significant improvements in both inversion and eversion at angular velocities of 30°/s and 120°/s for both the NPT and PPT groups (p < 0.05). However, no significant differences were observed between the groups for inversion and eversion at either velocity following the intervention (p > 0.05). Specifically, eversion strength improved from 39.8 ± 5.8 Nm/kg to 47.7 ± 4.8 Nm/kg (19.8%) in the NPT group, and from 37.8 ± 4.0 Nm/kg to 46.1 ± 2.5 Nm/kg (22.0%) in the PPT group at an angular velocity of 30°/s. Although the main effect of time was significant, there were no significant effects attributable to the group or interactions between time and group (Table 3).

3.4 Changes in dynamic balance and hop tests

Table 4 presents the results of the dynamic balance and hop tests. Data analysis showed that both the NPT and PPT interventions resulted in significant improvements from baseline. Notably, the PPT group demonstrated greater improvements in dynamic balance and hop tests compared to the NPT group. Interaction effects were observed in the posteromedial and posterolateral aspects of dynamic balance and in the single and crossover hop tests (p < 0.05). Specifically, the PPT group showed a 14.2% improvement in the posteromedial direction (from 68.9 ± 4.2 to 73.6 ± 3.8) compared to a 6.8% improvement in the NPT group (from 69.5 ± 3.6 to 79.4 ± 3.5). Similarly, the single hop distance increased by 33.7% in the PPT group (from 132.4 ± 20.3 to 152.0 ± 13.1) compared to a 14.8% increase in the NPT group (from 129.3 ± 17.0 to 172.9 ± 11.0) (p < 0.05).