Morphofunctional Characteristics of the Foot and Ankle in Competitive Swimmers and Their Association with Muscle Pain

Academic Editors: Claudio Belvedere and Fabio La Foresta Received: 19 February 2025 Revised: 24 March 2025 Accepted: 25 March 2025 Published: 29 March 2025 Citation: Jiménez-Braganza, C.; Sáez-Díaz, A.; Munuera-Martínez, P.V. Morphofunctional Characteristics of the Foot and Ankle in Competitive Swimmers and Their Association with Muscle Pain. Appl. Sci. 2025,15, 3755. https://doi.org/10.3390/ app15073755 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Article Morphofunctional Characteristics of the Foot and Ankle in Competitive Swimmers and Their Association with Muscle Pain Cristina Jiménez-Braganza 1, Antonia Sáez-Díaz 2and Pedro Vicente Munuera-Martínez 1,3,* 1 Faculty of Nursing, Physiotherapy and Podiatry. University of Seville, 41009 Seville, Spain; [email protected] 2Department of Statistics, Axioma Comunicaciones, 41016 Seville, Spain; [email protected] 3Department of Podiatry, University of Seville, 41092 Seville, Spain *Correspondence: [email protected] Abstract: The aim of this study was to analyze the joint characteristics of the foot and ankle in competitive swimmers aged 16–18 and 19–24 years and their relationship with the presence of muscle pain during swimming. A total of 74 swimmers were evaluated: 38 ‘junior’ (16–18 years) and 36 ‘senior’ (19–24 years). The following parameters were recorded: ankle dorsiflexion, rearfoot mobility, first metatarsophalangeal dorsiflexion, presence of hallux valgus, foot posture, first ray mobility, arch height, and plantar pressure. Additionally, the frequency and location of muscle pain in the triceps surae were analyzed. A cluster analysis was performed to identify variables that differentiated both groups. Ankle dorsiflexion was limited in both groups, with a greater restriction observed in adults (p< 0.001 with an extended knee; p< 0.014 with a flexed knee). The predominant foot type was the cavus foot. The most common pain was localized in the triceps surae, followed by the plantar musculature, with no significant differences between groups. Swimmers exhibited gastrocnemius shortening, which could limit ankle dorsiflexion and contribute to the onset of muscle pain in the leg and foot. These findings suggest the importance of incorporating lower limb flexibility strategies into the training of competitive swimmers. Keywords: swimming; foot; ankle; muscles; lower limb 1. Introduction Swimming is a sport that is widely practiced throughout the world. The athlete’s goal is to swim a distance in the shortest possible time. The improvement of speed is conditioned by the perfection of swimming technique and mechanical efficiency, with elite swimmers standing out for their efficiency, power, and performance in propulsive forces [ 1 ]. Researchers have tried to find out which determinants influence performance in order to try to make swimmers faster with minimal risk of injury, so it is considered a very important topic of research. Although it is a non-impact sport with a low risk of injury, approximately 75% of swimmers have been injured at some time [ 2 ]. Competitive swimmers are affected by musculoskeletal injuries mainly due to cyclic and excessive joint movement, with most injuries occurring during training [3]. Although the shoulder is the joint most frequently affected by injuries in swimmers, with a prevalence rate of up to 91%, the function of the lower limb is also crucial for performance and can be a common site of musculoskeletal discomfort [ 4 ]. The joints and musculature of the lower limb enable leg movements that generate propulsion, body elevation, and balance. While lower limb function has been associated with improved swimming performance, studies describing the joint characteristics of the lower limb in swimmers, particularly the foot and ankle, remain limited to date [ 5 ]. Due to this lack Appl. Sci. 2025,15, 3755 https://doi.org/10.3390/app15073755 Appl. Sci. 2025,15, 3755 2 of 13 of information, the aim of our study was to analyze the joint characteristics of the foot and ankle in competitive swimmers aged 16–18 and 19–24 years and their relationship with the presence of muscle pain in the posterior musculature and the plantar region during swimming. Due to this lack of information, the objective of this study is to determine the morphofunctional characteristics of the foot and ankle in competitive swimmers of different age groups (junior and absolute) and analyze their potential relationship with muscular discomfort in the lower limb during swimming. By identifying joint mobility patterns, foot structure, and plantar pressures in swimmers, this study aims to contribute to the understanding of how these factors may predispose athletes to musculoskeletal pain or dysfunctions. The findings may provide useful insights for optimizing training strategies, preventing lower limb injuries, and improving biomechanical efficiency in competitive swimmers. 2. Methods This is a cross-sectional study in which competitive swimmers from different swimming clubs in the province of Seville participated. The study was carried out between February 2022 and August 2023. All adult participants, as well as the parents of underage swimmers, gave their written consent to voluntarily participate in the study. The study was approved by the Research Ethics Committee of the University of Seville (internal code 250522). Seventy-four competitive swimmers voluntarily participated in the study, 38 ‘junior’ (age group 16–18 years, 20 men and 18 women) and 36 ‘absolute’ (age group 19–24 years, 19 men and 17 women). One hundred forty-eight lower limbs were analyzed. A total of 74 competitive swimmers were evaluated and divided into two groups according to their sports category. The “Junior” (JR) Group included 38 swimmers (20 men and 18 women ) aged 16 to 18 years, who are still in the process of physical and technical development within their discipline but already compete at the national level. On the other hand, the “Absolute” (AB) Group consisted of 36 swimmers (19 men and 17 women) aged 19 to 24 years, considered athletes with greater maturity and competitive experience, who have surpassed the juvenile development phase and may have reached a high level of performance in national and international competitions. The inclusion criteria were being registered in the junior (JR) and senior (AB) categories, having a minimum of five years of competitive swimming experience, and participating in national competitions in Spain. Swimmers who took part in triathlons, who had a bone fracture or surgery of the lower limb, or who had some degree of disability or functional diversity were excluded. Firstly, demographic and anthropometric characteristics were analyzed, recording age in years and body measurements such as weight in kilograms and height in centimeters. These data provide context for the sample and allow for the analysis of potential differences between swimmer groups based on their physical development. Information was collected on the general characteristics of the swimmers, such as age, sex, and body mass index (BMI). Specific variables related to sporting practice were also recorded, such as weekly kilometers swam, main stroke, and years of swimming practice. Next, several joint and biomechanical measurements of the foot and ankle were evaluated. Ankle dorsiflexion was measured in degrees using a goniometer (Fresco Podología, Barcelona, Spain), both with the knee extended and with the knee flexed [ 1 ]. Rearfoot mobility was assessed with an inclinometer, also expressed in degrees [ 2 ]. The range of inversion and eversion of the rearfoot was obtained by placing the athlete in the prone position with the knee in extension, according to the technique used by Taboadela [ 6 ]. The first metatarsophalangeal joint dorsiflexion (MPJ) [ 7 ] was measured using a goniometer [ 3 ], while first ray mobility was quantified in millimeters with a caliper [ 8 , 9 ]. The presence or Appl. Sci. 2025,15, 3755 3 of 13 absence of hallux valgus (HV) was quantified in degrees [ 10 , 11 ]. The presence and classification of HV were performed using the Manchester Scale described by Menz et al. [12] . Additionally, foot posture was evaluated using the Foot Posture Index (FPI) [ 13 ], a scoring system ranging from − 12 to +12, which allows for the classification of different foot types [5]. Regarding morphological and baropodometric assessments, the arch height was measured in millimeters using specialized calipers [ 14 ]. Type of footprint (high-arched, normal, or low-arched) [ 15 ]. Arch height was calculated by measuring Clarke’s angle on footprints [ 12 , 16 ]. To record plantar pressure, the GAI + VIEW AFA-50 pressure plate (alFOOTs Co., Seoul, Republic of Korea) was used. The percentage load of the forefoot and rearfoot and the total load of each foot were obtained. Furthermore, plantar pressure distribution was analyzed using a pressure platform, with values expressed in kilopascals (kPa), providing insights into the load distribution across different foot regions during standing posture [ 17 ]. An image of the footprint was digitized, and the angle was measured using AutoCAD 2022 software (Autodesk, San Francisco, CA, USA) [18]. Finally, muscle pain evaluation was conducted using the 11-point Numeric Pain Rating Scale (NPRS) [ 14 ], where 0 represents no pain, and 10 indicates the maximum possible pain [ 6 ]. The presence of pain in the posterior lower limb musculature and the plantar region was recorded, categorizing it based on location: triceps surae or plantar musculature [ 9 ]. To determine whether gastrocnemius tightness was present, the differences between dorsal flexion of the ankle with the knee flexed and extended were registered, according to the method described by Mil-Homens [19]. In Table 1, the data corresponding to the variables analyzed in both competition categories are presented, along with their respective units of measurement. Table 1. Variables and Units of Measurement. Variable Measurement Unit Group Junior (Media ±DE) Group Absolut (Media ± DE) Age Years 17.0 ±0.8 21.1 ±1.5 Weight kg 70.5 ±5.4 73.8 ±6.2 Height cm 175.2 ±6.1 178.4 ±5.8 Ankle Dorsiflexion (Knee extended) Degrees (◦) 10.5 ±2.1 8.7 ±1.9 Ankle Dorsiflexion (Knee flexion) Degrees (◦) 12.3 ±2.5 10.1 ±2.3 Rearfoot Mobility Degrees (◦) 10.1 ±2.3 7.5 ±1.6 Dorsal Flexion of the 1st MTP Degrees (◦) 50.4 ±5.2 47.2 ±4.8 Firs Ray Mobility mm 6.5 ±1.3 5.2 ±1.5 Foot Posture (FPI) Scale (−12 to +12) 4.0 ±2.0 3.0 ±2.0 Arch Eight Degrees (Clarke’s angle) 38.2 ±3.5 36.8 ±4.1 Plantar Pressure kPa 250.4 ±30.2 270.6 ±28.9 Pain Presence (NPRS) Scale (0–10) 3.8 ±1.2 4.1 ±1.3 Pain Location Category Triceps surae, plantar muscles Triceps surae, plantar muscles Data Analysis Each lower limb was analyzed independently since the joint range of motion, arch height, plantar pressure distribution, and other morphofunctional parameters can differ Appl. Sci. 2025,15, 3755 4 of 13 between the right and left sides of the same individual. Considering each limb separately allows for a more detailed and accurate assessment of the characteristics under study. For the descriptive analysis, the absolute frequency (N), relative frequency (%), mean values, standard deviation (SD), median and interquartile range (IQR) were calculated. For quantitative variables, normality was evaluated using the Shapiro–Wilk test. To determine whether there were differences between the JR and AB groups, the t-test for independent samples was applied if the variables were normal, or the Mann–Whitney U test if they were not. In the case of qualitative variables, the chi-square test was used. A two-stage cluster analysis was carried out using the silhouette measure of cohesion and separation coefficient, with the aim of finding out the values of the variables that differentiate the JR and AB groups at the global level. To determine whether foot type, classified according to the Foot Posture Index (FPI) and Clarke’s angle, influenced the 50 m swimming time, a one-way ANOVA test was conducted. A confidence level of 95% was taken into account so that the experimental p-value was compared with a significance level of 5%. To assess intra-rater reliability, a subgroup of 12 randomly selected participants underwent repeated measurements 10 to 15 days after the initial assessment. The intraclass correlation coefficient (ICC) was calculated to evaluate the reliability of the measurements. Statistical analysis was performed using the IBM SPSS Statistics 27 statistical package. A post hoc power analysis was conducted by calculating the sample size, and the following results were obtained. The variable ankle dorsiflexion with a flexed knee was taken as a reference, as it was the significant variable with the smallest effect size in this study. The sample size for the comparison of two means was calculated. The formula applied is: n= 2s2zα 2+zβ2 d2 s. Standard deviation estimation based on this study. A mean value of 3.8 standard deviations was obtained. α. Type I error. An a priori value of α= 0.05 was used. β. Type II error. 1 −βis the power, which is the value to be calculated. d. Minimum detectable difference. In this study, the mean difference was 2.7. Therefore, the final equation is: n= 2s2zα 2+zβ2 d2=2·3.82·1.96 +zβ2 2.72=36 Solving for the equation, a value of z_ β = 1.0545 is obtained, which results in β= 0.1469 , meaning that the statistical power is 1 −β= 1 −0.1469 = 0.8531. It can be concluded that the statistical power of this study is above 85%. 3. Results Seventy-four competitive swimmers voluntarily participated in the study: 38 in the junior category (16–18 years, 20 men and 18 women, mean age: 17.0 ± 0.8 years) and 36 in the senior category (19–24 years, 19 men and 17 women, mean age: 21.1 ±1.5 years ). At the time of examination, the JR swimmers had 8.0 ± 2.0 years of experience and averaged 35.3 ±8.4 km per week in their training. The AB swimmers had 12.3 ± 3.4 years of experience and averaged 37.3 ±9.8 km per week. The intra-rater reliability analysis, assessed using the intraclass correlation coefficient (ICC) for all joint range of motion measurements, as well as the straight leg raise test and Appl. Sci. 2025,15, 3755 5 of 13 the seat and reach test, yielded optimal results. The lowest ICC obtained was 0.912 for ankle dorsiflexion with the knee flexed. Specifically, absolute-category swimmers had a greater number of years of experience (12.3 ± 3.4 vs. 8.0 ± 2.0, p< 0.001), reflecting greater exposure to training load. However, no significant differences were found in weekly swimming distance (37.3 ± 9.8 km vs. 35.3 ± 8.4 km, p= 0.436), suggesting that training volume alone is not the only relevant factor in foot and ankle biomechanics. The number of participants who presented muscular pain is shown in Table 2. There was no significant difference between the sexes (p= 0.587). Table 2. Frequency (n and %) of muscular pain in triceps surae and plantar regions in both age groups, juniors, and adults. Competition Category Junior N = 38 (51.4%) Absolute N = 36 (48.6%) Chi-Square Tests N%N%p1Value df Sex 0.587 0.000159 1 Man 20 52.6 19 52.8 Woman 18 47.4 17 47.2 Right Triceps Surae 18 47.4 24 66.7 0.075 2.805 1 Left Triceps Surae 21 55.3 27 75.0 0.062 3.160 1 Right Plantar Muscles 17 44.7 23 63.9 0.078 2.730 1 Left Plantar Muscles 18 47.4 21 58.3 0.239 0.892 1 1Fisher’s Exact Test. Significant differences were observed for the right FPI and the left HV (Table 3). As can be seen, in the AB category, we found a greater number of feet with a neutral category FPI, while the JR category has the highest stage A for HV. Regarding the arch height, we observed no differences between the two categories, with the JR category having the highest number of neutral feet, although both groups have a higher percentage of cavus feet among their swimmers. Table 3. Frequency (N and %) of neutral, pronated, and supinated feet; absent (A), mild (B), moderate (C), or severe (D) hallux valgus; and normal-arched, high-arched and low-arched feet. Junior N = 38 (51.4%) Absolute N = 36 (48.6%) p N % N % Right FPI 0.030 Neutral 15 39.5 24 66.7 Pronated 10 26.3 8 22.2 Supinated 13 34.2 4 11.1 Left FPI 0.110 Neutral 15 39.5 23 63.9 Pronated 14 36.8 8 22.2 Supinated 9 23.7 5 13.9 HV Right feet 0.201 A 32 97.0 27 84.4 B 1 3.0 4 12.5 C 0 0 1 3.1 Appl. Sci. 2025,15, 3755 6 of 13 Table 3. Cont. Junior N = 38 (51.4%) Absolute N = 36 (48.6%) p N % N % HV Left feet 0.037 A 31 93.9 24 75.0 B 2 6.1 8 25.0 Arch height right Feet 0.676 Normal 7 18.4 4 11.1 Low 3 7.9 3 8.3 High 28 73.7 29 80.6 Arch height left Feet 0.327 Normal 10 26.3 5 13.9 Low 2 5.3 1 2.8 High 26 68.4 30 83.3 FPI: Foot Posture Index; HV: hallux valgus. The ranges of ankle dorsal flexion, rearfoot inversion, and eversion, first MPJ dorsal flexion, and first ray plantarflexion and dorsiflexion are shown in Table 4, as well as the results of the comparisons made between the two categories. As can be seen, dorsal flexion of the ankle with the knee extended was limited in both age groups. Table 4. Range of ankle dorsal flexion with the knee extended and flexed, rearfoot inversion and eversion, first metatarsophalangeal joint dorsal flexion, and dorsiflexion and plantarflexion of the first ray. All ranges are expressed in degrees, except for the first ray dorsiflexion and plantarflexion, which are expressed in millimeters. Competition Category Junior N = 38 (51.4%) Absolute N = 36 (48.6%) Mean SD Median IQR Mean SD Median IQR pEffect Sizes Ankle dorsal flexion with right knee extended 10.3 2.2 10 8–12 7.2 3.0 7 4.5–9.5 <0.001 20.530 4 Ankle dorsal flexion with left knee extended 10.3 2.4 10 10–12 7.2 2.8 8 4.5–10.0 <0.001 20.543 4 Ankle dorsal flexion with right knee flexion 18.6 2.9 20 18–20 15.9 4.7 16 12–20 0.014 20.287 4 Ankle dorsal flexion with left knee flexion 18.7 3.1 18 18–20 15.9 4.9 16 12–20 0.006 20.320 4 Right rearfoot inversion 27.1 5.8 28 23.5–30.0 22.3 4.7 22 20–26 <0.001 10.891 3 Left rearfoot inversion 25.4 5.2 24 22–30 20.8 5.9 21 18–24 <0.001 10.830 3 Right rearfoot eversion 9.7 2.8 10 8–12 7.6 3.0 8 6–10 0.004 20.336 4 Left rearfoot eversion 10.7 2.6 11 9.5–12.0 8.7 2.8 8 6.5–10.0 0.002 20.360 4 Right first MPJ dorsal flexion 58.1 7.4 58 52–64 53.2 11.3 54 50–60 0.088 2 Left first MPJ dorsal flexion 59.5 7.8 60 54.0–62.5 53.2 10.0 54 48–60 0.008 20.308 4 Right first ray dorsiflexion 3.5 1.0 3.5 3–4 4.2 0.8 4 4–5 0.002 20.364 4 Left first ray dorsiflexion 3.3 1.1 3 2–4 4.3 1.1 5 3.3–5.0 <0.001 20.401 4 Right first ray plantarflexion 5.5 0.9 6 5–6 5.4 1.0 5.5 5–6 0.550 2 Left first ray plantarflexion 5.7 0.8 6 5–6 5.7 0.7 6 5–6 0.869 2 MPJ: Metatarsophalangeal joint. 1 t-test for independent samples. 2 Mann–Whitney U-test for independent samples. 3d de Cohen. 4r de Rosenthal. Appl. Sci. 2025,15, 3755 7 of 13 In Table 5, we can observe the percentage of forefoot, rearfoot, and total load for each limb, with no statistically significant differences between both age groups. Table 5. Plantar pressure distribution (%) under the forefoot, rearfoot, and whole foot. Competition Category Junior N = 38 (51.4%) Absolute N = 36 (48.6%) Mean SD Median IQR Mean SD Median IQR p Right forefoot 17.6 7.4 16.1 12.8–20.3 19.3 6.2 18.7 15.4–23.1 0.063 2 Left forefoot 22.1 6.3 21.1 17.7–26.7 23.3 8.0 22.8 17.0–29.5 0.479 1 Right rearfoot 30.0 7.5 31.5 26.9–34.2 27.5 8.1 27.1 25.4–32.9 0.086 2 Left rearfoot 30.3 7.0 31.9 25.2–35.1 29.9 11.5 29.1 24.7–33.6 0.187 2 Whole right foot 47.6 4.3 47.1 45.6–50.4 26.8 10.1 48.3 46.9–50.4 0.312 2 Whole left foot 52.4 4.3 52.9 49.6–54.4 53.2 10.1 51.7 49.6–53.1 0.312 2 Both forefeet 39.6 12.5 37.4 31.5–48.2 42.6 11.9 42.8 34.3–51.2 0.144 2 Both rearfeet 60.4 12.5 62.7 51.8–68.5 57.4 11.9 57.3 48.8–65.7 0.144 2 1t-test for independent samples. 2Mann–Whitney U-test for independent samples. To know which variables differed most between the JR and AB categories and their values, a cluster analysis was carried out (Figure 1), considering that the silhouette measure of cohesion and separation coefficient had to be greater than 0.5 and the variables within the clusters had to include the right and left sides. The silhouette measure of cohesion and separation coefficient was 0.533, so the model was classified correctly. The variables that differed most between the two groups were dorsal flexion of the ankle with the knee flexed (more limited in the AB category) and muscle pain in the triceps surae (more frequent in the AB category). Table 6shows that the JR category is represented without triceps surae pain, and the measurement of ankle dorsal flexion with knee flexed is between 20 and 22 degrees . The AB category is represented by Triceps Surae pain, and the measurement of ankle dorsal flexion with knee flexion is between 13 and 18 degrees. Table 6. Values of the variables ‘triceps surae pain’ and ‘Ankle dorsal flexion with knee flexion’ in both age groups according to the cluster analysis. Competition Category Right Triceps Surae Pain Left Triceps Surae Pain Right Ankle Dorsal Flexion with Knee Flexion (Degrees) Left Ankle Dorsal Flexion with Knee Flexion (Degrees) Junior No pain No pain 19.98–22.01 18.03–22.02 Absolute Pain Pain 11.98–18.00 12.04–18.03 Appl. Sci. 2025,15, 3755 8 of 13 Appl. Sci. 2025, 15, x FOR PEER REVIEW 7 of 13 Left rearfoot 30.3 7.0 31.9 25.2–35.1 29.9 11.5 29.1 24.7–33.6 0.187 2 Whole right foot 47.6 4.3 47.1 45.6–50.4 26.8 10.1 48.3 46.9–50.4 0.312 2 Whole left foot 52.4 4.3 52.9 49.6–54.4 53.2 10.1 51.7 49.6–53.1 0.312 2 Both forefeet 39.6 12.5 37.4 31.5–48.2 42.6 11.9 42.8 34.3–51.2 0.144 2 Both rearfeet 60.4 12.5 62.7 51.8–68.5 57.4 11.9 57.3 48.8–65.7 0.144 2 1 t-test for independent samples. 2 Mann–Whitney U-test for independent samples. To know which variables differed most between the JR and AB categories and their values, a cluster analysis was carried out (Figure 1), considering that the silhouette measure of cohesion and separation coefficient had to be greater than 0.5 and the variables within the clusters had to include the right and left sides. The silhouette measure of cohesion and separation coefficient was 0.533, so the model was classified correctly. The variables that differed most between the two groups were dorsal flexion of the ankle with the knee flexed (more limited in the AB category) and muscle pain in the triceps surae (more frequent in the AB category). Table 6 shows that the JR category is represented without triceps surae pain, and the measurement of ankle dorsal flexion with knee flexed is between 20 and 22 degrees. The AB category is represented by Triceps Surae pain, and the measurement of ankle dorsal flexion with knee flexion is between 13 and 18 degrees. Figure 1. Graph showing the group comparison resulting from the two-stage cluster analysis. Figure 1. Graph showing the group comparison resulting from the two-stage cluster analysis. 4. Discussion The aim of this study was to analyze the joint characteristics of the foot and ankle in junior and young adult competitive swimmers and their relationship with muscular pain during training. Research on injuries in swimming has mainly focused on the upper limb [ 20 ], although from previous studies, it is known that leg kicking improves propulsion and speed by 31% [ 21 ], demonstrating that leg movement and muscle work is three to five times greater than arm movement [ 15 ]. To our knowledge, this is the first study to report joint characteristics of the feet and ankles of elite swimmers. On the prevalence of muscle pain, several researchers sustain that women tend to injure themselves more often [ 2 , 4 ], possibly due to their shorter limb length, which implies a greater cadence [ 22 ]. On the other hand, Abgarov et al. [ 23 ] suggest that men are more likely to be injured. We agree with Kerr et al. [ 24 ] and Prieto-Andreu [ 25 ], as no significant differences were observed between men and women. Several studies have indicated that men exhibit a higher incidence of injuries compared to women in various sports, including swimming, due to differences in biomechanics, body composition, and levels of strength and flexibility [ 1 ]. Specifically, research has reported that male swimmers may be exposed to greater muscular loads in the lower limbs, which could influence the prevalence of Appl. Sci. 2025,15, 3755 9 of 13 muscle discomfort and injuries in this population [ 26 ]. The hamstrings are a frequent area of injury in athletes [ 27 ]. Lack of flexibility of the gastrocnemius muscles has an impact on pathologies related to the ankle and foot [ 28 ], hallux rigidus, plantar arch stiffness, and plantar fasciitis. Tightness in the posterior musculature limits the dorsal flexion of the ankle and has an impact on the appearance of muscular pain, maybe due to the maintained position in plantar flexion that swimmers adopt. McCullough et al. [ 27 ] state that maintaining a greater degree of plantar flexion and ankle inversion increases speed by 29.3% and makes the swimmer more effective. There are several studies that express the importance of maintaining ankle and foot posture in PF on swimming speed [ 29 ]. Miller [ 30 ] states that more experienced swimmers work with a smaller joint angle than more novice swimmers. The present study confirmed this finding, as the adult swimmers had a smaller joint angle than the junior swimmers. Both junior and adult swimmers showed a decrease in ankle dorsal flexion below values considered normal (15–20 degrees with the knee extended) [31]. In this study, participant classification was based on age category, as the main objective was to evaluate the differences between junior and adult swimmers in relation to plantar characteristics and the presence of muscle discomfort. Previous studies have indicated that, in competitive swimmers, biomechanical differences between men and women may be less pronounced due to sports adaptation and the similarity in training loads [4,21]. Regarding the distribution of muscle pain, our results showed no significant differences between sexes (p= 0.587), suggesting that gender was not a determining factor in this variable within our sample. This finding aligns with previous studies that have analyzed the prevalence of injuries in high-performance swimmers and have not found clear differences between men and women [17,18]. With regard to the characteristics of the first ray in swimmers, as well as the dorsal flexion of the first MPJ, previous research has shown that the etiology of the plantarflexed first ray may be due to muscular misbalance [ 32 ]. The results of this work show an increase in plantarflexion of the first ray. This could be due to a shortening of the posterior musculature and possibly of the plantar musculature because of the maintained plantarflexed position of the foot and ankle during the sports activity. In addition, the swimmers in our investigation presented a range of first MPJ dorsal flexion under 60 degrees, which is considered the normal range [ 33 ]. Although this is commonly associated with excessive foot pronation, the FPI of many of the participants did not show pronation. The mild limitation observed might also be a consequence of an imbalance in the calf muscles and plantar musculature affecting the first ray. We observed that the most predominant type of plantar footprint corresponded to a high-arched foot, with no significant differences between the two age groups. López et al. [34] reported that overuse of the foot leads to biomechanical adaptations being different in preor post-competition. These modifications may become chronic with time, making the foot acquire morphological characteristics that depend on the sports modality. On the other hand, Martínez-Amat et al. [ 35 ] show that the cavus foot is the most predominant type of foot in athletes and swimmers, possibly due to the musculoskeletal adaptations that shape the plantar arch. Miguel-Andrés et al. [ 36 ] sustain that, in athletes, the prevalence of pes cavus is higher than in flat feet and that this finding is related to the demands of the foot towards plantar flexion. This could also affect the normal distribution of plantar pressure. In this work, the athletes presented a higher percentage of load in the rearfoot, both juniors and adults, which may be normal in unshod feet. Bernal et al. [ 37 ] mention that the distribution of plantar pressure in pes cavus is significantly greater in the rearfoot due to the fact that the arch elevation reduces the contact surface, increasing plantar pressure.