The role of morphology and function of intrinsic foot muscles in young handball players with chronic ankle instability

Abstract

Background

Strong evidence shows the importance of intrinsic foot muscles (IFMs) in balance and foot and ankle biomechanics; despite this, not much research has been published regarding the role of IFMs on chronic ankle instability. The aim of the study was to compare IFMs morphology and function between young handball players with and without chronic ankle instability. The secondary objective was to assess the association between the morphology or function of the IFMs and the foot morphology.

Methods

Cross-sectional case-control study. A sample of 100 young handball players were recruited and divided into athletes with chronic ankle instability (case group, n = 50) and without (control group, n = 50). Main outcome measures: morphology and function of abductor hallucis, abductor digiti minimi, flexor hallucis brevis, flexor digitorum brevis and quadratus plantae, which were assessed with ultrasound examination at rest and in active contraction. Foot Posture Index-6 was used to analyze foot morphology.

Results

Significant differences were only found in the abductor hallucis morphology (case group CSA 2.14cm2, control group CSA 1.84cm2, p 0.013) and in the flexor hallucis brevis morphology (case group thickness 1.31 cm, control group thickness 1.41 cm, p 0.005). A significant but weak positive correlation was found between foot morphology and abductor hallucis function (r = 0.32, p = 0.025).

Conclusions

Between-groups differences were only found in abductor hallucis and flexor hallucis brevis morphology; no differences were found in the rest of IFMs morphology and function outcomes. A significant positive association was only found between foot morphology and AbH function in young athletes with CAI.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13102-026-01605-z.

Introduction

Handball is one of the most popular sports in the world. The game is characterized by physical contact, sudden accelerations, jumps with hard landings and twisting actions, which are related with a high risk of injury [1]. Young handball players aged between 15 and 19 years have shown the highest injury prevalence (41%) among all the age groups [1]. Specifically, ankle sprain is the most common traumatic pathology in adolescent handball players, with a prevalence of 24.9–40% of all injuries registered in athletes from this age group [2]. This situation generates a high sporting and economic burden in handball teams [3].

Lateral ankle sprain (LAS) is defined as an acute traumatic injury of the lateral ligament complex after an excessive inversion of the hindfoot or a combined plantar flexion and adduction of the foot [4]. Around 40% of the young athletes with LAS may develop chronic ankle instability (CAI) [4]. CAI is characterized by multiple mechanical and sensoriomotor dysfunctions such as plantar flexor, inversion and eversion muscles weakness [5], restricted ankle joint dorsiflexion [6], joint position sense [7] and force sense impairments [8], static [9] and dynamic [10] balance alterations, and kinetic and kinematic changes [11]. Despite of mechanical dysfunctions in patients with CAI are widely investigated, a high proportion of young athletes do not fully recover after a LAS and have residual and chronic symptoms such as pain, swelling, ankle joint giving-way or recurrent injury [12]. CAI is therefore still considered a problem for which more research is needed.

Among the aspects that remain underexplored in the literature about CAI is the foot core. IFMs together with the extrinsic foot muscles form the active subsystem of the foot core, which plays a key role in the kinetics and kinematics of the lower limbs. In fact, a dysfunction of the IFMs can cause alterations in postural control and stability, changes in the distribution of plantar pressures, decreased foot stiffness, decreased mechanical efficiency during gait, and loss of motor control in the rearfoot and midfoot [13]; all these disorders can lead to increase the risk of developing LAS, which can result in the development of CAI. The role of the IFMs is also critical in foot hyper-pronation control [14]. Associations between functional or structural deficit of the muscles with the tendency to pronate the foot has been found [15–17]. There is strong evidence showing the relevance of IFMs in some conditions, such as talalgia [18], plantar fasciitis [19], Achilles tendinopathy [20], hallux valgus [21, 22] and other toe deformities [23] and sport injuries [24].

However, the role of IFMs in CAI has not yet been sufficiently studied and the results of some of the research are contradictory; that is, some authors have found differences in the morphology or function of IFMs between healthy subjects and those with CAI [25], while other studies have not described these differences [26]. These differences may be due to multiple factors, such as the characteristics of the study sample or the use of different methods of muscle assessment or the evaluation of strength through isometric or concentric contractions. There is also no agreement in the literature on which muscles of the foot are most important in CAI. Moreover, there is a lack of evidence about the morphology and function of IFMs of young athletes with CAI, which is the population group that has the highest incidence and prevalence rate of LAS and CAI. Further research on this issue may therefore be necessary. The hypothesis of the present study is that there would be morphological and functional deficits in the IFMs in athletes with CAI compared to healthy athletes. In this case, it could be considered to include IFMs training in the treatment of patients with CAI. Thus, the aim of this study was to compare IFMs morphology and function of abductor hallucis (AbH), flexor hallucis brevis (FHB), abductor digiti-minimi (AbDM), quadratus plantae (QP) and flexor digitorum brevis (FDB) between young athletes with and without CAI; these muscles were selected for assessment due to their importance in foot stability and mechanics and their ease of ultrasound examination. A secondary objective was to study associations between the foot morphology (assessed by Foot Posture Index-6 - FPI) and the morphology or function of the.

IFMs. These correlations are analyzed independently in athletes with CAI and healthy counterparts.

Methods

Design

A cross-sectional case-control study was conducted between January 2022 and March 2023. The study was designed following the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) Statement Guide [27]. Before patients´ enrolment, the Ethical Committee of the Hospital Clínico San Carlos (20/478-E) and the Ethical Committee of the European University of Madrid (CIPI/20/139) approved the study. The study was carried out following the Declaration of Helsinki (2013) and all the participants read the informant consent, agreed to participate, and signed the informed consent by themselves or by their parents or guardians depending on the age of the participant.

Sample size calculation

The sample size calculation analysis was performed using G*Power version 3.1.9.2 (G*Power©, University of Dusseldorf, Germany). The expected effect size was based on previous studies reporting medium effects for ultrasound assessments [15], corresponding to an estimated Cohen’s d of 0.501, for a one-tail hypothesis, a between group proportion of N/n = 1, and a power (1-β error probability) of 0.80 with an α level of 0.05, the minimum sample size required was 100 participants.

Participants

Patients´ recruitment was carried out between January 2022 and October 2022 in the Hospital of IMSKE (Valencia, Spain) following a sequential sampling method. A hundred young handball players participated in the study (50 athletes with CAI and 50 age-matched athletes without CAI).

Inclusion criteria

The sample consisted of handball players aged between 13 and 17 years, who practiced handball at least six hours per week. This sample was used because of the high incidence of LAS in this age group and because their physical characteristics may differ from those of adults. Participants were divided into two groups: the case group, which included young handball players with CAI, and the control group that included age and sex-matched athletes without CAI.

• Case group: Young handball players were included in the case group if they met the inclusion criteria stated by the International Ankle Consortium [28]: (1) history of at least one significant LAS in the previous 12 months or more prior to the study and resulted in inflammation and impaired physical activity; (2) the most recent LAS must have occurred more than three months prior to the study enrolment; (3) two or more episodes of ankle giving-way, or recurrent LAS or feelings of instability at the injured ankle in the previous six months; (4) a score of 24 or less in the Cumberland Ankle Instability Tool (CAIT); (5) in athletes with unilateral CAI, homolateral foot was assessed, while in athletes with bilateral CAI only one foot was selected by randomization in order to avoid systematic selection bias.

• Control group: Young handball players were required to meet the following criteria: (1) had no history of LAS or ankle instability; (2) were considered as coper (individual who is more than 12 months removed from the index ankle sprain, reports no or very minimal symptoms or deficit in self-reported function and perceives fully recovery) [29]; (3) had a score higher than 24 in the CAIT. As in subjects with bilateral CAI, only one foot was assessed, which was randomly selected.

Exclusion criteria

The exclusion criteria were: (1) young handball players with history of previous surgeries in the foot, ankle or other regions of the lower limb; (2) any acute injury of other musculoskeletal structures of the lower extremity (such as sprains, strains and/or fractures) in the previous three months; (3) or young handball players with visual, vestibular or neurological impairments that could influence their balance.

Socio-demographic and descriptive data

Descriptive data comprised sex (male or female), age, body mass index (BMI), playing position (goalkeeper, wing, back or line), use of foot insoles, use of ankle bandage or ankle brace and CAIT score.

Outcome measures

In this study, an ultrasound-based morphological and functional assessment of AbH, FHB, AbDM, QP and FDB was carried out. Morphology describes the muscle size at rest; thickness of AbH, FHB, AbDM, QP and FDB and cross-sectional area (CSA) of AbH, AbDM and FDB were measured (Fig. 1). On the other hand, function describes the capacity of muscle contraction; active contraction was request from each muscle (hallux abduction for the AbH, hallux flexion for the FHB, 5th toe abduction for the AbDM and lesser toes flexion for the FDB and QP). Participants were asked to perform a maximum active concentric contraction for five seconds; three repetitions were performed with a one-minute rest between them. Prior to this, a familiarization training session was conducted. Measures of active contraction were normalized to resting measures to obtain the activation ratio (AR) which is calculated using the formula (AR= active contraction measurement/resting measurement). If the value of the AR is greater than 1.0 indicates that muscle size increases when contracting [30].

Fig. 1.

Ultrasound image of intrinsic foot muscles. A: abductor hallucis thickness; B: abductor hallucis CSA; C: abductor digiti-minimi thickness; D: abductor digiti-minimi CSA; E: flexor hallucis brevis thickness; F: flexor digitorum brevis CSA; G: flexor digitorum brevis and quadratus plantae thickness

In terms of muscle function, ultrasound is considered as an indirect method of assessing muscle strength [22]. Moreover, ultrasound is a valid and reliable tool for the measurement of morphology and function of IFMs; according to several studies, ultrasound shows good to excellent test-retest (ICC 0.66–0.99), inter-tester (ICC 0.90–0.97) and intra-tester (ICC 0.79–0.97) reliability [31, 32].

A musculoskeletal ultrasound system with a 8-13 MHz linear probe (Logic-E BT11, General Electric Healthcare^®, Wisconsin, USA) was used in measuring IFMs thickness and CSA, both at rest and in active contraction. The ultrasound parameters used were as follows: B-mode image, frequency 12 MHz, depth 4 cm, dynamic range 75 and 2 foci; the parameters of gain and foci position were adjusted according to each subject and each muscle to be assessed. For all outcomes, three images at rest and three images in active contraction were captured [30]. In all ultrasound assessments, compression of the probe on the subject´s skin was avoid. All ultrasound assessment were performed by the same assessor, with more than 10 years of experience in musculoskeletal ultrasound. Ultrasound examination was performed based on the protocol described by Mickle et al. [33].

Once all the ultrasound scans had been completed, measurement of muscle CSA and thickness were conducted. ImageJ software version 2.0 (United States National Institutes of Health; Betseda, MD, USA) was used. Only the muscle belly was included in the measurement of muscle size and the perimuscular fascial tissue was excluded [34, 35]. Finally, the mean value of the three resting measurements was calculated to obtain the outcomes ´´morphology`` while the mean value of the three active contraction measurements was used to calculate the AR, to obtain the outcomes ´´muscle function``.

FPI was used to analyze foot morphology. FPI is a reliable tool (CCI intra-tester 0.928–0.937) [36] that assesses the foot morphology by visual inspection and palpation of six anatomical landmarks, to which scores are assigned. Each reference is scored between − 2 and + 2, so the final result can range from − 12 to + 12. Depending on the final result, the foot morphology is defined as neutral (0 - +5), pronated (+ 6 - +9), supinated (-1 - -4), over-pronated (+ 10 - +12) or over-supinated (-5 - -12). The FPI-6 test was administered by the same evaluator who performed the ultrasound scan, with more than three years of clinical experience using FPI-6.

Statistical analysis

Statistical analyses were performed using the Statistical Package for the Social Sciences (SPSS) version 25.0 for Windows (IBM Corp., Armonk, NY, USA). Qualitative variables were summarized as frequencies and percentages, whereas quantitative variables were expressed as mean ± standard deviation.

Normality was assessed separately for each quantitative outcome variable using the Kolmogorov–Smirnov test, and homogeneity of variances was evaluated using Levene’s test. Between-group comparisons were conducted using Student’s t-test when assumptions were met. A p-value < 0.05 was considered statistically significant. To control the family-wise error rate associated with multiple comparisons, the Bonferroni correction was applied according to the number of comparisons performed within each family of tests. Effect sizes were calculated using Cohen’s d and Minimal Detectable Change (MDC).

Correlations between foot morphology and IFMs morphology and function were analyzed using Pearson’s correlation coefficient. Correlation analyses were conducted independently within the CAI group and within the control group to explore potential group-specific associations.

Results

Socio-demographic and descriptive data

One hundred and seven participants were initially recruited for the study. After the selection criteria screening, seven participants were excluded. Four presented lower limb surgeries and three had acute musculoskeletal injury in the ankle or foot regions in the previous three months. Finally, one hundred young handball players met all the selection criteria (CAI group, n = 50; control group, n = 50); analysis were performed on the final sample of 100 participants. These results are shown in Fig. 2.

Fig. 2.

Flow chart. Sample recruitment

No statistically significant differences were found for any demographic or clinical variables except for the use of bandages and the total CAIT score; so the other demographic and clinical variables cannot be interpreted as confounding factors. In the CAI group, 24% (n = 12) of the young handball players reported using ankle bandages for training and/or matches; while the control group used no bandages. Significant differences were also found in the score of the CAIT, which was also established as an inclusion criteria for defining CAI. The total CAIT score was 19.82 ± 4.26 for the CAI group and 28.64 ± 1.54 (p < 0.05) for the control group. The detailed description of the demographic and clinical variables are shown in the Table 1.

Table 1.

Demographic and clinical variables at baseline

Demographic and clinical variables	Control group	CAI group	Test	Significance (p value)
Sex, n (%)
Men	25 (50)	25 (50)
Women	25 (50)	25 (50)
Age (years)	14.02 (1.91)	14.16 (1.77)	t(98) = -0.38	0.7
BMI1 (Kg/cm2)	21.24 (3.93)	21.76 (2.84)	t(98) = -0.76	0.45
Position, n (%)			x2(3) = 1.72	0.63
Goalkeeper	3 (6)	6 (12)
Wing	16 (32)	12 (24)
Pivoter	10 (20)	9 (18)
Backcourt	21 (42)	23 (46)
Use of foot insoles, n			x2(2) = 0.45	0.8
(%)	42 (84)	40 (80)
Never†	4 (8)	4 (8)
Only for sport‡	4 (8)	6 (12)
Sport and daily live§
Bandage, n (%)			x 2 (2) = 13.64	< 0.01
Never	50 (100)	38 (76)
For matches	0 (0)	8 (16)
Training and matches	0 (0)	4 (8)
CAIT2	28.64 (1.54)	19.82 (4.26)	t (98) = 13.79	< 0.01

¹BMI Body Mass Index, ²CAIT Cumberland Ankle Instability Tool

† athletes who have never used foot insoles

‡ athletes who only use foot insoles to play handball

§ athletes who use foot insoles both for sport and daily live

Bold values indicate between-group statistically significant differences (p < 0.05)

Outcomes

The results showed statistically significant differences between athletes with CAI and healthy athletes in AbH morphology (CAI group, mean CSA 2.14cm2; control group, mean CSA 1.84cm2; p = 0.013) and in FHB morphology (CAI group, mean thickness 1.31 cm; control group, mean thickness 1.41 cm; p = 0.005). Cohen’s d values for these variables were d = 0.62 (CI 95%= 0.09;0.89) for AbH CSA and d = 0.49 (CI 95%= 0.11;0.91) for FHB thickness. These Cohen’s d values represent a medium effect size. No statistically significant differences were found in the rest of IFMs morphology neither in muscle function outcome variables. Furthermore, these variables show a small effect size, and the differences cannot be considered clinically relevant according to the MDC. Regarding foot morphology, there are also no significant differences between groups. The mean value of the FPI-6 shows a neutral foot morphology, both in healthy players (FPI-6 score, 4.26 ± 2.66) and those with CAI (FPI-6 score, 4.24 ± 2.62). Cohen’s d values represent a low effect size (d = 0.01, CI 95%=-0.40;0.38). Furthermore, the MDC value showed that the difference between groups is also not clinically relevant (MDC 1.94). Table 2 shows the between-group comparison for all the outcome variables.

Table 2.

Ultrasound and foot morphology outcomes (between-group differences)

Outcome variable	Control Gr Mean (SD)	CAI Gr Mean (SD)	Means differences	Significance (p value)	D Cohen (CI, 95%)	MDC	SEM
AbH-T (M)	1.08 (0.18)	1.13 (0.22)	0.05	0.21	0.25 (-0.14;0.64)	0.1	0.03
AbH-T (F)	1.05 (0.06)	1.04 (0.05)	-0.01	0.3	-0.18 (-0.57;0.21)	0.03	0.01
AbH-CSA (M)	1.84 (0.55)	2.14 (0.67)	0.3	0.01	0.62 (0.09;0.89)	0.29	0.11
AbH-CSA (F)	1.06 (0.05)	1.06 (0.05)	0.00	0.93	0 (-0.39;0.39)	0.02	0.01
AbDM-T (M)	1.11 (0.2)	1.15 (0.22)	0.04	0.28	0.19 (-0.2;0.58)	0.1	0.04
AbDM-T (F)	1.06 (0.06)	1.07 (0.06)	0.01	0.54	0.17 (-0.23;0.56)	0.03	0.01
AbDM-CSA (M)	1.5 (0.44)	1.66 (0.47)	0.16	0.07	0.35 (-0.04;0.75)	0.22	0.08
AbDM-CSA (F)	1.15 (0.08)	1.16 (0.1)	0.01	0.7	0.11 (-0.28;0.5)	0.04	0.02
FDB-T (M)	0.81 (0.16)	0.83 (0.15)	0.02	0.57	0.13 (-0.26;0.52)	0.07	0.03
FDB-T (F)	1.12 (0.1)	1.14 (0.09)	0.02	0.37	0.21 (-0.18;0.6)	0.05	0.02
FDB-CSA (M)	1.79 (0.5)	1.89 (0.51)	0.1	0.25	0.2 (-0.19;0.59)	0.24	0.09
FDB-CSA (F)	1.1 (0.05)	1.09 (0.06)	-0.01	0.17	-0.18 (-0.57;0.21)	0.03	0.01
QP-T (M)	0.81 (0.16)	0.84 (0.17)	0.03	0.43	0.18 (-0.21;0.57)	0.08	0.03
QP-T (F)	1.08 (0.12)	1.11 (0.15)	0.03	0.35	0.22 (-0.17;0.61)	0.07	0.02
FHB-T (M)	1.41 (0.22)	1.31 (0.17)	-0.1	0.05	0.49 (-0.91;-0.1)	0.09	0.03
FHB-T (F)	1.08 (0.06)	1.08 (0.08)	0.01	0.52	0 (-0.39;0.39)	0.03	0.01
FPI-6	4.26 (2.66) -N = 26 -P = 21 -S = 3	4.24 (2.62) -N = 27 -P = 19 -S = 4	0.02	0.97	0.01 (-0.4;0.38)	1.94	0.7

SD standard deviation, T thickness, CSA cross-sectional área, Gr group, MDC minimal detectable change, SEM standar error of measurement, M morphology (thickness-cm, CSA-cm2), F function (ratio), AbH abductor hallucis, AbDM abductor digiti-minimi, FDB flexor digitorum brevis, QP quadratus plantae, FHB flexor hallucis brevis, FPI Foot Posture Index, N neutral, P pronated, S supinated

Bold values indicate between-group statistically significant differences (p < 0.05)

Regarding the correlations between foot morphology and IFMs morphology and function, a significant positive association was only found between foot morphology and AbH function in young athletes with CAI. In the experimental group, subjects with higher AR of the AbH tended to have a more supinated foot. However, this significant association is weak (r = 0.32; p = 0.025). Table 3 shows correlations between foot morphology and all the IFMs variables.

Table 3.

Correlations between ultrasound variables and FPI-6

IFMs morphology (control group)	AbDM (T)	AbDM (CSA)	AbH (T)	AbH (CSA)	FDB (T)	FDB (CSA)	FHB (T)	QP (T)
FPI (Pearson C.)	-0,038	-0,126	0,057	-0,1	-0,086	-0,191	0,062	-0,031
FPI (p value)	0,796	0,382	0,692	0,488	0,552	0,183	0,668	0,829
IFMs function (control group)	AbDM (T)	AbDM (CSA)	AbH (T)	AbH (CSA)	FDB (T)	FDB (CSA)	FHB (T)	QP (T)
FPI (Pearson C.)	0,102	-0,048	-0,173	-0,22	-0,02	-0,24	-0,094	-0,169
FPI (p value)	0,482	0,743	0,229	0,109	0,888	0,093	0,515	0,24
IFMs morphology (CAI group)	AbDM (T)	AbDM (CSA)	AbH (T)	AbH (CSA)	FDB (T)	FDB (CSA)	FHB (T)	QP (T)
FPI (Pearson C.)	0,016	0,051	0	0,119	-0,01	0,033	-0,03	-0,019
FPI (p value)	0,91	0,727	1	0,41	0,945	0,821	0,835	0,896
IFMs function (CAI group)	AbDM (T)	AbDM (CSA)	AbH (T)	AbH (CSA)	FDB (T)	FDB (CSA)	FHB (T)	QP (T)
FPI (Pearson C.)	0,074	-0,063	-0,019	0,318	-0,064	-0,053	-0,069	-0,006
FPI (p value)	0,61	0,663	0,897	0,025	0,657	0,715	0,635	0,969

IFMs intrinsic foot muscles, FPI foot posture index, CAI chronic ankle instability, T thickness, CSA cross-sectional area, C coefficient, AbH abductor hallucis, AbDM abductor digiti-minimi, FDB flexor digitorum brevis, QP quadratus plantae, FHB flexor hallucis brevis

Bold values indicate between-group statistically significant differences (p < 0.05)

Discussion

The aim of this study was to compare the morphology and function of the IFMs between young handball players with and without CAI. The results of this study only showed statistically significant differences in AbH and FHB morphology (athletes with CAI had higher resting AbH CSA and lower resting FHB thickness than healthy counterparts). For AbH (CSA) and FHB (thickness) morphology, the difference between groups may also be clinically relevant, since the MCD is greater than the difference in means in both cases. Furthermore, in both cases, a medium and significant effect size was reported, according to Cohen’s d at CI 95%. No significance between-groups differences in AbH thickness, AbDM CSA and thickness, FDB CSA and thickness and QP thickness were found. Regarding muscle function, no differences were found in any of the variables measured. Furthermore, in all variables, except for AbH (CSA) and FHB (thickness) morphology, the means differences between groups does not exceed the MDC, suggesting that these differences are not clinically relevant. For descriptive data, significance between-group differences were found in the use of ankle bandage and in the CAIT score. The difference in the use of ankle bandage could be explained by the fact that healthy players, with no history of ankle sprains, have no need to use taping. Whereas, 24% of the athletes in the CAI group usually use ankle bandage to prevent reinjuries. The between-group difference in CAIT score can be explained by the fact that a cut-off score is defined as an inclusion criterion. Subjects in the CAI group had to score 24 points or less; while subjects in the control group had to score more than 24 points. This means that athletes with CAI perceive less ankle stability than healthy athletes. The results of this study suggests that IFMs morphology and function may not be as relevant in young handball players with CAI as in other injuries. These results are in accordance with previous studies that found no differences in IFMs morphology and function when comparing a sample of active young adults with ankle pathology to healthy counterparts [30].

IFMs morphology

Regarding the findings of this study, the increased AbH CSA in subjects with CAI could be explained as an adaptive mechanism to compensate for joint instability; however, this morphological variation does not translate into improved function, as the AR is not better than in the control group; therefore, this compensatory mechanism to provide stability would not be effective. This result is consistent with those reported by Tas et al. [37] who reported that healthy subjects with less static stability in monopodal stance had a larger AbH on ultrasound assessment. Therefore, it may be interesting to further study the compensatory role that AbH may have on stability.

Regarding the reduction in FHB thickness in individuals with CAI, our findings are consistent with those of Feger et al. [5], who showed that subjects with CAI have less FHB volume than a group of healthy controls, measuring muscle size on Magnetic Resonance Imaging. Considering these results, it is reasonable to assume that atrophy of the FHB may play an important role in CAI. Other studies have also shown the relevance of this muscle in other foot and ankle pathologies. Romero Morales et al. [20] found significant differences in FHB thickness between healthy subjects and those suffering from Achilles tendinopathy (healthy 11.96 ± 2.97 mm; tendinopathy 10.11 ± 1.17 mm; p < 0.001). The results of this and previous studies seems to suggest that FHB morphology may present a particular relevance in diverse foot and ankle conditions; thus, it might be interesting to focus future studies on this muscle. Regarding the results achieved in this study, the morphology of the IFMs is not relevant to the development of CAI and no correlation between morphology and motor function of IFMs can be considered. Consistent with the findings of this study, Okamura et al. [38] demonstrated that IFMs strengthening produce several improvements in functional variables, such as foot morphology and gait kinematic parameters, with no change in muscle volume. Yin et al. [39] found no significant correlations between the morphology of the AbH, AbDM, FDB, QP and FHB with toes flexion strength. Kusagawa et al. [40] described that toes flexion strength was only significantly associated with adductor hallucis and FDB CSA, but not with FHB, QP, AbH and AbDM muscles size. In view of these results, it is reasonable to assume that stability may depend more heavily on the neural subsystem of the foot core and on the proprioceptive function of IFMs than on the muscles motor function. In this sense, some studies have shown that plantar skin sensitivity training produces improvements in static and dynamic stability [41, 42].

IFMs function

In terms of IFMs function, the AR of muscles ranges from 1.04 to 1.16; AbDM is the muscle with the highest activation index. These results are similar to those reported by Lee et al. [43] whose AR range from 1.03 to 1.16. Our results partially coincide with those shown by Fraser et al. [30] in which significant differences were found in the AR of the QP between healthy and CAI subjects (control 1.13, CAI 1.05, p = 0.08); however, no between-group differences were found in the AR of AbH, FDB and FHB. This finding supports the previously mentioned suggestion that the motor function of IFMs is not relevant in CAI. However, it should be noted that the methodological limitations of the study could explain the absence of differences between groups; therefore, further research in this area is needed.

AbH function is the only variable that has a significant correlation with foot morphology. This, together with the fact that there are between group differences in AbH morphology, may suggest that this muscle is of particular relevance in foot and ankle biomechanics. The absence of correlations between muscle morphology or function and foot morphology may be due to the homogeneity that exists in relation to foot morphology. As shown in Table 2, both the control and experimental groups have practically the same number of subjects with neutral feet, pronated feet, and supinated feet.

Limitations and strengths

This study presents several limitations. First, only young handball players were included, so the results cannot be extrapolated to other groups; furthermore, biological maturity was not taken into account, only the chronological age of the participants. Second, the cross-sectional design does not allow associating the cause-effect of the differences achieved. Third, a consecutive sampling may be more appropriate than random selection for the sample recruitment process. Fourth, foot dominance was not considered, which may have influenced the observed morphological and functional differences. Fifth, no gender analysis of the data has been carried out. Sixth, an intra-rater reliability for the ultrasound assessments of IFMs was not carried out. Finally, the difficulty in performing specific movements of the toes, especially abduction of the hallux and fifth toe [44], must be taken into account. This deficit in activation capacity should be considered as a confounding factor that could influence the functionality results of the MIP.

The main strengths of this study are the use of a large and homogeneous sample size; assessment of IFMs by ultrasound following described valid and reliable protocols; and the fact of including functional and not only morphological IFMs assessment.

Perspectives

Current findings do not demonstrate a clear association between IFM motor function and CAI; therefore, the need for IFMs strength training in subjects with CAI could not be justified. However, due to the study design, this causal relationship cannot be established. Therefore, clinical trials determining the efficacy of IFM training in patients with CAI would be necessary. Moreover, the results of the present study may have an impact on future research that focuses on assessing the role of the AbH and FHB muscles in athletes with CAI; in the present study, between groups differences have been found, but it´s unknown to what extent they may be relevant for the development of CAI. In addition, this study has only considered the motor function of the IFMs. According to the model proposed by McKeon et al. [13], the functionality of the foot core depends not only on the IFMs, but also on the extrinsic foot muscles and the passive and neural subsystems. Therefore, further research is needed to determine the role of these elements in CAI, with special focus on proprioceptive role of the IFMs and plantar cutaneous sensitivity. It may also be interesting to assess the association between perceived ankle instability (evaluated using CAIT) and the IFMs morphology and function in future studies.

Conclusions

Statistical differences were only found in AbH CSA at rest (in favour of the CAI group) and in FHB thickness at rest (in favour of the control group) with an effect size small-medium, between handball players with CAI and healthy counterparts. Positive correlation was found between the AR of AbH and foot morphology. No significant differences were found in the rest of outcome variables. These results should be considered for the clinical relevance of foot core training in subjects with CAI.

Abbreviations

IFMs

Intrinsic foot muscles

LAS

Lateral ankle sprain

CAI

Chronic ankle instability

AbH

Abductor hallucis

FHB

Flexor hallucis brevis

AbDM

Abductor digiti minimi

QP

Quadratus plantae

FDB

Flexor digitorum brevis

FPI

Foot Posture Index

STROBE

Strengthening The Reporting of Observational studies in Epidemiology

CAIT

Cumberland Ankle Instability Tool

BMI

Body Mass Index

CSA

Cross-sectional area

AR

Activation ratio

Authors’ contributions

D.G.G, C.C.L. and D.R.S. participated in data collection and execution of the field study. M.A.G, R.L.R. and I.L.R. designed the study´s protocol and carried out the data processing. L.C.L, J.J.A.O. and S.J.B. drafted the manuscript that subsequently was commented, revised and approved by all authors.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or non-for-profit sectors.

Data availability

Raw data could be sent after prior request to the authors.

Declarations

Ethics approval and consent to participate

The study was conducted in accordance with the Declaration of Helsinki and approved by Ethics Committee of Hospital Clínico San Carlos (20/478-E) and European University of Madrid (CIPI/20/139). All subjects signed the informed consent.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.