Physical Therapy Korea

In Taekwondo, an Olympic sport, points are earned by striking the opponent using various parts of the body, and the result of the match is determined based on these points [1]. Movements of the lower extremities (LE) in Taekwondo competitive matches are performed repeatedly and in a complex process within a short period [2]. The complex skills in Taekwondo consist of various movements, such as the double kick, axe kick, direction change, footwork, and roundhouse kick, and these movements are primarily executed and controlled by the LE [3]. When performing fast movements and sudden changes, the foot and ankles require stability and balance to maintain the dynamic posture of the body [4]. Among various sports, martial arts athletes use their LE much more frequently during training and competitive matches [5]. In Taekwondo, LE is one of the body parts with the highest injury rate, and 46% of athletes have reported experiencing a recurrence of LE injuries [6,7]. Previous studies have reported that the anatomical parts with the highest risk of injury in Taekwondo athletes are the foot and ankle [8-11].

Ankle sprains are one of the most common ankle injuries that occur during competitive sports and vigorous leisure activities [12]. Lateral ankle sprains (LAS) constitute approximately 80% of all ankle sprains and occur during sports activities with or without contact, often resulting from landing or changing direction [13,14]. The frequent occurrence of LAS is associated with a combination of plantarflexion and inversion with excessive supination of the ankle [15]. When the LAS occurs, the anterior talofibular ligament is often injured [16]. Among Taekwondo athletes, the most commonly injured areas include the anterior talofibular ligament (63%), bruising (10%), fractures (10%), and joint dislocations (7%) [16]. The rigorous demands of elite-level Taekwondo, both physiologically and psychologically, can lead to perceived exertion and reduced coordination due to the forceful combat skills used in sports [17]. Accordingly, as previously demonstrated in research, muscle imbalances experienced in contact sports decrease postural stability, leading to recurrent ankle sprains and associated ligament injuries [17,18]. Repeated LAS results in decreased muscle strength and ankle joint range of motion and reduced proprioceptive sensitivity, pain, and swelling, leading to chronic ankle instability (CAI) [15,16]. Among these factors, control of the muscles in the ankle and foot is necessary to cope with rapid movements of the ankle [19]. Other studies have focused on measuring the strength of ankle invertor and evertor. Willems et al. [20] and Donnelly et al. [21] reported that individuals with CAI had significantly decreased ankle evertor strength compared to that in healthy participants. According to Ahn et al. [22], the CAI group reported significantly reduced evertor strength and muscle activity during ankle eversion with toe flexion compared to that in the healthy group. In a study of Taekwondo athletes, the strength of the ankle evertor on the CAI side was lower than that on the non-side in isokinetic strength testing [23]. Both the ankle and toe muscles contribute to the inversion and eversion of the foot at the subtalar joint. The peroneus longus (PL), peroneus brevis (PB), and tibialis anterior (TA) are the primary muscles involved in ankle eversion and inversion [24,25]. In the event of an ankle injury, damage to the primary evertor and invertor muscles adjacent to the ankle results in compensatory actions by muscles such as the extensor digitorum longus (EDL), flexor digitorum longus (FDL), and flexor hallucis longus (FHL), which are other toe extensor and flexor muscles that contribute to ankle eversion and inversion of the ankle [22,24,25]. Therefore, to assess the ankle function of a participant with CAI, the ankle and toe positions must be modified to measure prime invertor and evertor strength [22,26].

Previous studies have measured ankle strength in athletes and individuals with CAI using various methods. However, to our knowledge, no detailed research has been conducted on the specific position of the ankle and toe during ankle muscle strength measurement in Taekwondo athletes. Given the continuous stepping techniques, rapid changes in movement, and striking the opponent player with the feet during Taekwondo sparing, investigating the characteristics of foot and ankle muscle strength in athletes with CAI can provide clinical guidelines for ankle rehabilitation, including strength measurements and training.

Therefore, this study aimed to compare the strength and strength ratios of ankle invertors and evertors in different ankle and toe positions between the CAI and uninjured sides in Taekwondo athletes. We hypothesized that the CAI side would demonstrate decreased evertor strength, particularly in the toe flexion position, and a higher invertor-to-evertor strength ratio than that on the uninjured side.

1. Participants

Fifteen male participants with CAI among university Taekwondo athletes participated in this study. To determine whether this sample size had adequate power, a prior analysis was conducted using the G*Power 3.0 program (Franz Faul, Kiel University). Analysis with an effect size of 1.04 and α of 0.05 showed a high power of 0.99. This study recruited participants who had experienced a unilateral ankle sprain at least a year ago, had not injured their ankle within 6 weeks of the experiment, scored < 85% on the Foot and Ankle Ability Measure Sports scale, and scored > 11 on the Identification of Functional Ankle Instability scale [21]. Furthermore, individuals with neurological diseases and sensorimotor dysfunction who experienced ankle sprains or previous lower extremity surgery were excluded from the study. The participants’ detailed demographics are shown in Table 1. Ethical approval for this study was obtained from the Institutional Review Board of Yonsei University Mirae campus (IRB no. 1041849-202302-BM-031-01). All participants completed a consent form after being informed of the study’s methods and purposes.

Table 1 . Participant demographics (N = 15).

Variable	Value
Age (y)	21.47 ± 0.52
Height (cm)	178.73 ± 5.48
Body mass (kg)	75.60 ± 14.22
CAI side (n)
Dominant/nondominant	9/6
FAAM-Sports (%)	69.67 ± 8.50
IdFAI	22.20 ± 5.17

Values are presented as mean ± standard deviation or number only. CAI, chronic ankle instability; FAAM, Foot and Ankle Ability Measurement; IdFAI, Identification of Funcational Ankle Instability..

2. Instrumentation

The ankle muscle strength was measured using a Smart KEMA Strength Sensor (KOREATECH Inc.). This sensor consists of a 5-cm-wide strap, tensiometer, nonelastic belt, and absorber that can serve as a fixed point when attached to the floor. The tensiometer of the sensor measured the force pulled in both directions. The measurement range was 0–1960 N [27]. After attaching an absorber to the floor to measure muscle strength, a strap was applied to the participant’s forefoot to adjust the tension of the non-elastic belt. The initial tension was adjusted to 2 kg to ensure that all participants could be measured under the same conditions [28]. The Smart KEMA Strength Sensor has good-to-high intra- and inter-rater reliabilities (ICC_3,1 > 0.85, ICC_2,1 > 0.85) [27].

3. Procedure

After the participant assumed a side-lying position, the forefoot was removed from the table. The hips and knees were then flexed to 90°. The participants were instructed to assume the assigned foot and ankle position and perform eversion or inversion with maximum force. During the measurement of the ankle muscles, the investigator fixed the participant’s distal tibial component to prevent hip and knee joint compensation (Figure 1). The participants measured ankle muscle strength at the following eight ankle and toe positions in random order: (1) Dorsiflexion with toe extension and eversion (DTEE) (Figure 2A); (2) dorsiflexion with toe flexion and eversion (DTFE) (Figure 2B); (3) plantarflexion with toe extension and eversion (PTEE) (Figure 2C); (4) plantarflexion with toe flexion and eversion (PTFE) (Figure 2D); (5) dorsiflexion with toe extension and inversion (DTEI) (Figure 2E); (6) dorsiflexion with toe flexion and inversion (DTFI) (Figure 2F); (7) plantarflexion with toe extension and inversion (PTEI) (Figure 2G); and (8) plantarflexion with toe flexion and inversion (PTFI) (Figure 2H). The ankle position was set to 10° in dorsiflexion and 50° in plantarflexion [21]. A standard goniometer was used to confirm the ankle position. The angle of flexion or extension of the toe was set as a self-selected comfortable angle. The order of measurement of the eight ankle and toe positions was randomized using site (www.randomizer.org). Before ankle strength measurement, the participants practiced the movements to familiarize themselves with each ankle and toe position. After a 5-minute rest, ankle muscle strength was measured. The ankle muscle strength was measured for 5 seconds at maximum force, and the average value of three measurements for each posture was used as the data [22]. A 3-minute break was provided between each measurement posture to prevent muscle fatigue.

Figure 1. A position using the SMART KEMA Strength Sensor (KOREATECH Inc.) to measure muscle strength in the ankle joint.

Figure 2. Ankle and toe positions for measuring ankle muscle strength. (A) Dorsiflexion with toe extension and eversion, (B) dorsiflexion with toe flexion and eversion, (C) plantarflexion with toe extension and eversion, (D) plantarflexion with toe flexion and eversion, (E) dorsiflexion with toe extension and inversion, (F) dorsiflexion with toe flexion and inversion, (G) plantarflexion with toe extension and inversion, and (H) plantarflexion with toe flexion and inversion.

4. Data Analysis

The ankle muscle strength was measured using the Smart KEMA application. The average value of the middle 3-second of the data collected during the 5-second ankle muscle strength measurement was used [21].

5. Statistical Analysis

A normal distribution of data was confirmed using the Shapiro–Wilk test. The paired t-test was used to compare the difference in ankle muscle strength between the CAI side and the uninjured side and the ratio of invertor and evertor strength on both sides. The level of significance was set at p < 0.05. Statistical analyses were performed using IBM SPSS ver. 25.0 (IBM Co.).

All data were normally distributed. The results of this study showed that the ankle evertor strength was significantly lower on the CAI side than that on the uninjured side across all tested ankle and toe positions (p < 0.05). The greatest deficit in evertor strength was observed in the PTFE position. In contrast, no significant differences were observed in inverter strength between the CAI and uninjured sides (p > 0.05) (Table 2).

Table 2 . Difference between CAI and non-CAI sides of Taekwondo athletes ankle muscle strength measurements.

Motion	CAI	non-CAI	t	p-value
DTEE	12.98 ± 4.21	17.36 ± 5.87	–2.95	0.010*
DTFE	11.26 ± 4.17	15.82 ± 3.81	–3.83	0.002*
PTEE	10.58 ± 3.96	15.66 ± 4.67	–3.73	0.002*
PTFE	9.26 ± 3.38	13.27 ± 4.07	–4.04	0.001*
DTEI	13.45 ± 4.65	15.86 ± 3.82	–1.80	0.093
DTFI	13.88 ± 3.42	15.78 ± 4.25	–1.68	0.114
PTEI	14.20 ± 3.34	15.02 ± 3.98	–0.79	0.442
PTFI	13.68 ± 3.58	14.45 ± 2.59	–0.85	0.407

Values are presented as mean ± standard deviation. CAI, chronic ankle instability; DTEE, dorsiflexion with toe extension and eversion; DTFE, dorsiflexion with toe flexion and eversion; PTEE, plantarflexion with toe extension and eversion; PTFE, plantarflexion with toe flexion and eversion; DTEI, dorsiflexion with toe extension and inversion; DTFI, dorsiflexion with toe flexion and inversion; PTEI, plantarflexion with toe extension and inversion; PTFI, plantarflexion with toe flexion and inversion. *Significant differences at p < 0.05..

Furthermore, analysis of ankle invertor-to-evertor strength ratios revealed significant differences between the CAI and uninjured sides in DTFE, PTEE, and PTFE positions (p < 0.05). The CAI side demonstrated higher invertor-to-evertor strength ratios in these positions than that on the uninjured side (Table 3).

Table 3 . Difference between CAI and non-CAI sides of Taekwondo athletes ankle invertor and evertor strength ratio.

Motion	CAI	non-CAI	t	p-value
DTEI/DTEE	1.14 ± 0.56	0.98 ± 0.35	0.91	0.377
DTFI/DTFE	1.36 ± 0.56	1.04 ± 0.33	2.17	0.048*
PTEI/PTEE	1.50 ± 0.64	1.03 ± 0.41	2.49	0.026*
PTFI/PTFE	1.63 ± 0.65	1.15 ± 0.29	2.58	0.022*

Values are presented as mean ± standard deviation. CAI, chronic ankle instability; DTEE, dorsiflexion with toe extension and eversion; DTFE, dorsiflexion with toe flexion and eversion; PTEE, plantarflexion with toe extension and eversion; PTFE, plantarflexion with toe flexion and eversion; DTEI, dorsiflexion with toe extension and inversion; DTFI, dorsiflexion with toe flexion and inversion; PTEI, plantarflexion with toe extension and inversion; PTFI, plantarflexion with toe flexion and inversion. *Significant differences at p < 0.05..

The findings of this study highlight the importance of considering the ankle and toe positions when assessing ankle muscle strength in Taekwondo athletes with CAI. The significant reduction in evertor strength across all tested positions on the CAI side suggests the global weakness of the peroneal muscles, which play a crucial role in providing lateral ankle stability. The most pronounced evertor strength deficit in the PTFE position may be attributed to the decreased ability of the peroneal muscles to counteract the combined plantarflexion and inversion forces, which are common mechanisms of LAS.

A previous study investigating the ankle evertor muscle strength in CAI with patients showed that the ankle eversion with toe flexion in the ankle joint neutral position was significantly lower in the CAI group than that in the healthy group (CAI: 15.33 ± 4.58 kgf/kg; healthy: 21.37 ± 3.43 kgf/kg) [22]. Other studies have reported that judo athletes have significantly lower ankle evertors 60 °/s isokinetic strength on the side with CAI compared to that on the healthy side (CAI side: 21.15 ± 3.05 kgf/kg; healthy side: 25.46 ± 6.92 kgf/kg) [19]. The results of this study demonstrated that evertor strength in all ankle and toe positions on the CAI side was lower than that on the uninjured side. Ankle eversion in the plantarflexion with toe flexion group showed the lowest strength. These results are related to the functions of the PL, PB, FDL, and FHL. PL and PB are the plantar flexor and ankle evertor, respectively. The FDL and FHL are ankle plantar flexor, invertor, and toe flexor. During the measurement of evertor strength in the toe flexion position, the FDL and FHL may work against the evertor, potentially reducing evertor strength in this position [29]. Therefore, the weakness of the ankle evertor in participants with CAI and the function of the FDL and FHL may explain why the eversion strength was lower in the PTFE position on the CAI side than that on the uninjured side in this study.

A significant difference was observed in the ratios of the ankle invertor and evertor strengths in the dorsiflexion with toe flexion, plantarflexion with toe extension, and plantarflexion with toe flexion positions. The altered invertor-to-evertor strength ratios observed in the DTFE, PTEE, and PTFE positions further support the notion of muscular imbalance in Taekwondo athletes with CAI. The higher ratios on the CAI side indicate the relative weakness of the evertor compared with the inverters, potentially increasing the risk of recurrent ankle sprains and prolonged instability. Other studies have reported similar results. Milanezi et al. [30] reported that the CAI group showed an invertor-to-evertor ratio that was 28% less than that of the control group during maximal isokinetic contractions during eversion at a speed of 60°. Pontaga [31] reported that the ratio of ankle invertor-to-evertor muscle torque in handball players with recurrent lateral ligament sprains at 60°, 90°, and 120° was significantly lower than that in uninjured joints. Several studies reported no significant differences in ankle evertor and invertor strength ratios among patients with CAI [20,32]. However, these studies did not consider ankle and toe positions when measuring ankle muscle strength. This may not have controlled for the compensatory actions of various foot muscles such as the FDL, FHL, and EDL when measuring ankle evertor strength. Therefore, according to the results of this study, ankle rehabilitation exercise programs for Taekwondo athletes with CAI should be designed by considering the ankle and toe positions. In this study, there was no significant difference between the DTEE and DTEI ratios. These results are related to the function of the EDL. The EDL comprises the ankle dorsiflexor, toe extensor, and evertor. When measurements are made in the toe extension and ankle dorsiflexion positions, such as DTEE and DTEI, the EDL is predominantly used to compensate for the injured PL and PB [22].

The results of this study have important implications for the development of ankle rehabilitation programs for Taekwondo athletes with CAI. Clinicians and trainers should incorporate exercises that target the ankle evertor across various ankle and toe positions, with particular emphasis on the PTFE position. Additionally, addressing muscular imbalance by focusing on evertor strengthening and ensuring proper invertor-to-evertor strength ratios may help prevent recurrent ankle sprains and improve overall ankle stability. Future research should investigate the effects of targeted ankle rehabilitation programs that consider ankle and toe positions on clinical outcomes, functional performance, and injury prevention in Taekwondo athletes with CAI. Furthermore, exploring the role of other factors, such as proprioception, neuromuscular control, and biomechanics, in the context of CAI in Taekwondo athletes would provide a more comprehensive understanding of this condition.

This study had several limitations. First, it was limited to Taekwondo athletes with CAI. Therefore, it is difficult to generalize these results to other populations. Second, the performance of Taekwondo athletes was not considered. Follow-up studies should investigate how ankle strength, according to ankle and toe positions, is related to athlete performance. Third, ankle eversion muscle strength was measured during isometric contraction. Muscle function included isometric, concentric, and eccentric contractions. Therefore, it is difficult to explain the results of this study on the concentric and eccentric contractions of ankle strength in Taekwondo athletes with CAI. Finally, further research is needed to investigate the effects of an ankle rehabilitation exercise program that considers toe and ankle positions in Taekwondo athletes with CAI.

This study investigated the effects of various ankle and toe positions on the evertor muscle strength of Taekwondo athletes with CAI. The results showed that ankle evertor strength significantly decreased at all ankle and toe positions on the CAI side. Ankle evertor strength was reduced in ankle plantarflexion in the toe flexion position. In addition, significant differences were observed in the ankle invertor to evertor strength ratio in the dorsiflexion with toe flexion, plantarflexion with toe extension, and plantarflexion with toe flexion positions. These findings underscore the importance of considering the ankle and toe positions when assessing and rehabilitating ankle muscle strength in this population. Targeted ankle rehabilitation programs that address these specific deficits may help improve ankle stability, prevent recurrent sprains, and optimize the performance of Taekwondo athletes with CAI. Additionally, to selectively strengthen the PL and PB, it is necessary to prevent the compensatory participation of the EDL when strengthening the ankle eversion muscles in the toe flexion position.

None.

None to declare.

No potential conflicts of interest relevant to this article are reported.

Conceptualization: BK, UH, OK. Data curation: BK, UH, OK. Formal analysis: BK, UH, OK. Investigation: BK, UH, OK. Methodology: BK, UH, OK. Project administration: BK, UH, OK. Resources: BK, UH, OK. Software: BK, UH, OK. Supervision: BK, UH, OK. Validation: BK, UH, OK. Visualization: BK, UH, OK. Writing - original draft: BK. Writing - review & editing: BK, UH, OK.