1. Introduction
Lateral ankle sprain (LAS) is the most common sports injury in athletes, and 60% of high school and college athletes have experienced at least one LAS [
1,
2]. Although LAS is most commonly reported in competitive athletes, a very high prevalence has also been observed in the general population [
3]. LAS impacts various aspects of physical activity and can significantly affect health-related quality of life [
4]. The International Ankle Consortium defines LAS as an acute traumatic injury to the lateral ligament complex of the ankle due to excessive inversion of the rear foot or combined plantar flexion and inversion of the foot [
5]. LAS is characterized by a high recurrence rate, and a considerable number of patients progress to chronic ankle instability (CAI) [
6].
In a previous study, 30–40% of individuals who experienced LAS complained of subjective ankle instability that persisted even after time had elapsed, and it has been reported that they progressed to CAI within 1 year of the initial sprain [
1]. Patients with CAI typically present with recurrent ankle sprains as a result of mechanical instability (pathological relaxation after ankle ligament injury) and functional instability (proprioceptive deficit and neuromuscular dysregulation) [
5,
7]. CAI is characterized by repeated perception of the ankle giving way and presents with low self-reported function and symptoms such as general weakness of the ankle, pain, limited range of motion (ROM), and decreased function. These problems contribute to impairments associated with reduced health-related quality of life and deficits in sensory–motor control [
8].
To date, numerous interventions have been investigated to ameliorate mechanical and functional defects associated with CAI. Among them, various studies have proven that rehabilitation is an effective nonsurgical intervention to improve defects related to functional instability. Most studies have focused on improving ROM, eversion strength, dynamic balance, proprioception, and neuromuscular control that was directly applied to the ankle [
9,
10,
11,
12,
13]. Improvement of these biomechanical and functional factors is ultimately associated with the restoration of ankle strength, peroneal reaction time, and postural stability to pre-LAS levels in patients with CAI [
13].
In particular, postural stability is greatly affected by ankle biomechanics and function, as well as sensory–motor control and core stability. As the core muscles are at the center of the body, they maximize the connections in the upper and lower extremity motor chains that are involved in strength, balance, and movement control. This creates a stable basis for adequate muscle recruitment and timing in sensorimotor control [
14,
15]. Core stability is related to the body’s ability to control the trunk in response to internal and external disturbances. This includes forces generated by distal body parts and the ability to respond to unexpected perturbations [
16]. Therefore, it is important to improve core stability to treat persistent ankle pain and instability associated with recurrent sprains after LAS, given the various movements of the body that are required for sports and physical activity.
However, there are not many studies that have applied core training to CAI patients. One study reported that 8 weeks of core training increased the postural stability of CAI patients [
17], and a review paper also suggested that core muscles play a positive role in postural stability of CAI patients [
18]. Conversely, other studies have published results that show a low correlation between core strength and CAI [
19]. These results suggest that further research is needed.
Therefore, we conducted this study because core stability and CAI-related studies are lacking, and the results were different for each study. The research hypothesis was established that core stability training would improve the biomechanical and functional deficits associated with CAI. In this study, Pilates training was applied for core stabilization. Previous studies have reported that Pilates training is effective in stabilizing the core and improving posture control [
20,
21,
22]. This study compared the changes in isokinetic ankle strength, functional hop tests, and dynamic balance, and subjective ankle scores were compared between college soccer players with CAI who received Pilates training (PT) and those who received balance training (BT).
4. Discussion
CAI is caused by LAS, which is one of the most common sports injuries; this study compared various kinetic variables by applying balance training, which has traditionally been performed for CAI, and the recently popularized Pilates training, over a 6-week period.
One of the main results of this study was that ankle eversion and dorsiflexion strength were improved after training in both the PT and BT groups in the isokinetic ankle strength results. These results imply that although only PT or BT training was performed, ankle strength training was effective in improving ankle muscle function in individuals with CAI.
In previous studies, several authors have reported that BT affects balance and also improves ankle strength [
13,
33]. BT stimulates the muscle fibers and nerves around the ankle during static and dynamic postural control in a closed kinetic chain. In addition, it has been reported that the balance equipment used to provide perturbation affects activation of the ankle muscles, such as the tibialis anterior, peroneus longus, and medial gastrocnemius [
34]. Interestingly, improvements in eversion and dorsiflexion strength were also observed in the PT group. However, it was difficult to determine whether this was an effect of training or a result of pain relief and natural recovery over time. Since no control group was established in this study, further studies should be conducted for more clear facts on this issue.
In addition, in both groups, there were significant changes only in eversion and dorsiflexion in this study, and the results were similar to those of a previous study [
35]. There was no significant difference between inversion and plantar flexion. This suggests that peroneus muscle weakness occurred in CAI, resulting in decreased strength in eversion and dorsiflexion involving the peroneus muscle, whereas inversion and plantar flexion were less affected [
36]. These data also show that the LSI of inversion and plantarflexion before training was approximately 90%, which means that the muscle strength was better than that of the unaffected side.
Hop tests are generally performance-based assessments that measure the functional performance of the lower extremities and are often used to evaluate the level of functional recovery after an ankle injury [
37]. In this study, hop tests were conducted based on this background information. In this study, both PT and BT improved significantly after the training. A previous study also showed similar results, in which all four hop tests improved after six weeks of balance training [
12]. In a study by Sonepat et al. [
38], training using a wobble board for 8 weeks increased the distance of the triple hop test. Similarly, a study by Aslan et al. [
39] reported that the distance of the triple hop test was significantly improved after 8 weeks of core training. Although PT and BT does not apply strong resistance, such as strength training, functional improvement would have been achieved by adequately stimulating the muscular nervous system. In this study, an increase in dynamic stability was also observed in both PT and BT, and this improvement in postural stability is believed to have had an effect on the performance improvement of the functional hop tests.
The hop test evaluates a combination of ankle strength, neuromuscular control, confidence in the lower extremities, and ability to withstand loads related to sports-specific activities [
25]. Depending on the study, only a single-hop test is performed owing to its simplicity and time efficiency, but since the single-hop test induces and evaluates only ankle stress in the sagittal plane, there is a limitation in evaluating control ability in various directions [
37]. Haitz et al. [
40] also reported that the sensitivity ratio of the test increased when two or more hop tests were evaluated together compared to when only one hop test was evaluated when assessing function of the lower extremities.
As both PT and BT play a role in improving postural control and stability [
13,
33,
41], it would have been improved in all hop tests in this study as well. Although both groups showed improved results in all hop tests, the interaction effect in the triple hop was perhaps due to the effect of core stability training, which is the main focus of PT. Core stability allows for the optimal generation, transmission, and control of forces and motion of the extremities in integrated kinetic chain activity [
15]. The theoretical background of the core stability training effect is as follows: Because core training shortens the amortization phase in the stretch-shortening cycle, it aids the efficient use of the elastic energy stored in the eccentric pre-stretch phase in the subsequent concentric shortening phase [
42]. This improved stretch reflex is associated with more effective plyometric performance and is thought to have a greater effect on a similar motion, that is, triple hop.
Dynamic balance refers to the ability to maintain postural stability while moving the body or changing the position of a limb and is an important component in most daily life and sports activities [
43]. In this study, dynamic balance was evaluated using the YBT. Both groups that received PT and BT improved significantly, but BT was more effective in posteromedial and posterolateral directions than PT. These results may be attributed to the characteristics of the training program conducted in the BT group in this study. Most BT programs conducted in this study comprised static and dynamic posture control training performed in a single-leg stance. This could have had an effect on the BT group, who showed greater improvement than the PT group.
Proprioception of the ankle maintains postural stability by receiving the joint position information of the extremities via the receptor and rapidly transmitting it to the central nervous system. Proprioception is mainly related to the sense of position of mechanoreceptors, which includes both static and dynamic positions. [
44]. Individuals with CAI experience repeated LAS or giving way because these mechanisms are impaired [
8,
45]. Terada et al. [
46] confirmed that patients with CAI showed decreased dynamic balance due to inhibition of tibialis anterior muscle activity in a single-leg stance, and Nanbancha et al. [
47] reported a decrease in dynamic ankle stability related to neuromuscular control in patients with CAI.
Subjective self-evaluation using questionnaires is not a kinetic analysis, but the FAOS, which is commonly used in clinical practice, was applied in this study [
27]. In this study, the subjective ankle score evaluated by the FAOS was significantly improved after training in both groups that received PT and BT. The improvement in subjective evaluation through training intervention has already reported the same results in previous studies [
12,
31]. A minimal clinically important difference (MICD) was identified to confirm whether the self-reported results indicated clinically significant changes. The MCID is defined as the smallest change in a measure that indicates a significant improvement in the symptoms of a disease [
48,
49]. In the present study, after training, both PT and BT groups reported high subjective scores exceeding the MCID. This suggests that PT and BT are effective interventions to improve the subjective pain, symptoms, and function associated with CAI, even in athletes with high activity levels and functional demands.
This study provides useful information that can be used in developing rehabilitation programs. Even if the load is not directly applied to the ankle, core-based Pilates training could improve muscle strength, one-foot hop function, dynamic balance, and improve subjective satisfaction in CAI. The Pilates method defines the core as the “powerhouse” of the body and emphasizes six basic principles: centering, concentration, control, flow, breath, and precision. Pilate progression is achieved by manipulating the effects of the base of support, gravity, length of levers, and center of gravity. Through this process, rapid reaction time, trunk stabilization, and improved postural control can be exhibited [
20,
50].
Our study had several limitations. We suggested that all participants adhere to the training presented. Moreover, additional individual or team ankle strength, balance, and core training that could affect the study results was indicated to be prohibited. Nevertheless, we did not have a system that fully controlled their exceptional training. In addition, although the female soccer population has been increasing recently, women were not included in this study. Moreover, there is a limit in evaluating the improvement of symptoms and kinetics over 6 weeks because the control group setting without any training intervention was not established. In addition, in this study, PT and BT allocations were not randomly assigned, and participants’ preferences were respected. This was because the opinions of the participants could not be overlooked in ethical research. In future studies, an experiment that compensates for these limitations should be conducted, and it would be very valuable to conduct a more detailed study on the local muscle activation among the core muscles.