Chronic ankle instability (CAI) is a subjectively reported phenomenon that has been defined as a tendency for the ankle to give way during normal or sports activities (Monaghan et al., 2006). A recent epidemiologic study reported that 23.4% of high school and collegiate athletes were identified as having CAI (Tanen et al., 2014). It could therefore be reasonable to infer that many elite-level athletes are participating fully in their various activities with mild to moderate CAI, although the exact prevalence of CAI among them has not been reported. Due to the prevalence of CAI and the disability it creates (Hiller et al., 2004; Ryan, 1994), considerable attention has been directed toward understanding the underlying cause of this phenomenon (Lahde et al., 1988).
Current evidence suggests that the dynamic control of ankle stability depends on feed-forward motor control of the central nervous system (Konradsen et al., 1997). In particular, inappropriate positioning of the ankle joint before ground contact during walking has been suggested to have important implications for ankle joint stability (Tropp, 2002). A recent study has shown that individuals with CAI exhibit altered ankle joint kinematics and kinetics during over ground walking at their self-selected velocities (Monaghan et al., 2006). In this study, the participants with ankle instability were found to have a more inverted position of the ankle joint before and immediately after heel strike (HS) compared with a control group. In another recent study examining the rearfoot/shank coupling at heel strike, individuals with chronic ankle instability also demonstrated a more inverted ankle than healthy controls throughout the entire gait cycle (Drewes et al., 2009). In jogging, this alteration was most apparent just prior to heel strike. This altered positioning may be related to an altered neuromuscular control of CAI, which results in an increased predisposition to suffering recurrent ankle sprain (Drewes et al., 2007).
Thus, identification of altered movement patterns in those with CAI compared to healthy controls may indicate adaptive changes attributable to CAI. Then, with the aim of improving and optimizing impaired movement control, specifically for athletes, rehabilitation training after ankle sprain injuries should focus on the restoration and enhancement of neuromuscular abilities (Zech et al., 2009). This approach may lead to improved therapeutic and prophylactic intervention for athletes with this condition in the future. However, to date, no study has investigated the patterns of ankle joint kinematics and the time-related changes and persistence of neuromuscular training effects in a group of athletes with CAI compared with a healthy control group. Therefore, the primary aim of this study was to identify rearfoot angle kinematics occurring at the early stance phase of the gait cycle during the walking, running, and initial jump landing phases, and to compare these to matched controls. The secondary aim was to examine the time-related changes and persistence of the effects of a 6-week neuromuscular training program, which specifically focused on the element of functional ankle instability, in elite women field hockey players with CAI who underwent the training program while they were participating fully in their training and competition.
Participants and training program
The present study was designed as a case-control group comparison and prospective observational study of elite women field hockey players who were drafted in as national players and were commencing their training at the national training center of the Korean Olympic Committee (KOC). The committee for ethics in research at our institute approved this study, and all subjects provided written informed consent. All study-related procedures were conducted in accordance with tenets of the Declaration of Helsinki.
Twenty-five women hockey players of the Korea Team were eligible for this study (Figure 1). Each participant completed a self-report questionnaire (Cumberland Ankle Instability Tool; CaIT, a 9-item 30-point scale), which was developed to measure the severity (cut-off score
Athletes who had reported functional instability of one ankle were included in the CAI group. Participants were excluded from the group if they reported bilateral ankle instability, a subtalar joint problem, medial ankle ligament lesion, syndesmosis injury, spring ligament injury, history of ankle fracture, ankle injury within the 3 months prior to participation, a history of anterior cruciate ligament injury, or participating in an ankle rehabilitation program 6 weeks prior to this study. Additionally, they were also excluded when there was a complete rupture of lateral ankle ligaments on ultrasound examination. Based on the aforementioned criteria, 4 subjects were excluded and the remaining 21 elite women hockey players (mean age 26 years) participated in this study. Thus, 12 women elite hockey players with CAI and 9 women elite hockey players without CAI as controls participated in the study. Among athletes in the CAI group, 10 participants in the CAI group and 3 in the control group exhibited partial tears of the lateral ankle ligaments upon ultrasound examination. Participants in the CAI group completed all follow-up assessments without any dropout, and there were no adverse events reported such as an ankle inversion sprain that necessitate immobilization during the study. The characteristics and CAIT scores of each group are presented in Table 1.
In addition to their daily training regimen for field hockey, athletes with CAI took part in a progressive 6-week (5 sessions per week) rehabilitation exercise program incorporating postural stability, strength, plyometric, and speed/agility drills (Table 2). Each exercise session lasted approximately 60 minutes and included gentle warm-up and cool-down phases. The exercise progression was designed to ensure that participants experienced continuous changes in intensity and demand throughout the course of the program on a weekly basis. Exercise sessions were conducted by two experienced athletic trainers, who emphasized the importance of maintaining the correct technique throughout the program.
Experimental Procedures and Kinematic Data Analysis
Kinematic data acquisition during gait and landing was performed using 12 infrared cameras and a visual 3-dimensional program (C-Motion, standard version 4.91.0; C-Motion Inc., Germantown, MD) was used to quantitatively analyze the collected data. Before collecting the kinematic data, all cameras were arranged and installed to smoothly measure the motion, and the global frame was created on the basis of the calibration frame using Wand calibration methods to set the 3-dimensional coordinate system. The X-axis was oriented in the right-left direction, the Y-axis in anterior-posterior direction, and the Z-axis in the superior-inferior direction. The sampling rate of images was set at 120 Hz. Before testing, fifteen markers were placed on the lower leg and foot in accordance with the protocols of previous studies (Figure 2) (Henley et al., 2008; Seo et al., 2014): knee (2 markers), tibial shank (3 markers), the medial and lateral malleolus (2 markers), the calcaneus (2 markers), navicular (1 marker), cuboid (1 marker), the metatarsal area (3 markers), and the hallux (1 marker). After the attachment of the markers, all participants were instructed to walk barefoot on a 10 m walkway at their comfortable normal walking speed and then to run at 7 km/h on a treadmill. Afterwards, participants were directed to stand on a 25 cm tall box and to jump and land on an indicated spot on the force plate (Pfile et al., 2013).
They were instructed to look at a distant mark to inhibit them from looking down at the floor and they practiced each task until they were acclimatized to the laboratory environment before starting the test. Then, 5 test trials were collected for analysis. The initial point of acquisition occurred once the participants were comfortable at the given speed and jump landing. The participants were not made aware of the precise period of data acquisition in order to allow them to assume their normal gait or landing patterns. A trial was terminated if a reflective marker or wand became loose; it was reapplied in the same position in accordance with markings made on the subject's skin before the test recommenced. All participants took a rest for 10 min between tests to minimize any carry-over effects.
During the normal walking, running, and landing, each event was calculated through the trajectory of the heel marker. Kinematic data were calculated by comparing the angular orientations of the coordinate systems of the adjacent limb segments. Joint angular displacements were calculated for the ankle joints in the frontal planes (inversion [+], eversion [-]) (Figure 3). When the foot was initially in contact with the floor, the point of HS was identified as the point at which the vertical acceleration of the heel marker crossed the horizontal axis (X-axis) of the graph fora testing gait cycle. The point of midstance (MS) was inferred from the point where the opposite ankle crossed the leg of which the foot is in contact with the floor on the sagittal plane (Mahmoudian et al., 2016). The point of toe touch (TT) was identified using the vertical component of ground reaction force using 15 N as a threshold for detection of impact. Kinematic variables in the frontal planes were averaged over time at each speed (walking, 7 km/h running, and landing) for each participant at HS, MS, and TT. The 4th-order Butterworth lowpass filter with cut-off frequency of 15Hz was used to eliminate the noises from skin movements or labeling errors and the data were processed using C-Motion software. Measurements of kinematic data for both the CAI and the control group at each gait speed and landing were...