Balance is the ability to maintain the condition of equilibrium in the body. In sports activities, balance plays an important role in maintaining posture and conducting given tasks, owing to a close relation between balance and motor performance capacity (Davlin, 2004; Hahn et al., 1999). Balance can be applied to a static condition while standing on a base of support (BoS) with minimum movement, and also to a dynamic situation while maintaining a stable posture and performing techniques (Bressel et al., 2007). Thus, in sports, both static and dynamic balances are related to performance enhancement (Paillard et al., 2006). In biomechanics, balance can be measured by the analysis of the antero-posterior (A/P) and medio-lateral (M/L) displacement of the CoP, and the CoP velocity of the body (Doyle et al., 2007; Lugade et al., 2011; Pai and Patton, 1997; Rocchi et al., 2004; Ryu et al., 2012; Yoo and Ryu, 2012).
Balance is an important aspect of Taekwondo Poomsae competitions because Taekwondo Poomsae is a form of self-practice, which is designed to be performed by following the lines of movement in a pre-planned manner by using Taekwondo techniques against an imaginary opponent (Kukkiwon, 2006). According to the rules of the Taekwondo Poomsae competition, balance is defined as the ability to maintain a straight posture without inclining or tilting (Kukkiwon, 2006). In terms of biomechanics variables, balance can be evaluated by the center of mass (CoM) and the center of pressure (CoP) movement in the course of performing individual motions and connecting these motions. The CoP is positioned within the base of support, and balance ability may appear differently depending on changes in such position (Lugade et al., 2011). Furthermore, the ability to adjust the body weight into a motion without losing balance in the process of releasing the force at the target point is important for the purpose of competition (Korean Taekwondo Association (KTA), 2015; World Taekwondo Federation (WTF), 2015). In particular, it is challenging to maintain one's balance while performing both the 'Hakdariseogi' of the "Kumgang" Poomsae, which includes standing on one leg, and the front and turning sidekicks of the "Pyongwon" Poomsae, which connect at high speed. Thus, a small mistake in "Hakdariseogi" may change the final outcome of a competitive event (Kukkiwon, 2006). Therefore, balance is an essential element in assessing the proficiency of Taekwondo Poomsae athletes.
In studies where the balance of skilled Taekwondo Poomsae athletes were compared to the balance of less skilled athletes, differences existed in joint coordination and a close relationship existed between increased balance skill and lower-limb muscle strength (Ryu et al., 2012; Yoo and Ryu, 2012). However, studies on Taekwondo motion balance improvement have yet to be conducted even though balance is an important assessing factor in Taekwondo Poomsae competitive event. Thus, there is a need to study training methods in order to improve the balance of Taekwondo Poomsae athletes.
Various studies on proprioceptive training programs aiming to improve awareness of body aspects such as posture, movement, and change of balance have been reported. Proprioception is defined as the sense or ability to comprehend the position and speed of movement or weight, as well as the resistance of the body (Hoffman and Payne, 1995). Studies suggest that proprioceptive training improves the balance of unstable ankles, adjusts postural control, and diminishes ankle sprains (Eils and Rosenbaum, 2001; Matsusaka et al., 2001; Mcguine and Keene, 2006; Osborne et al., 2001; Ross et al., 2007; Soderman et al., 2000; Verhagen et al., 2004). Moreover, proprioceptive training has improved the balance of healthy adults and the balance capacity of athletes (Hoffman and Payne, 1995). In addition, it has been reported that this training modality produces effective results in five weeks (Hoffman and Payne, 1995). On the other hand, muscular strength training is also reported to improve balance. Muscle activation of the lower limbs is related to balance and the ability to control posture (Patel et al., 2009; Tanaka et al., 2007). Studies have suggested that increasing the muscular strength of the lower limbs may improve the balance of the body (Hasselgren et al., 2011; Horlings et al., 2009; Pant et al., 2006). In addition, studies also found that muscular strength training improves static and dynamic balance (Ramsbottom et al., 2004; Sarshin et al., 2012). It has been suggested that strength training using a low load is the most effective way to improve capabilities for balance (Orr et al., 2006; Ribeiro et al., 2009). Thus, it is necessary to examine lower limb muscular strength training and proprioceptive training to determine whether they play a positive role in improving the balance of Taekwondo Poomsae athletes.
Therefore, the first purpose of this study was to investigate balance improvements of Taekwondo Poomsae athletes following proprioceptive training and low-load lower-limb muscular strength training. The second purpose of this study was to evaluate the relationship between CoP position within the BoS and balance variables. The first hypothesis of this study was that proprioceptive training and low-load lower-limb muscular strength training are both effective in improving the balance of Taekwondo Poomsae athletes. The second hypothesis of this study was that CoP within the base of support being in a more forward position would reduce the range and speed of CoP indicating improved balance ability.
Initially, 36 Taekwondo Poomsae athletes participated in the testing (Figure 1). However, 6 athletes were excluded from the study owing to injury or absence from training; therefore, the 30 athletes who completed the biomechanical analysis training were categorized as follows: proprioception training group (PG; n = 10; age = 20.0 [+ or -] 2.6 years; height = 171.8 [+ or -] 5.9 cm; body weight = 64.4 [+ or -] 6.5 kg), low-load lower-limb muscular strength training group (SG; n = 10; age = 19.2 [+ or -] 0.8 years; height = 171.9 [+ or -] 6.3 cm; body weight = 64.0 [+ or -]11.2 kg), and control group (CG; n = 10; age = 19.1 [+ or -] 0.7 years; height = 172.1 [+ or -] 7.3 cm; body weight = 65.7 [+ or -] 8.9 kg). All groups were assigned 8 male and 2 female athletes.
An initial sample size of 21 was determined using the M/L CoP range of previous studies (Ryu et al., 2012) with regard to balancing during the performance of the Taekwondo Poomsae "Hakdariseogi" stance. The mean and standard deviation of M/L CoP range from the aforementioned precedent study were used to calculate the effect size of 1.5 (G-power software), with a statistical power set to 0.80 and an alpha level of 0.05. However, considering that the athletes could be absent during the training period, 36 athletes with winning histories in the Korean National Taekwondo Poomsae competition, participated in the pre-test. All participants were recruited through various announcement and local meetings organized by the Department of Taekwondo at the Korea National Sport University. The participants signed informed consent letters, and the study was approved by the university's ethics committee. In the pre-test, all participants performed Hakdariseogi, which is the stance in which it is most difficult to maintain balance during the Taekwondo Poomsae competition. In order to accurately observe the effect of training, three groups were assembled by the adapted randomization method (Kang et al., 2008) using the factors of sex, M/L CoP range, M/L CoP mean velocity, and root mean square (RMS) of the ground reaction torque in the pre-test. The adapted randomization method can be effectively used to balance important covariates among control and training groups (Hedden et al., 2006). In covariate adaptive randomization, the method for assigning participants to either the control or treatment group included (1) calculating P values for each of the covariates using a t test and analysis of variance (ANOVA) for continuous variables; (2) assigning the participant to the group with the larger P value to avoid more imbalances in groups. Using this approach, a smaller P value represents greater imbalances among treatment groups.
The 60-minute training sessions were completed 3 times per week over a period of 6 weeks. Warm-up exercises were performed as stretching during a 10-minute period. Warm-up exercises consisted of neck tilts, neck rotations, neck stretch, triceps stretch, shoulder stretch, torso rotations, chest expansions, side arm raises, arm rotations...