As shown in Figures 4, 5, taking the point at which the kicking leg extends to 180° as a time reference, it is observed that both “miss” and “hit” roundhouse kick techniques in this study, which involve high kicks, have relatively late muscle activation times compared to the time reference. However, for both techniques, the activation of all muscles occurs to varying degrees before the time reference. There are also differences in the sequence of muscle activation between the two techniques:

For the “hit” technique, the muscle activation sequence of the swinging leg is as follows: Anterior Tibialis → Erector Spinae → Semitendinosus Semimembranosus → Gastrocnemius (lateral head) → Biceps Femoralis → Rectus Femoris → Gluteus Maximus.

For the “miss” technique, the muscle activation sequence of the swinging leg is as follows: Biceps Femoralis → Semitendinosus Semimembranosus → Gastrocnemius (lateral head) → Erector Spinae → Rectus Femoris → Anterior Tibialis → Gluteus Maximus.

The roundhouse kick is characterized by a combination of horizontal and vertical movement toward the target by the athlete. This movement involves rapid forward rotation of the pelvic axis, hip joint abduction, hip joint flexion, and knee joint extension (Kim et al., 2010). Kicking speed places high demands on the hip flexors and adductors, and well-developed hip joint muscles are essential for optimal activation during kicking, especially in techniques involving instep kicks (such as soccer shots and kickboxing roundhouse kicks) (Brophy et al., 2007).

It is interesting to note that while the final muscle activated in both techniques is the same (the gluteus maximus), the initial muscle activation differs significantly. In the “hit” technique, the tibialis anterior muscle is activated first, whereas in the “miss” technique, the biceps femoris muscle is activated initially. This difference may be attributed to the rich practical experience of professional athletes. The observation that athletes tend to perform the “miss” technique more in line with the standard technique, particularly during the knee-raising phase, indicates their utilization of core abdominal strength to control hip rotation to a position that is closer to horizontal and vertical to the ground (Ervilha et al., 2020). This position is maintained until the entire leg is fully folded and ready for the recoil phase. However, this phase necessitates the initiation of certain thigh muscles (Thibordee and Prasartwuth, 2014). Consequently, the muscles activated first in the “miss” technique are the biceps femoris and semitendinosus semimembranosus, with muscle activation times closely aligned. The muscles of the lower leg activate more slowly, which suggests that the thigh muscles are driving the lower leg muscles. In contrast, the “hit” technique often differs due to extensive practical combat experience, target training, and protective gear training. To ensure the effectiveness of the attack, athletes tend to not complete the knee-raising phase fully. As a result, the thigh muscles remain unfolded to a minimal angle, or the knee joint angle exceeds 60°, leading to a direct leg recoil. This observation explains why the rectus femoris muscle is the muscle activated first in the “hit” technique, as this technique follows a sequence where the lower leg muscles initiate the activation of the thigh muscles (Aandahl et al., 2018a). The observation that most muscles in the supporting leg are activated earlier than those in the swinging leg for both techniques implies that in the roundhouse kick, the muscles of the supporting leg initiate the force by pushing off the ground, with the left side muscles following suit. During the phase from raising the knee to recoiling, athletes are in an unstable state when they rely on single-leg support. The relationship between muscle activity in stable moments (preparation and completion) and unstable states follows a U-shaped curve (Dong and Yuan, 2019), and the transition between stable and unstable states is influenced by a combination of biological and dominant factors (Kim et al., 2017).

The duration of muscle activity to some extent reflects the level of contribution of the corresponding muscles to the completion of the action. As shown in Figure 6, in the “hit” technique, the left and right Rectus Femoris muscles have the longest working time, with the right Rectus Femoris muscle being particularly prominent, covering almost the entire duration of the “hit” action. It can be considered the core muscle for this action. Following this, the left Biceps Femoralis, left Rectus Femoris, right Gastrocnemius (lateral head), right Gluteus Maximus, and left Erector Spinae muscles also have relatively late activation times but continue to work until the end of the action, making them important muscles for this technique.

It is worth mentioning that the right Biceps Femoralis, with the shortest working time among the muscles, works for only 0.79 m. This reflects that the right Biceps Femoralis is not significantly involved in the “hit” action from the kicking phase to the end, suggesting that its contribution to this action is minimal.

Unlike the “hit” technique, in the “miss” technique, all the muscles have working durations that extend until the end of the action. The muscles with the longest working durations in this technique are the left Biceps Femoralis, left Anterior Tibialis, left Rectus Femoris, and left Gluteus Maximus, all of which are muscles of the supporting leg. These muscles play an essential role in this action. Except for the Semitendinosus Semimembranosus and Gastrocnemius muscles, all the muscles of the supporting leg have working durations longer than those of the kicking leg. The working duration of the swinging leg muscles is directly proportional to their activation time, with the muscles that are activated first having the longest working durations.

Through a comparison of the two techniques, it was found that the time period for the “hit” action is significantly shorter than that for the “miss” technique (Su et al., 2019). The author believes that this is because in the “hit” technique, athletes, after striking the target, experience a reactive force from the target on their instep, allowing the kicking leg to actively fold and retract promptly after hitting the target. In contrast, in the “miss” action, due to the unpredictability of when the target is withdrawn, the kicking leg fails to make contact with an object to generate a reactive force, preventing athletes from completing the “retraction of the leg” phase of the roundhouse kick and forcing the kicking leg to land directly.

Electromyographic values can directly reflect the force exerted by the corresponding muscles (Merletti and Lo Conte, 1995). As shown in Tables 1, 2, during time period 1, there were no statistically significant differences in the integrated electromyographic values of muscles between the “miss” and “hit” actions (p > 0.05). In the “miss” technique, the muscle with the highest electromyographic activity during this period was the left erector spinae muscle, and its activity significantly exceeded that of other muscles. This observation clearly reflects that athletes rely heavily on the left erector spinae muscle for support during the initiation phase of the “miss” action. Other muscles had relatively lower and uniform electromyographic activity, with some exceptions. Overall, the electromyographic activity on the supporting side was higher to varying degrees than that on the kicking side.

In the “hit” action during this period, the muscles with the highest activity were the semitendinosus, erector spinae, biceps femoris short head, and biceps femoris long head. Notably, the supporting side muscles such as the semitendinosus, erector spinae, and biceps femoris long head, as well as the swinging side’s biceps femoris short head, exhibited significantly higher muscle activity than their counterparts on the opposite side. These muscles play a crucial role in the initiation phase of the front leg roundhouse kick “hit” technique. The muscles with the least work in both techniques were the rectus femoris, which had the least contribution to the initiation phase.

During this period, all muscles were in an activated or pre-activated state. Both “miss” and “hit” techniques had relatively low overall electromyographic activity, as the supporting leg’s ground push and the kicking leg’s initiation occurred simultaneously. The folding angle of the kicking leg and the height of the knee lift determined the striking height and power (Aandahl et al., 2018b).

During time period 2, in both the “miss” and “hit” actions, the striking height is similar. The primary muscles involved in both actions include the bilateral erector spinae muscles, semitendinosus, and tibialis anterior. Furthermore, in the swinging side, the semitendinosus, gluteus maximus, and rectus femoris had integrated electromyographic values higher than those on the supporting side. This indicates that after the phase of muscle pre-activation, the muscles responsible for hip, knee, and ankle flexion and extension in the swinging leg are the primary force-producing muscles during this time period (Wąsik and Shan, 2015).

Based on the data from Tables 1, 2, it is observed that in time period 3, the left biceps femoris short head, right semitendinosus, right tibialis anterior, left tibialis anterior, right rectus femoris, right vastus lateralis, left vastus lateralis, right gluteus maximus, and left gluteus maximus exhibit significant differences between the “miss” and “hit” techniques. In these muscles, the integrated electromyographic values for the “miss” technique are higher to varying degrees compared to the “hit” technique, with the difference being most significant in the right semitendinosus (p < 0.01). Other muscles without significant differences are primarily on the supporting side, indicating that, apart from the moment of retraction, the supporting side muscles do not significantly differ in work and force production between the two techniques.

In this time period, the muscles with the highest work in the “miss” action are the erector spinae on both sides, vastus lateralis on both sides, rectus femoris on both sides, tibialis anterior on both sides, biceps femoris short head on the right side, and biceps femoris long head on the right side. For the “hit” action, the muscles with the highest work are the erector spinae on both sides, rectus femoris on both sides, right vastus lateralis, right biceps femoris short head, and right tibialis anterior. These muscles are considered the core muscles for this time period. It is clear that in the “miss” action, the integrated electromyographic values for various muscle groups are greater than in the “hit” action.

During the stages from the kicking phase to the retraction phase, the dominant muscle group for hip joint is the hip flexor (rectus femoris) during the preparation to knee lift phases. Subsequently, the hip extensors (biceps femoris long head and semitendinosus) take over as the dominant muscle group until the end of the kicking action. In contrast, the knee extensors (hamstrings) become the dominant muscle group from stage 1 (two-thirds of the entire action) onwards (Xu et al., 2020; Aandahl et al., 2018a; Miziara et al., 2019). The author believes that in the “miss” action, since there is no target to strike, the athlete’s core stability is somewhat compromised. This results in the kicking leg not being able to retract actively and landing smoothly. During this phase, various muscle groups need to continue generating force to control the inertia of the whipping action, leading to an extended action cycle and a slow landing of the kicking leg. This is also the main reason why the integrated electromyographic values of the erector spinae on the supporting side are greater than those on the swinging side.

Tables 1, 2 indicate that the integrated electromyographic values for various muscles are, to varying degrees, greater in the “miss” action compared to the “hit” action. Although the integrated electromyographic values are lower in the “hit” action, the action cycle is shorter, and the kicking leg retracts rapidly. The integrated electromyographic values for core muscles in the “hit” action are greater than those on the supporting leg, which is consistent with the results of HWANG (1986).

Since the introduction of electronic protective gear in world-class Taekwondo competitions, the World Taekwondo Federation (WTF) has made frequent rule modifications, gradually refining the high-tech fair competition environment system that records points using electronic protective gear and headgear. At each Olympic Games, they analyze rule deficiencies and make corrections. Starting from the London Olympics, they strictly enforced penalties for “back-avoidance” violations that affected the indomitable warrior spirit of Taekwondo. Today, they have strict penalties for stalling and negative behaviors that go against the spirit of sportsmanship, encouraging athletes to be more proactive (Zhang and Guan, 2017).

Due to rule-oriented considerations, counterattacks have gradually decreased in world competitions, especially in middle and lower weight classes. In high-level competitions, the proportion of proactive attacking techniques is much higher than that of counterattacks and defensive techniques. The front leg roundhouse kick described in this paper is a counterattacking technique primarily used when the opponent initiates an attack after a defensive lapse. It involves kicking in response to the opponent’s attacking motion (Qiao and Li, 2016). Because this technique is a high-level action that places less demand on power when striking electronic headgear compared to protective gear, it is less predictable for the opponent. Although it may not be used frequently, its scoring rate is remarkably high, making it an effective tactical surprise. Additionally, in overtime periods, the scoring rate for counterattacks becomes even more prominent. In conclusion, the judicious use of counterattacking techniques is crucial for athletes to steer the direction of the game in their favor.

In combat sports, it is often believed that a higher speed of execution leads to a higher scoring rate. However, the “miss” technique has a notably slower execution time compared to the “hit” technique. Both techniques involve a process where various leg muscle groups are lengthened and then rapidly contracted to generate force. These two components are critical factors determining the speed of these techniques. In the “miss” technique, the muscle contraction time during the recoiling phase is excessively long, leading to a slow execution. Therefore, whether in real combat situations or competitions, it is crucial to avoid the occurrence of the “miss” technique.

To address this issue, training should not only focus on the leg muscles but also pay attention to the training of the pelvic joint and associated muscles. In specialized physical training programs, the human body should be trained to utilize the small muscles of the lower leg to activate various muscle groups in the thigh through a lever system, thereby creating maximum force in the muscles and levers that are activated next in the kinetic chain. Furthermore, emphasis should be placed on the ability of muscles to contract maximally within the shortest time frame when in a relaxed state (Shi, 2013; Wang, 2013).