While previous studies have explored various aspects of kicks, a comprehensive comparison of key characteristics between the front kick (FK) and roundhouse kick (RK) is lacking. This aims to bridge this gap by comparing the impact forces, maximum velocities, and execution times of the FK and RK across different target distances, target types, and experience levels. By examining these performance attributes, this seeks to provide practical insights into the differences between and practical applications of the FK and RK, shedding light on the dynamic and kinematic variations at different technical levels and under different execution conditions.
The kick is among the fundamental techniques employed in combat activities to overcome opponents, which requires maximum strength and speed.
A powerful technique, such as a kick, depends, in part, on the coordinated momentum of multiple body parts
[11][22]. The front kick and roundhouse kick exemplify proximo-distal movements, where motion originates from joints and segments closer to the body’s center and progressively extends toward the extremities
[31]. For instance, to achieve the maximum foot velocity in executing a front kick, athletes must increase the velocity of the knee as it travels toward the target
[32]. On the other hand, for the optimal execution of a straight kick, the axial over-rotation of the hips in the sagittal plane becomes necessary to generate a higher impact force. Regarding the roundhouse kick, the velocity of the kicking foot results from the combined effect of linear motion at the pivot hip and the angular motion of the pelvis around the pivot hip, with the hip displacement contributing significantly in the initial phases
[25]. However, it was found that circular kicks are executed in less time and at a higher foot velocity than other types of kicks but that, in the sagittal plane, they exhibit similar kinematic data (temporal parameters) to other kicks
[14][25][33]. Moreover, differences in the execution technique, particularly in the kick’s initial stages, have been observed due to variations in stance positions
[34][35]. Therefore, it is desirable to compare the differences between the FK and RK across various execution conditions.
The impact force of kicks is examined in this section. The comparison was made for kicks executed at a middle distance from a target placed at a middle height for the FK and different distances for the RK. In the elite group, the normalized weighted mean of the impact force of the FK was 38% higher compared to the sub-elite group and 140% higher compared to the novice group (with only one selected study available for novices). Similarly, for the RK at a middle distance from a solid target, the normalized weighted mean of the impact force in the elite group was 20% higher than that in the sub-elite group and 60% higher than that in the novice group. Moreover, for the RK, the impact force decreased as the distance from the solid target increased, and this was observed across all participant levels (close, middle, and large distances).
From a close distance, the novice, sub-elite, and elite groups exhibited impact forces that were 20%, 5%, and 20% higher, respectively, compared to a middle distance. Furthermore, they displayed impact forces that were 39%, 10%, and 16% higher, respectively, compared to a large distance. However, the only significant difference was observed between the close and large distances in the novice group. This finding is consistent with a study
[14] in which no significant differences in impact force were found concerning the execution distance in expert competitors.
Contrary to the expectation of a higher impact force with a more extended time for action, the impact force decreased as the distance from the target increased. This finding is likely attributed to the stance position adopted by fighters when executing the RK from a long distance, which differs from the stance position when closer to the target. This supports the notion that when kicking from a long distance, the standing position is similar to a 90° stance position, limiting the RK’s effectiveness
[35].
The impact force of the kick may be related to the isokinetic strength of the hip flexors and extensors and the knee angular velocity
[2][36]. Therefore, fighters should focus on enhancing the impact force from a larger distance with the help of increasing their angular velocity during knee extension in the pre-contact phase
[10].
When comparing the normalized weighted mean of the impact force between the FK and RK, it was observed that at a middle distance from the target, the FK had a higher impact force than the RK: 47% higher in the novice group, 92% higher in the sub-elite group, and 120% higher in the elite group.
3. The Maximum Velocity of Kicks
The maximum velocity of kicks was examined in various studies, which presented different conditions for kick execution, including the target type, target height, distance from the target, stance position, and participant level. However, there were only enough studies to compare foot velocity during kicks executed into a solid target and into the air. The variations in conditions for kick execution across studies, such as target type, target height, and stance position, make it challenging to draw definitive conclusions.
When comparing the weighted mean of the maximum foot velocity between the FK and RK, it was found that the RK exhibited higher velocities than the FK at a middle distance from the target. Specifically, the RK was 44% higher in the sub-elite group and 48% higher in the elite group. This is consistent with the finding that circular kicks generate greater foot velocity at impact than linear kicks due to the rotational involvement of the segments in different planes
[37]. Both kicks are characterized by proximo-distal movement; however, while the FK is primarily executed through hip movement in the sagittal plane and foot velocity is largely influenced by knee velocity
[10][32][38], the RK achieves comparable hip velocity in the sagittal plane. However, when combined with the transversal and frontal planes, the RK attains higher velocity
[25].
Furthermore, the weighted mean of the maximum foot velocity during the FK into a solid target was 11% higher in the elite group compared to the sub-elite group. The maximum foot velocity range for the front kick varied from 7 to 9.98 m/s in the sub-elite group, from 8.2 to 10.32 m/s in the elite group, and 7.7 m/s in the novice group. Additionally, the weighted mean of the maximum foot velocity during the RK executed into a solid target at a middle distance was 13.3% higher in the elite group compared to the sub-elite group. Differences between elite and sub-elite groups of participants were examined for both kicks. Several were discovered when exploring why elite participants achieve higher speed in the FK. For instance, they exhibited smaller angular ranges of leg movement and higher activation of the rectus femoris, vastus lateralis, biceps femoris, and gastrocnemius muscles
[11]. Additionally, elite participants displayed lower variability in hip and knee joint movements
[10][39]. In the case of the RK, the elite group reached higher angular velocities during hip and knee extension and ground reaction force
[40][41][42]. Furthermore, better inter-joint coordination and muscle co-contraction were observed in the elite group
[18][30][43].
In terms of comparing the execution of the FK into a solid target or the air, there is a study that found a higher foot velocity during the execution of the FK into a target than into the air among karate practitioners at the levels of brown and black belts
[44]; however, the finding for the sub-elite group was consistent with a study
[38] showing that the FK into the air demonstrated higher foot velocities than that into the target. The weighted mean maximum foot velocity of the FK into the air in the sub-elite group was 31% higher compared to the foot velocity into the target. Conversely, for the RK in the elite group, the foot velocity during execution into a solid target was 14% higher than into the air. This finding could be related to the protective action on the knee joint when performing a kick without impact, as it was found that while kicking into a target, the maximum activation of vastus lateralis was observed during knee flexion (first phase of the kick), and during a kick into the air, the peak Vastus Lateralis activity was reached during the knee extension phase, along with the antagonist activation of biceps femoris
[42].
Regarding the maximum velocity of the knee in the FK and RK, the elite group exhibited a 6% higher maximum velocity of the knee during the FK compared to the sub-elite group. Furthermore, the elite group had a 13% and 20% higher maximum velocity of the knee during the RK compared to the sub-elite and novice groups during the FK, respectively. Unfortunately, due to different conditions of kick execution, such as varying distances and heights of the target, further comparison of hip and knee velocities between the groups was impossible due to a lack of available data.
The results reveal that sub-elite Taekwondo participants had a maximum foot velocity that was approximately 27% higher (9.695 m/s) compared to the sub-elite karate group (7.616 m/s) and about 24% higher compared to the sub-elite Musado group (7.812 m/s). These findings suggest significant differences in maximum foot velocity among the different combat styles, emphasizing the potential influence of training methodologies and techniques on achieving higher speeds. However, it is important to consider the study’s limitations, such as varying sample sizes and the focus on sub-elite participants, which may impact the generalizability of the results.
While exploring the data from selected articles, it was also found that to improve the maximum foot velocity in the execution of the FK, athletes need to increase the velocity of the knee traveling toward the target
[7][32][38]. Investigating the optimal kicking techniques, such as the positioning of the supporting leg, the angle of hip and knee flexion, and the coordination of joint movements, can contribute to a deeper understanding of how athletes can generate higher foot velocities.
4. Angular Velocity of Kicks
The comparison of angular velocities between sub-elite and elite groups revealed significant differences only for the RK. The elite group had a 37% higher maximum angular velocity of knee extension compared to the sub-elite group when executing the RK toward a target at a middle height. Additionally, a notable disparity was observed between the FK and RK, with the elite group demonstrating a 65% higher maximum angular velocity of knee extension in the RK compared to the FK. Conversely, within the FK, the elite group exhibited a 138% higher maximum angular velocity of hip extension compared to the RK. When examining differences within the RK, it was observed that the elite group achieved a 39% higher maximum angular velocity of hip extension when executing kicks into the air compared to a target. Unfortunately, there were not enough studies to compare maximum angular velocities for knee and hip flexion in the RK and FK, specifically within the sub-elite group.
In the context of other studies, it was found that the maximum angular velocity of knee extension can be influenced by agonist and antagonist activities
[33]. For example, the elite group exhibited clear antagonist activation of the biceps femoris during the extension phase of the FK, whereas such activation was not evident in the amateur group
[30]. The elite group also achieved higher maximum angular velocities of the hip and knee compared to the amateur group. However, it should be noted that the antagonist activity in the biceps femoris toward the end of the analyzed time interval does not significantly affect the hip joint moment during the FK
[31]. These findings partially explain the results showing an increase in angular velocity of knee extension after eight weeks of training focused on explosive lower limb strength without a significant increase in hip angular velocity
[2]. Nevertheless, the primary movement of the hip is cited as crucial for the overall effectiveness of both the FK and RK
[10][36]. Therefore, training to improve the angular velocity of the hip should also focus on functional exercises with an emphasis on core training
[2][45].
Analyzing the contributions of individual muscles and their coordination within the lower limb complex can provide valuable insights into optimizing angular velocities and improving kick performance.
5. Execution Time of Kicks
The execution time of a kick is typically divided into three phases: the pre-phase, the attack phase, and the return phase
[7]. The first two phases, known as the kicking time, were compared for the RK due to the selected articles. The findings revealed that the elite group had shorter kicking times compared to the sub-elite and novice groups across different distances. Specifically, the kicking time in the elite group was 5% shorter at a close distance, 10% shorter at a middle distance, and 12% shorter at a large distance compared to the sub-elite group. Similarly, compared to the novice group, the kicking time of the elite group was 7% shorter at close distance, 9% shorter at a middle distance, and 12% shorter at a large distance. Notably, the greatest differences in kicking time were observed within each group at different distances, with significant reductions in kicking time from close to the middle and from close to large distances. The elite group exhibited the highest differences between close and middle distances.
The total time of the kick is primarily influenced by technique, as elite athletes demonstrate faster hip flexion, knee flexion, and extension compared to non-elite athletes
[46][47]. Therefore, to achieve shorter kick times, focusing primarily on knee velocity in the FK
[32][38] and the angular velocity of knee extension in the RK is advisable
[18].