Soccer Scoring Techniques: How Much Do We Know Them Biomechanically?—A State-of-the-Art Review

Abstract

Biomechanics investigation on soccer scoring techniques (SSTs) has a relatively long history. Until now, there have been 43 SSTs identified. Yet, the body of biomechanical knowledge is still limited to a few SSTs. This paper aims to provide an up-to-date overview of idiographic biomechanical studies published from the 1960s to the 2020s in order to outline pertinent discoveries, investigation directions, and methodology progresses. Additionally, the challenges faced by SST studies are discussed. The main goal of the paper is to promote biomechanical investigation on SSTs through discussions on problem solving in the past, research progress in the present, and possible research directions for the future.

1. Introduction

The immense charm of soccer to millions of players and spectators can be traced back to the most prime idea of the game: to score goals—an idea that will always be captivating. This basic idea shapes the soccer scoring technique (SST) to be the crucial and final determinant of every offensive-maneuver fate of any team [1]. It is well known that goals are relatively rare in soccer. Various ways in which a team goes towards scoring a goal can be considered an extemporaneous show, where emotion increases over time and is suddenly released after a goal is scored [2,3]. As such, various SSTs for scoring goals are ultimately the source of excitement that make soccer the number one sport in the world [2,4]. Since diverse SSTs are the last destination that determines the outcome of every “emotional drama,” the quality of performing these SSTs is obviously an essential core of soccer coaching and training; of course, it should be a focus of biomechanical investigations on soccer. Unfortunately, biomechanical studies on various SSTs fall far behind the practice, resulting in a practical scenario that most participants acquire SSTs through personal experience without research-based instruction [3,5,6].

Nowadays, in professional games, airborne shots happen more often and the time for making a shot is becoming tighter. FIFA (Fédération Internationale de Football Association) has impressively portrayed the trend as “every nanosecond is special” [7]. These emerging airborne SSTs, such as bicycle kick, jumping side volley, dividing scorpion kick, and more, appear to be exceptionally complex and are widely regarded as the natural ability of soccer superstars [8]. However, one thing is clear: that these superb SSTs are trained motor skills. Research shows that biomechanical quantification on virtuosic humans can make these skills clearer and easier to be learned, i.e., knowledge gained from biomechanical studies can help us learn complex motor skills while reducing the risk of training-related injuries [9,10,11,12]. Clearly, relying on athletes’ talent to improve these superb SSTs can hardly be considered a viable learning strategy. Biomechanical studies will play an import role in helping practitioners launch science-based (not experience-based) motor learning to optimize their practice.

There are two fundamental aspects related to the establishment of a biomechanical SST training system: (1) the number of SSTs available in the current soccer and (2) the biomechanical know-how of increasing the kick quality of the available SSTs. The purpose of this paper is to provide an up-to-date overview on these two aspects via showing problem solving in the past, research progress in the present, and possible research directions for the future. To this end, studies from sound biomechanical investigations on SSTs and biomechanical reviews of some SSTs are summarized. Given its scope, this paper is not meant to be a systematic review.

2. Scope and Inclusion/Exclusion Criteria

As indicated in the title, this paper is a state-of-the-art review. A state-of-the-art review tends to elaborate contemporary themes and provides new points of view on a topic that needs even more investigation [13,14]. As such, the focus of this review paper is biomechanical analyses, permitting one to draw conclusions on the characteristics/parameters, which can be used for reasoning the research results related to SSTs. The articles included in the review had to satisfy the following criteria: (1) investigation of technology-based solutions for the assessment of gross and/or fine motor skill in soccer shots (e.g., motion-capture technology and biomechanical modeling), (2) clear purpose of the application of the research results for supporting quantitative motor-skill learning, and (3) full scientific papers in the English language.

A wide range of study types were eligible for the review, including kinematic analysis, kinetic analysis, and/or both on soccer shots.

There is a plethora of further studies that were excluded (i.e., were not considered further), for example, qualitative studies, repetitive studies on previous investigations, theoretical studies based on simulations, and studies that are not related to motor skills. Again, this paper is not a systematic review. Its sole aim is to provide an update for researchers who are going to launch studies in the area.

3. Identification of SSTs

In soccer, shots lead to goals and goals win games. Hence, it is logical for both researchers and practitioners to explore all means that could increase the chance and effectiveness of shots taken. Until now, practitioners have unearthed numerous SSTs that can be applied in various game situations; in contrast, scientific investigations (especially biomechanical studies) have only been conducted in a few of the SSTs seen in the game [3,5]. This scenario has resulted in the lack of science-based SST training in current soccer practice. Until 2021, neither researchers nor practitioners knew how many SSTs were available for the game [3]. As such, the names/terms of SSTs used in public broadcasting, documentations, and reports were confusing, e.g., the jumping side volley was often misreported in the professional leagues as a bicycle kick [15,16,17,18]. As matter of a fact, the former is a side-kick technique and the latter is an overhead-kick skill. In order to reduce the ambiguity and increase clarity in both research and training practice, every SST has to be clearly defined; therefore, we first need to know how many SSTs are available in current soccer practice.

In 2021, Zhang et al. established the first terminological system for clearly identifying various SSTs [3]. The research team collected 579 elite soccer goals from international professional tournaments for their study. The essential rule of their collection was that every goal was clearly repeatable, i.e., trainable. From the point view of biomechanics, different SSTs should be linked to certain anatomical parts and show distinct motor-control parameters. Therefore, their identification of SSTs was performed by applying both anatomical and biomechanical parameters. The anatomical parameters included segments and the anatomical landmarks on the segments used during shots. The biomechanical parameters covered the variables describing the dynamic instants of various shooting situations. These variables are associated with ball spatial position, impact style on the ball, jump style, body/trunk orientation at the instant of shooting, and shot control. Figure 1 summarizes the variables applied in the terminological system. Some examples of unique SSTs are shown in Table 1.

Zhang’s study identified 43 SSTs, which were divided into basic SSTs (14 out of 43) and advanced SSTs (29 out of 43). Among the basic SSTs, the widely trained ones were max instep kick, pass kick, chip/lob kick, curl kick, and header. The rarely trained ones were trivela kick and knuckleball kick [3,19,20,21]. Regarding the advanced SSTs, they are complex motor skills, normally known as air-attack or gymnastic-like techniques. Therefore, they are characterized as high risk and low reward. So far, they have been neglected by training books and/or manuals and almost overlooked by researchers [3,5].

It is worth mentioning that ~60% of the 43 identified SSTs are targeted at airborne attacks [3]. Although there are no statistical data available for an overall percentage of airborne-attack goals, two recently established databases present thoughtful results. The two database are (1) 579 brilliant goals selected by experts from soccer’s flagship tournaments, such as FIFA, UEFA (Union of European Football Associations), and the top professional soccer leagues, i.e., La Liga/Spain, Serie A/Italy, Bundesliga/Germany, Premier League/England, and Ligue 1/French [3], and (2) 132 nominees‘ goals of the FIFA Puskás Award 2009–2021 [22]. The two databases show that the percentage of airborne-attack goals is 49.6% and 56.1%, respectively. Given the current scenario, i.e., the lack of scientific research on airborne and/or acrobatic shots, the only way left for practitioners to learn is to impersonate and duplicate these complex shooting skills blindly. This non-systematical skill requisition can be hit or miss, depending largely on the player’s ability of cognitive understanding and associative performance related to the coordination and precision of a new skill, i.e., the so-called talent, as well as a great deal of repetition for the skill’s autonomy [23]. Regardless of the complicated motor coordination related to shot quality during airborne shots, only the high risk of injury during the landing discontinues the repetitive practice for skill autonomy [8,24,25]. Clearly, more biomechanical studies are inevitably needed to unearth elements necessary for systematic training toward the reliable execution of the complex SSTs.

Table 1. Examples of unique SSTs. Except for the bicycle kick [8,24], jumping side volley [8,25], and knuckleball kick [26,27], there are still no reports on the biomechanical studies of the rest of the unique SSTs.

SST	Anatomical Parameters	Biomechanical Parameters
Bicycle kick	Foot, instep	Airborne ball, hit-on-center, vertical jump, trunk lean backward horizontally, forward kick
Jumping side volley	Foot, instep	Airborne ball, hit-on-center, vertical jump, trunk lean sideward horizontally, forward kick
Jumping front volley	Foot, instep	Airborne ball, hit-on-center, vertical jump, vertical trunk, forward kick
Long-jump turning header	Forehead	Airborne ball, hit-on-center, long jump, vertical trunk, head twisting
Diving header	Forehead	Airborne ball, hit-on-center, long jump, trunk lean forward horizontally
Diving scorpion kick	Foot, heel	Airborne ball, hit-on-center, long jump, trunk lean forward horizontally, backward kick
Trivela kick	Foot, outside	Ground ball, hit-on-side, no jump, vertical trunk, forward kick
Jumping turning kick	Foot, outside	Airborne ball, hit-on-center, vertical jump, vertical and twisting trunk
Jumping breaking kick	Foot, plantar side	Airborne ball, hit-on-center, long jump, vertical trunk
Sliding kick	Foot, plantar side	Ground ball, hit-on-center, no jump, trunk lean backward horizontally
Knuckleball kick	Foot, knuckle area	Ground ball, hit-on-center, no jump, vertical trunk, forward kick

Table 1. Examples of unique SSTs. Except for the bicycle kick [8,24], jumping side volley [8,25], and knuckleball kick [26,27], there are still no reports on the biomechanical studies of the rest of the unique SSTs.

SST	Anatomical Parameters	Biomechanical Parameters
Bicycle kick	Foot, instep	Airborne ball, hit-on-center, vertical jump, trunk lean backward horizontally, forward kick
Jumping side volley	Foot, instep	Airborne ball, hit-on-center, vertical jump, trunk lean sideward horizontally, forward kick
Jumping front volley	Foot, instep	Airborne ball, hit-on-center, vertical jump, vertical trunk, forward kick
Long-jump turning header	Forehead	Airborne ball, hit-on-center, long jump, vertical trunk, head twisting
Diving header	Forehead	Airborne ball, hit-on-center, long jump, trunk lean forward horizontally
Diving scorpion kick	Foot, heel	Airborne ball, hit-on-center, long jump, trunk lean forward horizontally, backward kick
Trivela kick	Foot, outside	Ground ball, hit-on-side, no jump, vertical trunk, forward kick
Jumping turning kick	Foot, outside	Airborne ball, hit-on-center, vertical jump, vertical and twisting trunk
Jumping breaking kick	Foot, plantar side	Airborne ball, hit-on-center, long jump, vertical trunk
Sliding kick	Foot, plantar side	Ground ball, hit-on-center, no jump, trunk lean backward horizontally
Knuckleball kick	Foot, knuckle area	Ground ball, hit-on-center, no jump, vertical trunk, forward kick

4. Biomechanical Aspects Governing SSTs

Most soccer games are devoted to establishing or defending scoring opportunities. If an attacker gets a chance to shoot, his/her shot chance usually does not last very long. During this brief moment, the attacker needs to shoot quickly (temporal aspect of scoring) and accurately (spatial feature of scoring). These two parameters control the SST and ultimately determine whether the team’s efforts are rewarded. Therefore, a theoretical system incorporating spatiotemporal parameters of SSTs is needed to guide shooting practice to improve scoring chances. Such a system must explore both temporal and spatial parameters of SSTs, establishing science-based skill training, which can ultimately lead to improved scoring. Unfortunately, passing sequences (temporal) [28,29,30] and field geography (spatial) [30,31,32] related to successful shots in professional games have attracted the attention of researchers and coaches, and as such, soccer study and training have overwhelmingly focused on team tactics and strategy. As a result, consideration of the temporal–spatial aspects of SSTs is overlooked [5].

4.1. Temporal Factors Related to Shooting Effectiveness

Time is of the essence when preparing to shoot the ball. Even if in possession of a “free ball,” a player will likely not be free for long; defenders will attempt to thwart the shot. Until now there have been few studies on this aspect. A representative one [33] examined all goals of the 2008–2009 season in the English Premier League and, to show the temporal efficiency of the shots, the goals were classified into three categories: (1) zero-possession shot, i.e., a “one-touch shot” where a player shoots as a ball is passing by; (2) setting up a shot with one or more contacts of the ball; and (3) shot after individual dribbling. The study revealed that scoring with a one-touch shot accounted for 69.3% of goals, setting up a shot resulted in only 17.9% of goals, and a shot after individual dribbling had the lowest percentage (12.8%). A recent study [5] unveiled similar results: 56.8% for zero-possession shots, 13.6% for one-possession shots, and 29.6% for more-than-two-possession shots. These results suggest that, with an increase of ball-contact times, the scoring chance is continuously decreased.

4.2. Spatial Factors Related to Shooting Effectiveness

Space is a more challenging concept, but it must certainly consist of more than mere field geography [5]. A player’s proprioceptive abilities influence opportunity identification; body orientation and the spatial location of the ball have to be considered when an attacker tries to score. When the ball enters the attacker’s proprioceptive shooting volume, an instant body orientation of the player can be facing, side-facing, or back-facing the goal (Figure 2). The spatial location of the ball, can be horizontal—using the goal and the player as positional references, the ball can be between them, beyond them, or to the side of them, or vertical—the ball can be airborne or on the ground (Figure 2).

So far, there is only one study available that has explored the influence of spatial factors [5]. After analyzing all 132 Puskás goals between 2009 and 2021, the study found that 51.5% were achieved by facing the goal, 31.8% by side-facing, and 16.7% by back-facing. Horizontal ball-position statistics are comparable: 53.0% of goals occurred when the ball was between the player and the goal, whereas balls to the side and beyond accounted for the remaining 47.0%, 27.3%, and 19.7%. Regarding vertical ball positioning, 56.1% of goals were airborne and 43.9% were ground balls.

Based on the current literature, there seems to be existing research and coaching emphasis on shots taken (1) facing the goal and (2) with the ball between the player and the goal [19,20,21,36,37,38,39,40]. These explain why the current practice regimes concentrate on these variables: facing the goal and between-ball make up the highest percentages of goals. Yet, coaching practice should not neglect the remaining percentage of scoring opportunities. Biomechanical investigations on the SSTs contributing to the neglected percentage could help practitioners to master these SSTs.

4.3. Quantification of Attackers’ Proprioceptive/Effective Shooting Volume

The empirical evidence has obviously indicated that shots can be done in a 3D volume surrounding an attacker’s body [15,16,17,18]. The dimensions of this volume influence the recognition of goal opportunity and are limited by the STTs that players have trained—i.e., the more SSTs an attacker can perform, the larger the volume is. Currently, there are few, if any, studies on the manipulation/expansion of this volume. A significant increase in the volume requires advanced SSTs [5]. These SSTs are highly intricate, classified as acrobatic motor skills. Studies have unveiled that learning such complex motor skills relies on one’s proprioceptive ability, and effective learning can be gradually developed by applying structured repetitive trainings [41,42,43,44]. Accordingly, the shooting volume can be called the attackers’ proprioceptive shooting volume. Biomechanical quantification of this volume will help us to clearly identify a dynamic ball as a goal chance (within or through the volume) or not. A recent study has shown that, with or without some airborne/acrobatic SSTs, the difference of the proprioceptive shooting volume could reach sevenfold [5]. This result is based on anthropometry [45,46] and biomechanical modeling [47,48]. This number can be further increase/changed as future biomechanical research demystifies more and more SSTs that influence the effectiveness of proprioceptive shooting volume.

4.4. Focusing on Time in Space: A New Theoretical Framework That Integrates the Temporal and Spatial Factors

As elaborated above, the current practice emphasizes team strategies to win geographic advantage and neglects temporal–spatial factors of SSTs. Empirical evidence from the FIFA Puskás Award [12] has implied that a new theory system should be developed to supply novel perspectives for investigating the existing SSTs; as such, breakthrough studies can be launched to quantify the attackers’ proprioceptive/effective shooting volume and integrate the temporal–spatial factors of SSTs into coaching practice.

The most recent study [5] summarized and sorted the temporal–spatial factors of SSTs into four groups: one temporal and three spatial ones (Figure 3). The temporal group focused on the number of ball possessions before a shot, whereas the spatial groups emphasized an attacker’s dynamic posture at the shot as well as the instant horizontal and vertical ball position at the shot. The nexus for uniting the time efficiency and spatial effectiveness is to learn as many SSTs as possible. First, mastering more SSTs would increase the proprioceptive shooting volume, and therefore, more chances for shooting would be obtained. Second, mastering more SSTs would also increase the possibility for reaching a one-touch shot, as the kicking ability under various conditions would minimize the influence of ball spatial position and an attack’s dynamic posture on shots. Last, mastering more SSTs would increase an attacker’s ability to preclude a defense player from finishing a shot because more SSTs would supply more options for the attacker to select. An outstanding example for the last point would be Ibrahimovic’s scorpion kick, nominated for the 2014 Puskás Award [49]. In the game of Paris Saint-Germain vs. SC Bastia, Ibrahimovic was receiving an airborne pass when he was crossing, i.e., side-facing the opponent’s goal, the penalty area with two defenders on each side. Using perfect pre-judgement, Ibrahimovic purposely let the ball fall behind him with a forward trunk movement. The forward trunk movement excluded the defenders and freed the ball for a successful scorpion kick (Figure 4). It is clear that the condition for his success was to be able to perform such an acrobatic SST.

Unfortunately, due to limited scientific investigation on SSTs, the majority of the 43 identified SSTs cannot be systematically trained [3,5,25]. The SSTs overlooked by researchers and coaches are the airborne/acrobatic attack techniques. The common characteristic of the overlooked SSTs is that they are extremely complicated, labeled as high risk and low reward [24]. As a consequence, the spatial factors are insufficiently developed in most soccer-training regimes, and, as such, the temporal factor is negatively influenced, too [5]. The current practice seems to lose the sense of connection between temporal and spatial attributes of SSTs. The newly proposed theoretical framework, i.e., Focusing on Time in Space, has bridged the gap [5].

The cores of the new theoretical framework are (1) to indicate the way to increase scoring chance, i.e., to master as many SSTs as possible to increase shooting volume; (2) to improve players’ spatial awareness in terms of body orientation and ball position; (3) to learn various dynamic situations that can occur within the shooting volume; and (4) to select a proper SST for a one-touch shot depending on various dynamic situations, i.e., to train players for reach the highest temporal efficiency of shots. Briefly, the new theoretical framework signifies that the SST training should first increase players’ proprioceptive/effective shooting volume, and afterwards, aim at practicing one-touch shots within this improved volume in order to increase the temporal efficiency. These two cores are the meaning of Focusing on Time in Space.

Practically, the one-touch shot in many airborne-ball situations needs players to perform highly complicated SSTs [3,5,24,25]. It should be mentioned that, for most players, these skills can hardly be learned without insightful guidance. Therefore, it is time for biomechanical researchers to conduct quantitative studies to (1) demystify the complexity of the skills, (2) identify skills required for various ball positions and dynamic body postures, and (3) develop training programs for injury-free training, because the accurate performance of the airborne/acrobatic SSTs requires repetitive practice [23].

5. Current State of SST Research

Literature has shown that the biomechanical investigation on SSTs began as early as the 1960s, beginning with 2D motion analysis of maximal-instep kicks [50]. With advances in motion-capture technology, especially with the use of high-speed cameras, more 2D motion analyses were conducted between the 1970s and 1990s [51,52,53,54,55,56]. The 2D studies were mainly done in the sagittal plane and focused only on the kicking leg. Therefore, most data reported were results of partial-body analysis. The most common parameters used in this period were ball speed, joint-position changes, joint angle, and angular velocity [40].

The 3D biomechanical analysis of SSTs was initiated around 2000 [40,57]. The early 3D investigations still followed the path of previous 2D studies, i.e., using partial-body analysis to explore or examine biomechanical aspects of soccer kicking [57,58]. Therefore, the extension from 2D to 3D did not bring significant new aspects [40]. The full-body, 3D biomechanical quantification of SSTs appeared around 2005 [40,57,59]. It is known that full-body study represents a lot of additional time consumption. However, full-body studies have proven that failure to address the upper-body movement during kicking would lead to an incomplete understanding of the complex segments’ coordination and/or motor-control mechanism in soccer shots [57]. As such, the full-body biomechanical information can help coaches better develop training programs for accelerating skill acquisition and, at the same time, minimize risk of injury to athletes during training [2,9]. This section summarizes and classifies previous biomechanical studies on SSTs in order to show what has been done and where we might be heading in the future.

5.1. Maximal-Instep Kick

The maximal-instep soccer kick has received the most attention among researchers [2,36,40,56]. It was the first SST investigated by sports biomechanicists in the 1960s [50] and is still the most widely studied skill in soccer. In order to increase kick quality, one should pay attention to kick accuracy and kick power in skill-development training [35,60]. Accuracy ensures the precision of kick control, including the way the kicking foot moves toward the ball and the contact area of the foot with the ball. Power measures the momentum/speed of the kicking foot. The more accurate and powerful a kick is, the higher the quality of the kick. Yet, there is a practical challenge—i.e., accuracy and power are non-autonomous variables. This means that, in nature, the two parameters interact contrarily/work against each other [2,23]. Particularly for beginners, increasing kick power (i.e., increasing kick speed) can dramatically decrease kick accuracy. Previous studies have shown that training should begin with kick accuracy and afterward be directed toward improving kick power [2,40].

Previous studies have unveiled that there are four determining features that crucially influence kick accuracy and kick power; the four features are the approach to the ball, the supporting-foot placement, the positioning of the kicking foot, and the tension arc (dynamic posture) [2,35,40]. Skilled players perform the maximal-instep kick with an angle (β) ranging from 23.7° to 43.8° to approach the ball [34,61] (Figure 5, left). The main function of using the approach angle is to aid/increase trunk rotation (measured by α in Figure 5) to gain more kick power [34,57]. One should also pay attention to the last-stride/-step length in training (Figure 5, right). Studies have revealed that, under the condition of accurate shots, an increase in the last-stride length will increase the kick power [35,60,62], suggesting that the last-stride length can be used to evaluate the training progress or training effect in the practice.

The placement of the supporting foot in the last step and the placement of the kicking foot are closely related to each other. Studies have revealed that the supporting foot should be planted parallel to the intended direction of the ball release [2,60]. Furthermore, when placing the foot slightly behind the ball, one generates a rising ball, instead of a ball low to the ground. Additionally, the lateral distance of the supporting foot to the ball should be between 1 foot and 1½ feet. This approach can assure that the kicking foot will be at a roughly 45° angle to the horizontal plane at the impact between the kicking foot and the ball. Previous investigations [2,40] have proven that such a kick can enable one to properly strike the ball with the laces. The proper control has two advantages: (1) producing a more powerful kick and (2) preventing “footballer’s ankle,” a common injury among soccer players [63]. As unveiled in previous investigations, the contact between the kicking foot and the ball takes less than 1/100 of a second—an impact with high intensity [36,64]. If the impact takes place on the distal end of the foot (known as improper kick control), the highly intense loading can result in an over-extension of the kicking foot (i.e., plantar over-flexion). The effects of plantar over-flexion are an increased risk of foot injury along with a reduction in ball-release speed.

The kicking step (last step of the kick) is the core of the skill. It requires full-body control. Full-body 3D biomechanical studies have unveiled that the maximal-instep kick can be extracted as the formation of a tension arc (Figure 5, right) and the fast release of this tension arc [6,34,57]. The process of the former consists of three characteristic controls: (1) hip over-extension and knee flexion on the kick side, (2) trunk rotation toward the non-kick side, and (3) shoulder extension and abduction on the non- kick side, resulting in the non-kick-side arm pointing toward the rear–lateral direction (Figure 4, left). The latter one also involves three characteristic controls: (1) a whip-like control of the kicking leg, (2) upper-trunk flexion and rotation towards the kick side, and (3) the fast shoulder flexion and adduction of the non-kick-side arm, leading the arm to a quick forward swing towards the medial side of the body. These studies have found the advantage of the tension-arc control—namely, the control generates muscle pre-lengthening, a condition for producing an explosive muscle contraction. The explosive muscle contraction can make the kick more powerful. Additionally, these studies have also found that the last-step length influences the effect of the tension arc—i.e., an increase in last-step length can amplify the pre-lengthening effects of trunk–leg muscles involved in the kick and in turn give rise to an even bigger increase in kick power. In short, full-body control plays an import role in kick-power generation.

5.2. Knuckleball Kick, Curl Kick, Trivela Kick, and Chip Kick

The reason for the group review of these SSTs is that there is no full-body 3D biomechanical quantification of these SSTs available in the current literature. The current literature only provides a partial perspective on these SSTs. One relevant perspective—upper-body movement during these shots—is hardly addressed in existing literature, and as such, an understanding of segment coordination/motor control remains incomplete for these SSTs. Studies have shown that soccer shots belong to complex motor skills [3,6,24,25,34,57], and upper-body movement must not be neglected. Our knowledge of these SSTs will remain incomplete until upper-body movement is considered and integrated into our understanding of these skills.

The knuckleball kick looks similar to the maximal-instep kick, yet they are essentially different. The uniqueness of the knuckleball is the ball wavering or dropping unpredictably during its flight [65], known as a “zigzag ball” in practice. The unpredictability of the zigzag ball renders it fearsome for goalkeepers, who find it extremely difficult to judge its flight [66]. Pure mechanical tests have unveiled that the zigzag trajectories are associated with asymmetric and unsteady flows of air around the ball [65,66,67]. Practically, the unpredictable trajectory is produced by (1) shots without spin (or with very little spin) and (2) the random shedding of vortices in its trace. The obtention of a large knuckle effect requires the ball to be launched in a particular shooting range and initial speed corresponding to the drag crisis of the ball (25~30 m, ~26 m/s). These criteria, plus the absence of initial spin, explains the rareness of knuckleballs [65,66,67,68]. Studies have unveiled the important aspect of the dynamics of knuckleball: the style of striking the ball. This is also the main difference between the maximal-instep kick and the knuckleball kick [3,65,68] (Figure 6). The strike should be (1) hard with no spin induced at impact and (2) at the shortest contact time (i.e., minimizing the impact time). Some elite players have found that the best part of the foot for the knuckleball shot is the knuckleball area [3] (Figure 6, right), i.e., flat and rigid. Additionally, in attempting to generate such a knuckling effect, some elite free-kick specialists strike the valve side of the ball, where the ball is relatively stiff and uncompliant, so as to minimize the impact time [66]. Unfortunately, the biomechanical parameters related to the full-body control remain veiled.

Like the knuckleball kick, previous studies on the curl kick have focused predominantly on the kicking foot [66,68,69], and a few on both the kicking foot and the leg [70]. The mechanism governing the curve ball is the Magnus effect, which has already been written about in textbooks of sports biomechanics at the college level [71]. The Magnus effect explains the tendency of a spinning, translating ball to be deflected laterally in a direction perpendicular to its direction of flight. Therefore, setting the ball into a spin during a kick is the core of the shooting technique. It is commonly believed that the contact time would in general determine the ability of the shooter to control the ball; in particular, the longer the contact time, the more readily the shooter can impart spin to the ball when trying to bend it. Yet, foot–ball interaction during kicking impact at the elite level occurs over an extremely short time, ~10 ms [69]. Biomechanically, one can barely manipulate the contact time in such a brief moment; a trade-off of the kicking power for an increase in the contact time will dramatically decrease the kick quality [57]. As such, the kick accuracy must be considerably precise in order to achieve the maximum release speed with the desired spin. One decisive parameter is the moment arm of the impact force, i.e., the perpendicular distance from the ball’s center to the impact-force line (Figure 7). Under the condition of a maximal impact, the mechanical principles tell us that the larger the distance is, the faster the spinning that is produced. The distance in the maximal-instep kick is close to zero; hence, no spin/negligible spin is generated. Yet, previous studies have shown that the distance of the curl kick has a range of 40~100 mm [36,69]. Regarding the skill control, the approach angle (β in Figure 5) for the curl kick is more than 20° larger than the value for instep kicks [70]. Additionally, during the kicking movement, the kicking-leg swing angle (γ in Figure 7) for the curl kick is more than three times larger than the value in instep kicks. Furthermore, the kicking foot during the curl kick is more rotated to the right of the ball-release direction than during the instep kick [70]. The above differences between the curl kick and the maximal-instep kick can help practitioners identify the fundamental coaching points necessary to achieve a curved trajectory of the ball; however, full-body biomechanical analyses are still needed to determine the unknowns that are induced by the differences for the kick-power generation.

The trivela kick is, considering the contact side of the kick foot, a converse-shot technique of the curl kick. A shooter strikes the ball with the outside of the foot with the three outer toes (Figure 7, right) [3,66]. This shot imparts an opposite ball spin, as such curving in the opposite direction in comparison to the curl kick. Owing to the control change of the kicking-leg swing and the decreased contact area during the foot strike, the trivela kick is generally more difficult to perform than the curl kick. Unfortunately, the current knowledge of the kick remains at the fundamental level; a biomechanical investigation has not been found in the literature.

The chip kick is to lift the ball up into the air over a goalkeeper into the goal, characterized as a lofted kick to a target [3,56,72]. The technique is widely applied at all levels of soccer competitions, yet biomechanical studies are very limited. One relevant parameter influencing the lifting effect is the ball-release angle. One study revealed that the ball-release angle is influenced by the ankle angle of the kicking foot as well as its dorsiflexion at the foot–ball contact [72]. These results indicate that a rising ball will be produced by reducing the kick-foot ankle angle, i.e., more dorsi-position, and executing dorsiflexion at the shot. It should be noted that the chip kick belongs to a tricky kick that focuses on kicking accuracy, not on kicking power. A study has shown that players could perform equally well in terms of kicking accuracy by using both the dominant and non-dominant leg [73]. This is a unique aspect of the chip kick, which means that one could make the shot quicker, i.e., saving a possible process of setting the ball and shooting.

5.3. Header

Headers in soccer have been a hot research topic for a long time, but recent systematic review articles have demonstrated that the existing studies using biomechanical measurements are overwhelmingly epidemiological studies [74,75], not biomechanical quantifications of motor control and skill acquisition. The few existing studies related to performance biomechanics have investigated two types of header techniques: the jumping header and the jumping–turning header [76,77,78] (Figure 8). The results indicate that jumping headers consist of three phases: the jumping phase (the downward preparation and upward propulsion), the flight phase, and the landing phase. The two instants dividing the headers into phases are the take-off and the head–ball contact. The limited studies have not found an optimal segmental coordination for heading; however, they suggest that the trunk hyperextension followed by trunk and hip flexion just prior to ball contact with the forehead characterizes the successful execution of the jumping header [76,77].

Regarding the accuracy and power of the techniques, only one study has explored the accuracy of the jumping header [77]. The pilot study selected the ball-release angle for accuracy quantification and unveiled a critical value (i.e., 15°) that disguises a correct header (≤15°) from an incorrect one (>15°). The study also indicated that the critical value is linked to the jumping ability, a physical limitation of players. As for the power of headers, different segmental coordination is required for different techniques. For the jumping header, conservation of angular momentum plays an important role [76,77], i.e., the legs should be pulled backwards, as flexed and as quickly as possible after take-off, and be moved forwards fully extended just before heading the ball (action in Figure 8); the action will make the head–ball impact more powerful (reaction in Figure 8) than without this control action. For the jumping–turning header, the fast rotation of the trunk as well as the head should be executed quickly and during the head–ball contact to increase the hitting power [78]. It should be noted that the above studies are case studies in nature, i.e., the sample sizes are very small (≤5). Therefore, more studies are needed to justify the reliability of the results. Additionally, one possible effect of head–trunk rotation is to control the direction of head shots. Unfortunately, there are no studies available in the current literature.

5.4. Side Volley

A side volley is an asymmetrical body-control technique (Figure 9), with the kicking leg making an airborne attack while the other leg supports the body during kicking [3]. A systematic-review article [79] in 2019 showed that there are only three studies available in Google Scholar (none found in the Web of Science). Two of the three investigations quantified the influence of ball height on kick control by using a static ball [80,81], and the third one explored the skill-control variations through dynamic-ball test conditions [82]. One common aim of these studies was to determine the influence of the vertical kick height (i.e., ball height) on kick power/ball-release speed. Both the static and dynamic ball conditions achieved an identical result: The lower the ball height is, the more powerful the kick is. One static-ball study [81] further unveiled that, with an increase in ball height, the following control changes will occur: (1) a continuous decrease in the range of knee flexion–extension, (2) a successive increases in range of hip flexion–extension and pelvis twist, and (3) an unceasing increase in the trunk side lean. On the other side, focusing on the kick-leg control, the dynamic-ball investigation revealed the following control characteristics: for the high-and-fast ball condition: The kicking-leg control is performed in a more frozen way (rigid), whereas a whip-like control (sequential) is identified in the low-and-slow ball condition. The authors claimed that it is a trade-off effect—i.e., in order to make an accurate kick and, at the same time, to maintain the dynamic stability during kicking, elite players freeze the knee control to kick the high-and-fast ball. It is interesting that both the static- and dynamic-ball studies presented a similar phenomenon: As the ball height increases, the knee becomes more rigid, and as such, the kick power decreases.

Unfortunately, the kick accuracy and full-body control have not been fully explored. At least two relevant parameters related to both kick accuracy and kick power, i.e., the plant distance [2,40] and the ab-/adduction of the hip (Figure 9), are overlooked. These parameters could influence the kick-leg control, and as such alternate the kick accuracy and/or kick power.

5.5. Bicycle Kick

The bicycle kick is an acrobatic skill in which a player performs an overhead kick while the full body is airborne. Currently, only one research group was found in the literature that has published biomechanical-analysis articles on this SST [8,24]. The research team applied vertical projected balls and the full-body, 3D motion-capture technology to fulfill their investigation. The study unveiled that two events, the take-off and ball contact, divide the bicycle kick into three phases: (1) the jumping phase for creating optimal conditions for the kick, (2) the bicycle phase for accelerating the kick foot as fast as possible prior to the kick, and (3) the landing phase for minimizing landing impact to avoid injury (Figure 10). Since this SST is an airborne technique, one should pay attention to three aspects, i.e., kick power, kick accuracy, and injury prevention during landing, in training [8,24].

Regarding the kick power, the group identified that the instance angle between the two thighs at take-off (θ in Figure 10) and the asymmetric control of the legs during the bicycle phase influence the kick power. The results confirmed that the larger the θ is, the more powerful the kick is. The asymmetric control of the legs during the bicycle phase results in a difference between the moments of inertia of the two legs. The flexed kicking leg has smaller moments of inertia than the extended non-kicking leg. Based on the law of conservation of angular momentum, the flexion of the kick-side hip (upward movement of the kick leg) is faster than the simultaneous extension of the non-kick side hip (downward movement of the non-kick leg) due to the difference between the moments of inertia of the two legs. Taking advantage of the faster kick-side hip flexion and the flexed-knee condition, a player can execute an explosive extension of the kick-side knee shortly before ball contact, followed by kick-side ankle flexion, which forms a powerful whip-like movement of the kick leg [57]. Therefore, the asymmetrical control is crucial for the kick-power generation.

Timing plays a decisive role in kick accuracy. There is only one way to increase the accuracy: repetitive practice [25]. However, this airborne shot is considered high risk due to the inevitable fall to the ground after kicking. The biomechanical quantification shows that unusual landing happens on the arm and body. Hence, it is vital to know what the proper landing technique is. The biomechanical analysis suggested one: sequential multiple landings to share the impact loads. The first landing is the flexed arm–hand chain (like a spring), functioning as the first damping (Figure 10). The second landing is the hip landing, followed by body rolling, sharing the rest of the load among multiple contact points [24]. Due to the fact that repetitive practice will not happen without a proper landing technique to prevent injuries, the learning of this SST should begin with the last phase—landing. Mastering a safe landing technique is the foundation that can ensure repetitive practice to improve kick accuracy/timing as well as kick power.

5.6. Jumping Side Volley

The jumping side volley is also an acrobatic airborne technique. Like for the bicycle kick, only one research group has reported biomechanical studies on this SST in the literature [8,25]. Similar to the bicycle kick, the take-off and ball contact divide the SST into three phases: (1) the jumping phase, (2) the airborne phase, and (3) the landing phase (Figure 11). The results shows that it is a high-risk SST, too; that means that the learning/training should begin with the landing phase. The biomechanical mechanism for a safe landing is the same as the bicycle kick—multiple sequential-landing controls to share the landing impact. For a better understanding of the essential differences between the two acrobatic SSTs, an elaboration of the similarities or dissimilarities between the two SSTs is summarized below.

The jumping side volley is often mistaken for the bicycle kick because of one similarity—the scissor-like movement of the legs during the airborne kicking [3]. Consequently, one common goal in the jumping phase of both SSTs is to increase the angle between the thighs (θ in Figure 10) at take-off, a condition for the scissor-like movement and whip-like kick in both SSTs [8,24,25]. Notwithstanding the similarity, the motor controls of the two SSTs are considerably different. The distinct dissimilarity is that the bicycle kick is a back-facing-goal overhead-kick technique performed in the sagittal plane, whereas the jumping side volley is a side-facing-goal side-kick technique performed in the transversal plane [3] (Figure 12). This essential control difference results in two remarkable differences: jumping styles and the whip-like control. In order to execute a powerful overhead kick, the jump of the bicycle kick aims at positioning the trunk backward in a horizontal direction in order to make the overhead kick easier. As such, this jumping style can only result in a three-segment whip system (thigh–shank–foot) for the kick-power generation [8,24]. On the other hand, the jumping side volley does not have the limitation of the overhead kick; hence, a trunk twist could be added for the generation of the kick power [25]. Previous studies have revealed that, in the jumping phase, the trunk rotates first away from the goal, and during the takeoff, the trunk reverses rotational directions and comes to twist toward the goal; at the same time, the trunk approaches a more laterally horizontal position. This jumping style forms a four-segment whip system (trunk–thigh–shank–foot). Due to this change, the whip-like kick of the jumping side volley is actually initiated in the jumping phase, beginning with the trunk twisting toward the goal, followed by hip flexion, knee extension, and ankle flexion, showing a sequential flow of energy and momentum transfer [8,25]. Therefore, the jumping side volley is more powerful than the bicycle kick.

6. Challenges and Future Prospects

The present review has updated the biomechanical research on SSTs. The inferences that can be drawn from the updates pose several challenges for projecting the direction of future investigations. One main challenge is determining and/or increasing attackers’ proprioceptive/effective shooting volume in the future. So far, an increase in the volume is restricted by the current coaching practice with limited SSTs training, and clearly, it requires 3D biomechanical quantification of the 43 SSTs identified so far for its expansion [5]. Unfortunately, this state-of-the-art review shows that only about one quarter of the 43 SSTs are fully or partially quantified biomechanically, i.e., the biomechanical research on SSTs falls far behind the soccer practice. Obviously, many more investigations are needed in the future.

Another main challenge involves the inadequacy of using partial-body quantification in biomechanical analyses of SSTs. The current review article has shown that kick accuracy and kick power are the two basic aspects related to the kick quality of SSTs. Biomechanical studies focusing on kick-leg quantification are normally precise enough to identify parameters influencing kick accuracy, but not kick power. Notwithstanding the evidence, full-body biomechanical analyses of SSTs are still uncommon. What is more, an additional consequence of partial-body quantification for coaching practice is that complex SST learning will be negatively influenced without supplying a holistic picture of the skills’ control. It is known that the shaping of movement sequences is called kinematic knowledge of results [83], and this knowledge is required early in learning, especially for acrobatic skills, because learners have no idea of what the correct response is, and so it must be shaped by using full-body control [83,84]. The kinematic knowledge can only be obtained by a full-body motion analysis, i.e., biomechanical quantification.

The biomechanical quantification itself creates additional challenges with regard to identification of motor-control sequencing and dominant factors contributing to the control of the motor skill, as well as practical ways for applying the knowledge. One key issuer is part–whole transfer of training [84], i.e., whether there are ways in which a complex skill (e.g., bicycle kick, jumping side volley, diving header, and more) can be broken down into parts that can be learned more efficiently than the whole task itself. We do more part training than we realize, because some learning assignments are so complex that there is no alternative but to break them down. Therefore, future biomechanical studies should supply phase information of SSTs and identify the order of training related to phases of SSTs—e.g., the landing training should be the first one in learning/training the bicycle kick and the jumping side volley [8,24,25].

Since biomechanical quantification of SSTs requires application-oriented investigation, a typical challenge is how to gather realistic data. There are several factors contributing to this challenge; these factors include (1) that most SSTs are influenced by the free flow of the play, making context difficult to define in a laboratory setting, and (2) that it is difficult to collect unconstrained kinematic data due to limitations of both the laboratory and existing data-gathering techniques [25,57]. It is understandable that, in order to overcome the difficulties/limitations, biomechanical studies have necessarily made assumptions and/or simplifications. The key issue is how to establish/simplify test conditions to mimic shooting realities. Regrettably, there is no general protocol in existence. A few SSTs could be quantified by applying a static ball [36,57], but most SSTs should be quantified under dynamic-ball situations. The selection of the ball-flying condition should be made based on game reality. Two exemplary studies are the use of a vertically projected ball for quantifying the bicycle kick and the jumping side volley [8,24,25]. The vertically falling ball is one of the most common scenarios for performing these two SSTs in professional games, and studies using a vertically projected ball can reasonably be carried out in the laboratory environment.

Coupled with the above challenges is the need to develop a biomechanical data-base for the SST-training regime. Currently, most of the identified 43 SSTs cannot be systematically trained due to the lack of biomechanical investigations and understanding. Merely relying on the aptness of an athlete to master extraordinary SSTs would be hit or miss. Such an approach cannot be considered as a viable coaching strategy. A more biomechanically structured learning method should be developed in the future.

7. Conclusions

The most basic objective of soccer is to score goals; as such, learning various SSTs for gaining goals is considered one of the most essential training elements in soccer. Nevertheless, the scientific understanding of SSTs continues to fall far behind its practice, with most participants acquiring SSTs through individual experience rather than research-based instruction. To promote the biomechanical quantification of various SSTs, this state-of-the-art review has tried to update the research status and findings on the identification of SSTs, biomechanical aspects influencing SST performance, and relevant research progress related to SSTs. From these points, the review further highlights the challenges and future prospects that researchers might be facing. In addition, the review has demonstrated that biomechanical studies have great potential to develop trainable methods for effectively learning acrobatic SSTs. Finally, from a scientific point of view, the current SST-training system is still in its infant phase, and more innovative investigations are needed for its development.