# Soccer Scoring Techniques: How Much Do We Know Them Biomechanically?--A State-of-the-Art Review
## 5 Current State of SST Research
### 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, 57, 54], 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, 67, 66]. 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, \(\sim\)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, \(\sim\)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).