# Soccer Scoring Techniques: How Much Do We Know Them Biomechanically?--A State-of-the-Art Review
## 5 Current State of SST Research
### 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 (\(\beta\)) ranging from 23.7\({}^{\circ}\) to 43.8\({}^{\circ}\) 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 \(\alpha\) 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. Figure 5: Biomechanical aspects of the maximal-instep kick (the figure is adapted from the author’s previous studies [2; 35; 40]). 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 \(1\frac{1}{2}\) feet. This approach can assure that the kicking foot will be at a roughly \(45^{\circ}\) 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].