The Biomechanical Principles of Looping Technique

I. Mechanics of Loops

Loops are generated by forcefully "brushing" the middle or upper part of the ball with the bat, causing it to move in a looping curve. The quality of a loop is influenced by

several factors, including the magnitude and direction of the force applied, the duration of impact between the bat and ball, the point of impact on the ball, and the air

resistance encountered by the ball during its flight.

As the looping ball travels in a curve, it rotates at a constant speed while being brushed by the bat but experiences changing speed when resisted by air after leaving the bat.

The degree of spin of the ball can be measured by its angular acceleration.

The magnitude and direction of the force applied to the ball significantly affect its spin. Force (F) can be divided into two components:

and , as shown in Diagram 1. Here,  represents friction, and represents the force of impact directed at the centre of the ball.

Diagram 2 illustrates F', the force directed at the centre of the ball (O) to propel it forward, and M, the momentum affecting the ball's spin. Here, F' = F and

M = FL = FRcos, where R is the radius of the ball. M is also related to β, the angular acceleration, and I, the moment of inertia. Therefore, β = M/I.In essence, the greater the value of M, the higher the angular acceleration, resulting in increased spin of the ball.

As depicted in Diagram 3, the direction of F is correlated with the angle . A smaller value of (i.e., cutting the ball thinner with the bat) leads to greater momentum since

M = FRcos. Conversely, a larger value of (within certain limits) results in smaller momentum.

In summary, the spin of a ball is associated with M (or the magnitude of F and its direction of application). More precisely, it is related to the amount of friction (=Fcos) applied to the ball and the direction of this friction, while the angular depends on how thinly the ball is cut with the bat.

1. Duration of Impact with the Ball

From the equation M = βI, we can derive the equation Mt = ωI, where "t" represents the duration of impact and "ω" represents the angular velocity of the spinning ball. In other words, ω = FRt/I cos.

This equation illustrates that ω, the angular velocity of the ball at the moment it leaves the bat, is not only related to F and but also to t. Therefore, a longer duration of impact with the ball will result in increased spin of the ball.

3: Point of Impact on the Ball

As depicted in Diagram 4, the angular α remains constant as long as the ball is consistently cut at the same "depth." However, the direction of F varies with different points of impact on the ball.

A downward direction of F results in the ball traveling in a low trajectory and at a high speed (Diagram 5a). Given that faster shots are more challenging to return, it is

recommended to strike the ball at a higher point to lower the trajectory of its flight, provided, of course, that you possess the skill to do so safely.

4: Air Resistance

As illustrated in Diagram 5, a looping ball carries forward spin that influences the surrounding air. The velocity of the air moving alongside the ball is denoted by (V) (Diagram 6).

Conversely, as the ball advances, it displaces the air, resulting in air velocity (V') relative to the motion of the ball. Consequently, the air velocity above the ball reduces to

V-V', while that beneath it increases to V+V'.

Due to the inverse relationship between air pressure and its velocity, there is greater pressure (P) on the upper part of the ball and lesser pressure (P') on its bottom part. Consequently, the ball dips rapidly, and the range of its flight is shortened, thereby decreasing the likelihood of it going out of bounds.

II. Characteristic Features of Looping Strokes

Looping strokes in table tennis can be categorized into three main types: the accentuated loop, the forward-driving loop, and the side-spin loop. These variations differ in several key aspects, including stroke timing, racket-ball impact position relative to the body, point of impact on the ball, direction of the force applied to the stroke, and the length of the bat swing.

Table 1 provides a comparative analysis of the stroke movements associated with the three types of loops as observed in top players both domestically and internationally. It illustrates that each type of loop involves distinct stroke movements, leading to the exertion of force by different muscle groups of varying magnitudes.

Furthermore, within each stroke movement, the force exerted by a specific muscle undergoes constant changes in magnitude. This raises questions regarding whether these muscles are operating at their maximum power during each stroke and which stroke movements effectively utilize the strength of the muscles. Addressing this issue can facilitate the development of a player's muscular strength with specific goals in mind, consequently enhancing the quality of their loops.

Table 2 presents an analysis of the joint and muscle actions involved in performing the three types of loops. It's important to note that a complete looping stroke comprises three phases: the backswing, forward swing, and recovery. However, this article focuses solely on the backswing and forward swing phases for analysis purposes.

The backswing phase in all three types of looping strokes entails the extension of the shoulder and elbow joints, accompanied by the stretching of the working muscles. This stretching action reflexively enhances the contractive strength of these muscles, enabling a player to apply greater force on the ball.

Ideally, during the backswing, the muscles should be stretched to the point where the elbow joint is nearly straightened. A bent elbow restricts the full stretching of muscles and diminishes the potential for building up the speed of the bat swing. Conversely, a fully straightened elbow can lead to excessive tension in the extensors, negatively impacting the contraction of the flexors. Players are advised to maintain a naturally relaxed arm during the backswing to optimize muscle performance.

The spin of the ball is intricately linked to the speed of the arm swing, as represented by the equation M = ωI/t. The length of the arm swing significantly influences the speed of the arm swing; a longer arm swing generally results in a greater increase in speed. In Table 1, it is evident that a looping stroke, particularly the forward-driving variation, involves a longer backswing and, comparatively, a higher speed of the bat swing (ω). This leads to the imparting of greater force onto the loop, contributing to enhanced spin and trajectory of the ball.

Forward Swing of the Bat

Table 2 reveals that while the stroke movements in the three types of loops differ, they engage partially the same major muscle groups, except for sidespin loops, which involve forearm muscles. However, these muscles work at different times and exertion levels. Additionally, they recruit various minor muscle groups, such as the biceps brachialis (long head) in accentuated loops and coracobrachialis in forward-driving loops. To develop strength effectively, both major and minor muscle groups require adequate attention to produce high-quality loops.

During the forward swing of a looping stroke, the shoulder and elbow joints are flexed, followed by a radial wrist turn to achieve higher linear speed in the upper limb movement. As per the equation V = ωR, objects revolving around a fulcrum carry linear speed. In a stroke movement, bending the arm generates linear speeds V, V', and V'', respectively, resulting from the upper arm's rotation around the shoulder joint, the forearm's rotation around the elbow joint, and the wrist's radial turn (refer to Diagram 7). These combined speeds produce a total linear speed [V]. Without wrist flexion, the combined speed becomes [V'], which is smaller than [V]. Thus, wrist action contributes to increasing the bat swing speed.

It's crucial to understand when to exert force with the wrist. Since the bicep’s brachialis, responsible for elbow joint flexion, plays a significant role in generating a looping stroke, it's advisable not to exert wrist force until the forearm has reached its maximum strength. This ensures optimal coordination of muscle groups and maximizes the effectiveness of the stroke.

Generally speaking, a more powerful stroke can be achieved by engaging the wrist at the moment of racket-ball impact. To produce a loop, the ball is typically struck with the bat moving in an inward curve (refer to Diagram 8a). This inward curve helps increase the spin of the ball by facilitating the desired trajectory for its flight and fully utilizing the strength of the biceps. Moreover, an inward curve brings the bat closer to the ball's flight path compared to an outward curve (refer to Diagram 8b), theoretically lengthening the duration of contact (t) between the bat and the ball (although the increase in the value of t is minimal).

When all other factors, including the force applied to the stroke, are equal, the angular speed ω of the spinning ball struck with the bat moving in an inward curve is greater than when struck with the bat moving in an outward curve, as per the equation ω = FLt/I. Since an outward curve of the bat results in a smaller ω, it can be utilized to produce a feint loop with moderate spin.

Hitting the ball at the correct position relative to the body is crucial because it optimizes the exertion of force. The biceps brachialis, a major muscle involved in lifting a loop, is connected with both the shoulder and elbow joints. When both of these joints are bent simultaneously, however, it becomes challenging to fully utilize the power of this muscle.

As indicated in Table 2, the biceps primarily contribute to bending the elbow joint and secondarily to bending the shoulder joint. To fully leverage the power of the biceps, it's essential to bend the two joints at different times. This requires hitting the ball at the proper position. For instance, to produce an accentuated loop, the ball should be taken at a high point approximately half a meter to the front right of the hip.

Standing too close to the table when taking the ball can lead to simultaneous bending of the shoulder and elbow joints. Therefore, it's important to also focus on turning the waist during the stroke movement. This allows for greater abduction of the shoulder joint, reducing the likelihood of simultaneous flexing of the shoulder and elbow joints.

To summarize, achieving a more economical and efficient looping stroke requires attention to several key points:

1. Backswing: The racket arm should be nearly straightened and relaxed during the backswing phase.
2. Strike: When striking the ball, the arm should be bent, and the bat should be brought forward in an inward curve to optimize the spin. It's important to apply force with the wrist and coordinate the movement with turning the waist.
3. Speed and Force: By adhering to these biomechanical principles, a greater speed can be achieved for the bat swing, and a stronger force can be generated by the muscles.

Following these guidelines ensures a more effective and powerful looping stroke, maximizing the efficiency of the player's technique.

Based on the analyses provided, here are some suggestions for improving looping play:

1. Appropriate Wrist Action: Utilize your wrist action effectively during the stroke movement. The linear speed generated by your wrist action should complement the speed generated by the shoulder and elbow movements. Ensure that you exert force with your wrist at the moment of impact with the ball to enhance the power of your stroke.
2. Inward Curve Bat Movement: To maximize spin and achieve the desired trajectory for the ball's flight, consider hitting the ball with the bat moving in an inward curve. After the impact with the ball, continue to carry the bat forward in line with the flight of the ball. This action enhances friction and increases the angular momentum, thereby increasing the spin of the ball.

1. Training for Forward-Driving Loop: The forward-driving loop is recognized as the most powerful looping stroke. Therefore, in specific fitness training, it is essential to focus on developing the muscles involved in executing this type of loop. Refer to Table 2 for details on which muscles to target. Using a medium load, practice explosively contracting the upper arm (and forearm) while hitting the ball, ensuring to relax the muscles immediately after impact. This targeted practice will effectively condition your muscles for optimal looping play. Additionally, auxiliary exercises such as chin-ups, dips on parallel bars, and straight-arm side lifts (with dumbbells) can further enhance muscle strength and coordination.
2. Whole Body Coordination: In addition to arm movements, executing a looping stroke requires excellent coordination and reflexes throughout the entire body, including the legs, waist, and torso. Various methods should be employed to improve overall body coordination. Engaging in ball games such as basketball and badminton is highly recommended, as these activities help develop the necessary coordination and reflexes needed for effective looping play.

Conclusion:

The looping game has gained significant attention in recent years, becoming a major style of play in table tennis. Efforts to counteract and overcome looping techniques have shown initial success, prompting looping players to continuously improve and innovate.

Given the diversity of looping strokes and their varied purposes, players can benefit from incorporating a mix of techniques to create a game that excels in spin, forward drive, and stroke variation.

Despite its advantages, the looping game also presents challenges, such as the requirement for a large arm swing that can reduce the speed of play. Therefore, it is important to seek more efficient and economical looping techniques.

In conclusion, ongoing refinement and adaptation of looping techniques will contribute to the evolution of table tennis strategies.

Until next time, continue to refine your skills and play with precision.