Where does the force on the human body come from?

(1) In the process of playing table tennis, all the driving forces of human movement (three-dimensional rotation and three-dimensional translation) come from two external forces: ground reaction and gravity. Gravity is always downward, with the same size; The reaction force of ground support goes from the near-earth end to the far-earth end in turn.

(2) The forces between human body links are all internal forces, which appear in pairs, react with each other, have opposite directions, have the same life and interest, and act on two adjacent links respectively. Each pair of internal forces will not reach balance and can't cancel each other out, because they act on two limbs respectively.

The hitting force is transmitted from the ground to the racket, which in turn passes through the foot. Legs? Thigh? Lower body and upper body? Clap your hands. Upper arm? Forearm? Hands (including rackets) and other links. Let's take clapping the upper arm as an example to see what forces are related to its movement.

The proximal end of the human upper arm is connected to the shoulder joint, the distal end is connected to the elbow joint, and there are human tissues such as humerus, muscle, tendon and skin in the middle. In all kinds of movements of the upper arm, translation is driven by the shoulder joint, and the rotation around the shoulder joint mainly includes flexion and extension, flexion and extension, internal rotation and external rotation, plus compound 360? Circular motion of cone, etc.

1 Let's look at the ever-present external force-gravity. According to the national standard, the upper arm weight of a male weighing 70kg is 1.7kg, which bears about 17 Newton, and the point of action is at the center of gravity of the upper arm. The downward gravity has no effect on the horizontal rotation and translation of the upper arm, but has an effect on the up-and-down direction and oblique translation, in which gravity is the power when moving downward and the resistance when moving upward. When analyzing the motion of the upper arm, gravity and other forces combine into a resultant force.

Look again, the upper arm is driven by shoulder joint movement. Assuming that the shoulder joint does not rotate, what does clapping look like? Welding? On the shoulder. It moves with the shoulder joint, which means that the upper arm is driven by the shoulder joint. This force comes from the movement of shoulder joint, which comprehensively reflects the contribution of all the forces of trunk and lower limbs to the hitting force. Before sending the shoulder to greet, the arm moves forward, when sinking the shoulder, the arm moves down, when shrugging, the arm moves up, and when turning the shoulder, the arm rotates with it (Figure 1). It looks like this, doesn't it? Do you have a shoulder strap? Actually, than? With a belt in your hand? A more accurate description? One-liners, hands don't grow on the waist. From the point of view of force, it is that the shoulder exerts a driving force on the upper arm, and the upper arm simultaneously exerts a reverse resistance on the shoulder to brake (brake) the shoulder; From the perspective of momentum, it is the shoulder that transfers momentum to the upper arm, which increases momentum and speeds up, while the shoulder reduces momentum and speeds up. The shoulder can drive the upper arm to perform translation, rotation or compound motion.

3 Assuming that the shoulder joint does not move, swing the whole arm to hit the ball, that is, the upper arm is pulled by the muscles related to this link, resulting in circular rotation (Figure 2). The muscles that dominate the upper arm movement are mainly the shoulder, back and chest muscles of the upper body, such as deltoid, latissimus dorsi and pectoralis major, because many muscle tension lines are cross joints, which connect the two limbs at both ends of the joint. From the stress point of view, the shoulder is rotated by the tension torque of muscle contraction, and the shoulder is also subjected to the reverse torque. A link in muscle pulling is generally rotation.

Assuming that the elbow joint does not rotate, what is the forearm like? Welding? On the upper arm, the elbow angle and wrist angle are the same. The movement of the upper arm will naturally give the forearm (including hand and racket) a driving force, and how the upper arm moves will make the forearm and hand (without any force) move accordingly (Figure 3 and Figure 4). From the point of view of force, the upper arm exerts a driving force on the forearm and hand through the elbow joint to overcome the inertia of the forearm and make it move. At the same time, the forearm exerts reverse resistance on the upper arm to brake the upper arm; From the perspective of momentum, it means that the upper arm transfers momentum to the forearm, and the forearm momentum increases and the speed increases, while the upper arm momentum decreases and the speed decreases. The upper arm drives the forearm to perform translation, rotation or compound motion.

The rotation of the upper arm drives the forearm to swing and is subject to its inertial resistance.

The swing of the upper arm drives the forearm to move and is subject to its inertial resistance.

5? The forearm-related muscle mass force makes the forearm and hand move, and at the same time, it will produce a reverse reaction moment to the end of the upper arm through the elbow joint. The typical scene of this phenomenon in table tennis is to close the forearm and swing the wrist. Such as backhand cross, quick tear, pull the ball, etc. Are the most commonly used backhand series techniques (Figure 5). The * * * characteristic of its action is that the forearm and wrist swing and rub the ball to the right and front when swinging. This will produce a left reaction to the elbow. The elbow (including the end of the upper arm) will move slightly to the left, which will affect the stability of the elbow fulcrum.

To sum up, when playing ball, the movement of the upper arm is mainly influenced by four forces or moments when clapping hands (plus gravity is five forces): 1 shoulder strength (the point of action is at the upper end of humerus), 2 related muscle mass force of the upper arm itself (the point of action is at the attachment point of related muscle tendons on humerus), 3 reaction force when the upper arm drives the forearm to move (the point of action is at the lower end of humerus), and 4 reaction force of the forearm force (the point of action is at Generally speaking, 1, 2 is always greater than 3,4, that is, the whole arm still accelerates the swing movement after the strength overcomes the resistance. Because the body parts that produce 1 2 are lower limbs, trunk and clapping upper arms, only clapping forearms and hands produce 3,4. Compared with the two, there is a big gap between quality and strength. The principle of joint activity sequence is always that big joints drive small joints. Shoulder joint >: elbow joint > wrist joint The resultant force and moment of the five forces are the direct driving force of the upper arm movement (Figure 6).

Swing forward to hit the ball. In fig. 6, green arrows are power 1 and 2, blue arrows are resistors 3 and 4, and red arrows are resultant force. Gravity is downward, regardless of the resultant force.

The effect of strength (torque) is easy to observe (that is, swinging the arm to hit the ball), while the effect of resistance (torque) is often concealed or offset by strength (torque), which is not easy to observe. If four forces exist at the same time, the resultant force (torque) can be calculated; If they exist one after another, their cumulative effect on time, that is, momentum or moment of momentum, can be calculated separately.