Where is the place which it becomes painful after throwing a ball too much in baseball and so on?

In order to throw a thing or to spike in volleyball, we use a shoulder joint, an elbow joint, and the joint of a wrist.

There are muscles which connect a humerus and an ulna, a humerus and a radius in an elbow.

The main muscles to lengthen the elbow are shown below.
The right elbow is seen from the back side.

The main muscles to bend the elbow are shown below.
The right elbow is seen from the front side.

The right elbow is seen from the side. When the muscle shown in pink shrinks, the elbow lengthens.  When the muscle shown in red shrinks, the elbow bends. A well-developed biceps is made with this muscle
Baseball elbow
A baseball elbow expresses all the disorders of the elbow which happens by pitching motion.
In the child, a series of smooth pitching motions interlocked with the body are difficult since a bone, a cartilage, a ligament, muscles, etc. were underdeveloped
Therefore, a pitching form becomes in the state of a hand throw, and, it easily gives burden on the elbow.
Furthermore, if the wrong practice method and instructions, and overdoing of practice are added, the various disorders of elbows will occur.
Let’s try to remember cocking (The shoulder is turning to the maximum outward.)
The arm bends.

Although it is outside visually, the muscles are medically the inner sides of an elbow, and the muscles which adhere to the bone of a wrist and the bone head of a humerus are pulled in the direction of back.  They are muscles shown in blue in the animation below.

Power is added to the inside ligament shown in yellow. Let’s try to remember. This muscle is the one which bends a wrist.
Overdoing of pitching motion hurts inside muscles of the elbow, tendon, and ligament.

Inflammation sets to the bone head which is inside a humerus.
It is a part with the muscles which bend a wrist, and it is called an epicondylus-medialis-humeri inflammation.

 右肘上腕骨内顆

When time passes with having a pain in the inside, the outside of the elbow also becomes

painful.　
Why?？？？
Let’s see the pitching motion.

Although it is visually the inside of the elbow, medically it seems that the humeral bone hits the bone of radius in the outside of the elbow.

Do you understand that it approaches so that the bone of the radius hits the humeral bone?

Acceleration period, that is it is the time until it parts with a ball since it begins to throw a ball.
When an arm bends, do you understand that the thumb side becomes outward? That is, the power twisted outside works on the outside of an elbow. Let’s see it by the animation below.
Next, when the crooked arm is extended in front, the power twisted a little inside works on an elbow.  This is the normal smooth pitching motion. Let’s check it by yourself.
When there is much number of times of normal pitching motion, or smooth pitching motion cannot be done, what happens?
The picture below shows the normal bone.
It is photographed in the state that the open palm can be seen from the front.
（quotation from http://www.urban.ne.jp/home/tm512/sp3-1.html）

When the humeral bone hits the bone of the radius, and there is much number of times that the outward power becomes strong.

　　　　　 Right elbow
An outside bone changes and, occasionally it may come off.
Quote from http://www.urban.ne.jp/home/tm512/sp3-1.html

The bone which came off is also called the joint mouse.

Elementary school students should not throw a curveball. Why?

Please see an animation. In the motion which throw a curveball, an elbow is lengthened, twisting a forearm outside, in order to apply a rotation to a ball, isn’t it?

This is not a normal pitching method.
In a motion of a normal elbow, when lengthening an elbow, a forearm is twisted inside.
Repetition of this pitching motion which is not normal.
In the children period which bones grow, heat occurs in the muscles and tendon and ligament which surround an elbow, and becomes arthritis. If it continues for a long period of time, a bone will become deformed. It is crooked into "the character of く" peculiar to a pitcher elbow.
In the 60% of the baseball elbow, there is a pain inside of the elbow.  A bone often comes off in the outside.  As for the above photograph, a bone comes off in the outside (L) of the right elbow, and, in the inside (M), a bone comes off as well. It is considerably seriously ill.
How about the back side of the elbow?
The follow-through period　(It is to finish throwing a ball, and movement is over)
　 When the muscle shown in pink shrinks in the back side of the elbow, it lengthens. This muscle is called triceps brachii muscle.

Before throwing a ball, an arm bends, and the muscle in pink and tendon lengthen.

When an elbow lengthens, this muscle shrinks, and a ball flies off. This muscle becomes hot when we repeat that we suddenly shrink the elbow from the state in which we lengthened it.
Triceps brachii muscle inflammation occurs. Moreover, the bone on the backside of an elbow comes off or the epiphyseal line with which a bone grows opens.
Number of pitches in order not to become a baseball elbow
In elementary school students, the number of pitches a day should be within 50 pitches, and the number of hours of practice is until 2 hours a day.The practice time should be until three days a week, within 300 pitches a week, and not throw a breaking ball.
In junior high school students, the number of pitches a day should be within 70 pitches a day, and until 6 days a week.
In high school students, the number of pitches a day should be within 100 pitches, and until 6 days a week.