1. The arm simulator provides 8 positions for intramuscular injection training,four of which have the red skin-hill-marks. And the hill marks will come to work during practice. 2. If the injection is in right position and strength, a hill mark will come out. Then when draw out the injected liquid, the hill mark will disappear. 3. Every position can be practiced for few hundreds times. When it is broken, we can use glue to agglutinate it and then it can be practiced few times again.