Some babies in the four or five months, do not want to dip their feet on the ground, if you stand, the legs are weak, all at once fell to the ground, feel the legs have no strength, what is wrong?

If this is the case, I suggest that as a parent, it is best to look at how the child's thighs are.
What is a corner? If the child is lying on his back on the bed, if you can easily separate the child's legs with both hands, line into a word horse, or even more, reach about 200 degrees, such a child, often low muscle tone.
Low muscle tone, muscles do not have the strength to support the weight, the time to be able to support or stand with the hand will become, which is why other children can already stand, but your children can not sit alone.
So, how about a child with low muscle tone and what should be done?
First of all, the child should be given a proper massage, the strength should be greater than usual, so that it can effectively stimulate the muscles, promote muscle growth, but also give the child's brain information, so that he can reasonably allocate the ability required for what parts to grow.
These tasks are not achieved overnight, they require long-term persistence, and for parents, the most important thing is patience.
If you feel that the progress is limited, it does not meet expectations, or you feel that your massage techniques are not professional, for fear of forming side effects, then you need to go to a professional institution, preferably the maternal and child health hospital in the city, where there are professional child health care practitioners, the most professional therapists.