Does the muscle on our knee and elbow have some speciality ? Why are we unable to feel any sort of pain when I pinch it ? I also did it with my Mom and Dad and result was same .
There is several types of nerves in the body. Some nerves are specialized in sensing cold or hot temperature, some other to cuts and cell damage and others feel pressure and touch.
The answer to your question is that the nerves in your elbows and knees have less pain detectors and are in fewer number, thus with they are less sensitive.
Here's a funny little poster on BBC: Can you pinch yourself without it hurting?