Please help me. I have looked up in Oxford dictionary but I couldn't find out the meaning of this phrase. I saw it in this sentence and I don't understand:' It is particularly ironic in light of the large number of old adults who are physically independent, that this group falls between the cracks in terms of understanding their physical capabilities'. What does' fall between the cracks' mean? Moreover, I'm confused about 'in terms of understanding their physical capabilities', does the write  want to say ' in term of the old adults' abilities to understand the capabilities of themselves' or' in term of researchers' abilities to understand the capabilities od the old adults'?

Thank you, Phillip. By the way, I'm sorry for the errors in my question. I typed it in a hurry and didn't check it carefully ( the writer, of the old adults) 

So if 'fall between the cracks' means' goes unnoticed', the idea of this sentence is:' It is ironic that although there are many physically independent adults, there has not been much information about the physcical capabilities of this group', isn't it?

Please help me since this sentence is the 'key sentence' in a paragraph that I have to translate. Thank you very much.