What do "heart", "nerve" and "sinew" (line 21) individually symbolize?
Any interpretation of a poem, a line in a poem, or an individual word in a poem revolves around a person's individual thoughts about it. Poetry is often analyzed as a subjective text. What this means is that the meaning of a poem may be derived through the reader's own feelings, stereotypes, emotions evoked, and thoughts on the subject. Therefore, any one poem may bring about multiple interpretations. Each interpretation is, therefore, justifiable based upon a reader's support.
This being said, the words "heart", "nerve" and "sinew" in the poem "If" represent deeper levels of "Will which says to them: 'Hold on!'"
The heart represents the whole of love. The nerve (transports signals from the brain and spinal cord to other parts of the body) and sinew (tissue which holds bone to muscle or another bone) represent pieces which deepen the "Will." What this provides, for the reader, is an understanding that will does not only take place in a singular place in the body. Instead, it can be ruled by many parts of the body. The use of heart, nerve, and sinew deepen the effect by which the reader can come to understand the whole effect of will on even the smallest, but most functionally relevant, parts of the body.