your HEART, your SPIRIT.

The heart of man is the spirit of man. The very core of a man, who ordinarily is made up of spirit, soul and body.
The heart here doesnt refer to your physical heart that pumps blood to your whole body. But just as the word heart is used in the sense of, for example "the heart of nike is shoes, the heart of of sony is music" meaning that they are the very core of such things. Such is the same sense is used as the heary of man- the very core of man which is his spirit man, inner man, inward man, the real man.
Now, the human spirit is the means via which we reach God who is a spirit being. Remember "the true worshippers of God must worship in spirit and in truth" only with your heart i.e your spirit can you worship God.

So people, there is a need to wax strong in spirit by the help of the holy spirit anyways. See some scriptures. Luke 1:80, Luke 2.40. How that Jesus as a boy child waxed strong in spirit.
Because of the need to be able to worship God not with sense knowledge, the mind, but with the spirit, the heart.
Even so because "a man that hath no rule over his spirit is like a city without walls Prov 25.28
And that "he that ruleth his spirit being better than him that taketh a city"
Thanks.