The heart is the machine that drives all acts of worship. It is what moves the entire body! As long as the heart is alive, then the limbs will be alive, and the soul will open itself up to worship. However, if the heart becomes diseased, then worship will become too heavy on the soul, leading to it eventually disliking and hating - and we seek refuge with Allah from this - worship. Because of this, Allah - the Glorified and Exalted - said, regarding the prayer:
"...and truly, it is extremely heavy and hard except on those who are submissive..." [al-Baqarah; 45]
The prayer is heavy, because one's legs and hands are not what get up for the prayer. What gets up for the prayer are the heart and the soul.
"Verily, the hypocrites seek to deceive Allah, but it is He Who deceives them. And when they stand up for the prayer, they stand with laziness and to be seen of men, and they do not remember Allah but little." [an-Nisa'; 142]
Because of this, it is the heart that stands up for worship. The limbs are simply slaves of this heart, carrying out what it commands them. If the heart is alive, then the soul will be alive, and worship will become beloved and sweetened to the hearts and the souls, and they will open up for it.
However, if the heart becomes diseased, then worship becomes too heavy on it.