Joshua's heart was committed to the Lord.  He spoke from his heart and he followed the Lord fully.
He reminded the people to love God, to walk in His ways, to keep His commands, to hold fast to Him and to serve Him with all their hearts and souls.  He told them to incline their hearts to the Lord.
To "stretch" towards the Lord and away from idols.  To freely and willingly pledge their hearts to the Lord.

In Judges we find that Deborah had a heart of compassion towards the commanders.  We find that the leaders had resolved hearts and that there was much searchings of heart among those who did not come to help their countrymen.

Samson told Delilah all that was in his heart - and then she betrayed him.
The heart can be merry or glad.  The heart also refers to "self".

Lord God, rule in my heart today.  Incline my heart to Yourself.  Keep Your Word in my heart and may my every intention and thought be stretched towards You always.  I commit myself willingly and freely to love and serve You and to obey Your Word.  Help me to do so Lord!  Thank You.  Amen