Chapter 7 Gnana-Vignana Yoga Verse 9
Shloka
Balam balavataam asmi kaamaraagavivarjitam;
Dharmaaviruddho bhooteshu kaamo’smi bharatarshabha.
Translation
Of the strong, I am the strength devoid of desire and attachment,
and in all beings, I am the desire unopposed to Dharma, O Arjuna!
Interpretation
Lord Krishna continues to talk about Himself in this sloka also, continuing from the previous one where He describes Himself as the eternal seed of all beings. In this sloka Lord Krishna says that He is the strength of those people who are strong. But He is the strength only those strong people who have abandoned the desire and attachment. Then Lord Krishna also says that amongst all the living beings on this planet He is the desire of all those who follow their dharma without fail.
Extending Interpretation to Mankind’s Life!
The point we need to understand here is the strength of the strong ones. We have different kinds of people and everyone thinks that they are strong. But are they really strong? Strength of the person is his ability to hold onto things even in adverse situations. For that to happen a person has to be strong without any desire or attachment. If he wants  result or if he is attached to the result, then he will not be able to face adversities in life and though he thinks himself to be strong he is not actually a strong person.
The other point we need to understand here is, if an individual becomes strong by giving up desire and attachment then why Lord Krishna says that He is the strength among the strong people? It is because, the moment you renounce desire and attachment, you walk the path of spirituality, which is nothing but discovering God. Once a person starts to walk that path then automatically God becomes his strength and that gives him the ability to walk the path of spirituality and attain salvation.