Strength

is,

Firmness; solidity or toughness; power or vigor of any kind;
power of resisting attacks;
power of mind;
intellectual force.
And Jesus answered him,
The first of all the commandments is,
Hear, O Israel; The Lord our God is one Lord:
And thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength:
this is the first commandment.
And the second is like, namely this,
Thou shalt love thy neighbor as thyself. 
There is none other commandment
greater than these. 

Mark 12:29-31