Strength in this vocabulary is vital force rather than muscle, and it has two significators because the tradition distinguishes two things a person can mean by the word.
Mars supplies the capacity to act against resistance. It is hot and dry, quick, and the tradition gives it iron, fire, the sharp edge, the soldier and the surgeon. What Mars measures is not how much a person can lift but whether they will push when something pushes back.
The Sun supplies the vitality itself. It is the light of the chart by day, the heart of the cosmos in the classical picture, and in the body it is given the heart and the vital spirit. A weak Sun is read as a constitution short of force rather than as a person short of courage, and the two conditions are not the same and do not always occur together.
The first house is where both are examined for the body. It is the house of the native's own person, stature and complexion, and its lord, along with any planet placed in it, is read for physical constitution. Mercury has its joy in the first, which is one reason the tradition treats the body and the presentation of the self as continuous.
Aries carries Mars by sign and Leo carries the Sun, and both are read for the outward, forward version of the quality. The fixed signs supply endurance instead, which is a different capacity again, and is recorded under stability in this vocabulary rather than here.