A year after taking up Shukokai- when I was 16- our sensei once asked me to open up the top of my gi. What he wanted to show the class was how my abdominal and pectoral muscles have toned and developed since I started. This- he said- meant that I have been doing it right.
I remember hearing the saying "nan quan bei tui". It means "Southern fists Northern legs." It referred to the fact that Southern Chinese martial artists relied relatively more on upper body strength than Northern martial artists. Naturally- the style you would practice would influence whether you have strong legs or strong upper body.
The difference between a martial artist and one with knowledge of fighting techniques (or someone who has attended a self defense course) is training. We train to be stronger, faster- to hit harder to kick higher.
In the fire of that training our bodies get forged.