At ease soldier

So, there I am, marching like a good 'un across the park, when I realised that my left arm was going forward in tandem with my right leg. I decided to experiment as I'm the adventurous inquisitive type. I put my left arm forward at the same time as my left leg.

Picking myself up I realised this was never going to work, but I found myself wondering why.

Can you help?

A X