Why do we lift upper body when we primarily use our legs for hiking and trail running?
It's pretty obvious that you're primarily using your legs.
One foot in front of the other.
I can feel my glutes and hams firing as we're even discussing this haha.
What might not be as obvious is the momentum that your arms carry through each step. (You do this while you're walking, too. Otherwise your arms would naturally stay stick straight through the movement, right?)
Your arm swing comes from your shoulders.
Your back and shoulders carry much of your pack weight.
Your back, shoulders, and core provide stability, balance, and strength to maintain posture.
Your upper body does a lot more work than you may give it credit for!
It's so important that you continue to include Rows, SA Presses, Push-Ups, Pull-ups, etc. in your program and not omit them. They're included within KATHLETICS Trail for a reason ;).
Building a strong back, chest, shoulders, and core WILL help you become a stronger, faster, and more efficient hiker and trail runner.