@Aprilvanert, are you using a specific training plan? What you describe sounds like a Max Strength training block as described in Training for the New Alpinism. And then in Training for the Uphill Athlete it appears in Stage 3 of General Strength (the earlier stages look like they use higher reps to increase muscle mass to get to a baseline level.)
The focus of max strength is a neuromuscular one that recruits more muscle fibers so that you get stronger without adding mass.
I took a look at the Intro to Ultras plan since I’m currently using it myself. The max strength shows up in the form of hill sprints. You could add in upper body max strength as well. I’ve done it in the past with pushups and chin-ups/assisted pull-ups but there are lots of options. I’m not sure about the other plans but I’m sure someone will chime in if you let us know what one you are following!