I don’t know how else to describe it. I googled, “Pumped Muscles” and all I get is weight lifting stuff about bigger muscles after a workout. I hesitate to say it’s the lactic acid burn but it might be. It just feels like they are swelling up, I just can’t keep the power down, they stiffen up and my form falls off, a lot like the pumped feeling when lifting weights. I’ve done a handful of workouts at this FTP with and without Stages that haven’t produced this feeling.
I rock climb and ‘pumped’ is a term that is used in every other sentence in a climbers vocabulary. The muscles get ‘pumped’ and your arms almost lock up and you lose mobility. It feels the same.