likely you need to hold on less. Be more relaxed.I went to gisburn yesterday I think its the best trail centre i've done so far did struggle a bit on the 'Simons Swamp' section of it on my e-hardtail though (lots of rocks boulders) and 'bottoms beck' was a bit tricky but the flowy sections were loads of fun, thinking of hiring a FS electric the next time i go to see what the difference is like
the biggest thing I struggle with is holding my handlebar over bumpy terrain do I need some thinner grips? (have DMR deathgrip thick on them)
the more tense and grabbing the brakes you are the more fatigue you will feel. If you are relaxed your arms will do their thing.
