Leaning forward on a bike & adding in very bumpy ride is a big factor in neck /arm tension/stressors..
Consider how much your head weighs, then add in the factor of how it hangs out in front of the body on our neck..
There are calculations on the added weight & stress to our neck muscles, shoulders , even upper back adds in to it..
A web search can find that info..
Plus you naturally would tense up from the bumpy ride, so that stifles the free flow of good blood/oxygen to the areas involved..

Cyclist's Neck - Physiopedia
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how much head weight is added to spine when riding a bike - Google Search