i raced 'cross last weekend, but - aside from a could of quick trips over the barriers - the only time i ran was straight up some really steep hills.

from the physiology point of view, i guess it's not super-surprising - i'm usually able to hit slightly higher HR on the run than the bike, and am usually at a slighter higher HR for a given effort. maybe also there's an HR cost to switching to slightly different muscle groups?

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https://lshtm.academia.edu/MikeCallaghan

http://howtobeswiss.blogspot.ch/

Lists more muscles in play when you run…iirc heart rates for z1,z2…. are 8-10 bpm higher for running vs cycling across the board.

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" I take my gear out of my car and put my bike together. Tourists and locals are watching from sidewalk cafes. Non-racers. The emptiness of of their lives shocks me. "
(opening lines from Tim Krabbe's The Rider , 1978

When I look at my watch/HR data from my races last weekend, and then compare to say Road rides or Runs with intervals, clearly different.

My average HR 162, max 200, and I spent 61% of time at Anaerobic Power with HR > 168.

I think that even at skills practice, the HIT like phenomenon has you overall higher O2 demand/consumption. Swinging leg, using arms and core for balance, and hopping off bike and then breaking into run, in cleats (not flat sneakers) all factor in to more muscle use than simply upping a pace.