It’s mainly to do with the size and number of muscles being recruited
The general order of maxHR is swimming < cycling < running < cross country skiing (lowest to highest)
- With swimming, it is mainly upper body (smaller muscles) and the legs aren’t working very hard.
- With cycling, you are mainly working the legs and glutes, with a little bit of core stabilization.
- With running, you are working the legs, and a fair bit of core and upper body to keep you stabilized.
- With cross-country skiing, the legs are working, and the arms are doing a lot of ACTIVE work as well (as against just stabilizing work)
The heart won’t beat faster than the arteries delivering blood to the muscles involved can actually handle. There’s only so much blood you can force down a pipe at any one time. The quads and glutes are the largest and most powerful muscles in the body, and the femoral artery that supplies them is pretty large, so you can get pretty close to max HR with just those, but there are limits. But add in some upper body work, and there are some additional arteries involved, and they can take delivery of more blood supply than just the legs alone, so the heart is called on to supply a bit more.