My HR spikes when I start running. Why doesn't this happen for cycling or swimming?

When we go from walking to running, we create what is called a “flight phase”—a moment when our body lifts off the ground. As we land back on the ground, we have to first absorb the shock in order to take the next step. This shock-absorption component requires a lot of effort, meaning that our heart rate must rise to accommodate it.

Even if we decided to jog at the same speed that we walked, our heart rate would be higher while jogging due to the shock absorption component.

Cycling and swimming are different. No changes in the gait pattern are necessary to go from a lower speed to a higher speed, so you only see a gradual rise in heart rate as you increase speed.

Although your running heart rate is higher than your walking heart rate, the spike that takes you even beyond the running heart rate should be eliminated.

So why does it happen?

Because your aerobic system isn’t warmed up enough for you to be able to absorb shock using energy provided by it, and so the body has to employ the anaerobic system until your aerobic system is up to speed. The fix? Do longer warm-ups—no less than 15 minutes—by starting well below your MAF heart rate and just above resting, and very slowly reach your MAF HR.