Your body goes through some serious changes when pushing up those steeps - here's a brief list.
As the road tilts up, your body undergoes a cascade of metabolic reactions that may kind of hurt in the moment, but are oh so good for you long-term. Here’s a breakdown of what happens to your body on a climb.
Hills are nature’s gym - they help build strength. As you pedal up a grade, gravity tries to pull you back down. The steeper the pitch, the more forceful gravity’s pull. That means you need to recruit more muscles to maintain forward momentum.
Climbing hills, especially seated climbing, engages your glutes, quads and calves to a larger degree than when spinning along the flats. Climbing not only builds neuromuscular connections - so you have more muscle fibres turned on and at your disposal - but also breaks down the small fibres in your muscles, which rebuild stronger when you rest, making that same monster climb just a little easier over time.
Your pedalling cadence will naturally slow on climbs, but aiming for a cadence of about 75 to 80 rpm will help keep your legs from fatiguing before you reach the top.
With more muscles being called to action, your heart has to work harder to supply oxygen and nutrient-rich blood where the’re needed. All that work also creates heat, which means more work for your heart, as blood is also needed by the skin to help you sweat and keep cool.
Your heart rate will be 30 to 40 bpm higher when you’re cranking up a hill than when you’re cruising along the flats. Standing and climbing out of the saddle drives up your heart rate about five to 10 beats further because you’re engaging your upper body in the movement: Those arms, shoulders, back need to consume more oxygen and therefore demand a faster heartbeat.
All of this effort makes your heart stronger so it can squeeze out more blood with every beat. (This is why fit cyclists have resting heart rates 10 to 15 beats lower than their unconditioned peers).