Think of any training montage involving running, and you'll likely see a monumental stair climb. The plyometric motion you go through when climbing stairs strengthens the same muscles as lunges and squats. The added exertion also makes you breathe heavier, and your heart pumps faster as you work against gravity.

A study conducted on sedentary (inactive) people who walked upstairs a couple of times per day, in a controlled environment, saw a 17% improvement in their Vo2 Max. It isn't the best reference for runners who are already active. Think about it - if you introduce any exercise to an inactive individual, you will almost definitely see improvements. That said, you can expect that improvement as a runner who introduces stairs to their program.

Stair workouts fall into the category of plyometrics or neuromuscular training. These programs have demonstrated excellent results, including decreased injury rates, improvements in speed and agility, improved vertical jump, and enhanced ground contact times.

The fight against gravity improves your strength and power. Both are needed at the start and the end of the race. Climbing them will accelerate your heart rate rapidly, your breathing will increase in speed to take in more oxygen, and you will, in turn, improve your Vo2 max - the maximum amount of oxygen you can use during a hard workout.