Interval training enhances stroke volume. You increase heart rate and blood flow during your workout interval, but your rest interval is the most crucial aspect. Your heart rate drops fast, which forces your ventricles to fill more fully. Stroke volume is one of the greater determining factors of successful running.
Unfit runners reach max stroke volume while jogging. In contrast, fit/trained runners reach max stroke volume up to 5K pace or faster.
Train your heart and it will do what human tissue does best, adapts. Cardiac muscle fibers get bigger. Your heart's connective tissue gets stronger. Your ventricle chambers grow larger. Your stroke volume goes up and resting heart rate goes down.
Train your heart
30 to 90 seconds at 1500m to 3k pace is good heart training. Hill repeats, tempo and cruise intervals are also great options.
Capillaries transport blood and nutrients to your muscles. The more capillaries you have serving each muscle fiber, the more oxygen you can transport to that muscle fiber, and the more carbon dioxide and waste products you can get rid of.
New capillaries begin development in the first week of training (only for muscle fibers that are being recruited). While riding a bike may be good exercise, it won't develop capillaries around non biking muscle fibers. In the same manner, if you run slow distance only, you'll develop capillaries around slow-twitch muscle fibers, not fast -twitch fibers.
5 Ways to stimulate capillary growth:
1) Increase muscle fiber contractions by volume (number of contractions) or rate of contraction (speed at which your fibers contract)
Long runs are a workout that stimulates capillary growth by volume. A tempo run simulates growth by both volume and rate of contraction.
2) Increase blood flow
Fast blood flow puts great stress on capillaries. When stress reaches a critical point, your capillaries either divide or sprout new ones. Interval workouts (5k pace or faster) are a great workout for increasing blood flow.
3) Increase pressure on capillary walls
Constant pressure against capillary walls can increase capillary diameter. Sustained distance runs (slow or fast) are good workouts.
4) Increase pace beyond aerobic threshold
Running at pace just above oxygen-fueled running can stimulate capillary growth. 5k pace intervals are a good workout for this.
5) Increase lactate
When you stop training, you lose all your new hard-earned capillaries in as little as seven days. When you reduce training, you reduce blood flow, you reduce capillaries. Exercising too hard also damages capillary growth. By training capillaries, your mitochondria also benefit (powerhouse inside muscle cells that convert glycogen into fuel).
Train Your Lungs
Stronger respiratory muscles lower psychological stress from sucking wind and lower energy use. At rest, breathing accounts for 1% of energy use. During running, energy usage can increase to 9%. Cutting that down by a few percent leaves more energy for the rest of your body to use.
Training respiratory muscles requires fast running (moderate to high intensity intervals):
-5k effort road and trail repetitions
-Fast tempo runs
-Slow tempo runs
Train hard, but not too hard. Above all, remember that rest is also a crucial part of training.
Be well and run on!