I noticed from last week's long run that my legs felt stronger and more coordinated. I was able to land comfortably and my bones felt cushioned as I ran. This was clearly a benefit of the speed runs I had been doing.
There are several forms of running exercises, including endurance training, stamina training, speed training and sprint training. But for simplicity, I will group them into two main types of exercises: long runs (endurance training) and speed/stamina training for a proper treatment of the subject, please visit McMillan's Six-Step Training System from which I borrow the ideas summarized below. Below are the different purposes of each training:
Endurance Training/Long Runs
These are carried out with your heart rate between 60 and 75% of maximum and your oxygen consumption between 55-75% of your VO2max. In this zone, your breathing is comfortable and the effort is easy. Your lactate level is low or only slightly above resting levels.
Appropriate paces can be as fast as your marathon race pace plus 30 seconds or as slow as marathon pace plus two minutes, depending on the workout.
"The key cardiorespiratory or "central" adaptations that result from Endurance training include an increase in your stroke volume -- the amount of blood that is pumped with each heart beat. The result is that fewer heartbeats are needed to deliver the same amount of blood to the working muscles. You experience this as a slower resting pulse and lower heart rates at a given pace.
In the muscles, there is a corresponding increase in the number of tiny blood vessels (capillaries) to deliver this greater volume of blood per beat. The number and size of mitochondria, the power plants of the muscle cells, also increase. You become more efficient at using fat as a fuel source, decreasing your reliance on your limited carbohydrate stores (muscle glycogen). Speaking of glycogen, Endurance training stimulates the muscles to store more glycogen making this fuel readily available for long duration efforts as well as high intensity workouts.
The nervous system becomes very coordinated in its recruitment and use of your slow-twitch muscle fibers, which helps improve your running economy. There's even a stimulus for your fast-twitch muscle fibers to become more "endurance-like"."
Speed/ Stamina training
Under this, "your heart rate and oxygen consumption go from 90% up to maximum. Your breathing is fast and labored. The effort is hard and your lactate level is high."
"...Training in the Stamina zone helps push several critical thresholds (lactate, ventilatory and anaerobic) to faster paces. The result is that you can run faster before crossing these thresholds. The key cardiorespiratory adaptations that result from Stamina training deal with what scientists call the "Lactate Shuttle". While we used to think that lactate simply started being produced and eventually accumulated to the point where fatigue sets in, we now know that lactate is always being formed, just at different rates. At rest and during light exercise, only small amounts are formed. During heavy exercise, large amounts are produced. Once formed, the body has mechanisms whereby the lactate is "shuttled" to other tissues to be used for fuel, sort of like recycling. This recycling or shuttling has a maximum capacity, however. Once reached, the production of lactate outpaces its removal resulting in the accumulation of it in the blood. Thus, the lactate threshold is reached."
"Speed training works to actually increase the capacity of several of your body's systems. Research shows that Speed training increases the enzymes that help liberate energy from our fuel sources, improves the lactic acid buffering capacity, provides a greater stimulation and training of the fast twitch muscle fibers and results in a greater ability to extract oxygen from the blood as it perfuses the muscles.
You experience this as increased speed-endurance, the ability to run fast for a long period of time. The running motion becomes more consolidated as all errant form changes (like flying elbows, funky foot plants) are eliminated. They require too much energy. Your breathing acclimates to fast, constant efforts and your legs begin to feel fast and strong.
"Research has found that during this fast sprinting, groups of individual muscle fibers become more coordinated in their "firing" (contracting) so that you can achieve greater power and speed. Likewise, different whole muscle groups (like the quadriceps, for example) get "in sync" with each other resulting in faster turnover and a smoother stride. Basically, the body becomes efficient and coordinated at turning your legs over very fast. Your running economy improves.
The second adaptation affects the bicarbonate buffering system that we discussed in the Stamina section. Since training at this pace creates large accumulations of lactic acid (lactate and its compatriot, the hydrogen ion), it challenges the body's ability to remove these by-products. With repeated exposure to elevated lactate (and associated hydrogen ion) levels, the body improves its ability to quickly remove it."
There you have it you gain three main things:
1. Physiological adaptations - more mitochondria, more capillaries and slower pulse rates.
2. Better running economy - better coordination of hands, feet and use of stomach, pelvis and hips to generate running power.
3. Higher AT* and LTs*
*AT is aerobic Threshold and LT is Lactate Threshold