Type II muscle fibers oxidize lactate at a very fast rates.  When muscle contraction produces a significant amount of lactate, it is then released into the central circulation of the blood, and within seconds it is made available to that muscle for energy.  Therefore, 75% of the lactate produced from high intensity exercise is made available for energy production in type II muscle fibers.  The remaining 25% of lactic acid is used for energy in the heart, the make up of liver glycogen, and the supply of energy to inactive muscles.  A good example of this would be a runner who is exceeding his or her planned race pace in a 10k.  The excess lactic acid accumulated in the contracting muscle from insufficient oxygen is then made available to inactive muscles (e.g., the arms) from the central circulation of blood.

The remaining lactic acid that is not directly oxidized for fuels is sent to the liver, where it is stored as glycogen.  In the process of exercise, glycogen is released into the blood stream to form glucose.

Lactate is Good

As coach and athlete you must learn how to teach the body to handle lactic acid.  It is imperative, if you want successes in today's highly competitive field of athletics to train your muscles, body and mind to accomplish gains in performance even in the presence of lactic acid.  Coaches and athletes should design training programs with this being a primary focus.  This is done by two basic components of training.

Long Slow Distance (LSD) training beyond the normal racing distance, will develop tissue enzyme adaptations that will rely upon the use of free fatty acids for energy production, which will result in less lactic acid being produced.  LSD training will also increase the rate of lactic acid removal from the blood and muscles.  During continuos steady state exercise, you increase capillary density and mitochondria function in skeletal muscle,  These two peripheral adaptations brought on by LSD training will enable your body to handle lactic acid much more efficiency. 

High intensity training will develop the cardiovascular system to increase the rate of oxygen transport to the contracting muscles so there is less reliance on carbohydrate breakdown to lactic acid.  High intensity training such as intervals.,  and variable pace workouts, will increase your functional capacity (Max VO2).  This means that in actual competition you will produce less lactic acid, because your muscles are relying mostly on the use of free fatty acids for fuel.  The lactic acid that is produce will be removed by the tissues that can use it as fuel, such as the heart and type II muscle fibers.

More On Aerobic Type Training

When it comes right down to it, the main way to increase oxygen uptake is to do distance, plain and simple.  Generally speaking, Type I muscle fibers (the endurance fibers) are the fibers which must be trained in order to raise VO2 Max,  But, what exactly does raising VO2 Max entail and what is happening.

VO2 Max is comprised of several factors:  VO2 = Q X (A-Vo2 difference) where Q = Heart rate * volume of blood pumped per beat and (A-Vo2 difference) is the amount of oxygen extracted by the muscle.   So, there are three things involved.  Well, since, by definition VO2 Max will be highest at maximum heart rate, it would be great if you could raise this.  Unfortunately, maximum heart rate is genetically determined and does not change with training (although it does decrease with age).  

One of the effects of prolonged endurance training is an increase in heart size and pumping strength.  These two factors see to raise the volume of blood which can be pumped per beat.  Incidentally, this is also part of why aerobic athletes have very decreased resting heart rates.  Actually, their Q values are the same as sedentary people, it's just that since their heart can pump more blood in a given beat, the heart doesn't have to beat as often at rest.

Well, another adaptation to endurance exercise is an increase in aerobic enzymes in the muscles and an increase in mitochondria density and number.  This serves to increase the (A-V02 difference) as the muscles are now capable of extracting more oxygen from the blood.  This increase generally occurs the most in the Type I fibers which have the most aerobic enzymes and mitochondria to begin with.  Hence, in order to improve VO2 Max, it is necessary to both stress the heart to improve stroke volume and also the Type I muscle fibers to get the necessary enzymatic changes to occur.  Basically, the key to both of these is duration. 

Also, note that, in order to be competitive in endurance events, you cannot do only distance work.  Although much of the race may be done at low intensities, there will be times (hills, final sprints) when you need to go faster than your easy pace.  If all you've done is distance work, you'll get blown away during these times. That's' why its imperative to include an anaerobic component to your training (intervals) to compliment the aerobic conditioning to race successfully.

 AT Training 
 
Anaerobic Threshold Conditioning works in two ways.  The first is an increase in maximal heart rate due to a rise in cardiac output and stroke volume. The second is the specific training of Type IIa muscle cells, which have some aerobic capacities, unlike Type IIb (which is also a Fast Twitch muscle fiber, but has no aerobic capabilities).  AT training enhances aerobic enzyme activity and muscle density in the Type II cells, giving them a greater ability to supply O2 for the energy you need when racing at high speeds. The way to achieve these gains is through intervals. Intervals are designed to bring heart rate, cardiac output  and 02 intake to maximal values, which help you obtain those last little gains in VO2 your economy. We all reach a point in our fitness  where it is very difficult to improve our performance even the slightest bit. Intervals will work on your cardiovascular system, improving O2 delivery and efficiency rates, giving you a boost in ability. More importantly, they will allow you to exercise continually at a higher percentage of your functional capacity.

AT Training Can Improve Performance By 7-8%

Researches have shown that a trained endurance athlete can improve his or her performance by 7-8% by including AT work in the weekly workout.  A 7% increase in aerobic capacity could mean a considerable time difference in a 10k performance.  For example, if you complete the 10k in 38 minutes, a 7% increase in your aerobic capacity could allow you to better your best time by a little over 2 minutes.  The reason for this increase is that with AT training you train your 02 system to work more efficiently.  In order to improve your oxygen delivery system adequately enough to get these benefits it's necessary to perform anaerobic conditioning which, contrary to popular belief, does not necessarily mean exercising without oxygen.  Anaerobic simply means that you can't supply enough 02 or energy to the active muscles in order for them to constantly rely upon fat and glycogen as their sole energy source.  At high intensity exercise there is not enough 02 provided.  Thus you slip into anaerobic metabolism, which involves the breakdown of glucose to pyruvic acid and lactate acid without the presence of 02.  This makes up the difference and supplies additional energy to your muscles to keep them going.  There are several different types of AT training that one can do to enhance the oxygen delivery system, including Long Fast Distance Variable Pace, Fartleks, and Tempo runs.