Basically there are two energy delivery systems that we must improve if we are to achieve our optimal performances. There are two ways that your muscles can get the energy necessary to run a 5000 meter race. The first system (the Aerobic) utilizes oxygen inhaled on the run, transported via the lung and capillaries to the muscles, and burned along with stored glycogen in energy factories at the cellular level called mitochondria. The byproducts of this form of exercise are carbon dioxide and water which do not interfere with the future production of more energy. The second system (the Anaerobic) also utilizes energy stored within the muscle itself in the form of glycogen but when burned at the cellular level without oxygen it leads to accumulations of waste products, including but not limited to lactic acid, that cause us to 'tie up' (the muscles feel tired and cannot produce energy as efficiently). In cross country we race over 5000 meters. The primary focus of our training will be within the aerobic energy system, since it is much more important than the anaerobic system. Most estimate that a 5000 meter race uses about 80% aerobic and 20% anaerobic.
Obviously both systems have limits. Our aerobic system is limited by how much oxygen we can inhale and our body can utilize. This upper limit is referred to as your VO2Max. For most of us, this limit is reached at just a little bit slower than the pace of our 5000 meter race. When you begin running at race pace (or faster, if you get sucked into a foolish start by the masses at the start), your 'shortness of breath' indicates that your aerobic system can't keep pace with your muscles' demands. At this point, your muscles are required to dip into their anaerobic reserves to satisfy energy requirements not being met aerobically. Since the anaerobic system has limited resources and is grossly inefficient relative to the aerobic system, it is crucial for us to delay racing into "oxygen debt". The key to our preparation then is to increase our ability to deliver oxygen to the muscles. In other words, we want to raise our VO2Max. Research demonstrates that one way to do that is to run distance of between four and thirty miles continuously at a variety of paces. Exercise physiologists disagree on the optimal distance and pace, but most would agree that high school athletes running between four to ten miles at above 70% of VO2Max (Remember that 100% VO2Max is about your 5000 meter race pace- so 70% is about 3500 meters in your 5k time. For example: If you are an 18:00 5k runner, your distance training pace should be faster than 18:00/3500 or 8:10/mile) on a regular basis are going to significantly increase their VO2Max. Additional and important gains can be made through running right at 100% VO2Max. Obviously we can't run long distances at this level, so we run mile repeats at about our 5000 meter pace. Both of these training methods result in aerobic gains. Our capacity is increased through the opening of additional capillaries within all of the exercised muscles that will deliver additional oxygen, the development of more mitochondria to burn the additional oxygen, and a more efficient and stronger cardiopulmonary (heart-lung) system to deliver more oxygen through the capillaries to the mitochondria. This system develops over years and is one of the factors in distance runners still achieving personal bests well after their 30th birthday.
Another means of improving our aerobic system is to raise our anaerobic threshold (AT). That is to increase our running speed at which we begin to accumulate large amounts of lactates in the blood. This pace is well below our VO2Max, but through training we might be able to raise it to 90% of our limit. In other words, in the middle of a 5000 meter race, if you have a higher AT pace than your competitors you will be producing fewer lactates even though you both have the same VO2Max. To work on improving our anaerobic threshold, you need to run some of your distance at this pace. For most of us, our AT pace is about 10 to 15 seconds slower than your 10k pace or at about your 15k pace, or at a pace that feels "hard, but not uncomfortable." Research indicates that runs at this pace of about25 minutes of duration are very effective. You get benefits without over stressing the system.
Also important in our training is developing our anaerobic system. Specifically, we want to raise our muscles' tolerance to lactates by improving the ability of the body to remove this lactate from the muscles through the circulatory system and to increase its ability to buffer the high acidic level that results from anaerobic work. As our muscles become more efficient at dealing with higher levels of lactates, they will be able to perform efficiently longer into the race even at faster paces. There are limits to how much or how long you can tolerate high levels of lactates and there are limits to the amount of anaerobic energy you can store. Workouts designed to improve this ability are referred to as lactate workouts. I feel it's important to mention at this point that our workouts must not only improve our aerobic and anaerobic delivery systems. We also need to become more efficient runners at race pace (both through improved form and increased neuromuscular efficiency) and stronger runners (through increased leg strength). As a result, we will do our lactate work in the form of 400 meter intervals at no more than 5 seconds faster than our 5k race pace. This will be fast enough to create high lactate levels and slow enough to approximate 5k race pace so that we will improve our running economy. By running at faster than our AT pace we generate lactate levels that our bodies will adjust to and better tolerate in future encounters.
In addition, we will spend time running hills and doing bounding for lower body strength and sit-ups, chinnies, pushups, dips, pull-ups, & back curls for upper body and pillar strength.
Finally, we must develop a sound sense of pace and a basic understanding of racing strategies. These can be accomplished through races themselves and paying close attention to the pace and 'feel' during our 400 reps and 1600 intervals.
(Obviously, this subject is much more complex than I have taken the time and space to detail here. If you wish more information, I can refer you to a number of articles or books.)