Energy Zones and Swim Training Systems

Part one of three

By Coach Tom Phipps

In this series of articles, we will explore some of the basic science of exercise physiology and the training models that have developed from that science. The discussions begin with the abstract and progress to the concrete or, put another way, from the science of exercise to the art of training. In this first section we will discuss the science of muscle energy systems. If science gives you the hives you can stop here but if you carry on you will, hopefully, gain some insights into the following sections. Later we will discuss ways to try to translate science into practical measures in the pool and then to a couple of not entirely inconsistent training systems we will be employing in the pool, once the world reaches a point when we can have organized workouts again.

Energy Systems and Muscle Metabolism

There are fundamentally three systems in play to supply the energy required for muscle contraction (actually the energy is expended in getting the muscle fibers to relax but that is a whole different discussion). There is an immediate system that can fuel explosive efforts lasting less than about six seconds and it is important in sports like football but in swimming it is limited to movements such as starts. Glycolytic metabolism support high intensity efforts lasting 6-60 seconds. These systems are often referred to as anaerobic although, if you want to get all technical, they use oxygen and are not truly anaerobic. These processes are important in sprint events. Aerobic metabolism fuels intensive long-term effort lasting more than a minute and, in theory, as long as oxygen and fuel persist.

The immediate systems represent the final pathway of energy metabolism at the cellular level. The ultimate fuel for all cellular activity is the phosphate bonds of a molecule called ATP and the source of that phosphate, creatine phosphate. The reserves available at any given moment are quickly depleted although multiple systems exist to replenish those supplies.

The aerobic system is located in cellular organelles called mitochondria. It is a very efficient system that can use fats, proteins or carbohydrates as fuel and leaves only CO2 and water as waste products. Unfortunately, the system is limited in capacity. That limit can be modified by training. The number of mitochondria in your cells and that, combined with the status of heart lungs and blood vessels can determine the amount of oxygen you body can absorb, deliver and process. The upper limits of this system, the point where energy demand exceeds the aerobic capacity, is called the lactate threshold.

Beyond the capacity of the aerobic capacity the body can produce energy through a system that burns carbohydrates in a process called glycolysis. Glucose in the blood and glycogen in liver and muscle are converted by a less efficient process to lactic acid and the related compound lactate. Lactate can be further broken down by the aerobic system to produce energy but beyond the limits of that system it accumulates and ultimately limits energy production and muscle activity.

The ultimate limit of the capacity of these systems is the ability to deliver oxygen to cells, the VO2max. Activity at this limit is not sustainable due to lactate accumulation but is important in sprint events.

All of these energy systems are constantly in action but at different intensities of activity one may be more important than the others. At low to moderate activity mitochondrial activity is most prominent, burning fat and later carbohydrate. At about 60% of VO2max glycolysis becomes the primary source of energy with carbohydrate as the main fuel and at lactate threshold waste products begin to accumulate. As VO2max is approached activity can only be sustained for brief periods.

In addition to these changes at the molecular level it is important to remember that other systems are influenced by training. While not the focus of these articles we would anticipate central adaption, that is increased delivery of oxygen to the muscles, and peripheral adaption, increased utilization of oxygen by working muscles by increases in mitochondria and capillaries.