Training for all sports and athletic endeavours focuses predominantly on developing physical skills and capacities that are specific to the unique demands of the sport. Much effort, particularly in track and field, goes into developing our body’s ability to produce energy. While it is generally well understood that energy is produced either anaerobically or aerobically, it is often misunderstood how these energy systems interact. Coming to terms with these concepts can be difficult, particularly given that many traditional beliefs are currently being questioned and appear in need of re-evaluation.
Overview of the Energy Systems
The energy for muscle contraction during exercise comes from the splitting of a high energy compound called adenosine triphosphate (ATP). Unfortunately the stores of ATP in muscles are limited. The body has developed three distinct, yet closely integrated processes or systems which operate together to satisfy the energy needs of the muscles.
The first process involves the high energy fuels ATP and phosphocreatine (PC), which are stored in small amounts in the muscles. The second process involves the non-aerobic breakdown of carbohydrate to lactic acid, and is often called the lactic acid system. These two processes occur without the use of oxygen and together make up the anaerobic (without air) energy system.
The anaerobic system is capable of producing energy very rapidly and can result in large muscle power outputs during brief intense events such as in jumping and sprinting. It is limited, however, by the amount of energy it can produce. The build-up of lactic acid and a quick depletion of ATP and PC will bring about a reduction in power and a drop off in speed.
The aerobic energy system, on the other hand, is capable of producing extremely large amounts of energy. The down side is that it cannot produce energy quickly, being limited by the muscle’s ability to breakdown carbohydrates and fats with the aid of oxygen, and the body’s ability to deliver oxygen to the working muscles.
In summary, the anaerobic system produces energy very rapidly yet is limited in its capacity. In contrast, the aerobic system has an enormous capacity to produce energy yet is somewhat hampered in its ability to deliver energy quickly. Together these two systems are well suited to cope with the high, often sustained, and usually diverse energy demands we place on them during our sporting exploits.
It is in this aspect that our understanding of the interaction between the energy systems is sometimes lacking. There is no doubt that each of the three energy processes is best suited to providing energy for a different type of event or activity. For example, the ATP-PC system is suited to lifting, jumping, throwing, and short sprints. The lactic acid system is more suited to sustained sprints or repeated intense efforts, and the aerobic system to endurance events. It is important to remember, however, that virtually all physical activities will derive some energy from each of the three energy systems.
The energy systems do not function like a set of lights switching from one to the other as the duration of exercise gets longer. It is often written that the ATP-PC system works from 0 - 10 seconds, the lactic acid system from 10 seconds to 2 - 3 minutes, and the aerobic system thereafter. While this might indicate which system predominates during these times, this is far too simplistic an approach. The energy systems should be viewed as contributing sequentially but in an overlapping fashion. We know, for example, that considerable lactic acid is produced in a 10 s sprint. A marathoner, in sprinting for the finish line, will derive some energy from the ATP-PC and lactic acid systems. Perhaps of more significance, and this is where many of our traditional beliefs are being questioned, is the considerable contribution of the aerobic energy system during short duration, high intensity exercise.
It has generally been accepted that the aerobic energy system responds slowly to the demands of high intensity exercise, such as in sprint and middle distance running. Recent evidence from a number of laboratories around the world now suggests that the aerobic system responds quickly during this type of exercise and in fact plays a significant role in determining performance. In 400m running, for example, approximately 45% of the energy required comes from the aerobic system. After only 30 seconds of exercise, the oxygen uptake can be as high as 90% of the athlete’s maximum.
Energy System Contribution
The principle of specificity of training is one that is strongly recommended to coaches and athletes. The guiding theory that often dictates this principle is an estimation of the predominant energy systems involved in various events. Figure 4-5 and Table 4-1 of the Australian Sports Commission’s publication ’Better Coaching: Advanced Coach’s Manual’ (1991), for example, provide a breakdown of the energy system contributions towards various events. This information, however, should be viewed cautiously as it is derived from data that is somewhat outdated and based on the use of inappropriate methodology to assess energy system contributions.
More recent data obtained using the accumulated oxygen deficit method, which separates the aerobic and anaerobic contributions during exhaustive exercise, suggests that information often presented in the coaching and exercise science literature significantly underestimates the contribution of the aerobic system and, in turn, overestimates the contribution of the anaerobic system. Table 1 provides a more up to date summary of anaerobic and aerobic energy system contribution during increasing durations of exhaustive exercise.