Explain the three energy systems, their interplay during activity, and how training principles produce physiological adaptations

What this dot point is asking

WACE wants you to explain how energy is supplied for different activities and how the body changes in response to training. You should know the fuel, by-products, duration and rate of each system, how they interact, and which training methods and principles target each system.

ATP and the three energy systems

All muscle contraction is powered by adenosine triphosphate (ATP). Cells store only a few seconds of ATP, so it must be continually resynthesised by three systems.

The ATP-PC (alactic) system splits stored phosphocreatine to rebuild ATP. It is anaerobic, needs no oxygen, produces no fatiguing by-product, and supplies energy at the highest rate but for only about 10 seconds. It dominates in maximal efforts such as a short sprint, a jump or a throw.

The anaerobic glycolytic (lactic) system breaks down glucose and glycogen without oxygen to resynthesise ATP. It supplies energy quickly for high-intensity efforts of roughly 10 seconds to 2 minutes, such as a 400 metre run. Its limitation is the accumulation of hydrogen ions associated with lactate production, which lowers muscle pH and contributes to fatigue.

The aerobic system uses oxygen to break down carbohydrates, fats and (in extreme cases) protein in the mitochondria, producing a large amount of ATP with only carbon dioxide and water as by-products. It is the slowest to supply energy but can continue for hours, so it dominates in endurance events such as distance running.

Interplay of the energy systems

The systems do not switch on and off; they all contribute at all times, with one predominating depending on intensity and duration. At the start of any activity the ATP-PC system leads, the glycolytic system takes over as efforts continue at high intensity, and the aerobic system becomes dominant as intensity drops or duration extends. In a team-sport game the player relies on the ATP-PC and glycolytic systems for sprints and tackles, and on the aerobic system to recover and resynthesise phosphocreatine during low-intensity periods. The crossover point is set by intensity, not a fixed clock.

Training principles

Adaptations only occur if training is structured. Specificity means training must match the energy systems, muscle groups and movements of the sport. Overload means training above the normal demand to force adaptation, usually applied through the FITT variables (frequency, intensity, time and type). Progression means increasing overload gradually as fitness improves so the body keeps adapting without injury. Reversibility means adaptations are lost when training stops (use it or lose it). Individuality means programs must suit the athlete's starting fitness, age and goals. Variety keeps training engaging and reduces overuse, and a tapering or recovery principle ensures adaptation is realised before competition.

Training methods and their target systems

Continuous training (sustained submaximal work) develops the aerobic system. Interval training alternates work and recovery and can target any system depending on the work intensity and rest ratio: short maximal intervals with full recovery develop the ATP-PC system, while longer hard intervals develop the glycolytic and aerobic systems. Resistance training develops strength and power and stresses the ATP-PC and glycolytic systems. Fartlek and circuit training blend systems.

Physiological adaptations

Aerobic training increases stroke volume and cardiac output, lowers resting and submaximal heart rate (bradycardia), increases capillary and mitochondrial density, raises maximal oxygen uptake (VO2 max) and increases the muscles' ability to use fat as fuel. Anaerobic and resistance training increase muscle cross-sectional area (hypertrophy), increase stores of ATP, phosphocreatine and glycogen, improve tolerance to and buffering of hydrogen ions, and improve neural recruitment of motor units.

How this maps to the exam

Energy systems and training are heavily examined. Practise describing each system by fuel, by-product, duration, rate and dominant activity, then justify a training program by linking the activity demands to specific systems, methods and principles.