Intensity is how hard you work for and duration is the length of time you exercise for. The energy continuum is used to show how the energy systems interact, to provide energy for the resynthesis of ATP and highlights the predominance of each of the 3 energy systems related to duration and intensity of exercise.
When we look at high intensity and short duration such as 100m sprinting or 1500m we say the exercise intensity is anaerobic. This is because it takes place without the presence of oxygen. This type of exercise will use ATP/PC and LA systems will be predominant systems. A threshold represents the point at which one energy system is over by another as predominant energy systems to provide the energy to resynthesis ATP.
The ATP/PC also known as the alactic system has a duration of 3-10 seconds during high intensity activities. It takes place in the sarcoplasm of the muscle cell and produces 1ATP molecule with aid of the enzyme Creatine Kinase. This is why in activities such as the 100m sprint it is predominant as it is a short duration with high intensity.
The Lactic acid system uses glucose as the energy source to produce ATP from ADT and Pi, through various processes (phosphorlation dehydration and cleavage). The whole system provides 2ATP molecules with the aid of the enzyme PFK, this system also takes place in the sacroplasm of the muscle cell. This system has a maximum duration of 3 minutes, as during this process pyruvate is produced eventually leading to a build up of lactic acid. Therefore this system is most predominant during activities such as the...