Alactacid System (ATP-PC)
The alactacid system is used by the body to produce ATP when there is insufficient time to break
down glycogen in the presence of oxygen for the replenishment of ATP. Residual supplies of ATP in
the body are very limited. In fact, we have sufficient for only one explosive muscular contraction.
An explosive movement causes the ATP molecule to split, providing energy for muscular contraction.
Further muscular work relies on creatine phosphate (CP) breaking down to creatine and phosphate
and releasing energy in the process. The energy is used to push free phosphate back to ADP so it can
become a triphosphate once again. Once reformed ATP can break down again and so the cycle
continues. The problem, however, is that CP supplies are exhausted within 10 -12 seconds and take
two minutes to be restored.
Source of fuel
We have about 90grams of ATP in our body that equates to one or two seconds of hard work.
Following that, we rely on 120 grams of reserved CP stored in our cells. Creatine phosphate is then
the source of fuel for the ATP-PC system
Efficiency of ATP production
The alactacid system functions to make ATP available at a rapid pace making this system very
efficient at producing ATP. However the supply is very limited and cannot sustain long durations of
effort.
Duration that the system can operate
In the alactacid system ATP stores are exhausted after two seconds of hard work and CP supplies are
exhausted in a further 10 – 15 seconds. However, at rest CP supplies are almost fully restored after
two minutes rest.
Cause of fatigue
At maximal or near maximal effort, fatigue is caused by the inability of the system to resynthesis
ADP from CP because CP supplies are quickly exhausted.
By-products of energy production
Although there are no real fatiguing by-products, heat is produced during the process of muscular
contraction.
Process and rate of recovery
The ATP/PC system recovers quickly from exercise. Within the first two minutes, most of the ATP
and CP supplies have been fully restored, with 50% of creatine phosphate replenished in the first 30
second of recovery.
Lactic Acid System
Following 10 – 15 seconds of maximal exercise CP stores are exhausted. ATP still needs to be
produced to provide energy, assuming that the activity requires effort for longer than this, such as a
400m sprint. Sufficient oxygen is not available as it takes up to 2 minutes for the blood to move from
the lungs around the body and then to the working muscles where oxygen is in high demand.
The lactic acid system functions by breaking down carbohydrates to the form glucose. Excess glucose
is stored in the muscles and liver as glycogen. The breakdown of glycogen produces two products,
ATP and pyruvic acid. Without oxygen pyruvic acid becomes lactic acid and builds up in the muscles
causing fatigue.
Source of fuel
The only fuel that can be used by the lactic acid system is carbohydrate. This exists in two forms in
the body:


Glucose- stored in the blood
Glycogen- stored in the muscles and liver
Efficiency of ATP production
The lactic acid system produces ATP quickly, but this requires large quantities of glucose. In other
words, ATP is rapidly available but at considerable cost.
Duration that the system can operate
The duration of the system depends on the intensity of the workload. A near maximal activity may
cause fatigue in 30 seconds whereas an activity performed at 70 -80 % may last 2 -3 minutes and
much longer for an activity performed at a moderate intensity. Generally speaking it is the dominant
energy system for all maximal activities between 30 seconds to 2 minutes.
Cause of fatigue
Fatigue occurs when lactic acid levels build up within the muscle cells. It is suggested that fatigue is
not caused directly by lactic acid but rather by its rate of removal and this varies from person to
person. Lactic acid is produced whenever the body uses carbohydrates as fuel (which is almost all
the time, even at rest). The speed of lactic acid production depends on the intensity of the activity,
the greater the intensity the greater amount of lactic acid produced. Excessively high levels of lactic
acid prevent muscle fibres from contracting causing fatigue. The cause of fatigue in this system is
predominantly the accumulation of lactic acid in quantities faster than it can be removed,
By-products of energy production
Lactic acid
Aerobic System
Physical activity lasting more than a few minutes requires the presence of oxygen to ensure the
continuation of muscular contraction. Oxygen is not immediately abundant to the muscles when we
begin exercise; rather, it gradually becomes available as the oxygen rich blood fills the muscle cells.
This allows the third energy pathway to become the predominant supplier of ATP. Although this
system also uses glucoses for fuel, it does not produce lactic acid as oxygen is now available.
Source of fuel
The aerobic system can use carbohydrates, fat and even protein as fuel. During the early stages of
endurance work, carbohydrate is the preferred fuel, however, if exercise continues beyond an hour
or so fat becomes increasingly important as a fuel and reigns as the dominant energy source if
glycogen supplies become exhausted.
Efficiency of ATP production
The aerobic system is extremely efficient in the metabolism of fuel and the provision of energy. In
effect, it enables the production of much more energy from glycogen enabling us to continue
sustained work for longer periods of time.
Duration that the system can operate
As the aerobic system uses a combination of fuel sources there is almost an endless supply at low to
moderate levels of intensity activity (less than 70%) and at near maximal effort the system can
produce enough energy for an hour
Cause of fatigue
Fatigue is due to the depletion of glycogen and fat stores in the body.
By-products of energy production
There are two major by-products of the aerobic system, Carbon dioxide which is expired and water
which is normally cleared through sweating and urination.
Rate of recovery
The recovery rate of the aerobic depends on the duration of use. If used for a short time it will
recover quickly as glycogen supplies have not been depleted. If used for hours glycogen supplies
could be depleted which could result in glycogen levels taking days to fully recover.