Download Energy Systems

NISMAT Exercise Physiology Corner:
Energy Supply for Muscle
Adenosine triphosphate (ATP) is the source of energy for all muscle contractions. Energy
is released when ATP is broken into ADP+Pi (adenosine diphosphate and phosphate
group). Maintaining the availability of ATP for muscle contraction is the limiting factor,
since ATP is not stored in large amounts in skeletal muscle. Viable sources of ATP come
from both anaerobic (does not require O2) and aerobic (requires O2) means. The primary
energy source for a given activity will primarily depend on the intensity of muscle
contractions.
Anaerobic Metabolism
The two main anaerobic sources of ATP are from Phosphocreatine (PCr) and
Anaerobic Glycolysis. Intramuscular PCr stores are used for rapid high intensity
contractions but are depleted in less than 30 seconds and take several minutes to
replenish. For example, PCr provides the majority of the energy for a 100 m sprint.
Additionally, the ability to perform repeated bouts of near maximal effort is largely
dependent on PCr stores. Augmenting PCr stores by Creatine Supplementation can
increase the amount of work that can be performed in repeated bouts of high intensity
exercise.
Anaerobic Glycolysis refers to the breakdown of glucose (glycolysis) to pyruvate, which
in the absence of O2, is converted to lactic acid. In muscle fibers, glucose is made
available through the breakdown of muscle glycogen stores. Anaerobic glycolysis is not
limited by the availability of glycogen; instead, the accumulation of lactic acid and other
metabolites is the limiting factor. High intensity exercise with a duration of 1-3 minutes
(e.g. 800 M race) will rely primarily on anaerobic glycolysis, resulting in a large
accumulation of lactic acid.
Aerobic Metabolism
Aerobic glycolysis occurs when O2 is available to breakdown pyruvate, which yields
ATP through chemical reactions that occur in the Krebs Cycle and the Electron
Transport System. As in anaerobic metabolism, glucose may be obtained from stored
glycogen. Glycogen stores are plentiful, and therefore glycogen depletion is only a
concern for athletes who are continuously exercising for more than 90 minutes or
intermittent exercise over substantially longer periods of time. For example, it is not
uncommon for endurance athletes to become glycogen depleted. In marathon races this is
referred to as "hitting the wall". In order to reduce the chances of depleting glycogen
reserves during a contest, athletes often "carbo load" prior to the event. This involves
manipulating the carbohydrate content of one's diet in order to maximize glycogen stores.
The most abundant energy source available to the muscle fiber is fat. The breakdown of
fat to yield ATP is referred to as lipolysis. While the supply of fatty acids is essentially
unlimited, the rate at which lipolysis occurs is the limiting factor in obtaining ATP.
Lipolysis is responsible for resting muscle activity, but its contribution to the overall
muscle energy supply will decrease as contraction intensity increases. For example,
glycogen depletion occurs when the rate of lipolysis cannot meet the energy demand of
the exercise, and the reliance on glycolysis expends the available glycogen stores. Once
glycogen depletion occurs, exercise intensity will be reduced dramatically. However, a
small decrease in intensity (e.g. slowing the pace) earlier in the exercise bout would spare
glycogen sufficiently to avoid depletion. In turn, the importance of facilitating lipolysis
during endurance events cannot be overemphasized.
Energy Systems Versus Running Speed
Based on world record times, humans can maintain maximum sprinting speed for
approximately 200 m. The average speeds for the 100 m and 200 m world records are
similar (21.6 mph and 22.4 mph, respectively). However, with increasing distances,
average speeds decline. The average speed for the marathon world record is 12.1 mph,
which is 55% of the world record sprinting speed. This is remarkable since the marathon
is more than 200 times the length
of a 200 m race. Although natural
selection plays a crucial role in
elite sprinting and marathon
performance, the energy systems
also must be highly trained and
exercise-specific to be successful.
For example, the energy needed to
maintain an average sprinting
speed of 22 mph for 200 m or less
and an average running speed of
12.1 mph for the marathon are
acquired by two very different
systems (the predominant energy
systems required for running at
different speeds are shown in the
first figure). The primary energy
source for sprinting distances up to 400 m is PCr. From 400 m to 1,500 m, anaerobic
glycolysis is the primary energy source. For distances longer than 1,500 m, athletes rely
primarily on aerobic metabolism.
The rate of glycogen and fat utilization will vary according to the relative running speed.
Although the rate of glycogen utilization is low while running a marathon, the duration of
the event increases the possibility of depleting glycogen stores. In contrast, the rate of
glycogen utilization is substantially higher during a 5,000 m run, but glycogen depletion
is not a concern because of the short duration of the event.
Maximum maintainable speed drops by approximately 7 mph as running distance
increases from 200 m to 1500 m (about 1 mile). However, as the distance increases from
1 mile to 26 miles, maximum maintainable speed only drops an additional 3.5 mph. On
average, a healthy, fit, non-elite, male athlete can be expected to sprint at an average
speed of 16-18 mph for 100-200 m and approximately 6-8 mph for a marathon.