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The Neuromuscular System Part II:
What A Weight Trainer Needs To Know About Muscle

by Casey Butt

The Energy for Muscular Contractions

There is only one source of energy for muscular contractions: ATP. Energy, to power muscular contraction, is released when ATP is broken down to adenosine diphosphate (ADP) and phosphate (Pi). The body has several different paths by which it produces ATP; and it is vitally important for the body to have these ATP production mechanisms because only a very small amount of ATP can be stored in the muscle (enough for only a few seconds of maximum muscular effort). For this reason, ATP must be supplied to the muscles on a continuous basis during muscular exertion. There are three ways by which the body produces ATP, all of which take place predominantly at the mitochondria. They are:

The phosphagen system: PC is stored in the muscle. A rested muscle contains ~5 times as much PC as it does actual ATP. When PC is broken down, the energy released is used to recombine Pi and ADP to form ATP again. This process can happen in merely a fraction of a second, and so provides a source of quickly replenishable energy. Because of the nature of this mechanism, ATP stores remain fairly constant during the early stages of muscular contraction, but PC stores get depleted. As contraction continues, there is not adequate CP left to continue fueling the necessary ADP -> ATP conversion, leading to the depletion of ATP stores also. This contributes to fatigue of the fiber. In total, within 30 seconds or less of maximum muscular contraction the ATP and PC stores in the muscle are exhausted. It should be noted that the phosphagen system provides the largest power source of any muscular energy mechanism.

The chemical breakdown of PC cycle

As you can probably figure out, the phosphagen system is the primary energy source for short-term, high-intensity work, such as heavy weight training.
NOTE: PC stores cannot be used to provide energy for long-term, low-intensity work.

Anaerobic glycolysis: Glycogen (the form of glucose that is stored in muscle) is broken down to provide the energy for ATP formation and and also the formation of pyruvic acid. Additionally, some blood glucose may be used in this process, along with the intramuscular glycogen. One of the end products of this mechanism is lactic acid, which is made by the eventual conversion of pyruvic acid.

This mechanism can provide more total energy than the phosphagen system, but not as quickly. This being the case, anaerobic glycolysis is the major energy pathway for muscular contractions lasting from ~30 to ~60 seconds.

The effects that the lactic acid (which is produced during this process) has on muscular contraction must be considered here. Lactic acid build-up in the muscle cells make the interior of the muscle more acidic. This acidic environment interferes with the chemical processes that expose actin cross-bridging sites and permit cross-bridging. It also interferes with ATP formation. So, these factors, along with depleted energy stores, contribute to muscle fiber fatigue. Contrary to what was once believed, lactic acid does not cause delayed onset muscle soreness (the soreness that you feel in an exercised muscle the next day or so). High lactic acid concentration does, however, contribute to the sensation of pain in motor nerve endings during muscular contraction.

Oxidative phosphorylation: In this mechanism the body metabolizes carbohydrates and fats (and protein when under starvation conditions or during very long duration exercise sessions) to create energy. Carbohydrates are used more extensively during intense aerobic work (at near 100% capacity levels carbohydrates are used almost exclusively as the energy source) and fats become the primary energy source during low-intensity, long duration exercise sessions. The process of energy release from these substrates is much more complex than we need to get into, and as this process is the least crucial for most weight training activities, it will suffice to say that they require oxygen. Hence, your breathing rate increases during aerobics.

This mechanism provides virtually endless amounts of energy (well, until you collapse) as your body will actually begin to cannibalize itself in order to keep the process going. It does, however, require time and so is not a major player in supplying energy for intense muscular contractions.

As was mentioned above, certain fiber types are optimized to utilize each of these energy production mechanisms.
Type Is (slow twitch (ST), slow oxidative - called red fibers) utilize primarily the oxidative phosphorylation mechanism.
Type IIAs (fast twitch (FT), fast oxidative - called white fibers) utilize both the phosphagen system and the anaerobic glycolysis mechanisms primarily.
Type IIBs (fast-glycolytic - a kind of white FT fiber) utilize primarily the phosphagen system.

It should also be noted that all three of these mechanisms begin at the start of muscular contraction, but because of their natures, and the natures of the muscle fibers being used for the activity, they only become prominent during the time frames given above. This is illustrated in the graph below.

Time Course of Contributions from Different Energy Sources
Taken from Gleim, Anaerobic Testing and Evaluation, Med Exerc Nutr Health 1993;2:27-35

ATP Replenishment

Oxygen is not only used during the processes of oxidative phosphorylation. It is also required in mechanisms which replenish ATP, PC and glycogen. This is one of the reasons why, even if you only do lows reps, you breath heavy between sets of Squats (or any other exercise that utilizes a lot of muscle mass). ATP replenishment occurs roughly in the time frames presented below.

ATP replenishment times

time since activity ended percentage of ATP replenished
20 sec
50.00 %
40 sec
75.00 %
1 min
87.50 %
80 sec
93.75 %
100 sec
96.88 %
2 mins
98.44 %
140 sec
99.22 %
160 sec
99.61 %
3 min
99.81 %

These times assume that the fibers recovering are at rest. If you do anything during this period that depletes ATP then the process would be impaired and the time needed for replenishment lengthened.

If the activity that the muscles were doing generated a lot of lactic acid (anaerobic glycolysis mechanism) - such as intense weight training in the 12 rep and above range - then light activity of the muscles during the replenishment period may actually be of benefit. This is because some of the lactic acid would be used to fuel the light activity and, hence, the activity would help clear lactic acid from the muscle. Care must be taken, though, to ensure that this light activity is not intense enough to require the use of the phosphagen or anaerobic glycolysis mechanisms for energy - this would deplete ATP as warned of above.

Once glycogen stores in the muscle are depleted (from prolonged anaerobic glycolysis) they may take several days to be restored. As this is getting into the realm of nurition, the subtleties of the practice of replenishing glycogen between weight training sessions will be covered in an article on the 'Nutrition And Supplementation Articles' page.

In Part III of this series we'll take a look at the nervous system and the processes that occur to 'set off' a muscular contraction.

The Neuromuscular System Part III: What A Weight Trainer Needs To Know About The Nervous System

The Neuromuscular System Part I: What A Weight Trainer Needs To Know About Muscle

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