You've gotta shake your head in awe and wonder when you realize what muscles do, either when going about the regular tasks of daily life or exercising at your limits.
Truly one of the great miracles of life is how chemical energy, stored in our bodies as glucose or fat, is converted into power and motion - lifting, running, catching, kicking, throwing, etc. To accomplish these, our bodies have multiple energy systems at work, often simultaneously.
To move, muscles must contract, which requires what is called adenosine triphosphate (ATP). When a single phosphate molecule breaks away from an adenosine molecule in the muscle cell, energy is released that causes a muscle to contract. Each time this happens, another phosphate molecule must replace the previous one for the muscle to contract again. That phosphate must come from delivered from outside the cell. The harder and longer we exercise, the more ATP is needed. When you run out of ATP, you stop moving.
Depending on whether you are jogging slowly or sprinting hard, a different energy system will perform the job of providing the phosphate to form ATP. If you're jogging slowly (aerobic), you use oxygen with glucose or fat. If you are doing high intensity exercise (anaerobic), you will use creatine phosphate for a few seconds, then switch to pure glucose, which won't last much longer than a few minutes.
Few of us, even in our most intense workouts, ever discover the limits of our energy systems because if we push our muscles hard enough, our willpower surrenders to the the burning sensation caused by the buildup of lactic acid and we slow down or lower the barbell. On rare occasions, such as shown in the video below, we reach our chemical boundaries, sometimes in ways that are admirable and comic at the same time. You can also learn a lot about how muscles work by watching someone, like these triathlon competitors, enter the twilight zone of muscle failure.
What you just saw was somenne who can no longer deliver phosphate to the ATP in their muscle cells. The commentator is wrong that the person has run out of calories. The energy is present in the body, but in forms that can no longer be delivered and converted fast enough. Actually these women were probably starting to produce glycogen by metabolizing their own muscle tissue through a slower process called gluconeogenesis, a last ditch effort by the body to respond to signals from the brain to get fuel to the muscle cells. But it's too little, too late. The people who finished ahead of them, through a combination of genetics and training, were able to keep their energy flowing.
What's also interesting to watch is how the body naturally recruits other muscles to accomplish a task that muscles usually used in the task can no longer perform. If you are trying to open the tight lid on a bottle of ketchup, sometimes you cannot do it just with an easy twist of your fingers, but must enlist your hardest grip and the action of your forearm muscles. Often when you watch the finish of a running race, you'll see people running in abnormal or awkward ways, throwing their arms side to side or lifting their hips to pull their legs forward. When one set of muscles fail, another set will come forward to try to get the goal accomplished. In the case of the video, these womens' leg muscles were depleted of ATP, but they were still able to recruit their fueled upper bodies, still rich with ATP, and crawl to the finish.
Shakespeare was right: we are such stuff as dreams are made of.
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