Energy in Humans – Thoughts on Energy

Energy and Exercise

Introduction

The topics of health and exercise are never far from many of our minds. It is definitely late enough in the year now that new year’s resolutions about exercise probably haven’t lasted this long. We are now well past the January spike in interest in gyms and gym memberships (as pointed out in this interesting article about the economics of gyms). But it is still a good time to explore the topic of the energy of exercise.

Hopefully most of us realize that food, exercise, health, and weight are all intertwined. Of course, those interactions and discussions get very complex and I can’t begin to scratch the surface of all that that entails. Thus, my purpose in this post is not to attempt to comment on all of the implications of energy and exercise. Rather, I will simply consider a few seemingly simple questions just for the joy of thinking about energy. The questions:

How much energy is used during exercise?
How is energy measured during exercise?

How Much Energy is Used during Exercise?

Often, this question is asked in the context of attempting to burn as many calories as possible so a person can lose weight. To answer this question, various tables have been put together with a quantification of the amount of energy “burned” by humans performing various activities including different forms of exercise. And not only exercise, but different types of activities in general. Of course, a person still burns some amount of energy even when sleeping or watching TV.

The following table shows how many food calories (about 4,184 joules per food calorie) a person weighing 155 pounds “typically” burns in half an hour of performing the listed activities. These calorie values are based on studies performed of people doing these exercises while metabolic rates are monitored. I found two sources for the information in the table, which mostly agreed, so may be from the same original source, but I included both just to show the comparison. The two sources were Harvard Medical School and website VeryWellFit.com.

Of course, I just selected a few of the many, many activities listed on these web sites, just to give a flavor. Obviously, there is a great deal of variation in the energy requirements of various exercise undertaken by humans. It is important to note, however, that the thing about a “typical” person performing “typical” exercise is that there is actually no such thing as “typical”. The actual energy burned during exercise varies significantly based on a variety of factors, some of the most prominent of which are the following:

Weight
Relative amount of different types of body mass
Actual activity rate

The first factor is weight. The more a person weighs, the more energy they burn off during exercise. In fact, for a typical person, energy burned is directly proportional to body weight. So a person who weighs 200 pounds would just need to multiply the numbers in the table by 1.3 (200 divided by 155) in order to get the number of calories they would burn performing that same activity. So, for example, a 200-pound person would use about 408 calories in half an hour bicycling compared to the 316 calories a 155-pound person would burn. This makes sense. It takes more energy to move more mass around, and it takes more energy to support more body tissue.

However, the second factor accounts for different types of body mass. There is a perception out there (which I maintained as well before doing some research) that a person with more muscle mass burns a lot more calories at rest than a person with more fat. It turns out that this isn’t as big of a factor as one might be led to believe. In fact, for a person at rest, typically at least 80% of energy consumption occurs in the major organs of the body (heart, lungs, kidneys, brain, and liver). See a good summary presenting this information in this article. Another good reference can be found here. Thus, the proportion of the body that is muscle tissue versus fat tissue is a relatively insignificant factor for total energy consumed at rest compared to the impressive metabolic rate of organ tissue.

The third factor is the actual activity rate maintained for a particular exercise. While the ratio of muscle mass to fat mass does not significantly effect resting metabolic rate, it can significantly affect daily activity rate. That is, a person with higher muscle mass will be more likely to have more active periods during the day and have higher metabolic rates during those active periods. It is pretty simple: the more muscle a person has, the more easy it is to be more active.

So, while a “typical” person weighing 155 pounds will burn 316 calories in a half hour of playing basketball, not all basketball is equal, and those casual playing will burn less energy than those involved in an intense game. Thus, the actual calories burned will vary significantly from the listed amounts in the table.

Of course, elite athletes will of course burn more energy than casual exercisers. This can be shown in an example from a book I read recently. While the table indicates that “vigorous” rowing still burns significantly less calories than running at a seven minutes per mile pace (316 versus 539 calories per half hour), rowing by an elite athlete might just be the most energy-demanding activity there is, as described in a quote in the book The Boys in the Boat:

The result of all this muscular effort [of rowing]… is that [a rower] burns calories and consumes oxygen at a rate that is unmatched in almost any other human endeavor. Physiologists, in fact, have calculated that rowing a two-thousand-meter race – the Olympic standard – takes the same physiological toll as playing two basketball games back-to-back. And it exacts that toll in about six minutes.
A well-conditioned oarsman or oarswoman competing at the highest levels must be able to take in and consume as much as eight liters of oxygen per minute; an average male is capable of taking in roughly four to five liters at most.
The Boys in the Boat, Daniel James Brown, Penguin Books, New York, 2013, pages 39-40.

While we all may not be able to become Olympic rowers, we all can control to some extent our metabolic activity rate through each day. This takes us to our next question, how is energy consumption during exercise (or any other activity for that matter) measured?

Measuring Metabolism

As one can imagine, the topic of measuring human body metabolism is vast and covers a lot of ground. We don’t have time to explore all of it in this blog post, but one useful topic for our consideration is the concept of the metabolic equivalent of task (MET). In order to understand the concept of MET, it is helpful to know that the average energy used by a person at rest is about one food calorie per kilogram of body weight each hour. Thus, a person who weighs 155 pounds (70 kilograms) uses 70 calories each hour. For 24 hours, that equals about 1700 calories.

Every nutrition facts label contains the statement “Percent Daily Values are based on a 2,000 calorie diet.” It is easy to see that a 2,000 calorie diet is derived from a typical person at rest for a day (1,700 calories) plus a few hundred more calories for being active beyond resting.

So while a MET can be defined in various ways, the most useful definition for our purposes is “the ratio of exercise metabolic rate to the resting metabolic rate.” So if a person is a couch potato all day, he would have a MET of 1.0 all through the day (and a MET of about 0.9 while sleeping, as activity level then is below the “resting rate”). So let’s look at our chart from earlier, this time in terms of METs instead of calories burned.

It is pretty astounding, at least to me anyway, that the human body can increase its rate of energy use by at least a factor of 15. Human bodies can be amazing energy-consuming machines when needed (or if that is your idea of fun). Of course, not everyone needs to be an elite athlete or maintain such high metabolic rates. The key to beneficial exercise is increasing METs in whatever ways are available.

The graph below shows a possible scenario for a person going throughout the day with different MET levels. Getting some exercise in first thing in the morning is a great idea. One could run for 10 minutes, or bike or row for 20 minutes, or do weightlifting for 30 minutes, or walk for 45 minutes. Any of those scenarios (assuming the remainder of the hour is spent “resting”) would result in an hourly average for the 6:00 to 7:00 a.m. hour of about 3 MET. Then maybe this person has a few opportunities in the day to take a walk or climb some stairs. Then when the person gets home from work, he does some chores or plays with the kids to get some increased metabolic activity.

The end result of the day shown in the chart is a total of 27.5 MET-hours, or about 3.5 MET-hours above the baseline level of 24. Increasing daily METs is a powerful method for overall better health and life satisfaction.

What about Fitness Trackers?

Maybe some of you have been thinking this question all along. Many these days wear little devices that can simply tell them exactly how much calories they have burned in a day. We can simply use a fitness tracker to tell us how much energy we are using in real time all the time, right?

Maybe. As with any data, we need to be cautious how we use it. As noted in this spot on National Public Radio, fitness trackers can have significant error in calculations of calories burned. Another study (summarized in another article on NPR) noted that people wearing fitness trackers actually lost less weight than those that didn’t, presumably due to the “look how many calories I burned, now I can eat a donut!” effect.

Conclusion

The human body has an amazing capacity for converting food into energy to be used at rest (mostly by our internal organs) and during all the various activities that people enjoy. I hope some of you will have a line graph of hourly average METs pop in your heads sometimes to remind you to do something enjoyable to get those METs up!

Kinetic Energy of Human-propelled Objects

Introduction

My goal with this post, as with many of my future posts, is to fill a hole in the Internet. That hole is readily accessible information about the kinetic energy of human-propelled objects.

You see, when I watch sports, I often think about physics. I think about energy and forces and friction and velocity. I’m not the only one that thinks about physics in connection with sports. In fact, physics of sports is getting to be big business these days. With professional sports being a multi-billion dollar industry, any possible advantage in competition is worth money. Sometimes science is utilized to help generate competitive advantages in sports. And on the other hand, teachers of physics all around the world have used sports for years to attempt to entice unsuspecting students into being interested in science. I found a great variety of web pages related to the physics of sports while researching for this post.

At any rate, one day while thinking about physics and sports, I thought of a question: what is the object that can be propelled by a human that has the most kinetic energy? There are various objects in sports that are propelled in various ways (thrown/ hit/ kicked/ rolled). What is the method that produces the most kinetic energy? What is the object that, when propelled, has the most kinetic energy?

After having this question, I performed an Internet search to find the answer. I couldn’t find it. I performed various searches, but was never successful in finding the answer. So I determined I would need to calculate the answer myself.

The math to calculate kinetic energy is not difficult. The equation is simply kinetic energy equals half of the value of the mass multiplied by the velocity squared.

So it becomes a matter of simply finding all the values and plugging them in to solve for kinetic energy. I selected a variety of possible winners for objects in sports and plugged in the numbers. The results are shown in the table below, sorted by most kinetic energy on top to the least kinetic energy on the bottom.

Results

First, I will discuss the results, and then make some disclaimers and notes about the data included. I do not include my sources for all of the numbers because the information is fairly readily verifiable except in a few cases as noted below. The first note I must add for consideration in discussing the results is that the numbers shown in the table are mostly maximum values. For example, the fastest speed for a human sprinting is about 12.3 meters per second, which represents Usain Bolt at his peak speed in an Olympic sprint event. This is obviously a lot faster than the average person can run. I applied the same rule to the different objects to find the fastest speeds they can go. So the average person will not achieve similar results to those shown in the table, and maybe not even close. But the maximum results give us a way to compare the different objects.

In looking at the results, the top entry is for a human sprinting, which I don’t consider to be in the category of “human-propelled object”. Thus, that entry is shown only for comparison purposes. Maximum kinetic energy of a human sprinting is approximately eight times the highest kinetic energy of a human-propelled object. This makes sense because when running, a person can continue to add energy to the “object” (their own body) as they pick up speed. The energy doesn’t need to be added all in one short burst as with any propelled object.

The rest of the results in the table were initially surprising to me, and perhaps are surprising to you as well. If I had thought about it some more and done some rough numbers in my head first, the results would not have been surprising (but sometimes one must simply calculate first and think later!). Let me explain. In the kinetic energy equation, the velocity is multiplied by itself while the mass is not. Thus, it would appear that the velocity of an object will be the dominant factor in determining total kinetic energy.

Thus, I figured a golf ball, which I knew must get to some pretty impressive speeds, would yield a high value for kinetic energy. A baseball also gets to relatively high velocities. As you can see, however, these are at the bottom of the table. It is the shot (which is the name of the spherical object that is “put” in a shot put try), with a relatively low velocity, that wins for the propelled object with the most kinetic energy. This is because the shot has a mass that is about 160 times that of the golf ball. While the golf ball does reach speeds five times higher than the shot (resulting in the velocity factor of the kinetic energy being 25 times greater), the shot’s overwhelming mass compared to the golf ball makes a greater difference in yielding a high kinetic energy value of about 750 joules.

The other Olympic throwing sports come right behind the shot put, with the discus around 600 joules and the javelin around 400 joules. A bowling ball, the first real object in the kinetic energy rankings that the average American sporting enthusiast has access to, comes next with about 350 joules. However, the bowling ball spends most of its time limited to rolling along the ground. Along with the preceding objects, the bowling ball is simply propelled into a field of play with no opportunity for further interaction by another player.

Thus, the soccer ball is the first object in the list that is actually “in play” during a sporting competition. That is, a soccer player has the opportunity to absorb the full impact of the almost 300 joules of kinetic energy of the soccer ball. Good thing soccer balls are relatively soft and have a large area of impact!

An arrow, at about 250 joules, and a bullet (typical 22 caliber rifle) at about 100 joules are included in the table for comparison. Obviously, the kinetic energy of a bullet depends significantly upon the type of weapon and bullet (as shown in a table on this blog post, which also includes the kinetic energies of some of the objects included here). However, it is interesting to consider that a soccer ball at high speed can have almost three times the kinetic energy of a typical 22 caliber rifle bullet!

Interestingly, a football and a baseball, when thrown at their respective highest speeds, have approximately the same kinetic energy at about 150 joules. While a football has approximately three times the mass of the baseball, the baseball can be thrown at speeds almost 70% faster than a football, resulting in approximately the same kinetic energy.

Further Discussion and Notes

When I first put the table together, I figured I would need to have separate categories for objects that utilize an external tool to be propelled (such as a golf ball, which is normally propelled by means of a golf club) and those that do not require an external tool, since I figured it would give objects an unfair advantage to have a tool to propel them. However, with the final results, it is clear that the winner, the shot, is an object that does not require an external tool.

One interesting find I had while researching the maximum speed of a football was this article about physics in football with some gross physics errors. The article mentions that the work done by a football quarterback is the force multiplied by the distance, which is approximately correct; however, the distance used in the article is the total distance traveled by the football instead of the distance that should have been used, that traveled by the football during the quarterback’s arm motion. Thus, the work done on the football is calculated at 67,000 joules instead of about 150 joules as it should have been (incidentally, the value used for the force applied is also incorrect), as the work done on the football should approximately equal the kinetic energy of the football as it leaves the quarterback’s hand. This just goes to show you can’t trust everything you read about physics on the Internet.

For baseball, you might wonder if balls can be batted faster than they are pitched. The answer is yes. The maximum speed of a batted ball can get upwards of 120 miles per hour (compared to maximum pitch speed of 100 mph). The forces involved in the collision of baseball to bat can be pretty spectacular, as explained in an article from Popular Mechanics.

It was somewhat difficult to find the maximum speed of a shot put. I ended up just using the world record shot put distance and the fact that a typical shot put launch angle is 40 degrees to solve for shot put speed. There is a related physics homework problem (with fictional data), but the result is that the equation to solve for velocity is:

This yielded a velocity of 14.4 meters per second, which makes sense with a related article that solves for a female Olympic athlete’s shot put speed at 13.5 meters per second.

An athlete putting some serious kinetic energy into a shot (the round metal ball)!

One object I considered including was an atlatl, but I found it to be not significantly different from an arrow. I thought perhaps hunters before firearms were available would have found a way to get a lot of kinetic energy into an object for survival purposes, but it appears that maximum kinetic energy is not the only consideration in launching a projectile for hunting purposes.

Another object I considered including was a curling stone. However, the speeds of curling stones in competition are so low (two to three meters per second) that the kinetic energy ends up being pretty low as well. I suspect with the right conditions, an athlete could find a way to propel a curling stone at significantly higher rates of speed. Since a stone has a mass of about 18 kilograms (about 40 pounds), the kinetic energy could get pretty high.

I learned a few things about archery. Increasingly sophisticated compound bows have been consistently topping previous arrow speed records in recent years. Also, there is a lot of thought put into the mass of an arrow, and the experts have a lot to say about that. Thus, it is difficult to come up with a good “average” number for arrow mass, but I put what I thought was reasonable based on my research.

Another interesting tidbit I found was that the javelin was redesigned in 1986 to keep throwing distances down. The world record had gotten over 100 meters, which was unsafe for some competitions. I just found it interesting that the record distance was intentionally limited when the usual purpose of Olympic competition is to showcase maximum human abilities.

In calculating the kinetic energy of the various objects, I have neglected rotational kinetic energy. Some objects have a significant amount of spin on them, such as a golf ball with up to 2000 rpm and a baseball with up to 1800 rpm. I won’t go further into that realm in this post, but it may be something to explore in a future post.

Conclusion

So the winner is the shot of the shot put event! So if anyone ever tells you they will give you a dollar for each joule of kinetic energy you can produce by propelling some sports-related object with your own power (I know, this happens to me all the time, too), the shot put is a good choice. You might want to practice your form so you will be ready. And if that circumstance never happens, I hope you enjoyed reading this post and considering the energy of human-propelled objects anyway.

If you are interested, here is the spreadsheet for calculating the kinetic energy of the various objects. You can expand to include your own objects if you want. Let me know what you discover!

Kinetic Energy of Human-propelled Objects Download