Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Human Power

  1. Dec 29, 2004 #1
    How can a human produce the greatest amount of physical power?

    One method of producing a great amount of power is a bicycle. The strongest humans can output several horsepower for a while using a bicycle. But this seems limited because it involves primarily the legs without involving other body parts. Another method is the sculling set-up. This seems fairly whole-body but is every limb working as hard as it can? And it seems power is wasted in bringing the oars back to the front. Similar to the sculling set-up is weightlifting. While the power here is only exerted over a small amount of time it must be considerable, and weightlifters tend to have great whole-body strength. Maybe the weightlifting motion could be adapted to produce power over a longer amount of time.

    Optimally a machine should engage every limb of the body simultaneously to the greatest extent possible, as well as the trunk strength, and it should not be burdened by inefficiency of motion. For example, a bicycle pedal can only move in a circle--is that the most efficient way for the leg to exert power or would it work better by moving up and down or in an oval? Whole-body motions like weightlifting are naturally limited by one or two groups of muscles. Weightlifters tend to have thin arms compared to the rest of their physique--the arms can only usefully be strong enough to keep up with the legs, which apparently do most of the real work.

    Can the human arms become as powerful as the human legs, or is there a fundamental biological difference? I know bodybuilders can have arms thicker than the average person's legs, but can they exert continuous power over long periods of time, like legs can?
     
  2. jcsd
  3. Dec 30, 2004 #2

    Ivan Seeking

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    For the record, the average adult can produce about 100 watts constantly. The best athlete can produce about 1000 watts [1.3 HP] for a few seconds.
     
  4. Dec 30, 2004 #3
    http://www.wisil.recumbents.com/wisil/whpsc2001/Blue_Yonder_Team.htm

    This contains a statement that Jason Queally was measured as producing 2200 watts of power. I don't know how long he kept it up. Weightlifters can produce several times even that power, though only over very short periods. If a weightlifter lifts 200 kg to a height of 2 meters in half a second, a reasonable feat, he has exerted about 8000 watts.
     
  5. Dec 30, 2004 #4

    Ivan Seeking

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The key is the time. Like I said, the 1000 watts was for a few seconds. 2200 watts must have been only for a fraction of a second or little more [or drugs may play some role?]. From my own observations which include many hours of testing, most adults poop out at about 300 watts continuous. IIRC, the ride across the english channel in the Gossamer required about 400 watts continuous. This nearly killed the guy pedalling before he made it. Here is one bit found about this.

    http://www.progressiveengineer.com/PEWebBackissues2003/PEWeb 42 Sep 03-2/MacC.htm

    note that 0.4 Hp is only 298 watts.
     
  6. Dec 30, 2004 #5
    Well, okay, for long periods of time, the power is relatively low. But that's the point: How much COULD you exert, continuously? If you used every muscle in your body to greatest effect what would be the theoretical limit for, say, an hour? It would be greater than 0.4 hp, possibly greater than 1 hp, maybe even greater than 2 hp. You could probably figure it out from muscle properties (for example, 150 pounds of slow-twitch muscle contracting aerobically and sustainably equals how much power?)

    Edit: Also, the guy in the plane had to be very light and small to produce the most power for his weight. If you had someone weighing twice as much, he'd be able to produce a lot more power.
     
    Last edited: Dec 30, 2004
  7. Dec 30, 2004 #6

    Ivan Seeking

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    As I understand this, I would think that the limitation is found in the heart and oxygen supply, rather than in the muscles in the arms and legs. You have a few seconds of stored, anaerobic, chemical energy within the muscles, but for sustained periods and minor to moderate loads, the aerobic muscle fibers take over. The supply of oxygen to these fibers sets the limit for the total power that can be produced. IIRC, by measuring one's oxygen intake and the corresponding exhaled gases, the maximum energy available per breath can be calculated. The average volume of oxygen inhaled per second sets the theoretical limit for the average power produced.

    Anyway, I'm already in over my head so I'll stop there.
     
    Last edited: Dec 30, 2004
  8. Dec 30, 2004 #7
    Inreasing RBC counts for enhanced athletic performance

    There is also an aerobic limit based on how many red blood cells you have. Two outlawed methods of increasing sports performance involve increasing red blood cell counts. One of those is for an athlete to "give" a pint of blood six weeks before an event, and then - just before the event - inject the same pint of blood back into himself. Another method is to take a drug that increases red blood cell production:
    http://www.chclibrary.org/micromed/00047260.html

    • Erythropoietin, also called EPO, is a type of protein called a glycoprotein that is formed mainly in the kidneys to stimulate the production of red blood cells.

      EPO is unique among the blood cell growth factors, because it is the only one that behaves like a hormone.

      Some athletes use EPO to enhance performance, as the increased red cell volume adds more oxygen-carrying capacity to the blood. Adverse reactions to this practice can include clotting abnormalities, headache, seizures, high blood pressure, nausea, vomiting, diarrhea, and rash.
     
  9. Dec 30, 2004 #8
    If the limit is aerobic, the solution is to use muscles which are close to the heart so that blood can be supplied more easily. If blood has to travel only two feet from the heart to the chest muscles and back, as opposed to five or six feet from the heart to the legs and back, then far fewer red blood cells are needed and the heart has to work much less hard. I imagine an athlete with his forearms tucked back flapping his elbows, which are connected to a power-measuring machine, as hard as he can.

    He would be limited by how much he could develop his chest and back muscles and also by how selective the body is at allocating resources. Blood can certainly be pumped selectively to one region or another, but when you take it to extremes how MUCH blood can be pumped to one region at the exclusion of others? If he runs into these limits then he could use leg muscles in the same way--ankles tucked back and thighs moving back and forth--in addition to elbow motion. This would probably be less efficient than using the shoulder and back muscles because the blood has to travel a little farther, but it's the next best choice.
     
  10. Dec 30, 2004 #9

    Ivan Seeking

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Just a couple of comments in case anyone is interested. I found that the response to load is extremely non-linear. The perceived change in difficulty between 100 and 200 watts is negligible - a 100% increase in load - but the change from 200 to 300 watts - a 50% increase in load - is enough to overwhelm most people in very little time; a matter of 1 to 5 seconds. Also, humans are much more efficient at producing slow, high force motions than relatively quick, low force motions. Even though the same power is produced, we tire more quickly with more motion. This becomes obvious when one considers that in addition to the load we must move our own mass, legs, arms, etc. At the same time, high torque motion met with impulse loads can cause almost immediate exhaustion. The person often feels overwhelmed after only fractions of a second. For example, when the pedals of a bicycle are in the vertical position yielding zero mechanical advantage, the rider is easily overwhelmed if near his or her maximum ability already. Even if they can power through, the riders power output drops off drastically.

    In addition to the two muscle fibers mentioned, aerobic and anaerobic, there is an intermediate group that uses both mechanisms to produce motion and force. I always found it interesting to try to understand this interplay of the three different types of muscle tissues, given that discussed above.
     
    Last edited: Dec 30, 2004
  11. Dec 30, 2004 #10
    Bicycle generator

    I have a fixed standing 10 speed bicycle with a full sized car alternator belted to the rear wheel. I can make 10 A x 14 V for ten seconds and 6A for 20 seconds and 4 A for maybe a minute. The power (1KW) quoted for others is probably input power. I probably loose half in the belt because it warms.
    I have a 5W solar panel that can make three times as much energy in a day as me.
    Different gears don't help. At a higher speed gear the gears make the torque go up and the alternator makes the torque go down. So it stays the same.
    Generally the procedure is 20 seconds work, 5 minutes to catch my breath and fifteen minutes to cool down. Four sessions and I am sick.
     
    Last edited: Dec 30, 2004
  12. Dec 31, 2004 #11
    Humans don't work well with either particularly high gearing or particularly low gearing. On a bike, you're inefficient on a flat stretch in a low gear, and you're inefficient on a hill in a high gear. There is an optimum force and speed of motion that lets people exert the most power. I guess weight of limbs and muscle contraction rate limits it at one end, and there's just a force maximum that muscles and bones can exert, no matter the speed of motion, at the other end.
     
  13. Jan 1, 2005 #12

    Ivan Seeking

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    We measured true power - factoring in the measured mechanical and electrical losses. Automotive belts are terribly lossy. How long can you pedal with no load or battery connected to the alternator?
     
  14. Jan 25, 2005 #13
    I cranked it dead circuit but a bit slow for five minutes and then cut on the alternator and went normal power for twenty seconds. I think any normal person could crank dead circuit indefinitely.
    It feels uncomfortable to go fast with dead circuit because there is no torque increase with lower speed. But with the load on the torque goes up very fast if the speed is lowered. I think too much power is being wasted on the field coil at low speeds.
    I tried a short run with the alternator energized and no battery load. The torque increases at lower speeds than with load but it still rises fast and at low enough speed is more than I can do. The system drags to nearly stopped, the field collapses and then it gets real easy to turn.
    It is a shame I can't instrument this thing. I have no way to measure torque and if I put in an ammeter the current decreases.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Human Power
  1. Are humans animals? (Replies: 6)

  2. A Human or Not? (Replies: 2)

  3. Frequency in humans (Replies: 4)

  4. Human Evolution (Replies: 5)

Loading...