# Why does the main muscle use the most energy in body movements?

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• John Constantine
John Constantine
TL;DR Summary
Energy consumption and transmission of muscles
This may seem like an obvious and foolish question intuitively, but I am curious about something. Let's assume we are performing a biceps curl exercise. To properly analyze human movement, a much more complex process is needed, but I will simplify it as much as possible. As the name of the exercise suggests, most of the energy required for this exercise will come from the biceps. However, there are also the forearm and hand that support the object, and the shoulder that stabilizes the entire arm and object.

The work done by the forearm and hand on the object is the product of the force supporting the object and the distance the object has moved. This will undoubtedly have a specific value. The shoulder also does work on the entire arm. This is defined as the force the shoulder exerts to support the arm multiplied by the distance the center of mass of the arm has moved. Thus, the work done by the forearm and hand on the object, and the work done by the seemingly stationary shoulder on the entire arm, can both be defined.

If A does work on B, B's kinetic energy increases, but it does not always mean that A has to lose the same amount of energy. Therefore, even though there is work done by the shoulder and forearm on the object, they do not need to use a corresponding amount of energy. It is intuitively clear that the forearm, hand, and shoulder do perform work on the object, but their energy consumption is low, and most of the energy comes from the biceps.

However, aside from intuitive methods, is there a more physical approach to understand why the biceps primarily consumes energy? I do not want specific numbers or diagrams, but I am curious about how energy transfer or consumption occurs.

You don't necessarily have to use human exercise as an example. For instance, in an excavator, most of the mechanical energy is converted from electrical energy, but the arm lifting the object and the body providing support also do work, which can be defined. Even if A does work on B, A's energy consumption can be minimal, or A might consume more energy than the work done on B. How do we distinguish this?

John Constantine said:
Even if A does work on B, A's energy consumption can be minimal, or A might consume more energy than the work done on B. How do we distinguish this?
It's called efficiency, which can even be negative. Especially for muscles, which often absorb mechanical energy while also consuming chemical energy, coverting both to heat.

PeroK, russ_watters and John Constantine
A.T. said:
Especially for muscles, which often absorb mechanical energy while also consuming chemical energy, coverting both to heat.
Energy is invisible and does not have a specific location but truly exist! ; it cannot be seen transferring from one place to another. Is there even a way to know how much energy each part uses or receives in such complex movements?

A chain of links, that moves against a force, transfers energy. If a link is locked in one position, while another link in the chain moves, the locked link requires very little energy to remain locked.

John Constantine
Baluncore said:
the locked link requires very little energy to remain locked.
This seems intuitively very correct. In interactions between objects or within organisms, if the movement is minimal or fixed, why is the energy lost also minimal? For instance, when we do rowing exercises for our back, the arms move much more than the back. Similarly, during push exercises, the arms move much more than the chest. When doing arm exercises, the shoulders barely move, though this situation is slightly different. My question is, is there a way to determine how much energy each part is using?

John Constantine said:
within organisms, if the movement is minimal or fixed, why is the energy lost also minimal?
It's not in general. Muscles can consume lots of energy while applying a static force.
John Constantine said:
My question is, is there a way to determine how much energy each part is using?
In a living organism that's difficult. Simpler in robotics.

PeroK, russ_watters and John Constantine
A.T. said:
In a living organism that's difficult. Simpler in robotics.
When considering a robotic arm lifting an object, the arm itself merely transmits force, so the actual work is done by the motors attached to the arm. The electrical energy is converted into the mechanical energy of the object. Nonetheless, by the definition of work, the arm does perform work on the object. As the object gains kinetic energy, energy must be lost or consumed somewhere. We know that the energy is not generated by the arm but is instead converted from the motor's electrical energy to mechanical energy. The arm itself does not consume any energy. How can we be sure that "the arm itself merely transmits force"? If we could see energy or its transformation, this fact would be clearly proven, but we cannot.

John Constantine said:
The arm itself does not consume any energy.
It can temporally store or disspate some energy due to deformation.
John Constantine said:
How can we be sure that "the arm itself merely transmits force"?
You cannot be sure of anything. If we make observations that contradict some parts of physics, we modify those parts.

russ_watters and John Constantine
John Constantine said:
My question is, is there a way to determine how much energy each part is using?
Of course. Just not very easily when talking about muscles since they use energy even when not moving an object. Hence why holding yourself off the ground a few inches while in a push-up position is extremely tiring.

A simplified model would look at the force applied by each simplified body part vs how far it moved its load to determine the amount of work done. That should get you into the ballpark for an estimation.

John Constantine, berkeman and Lnewqban
John Constantine said:
How can we be sure that "the arm itself merely transmits force"?
Note that EVERYTHING that performs work on something else does so by transmitting/exerting a force. I think you're running into an issue of trying to find the exact component in a system of components that does the actual work/transmits the energy.

We could point to the end of the robotic arm, at the attachment point, and say that the attachment point transmits the force to the object, so it must do the work. Which would be true. But then the end of the arm has to transmit a force to the attachment point to make it and the object move. And the middle of the arm must transmit a force to the end of the arm. Etc, etc, etc. The entire system is one long chain of force-transmission.

And in a real robotic arm energy is lost through slight bending of the material and through various other losses like friction (all ultimately lost as heat), so it becomes even more problematic to trace where all the energy is going and where it came from. Hence why it's often easiest to simplify things and look at the source of the energy in the system, the major components where it is converted, and whatever you're interested in performing work on. So the motor moves the arm, the arm moves the object, and the object gains energy.

John Constantine and Lnewqban
John Constantine said:
My question is, is there a way to determine how much energy each part is using?
Energy, or work done, is the integral of the muscle tension force, multiplied by the change in muscle length.

John Constantine said:
You don't necessarily have to use human exercise as an example. For instance, in an excavator, most of the mechanical energy is converted from electrical energy, but the arm lifting the object and the body providing support also do work, which can be defined.
Not electrical. Strictly speaking, a static force does no work.

In an excavator, a diesel engine continuously drives a hydraulic oil pump. When work is not being done, the fluid circulates through the pump and an open spool-valve assembly, with very little pressure drop. When work is being done, fluid is diverted by a spool valve to flow first through some actuator, then back into the line of spool-valves. Pressure is developed when the flow is obstructed. Spool-valves are area balanced, so the work needed to operate them, to divert the oil flow, is independent of the developed oil pressure.

The work done by an actuator, is proportional to the oil pressure, multiplied by the volume of fluid that flows.
Pressure drop in pascals * metres cubed = joules.
Pressure drop in pascals * metres cubed / second = joules / second = watts.

Rotary actuators are hydraulic motors that, when needed, drive the tracks or rotate the body.
Linear actuators are hydraulic rams, that can extend or retract, push or pull.

Actuators that are not actually doing work, can be locked in place for support, by a closed spool valve, or can be free to swing, by having an open cross-flow valve.

The flow rate of the oil is proportional to the fixed diesel engine RPM. The hydraulic oil pressure is determined by the actuator forces, controlled by the spool valve assembly.

Hydraulic power is quite the inverse of traditional electrical distribution networks, where the supply pressure (voltage) is fixed, and the fluid flow (current) is varied.

nasu, John Constantine and Lnewqban
John Constantine said:
How can we be sure that "the arm itself merely transmits force"? If we could see energy or its transformation, this fact would be clearly proven, but we cannot.
The weight of a parked car on a bridge is transferred all the way down to the ground via the structural elements of that bridge, which “merely transmits force” and moments.
No energy is cosumed there, as long as the bridge does not collapse.
Let’s say that, when standing, our spine and bones work in similar way.

I would compare the case of muscles holding a weight in static condition with a statically hovering helicopter which is keeping the same car (previously parked on a bridge) lifted in the air (no movement related to ground).

The same weight of that car is now transferred all the way down to the ground via the working engine-propeller-air mass, which also transmits force, but at the expense of contant consumption of energy (for as long as the car is kept in the same spatial position).

In the first case, we are using solid columns.
In the second case, we are using impulse of a non-solid element (air).

In the same way, a muscle (non-solid nature) can only act as a solid structural element (like a bone does) by constantly consuming energy (electrical impulses-oxygen-glucose).

John Constantine
The definition of energy is the ability to do work. To do work, force must be generated. Motors generate force. Robotic arms do not generate force; they merely transmit force. (Is it correct to say they "serve as conduits for transmitting energy"? This is still uncertain.)

If we assume the robotic arm is a rigid body, there would be no energy consumption due to deformation, so it is indeed the motor that delivers energy to the object. The robotic arm does actual work, but it does not consume energy. This implies that something performing work on an object does not necessarily consume energy.

What I'm curious about is how to identify and distinguish the elements of a system that actually use energy from those that merely transmit force without consuming energy. Statements like "the motor generates force," "the robotic arm merely transmits force," and "bones simply support the body" are correct but seem to be conclusions rather than explanations.

How can we precisely determine which components in a system consume energy and which ones merely transmit force without energy consumption.
Baluncore said:
Energy, or work done, is the integral of the muscle tension force, multiplied by the change in muscle length.

Drakkith said:
We could point to the end of the robotic arm, at the attachment point, and say that the attachment point transmits the force to the object, so it must do the work. Which would be true. But then the end of the arm has to transmit a force to the attachment point to make it and the object move. And the middle of the arm must transmit a force to the end of the arm. Etc, etc, etc. The entire system is one long chain of force-transmission.

Lnewqban said:
The weight of a parked car on a bridge is transferred all the way down to the ground via the structural elements of that bridge, which “merely transmits force” and moments.
No energy is cosumed there, as long as the bridge does not collapse.
Let’s say that, when standing, our spine and bones work in similar way.

John Constantine said:
How can we precisely determine which components in a system consume energy and which ones merely transmit force without energy consumption.
Energy is not consumed, it flows, a bit like a fluid and so is transferred from one form to another.

John Constantine
Baluncore said:
Energy is not consumed, it flows, a bit like a fluid and so is transferred from one form to another.
Yes, according to the law of conservation of energy, the total energy is not consumed but merely transformed into different forms. For example, in the case of a robotic arm, the electrical energy from the motor is converted into mechanical energy. It is not the energy that the robotic arm inherently possesses that is transformed into mechanical energy. This statement is correct, but it seems like a result-oriented answer when trying to analyze why this happens or where the energy transformed into mechanical energy originates from. Perhaps I am asking something obvious.

John Constantine said:
The definition of energy is the ability to do work. To do work, force must be generated. Motors generate force. Robotic arms do not generate force; they merely transmit force. (Is it correct to say they "serve as conduits for transmitting energy"? This is still uncertain.)
I dislike this "ability to do work" definition. It is devoid of real content. Force is not "generated", it is the mechanism indicating energy flow. There is no "amount of force in the universe". Newton (3rd) tells us that force is always "merely" transmitted, so your statement is not productive. How does one "create" force.
John Constantine said:
This statement is correct, but it seems like a result-oriented answer when trying to analyze why this happens or where the energy transformed into mechanical energy originates from. Perhaps I am asking something obvious.
What does "why does this happen" mean???? Inimitable Feynman on Why
Are there valid physics questions that are not results-oriented? The proof of the pudding is in the experiment.

John Constantine and gmax137
IMO it is better to think of energy as a calculational tool, bookkeeping if you will, rather than as a "thing" in itself.

russ_watters
John Constantine said:
What I'm curious about is how to identify and distinguish the elements of a system that actually use energy from those that merely transmit force without consuming energy.
...
How can we precisely determine which components in a system consume energy and which ones merely transmit force without energy consumption.
What about static versus dynamic situations?

“Elements of a system that actually use energy.”
In that statement, I would change the word use with transfer, which implies movement or flow, as explained above.

The same concept could be used for manifestations of energy transfer other than mechanical, such as thermal.

“Elements of a system that... merely transmit force without consuming energy.”
In that statement, I would change the word transmit with transfer a finite amount of energy once, which implies movement or flow first, followed by a static condition that internally accumulates that transferred finite amount of energy.

That is the case of the molecules of a bone or a concrete column which are compressed by a load, accumulating certain amount of elastic potential energy while reducing their natural inter-molecular distances.
The process will be reversed once the load is removed by an external force, which will be helped by that accumulated internal stress.

russ_watters and John Constantine
gmax137 said:
IMO it is better to think of energy as a calculational tool, bookkeeping if you will, rather than as a "thing" in itself.
Energy flows through the physical system, in the same way that monetary funds flow through an accounting system.
"The cost of energy", makes money and energy equivalent in our economy.

Lnewqban and John Constantine
It seems to me that in this video, energy flows through the following path:
Electrical - mechanical - pressure - mechanical - molecular - heat and kinetic

The rate of flow of energy decreases as the resistance increases.
Sometimes the resistance is high enough to stop the flow (Earth resists the penetration from the columns of the bridge).
Sometimes the resistance is overwhelmed by the persisting flow of energy (huge pushing force in this case).

For hard and fragile materials, a huge amount of elastic potential energy can be accumulated at molecular level, while the evidence of distance displacement or movement is barely measurable.

I think analogies are best left to other fields of study. Regarding energy in physics, we have very good equations and procedures, IMO they speak for themselves without requiring analogies.

As an example, "energy" is frame-dependent. Your bank balance is invariant, we all see the same value regardless of our velocity relative to your bank.

John Constantine said:
This seems intuitively very correct. In interactions between objects or within organisms, if the movement is minimal or fixed, why is the energy lost also minimal? For instance, when we do rowing exercises for our back, the arms move much more than the back. Similarly, during push exercises, the arms move much more than the chest. When doing arm exercises, the shoulders barely move, though this situation is slightly different. My question is, is there a way to determine how much energy each part is using?
"is there a way to determine how much energy each part is using?" I think a biologist could develop some kind of measuring scheme (bloodflow, chemical changes in the muscle tissues, etc.) that would measure energy use in particular areas. Maybe I'm wrong*, but I don't think trying to back-calculate it from the movement of the rowing machine isn't going to provide very useful results.

*happens a lot.

John Constantine said:
The definition of energy is the ability to do work. To do work, force must be generated. Motors generate force. Robotic arms do not generate force; they merely transmit force. (Is it correct to say they "serve as conduits for transmitting energy"? This is still uncertain.)

If we assume the robotic arm is a rigid body, there would be no energy consumption due to deformation, so it is indeed the motor that delivers energy to the object. The robotic arm does actual work, but it does not consume energy. This implies that something performing work on an object does not necessarily consume energy.

What I'm curious about is how to identify and distinguish the elements of a system that actually use energy from those that merely transmit force without consuming energy. Statements like "the motor generates force," "the robotic arm merely transmits force," and "bones simply support the body" are correct but seem to be conclusions rather than explanations.

How can we precisely determine which components in a system consume energy and which ones merely transmit force without energy consumption.
russ_watters said:
So:
1. Yes, these principles are true.
2. You have to properly define your system and not play bait-and-switch games with it. Keep track of the work/energy faithfully and you'll never have a problem.
3. The efficiency of the human body is exceptionally poor (even sometimes negative) and isn't great for this tracking.
But I'll add this: you are using a lot of words (many handwavey) to describe vaguely what can only be described clearly with math and diagrams. To put it another way: if you want a clear/specific answer, you need to define a clear/specific problem to solve. Once you do, the answer will become unambiguous.

berkeman, John Constantine and gmax137
russ_watters said:
But I'll add this: you are using a lot of words (many handwavey) to describe vaguely what can only be described clearly with math and diagrams. To put it another way: if you want a clear/specific answer, you need to define a clear/specific problem to solve. Once you do, the answer will become unambiguous.
I apologize for causing confusion. I am not a physics major, just an ordinary person with a keen interest in physical phenomena, so I often use vague expressions unintentionally. I will try to clarify the problem more specifically.

russ_watters
Baluncore said:
A chain of links, that moves against a force, transfers energy. If a link is locked in one position, while another link in the chain moves, the locked link requires very little energy to remain locked.

Drakkith said:
Note that EVERYTHING that performs work on something else does so by transmitting/exerting a force.
Trying for a good mechanical analogue of the human body is pretty fraught. Standing up, holding a 5kg mass involves the expenditure of energy within muscles, despite zero force times distance. The energy expended by the body during exercise can't be described in terms of just forces times distances.

It is true that no work is done on the 5 kg mass, unless that mass is moved.
In neither quote was it claimed that muscles are 100% efficient.

It has been a long day. I expect you will deliberately lie down, before you relax and rest your postural muscles.

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