Why do my arms get tired when I hold a weight in front of me?
If I load cement blocks onto a truck my muscles get tired. This makes sense; I am applying a force over a distance and that is work so I am expending energy; I expect my arms to get tired.
But if I just hold a cement block in front of me for any length of time, I am not moving the weight over any distance. So, I’m not doing any work. But my arms still get tired. They feel the same as if I’d been lifting blocks.
I know muscles get tired due to building up lactic acid; but can that happen without expending energy?
What mechanism is going on here? Why am I experiencing the same tiredness even though I’m not doing any work?
You are doing work. Your muscle must contract to hold the weight up. The reason your arm doesn’t move is you’re only exerting enough force to counteract the pull of gravity on the weight.
You may not being doing any work, but your muscles are. Your muscles are literal motors, they whiz along at about 240 rpm so long as they are loaded. So of course they use energy when they are under load.
Imagine that you park your car on the side of a really steep hill and use the engine to “balance” there. The car itself isn’t going to move because the motor is only balancing out the effect of gravity. But the car will use a lot more fuel than it would if it were simply idling. Despite the vehicle standing still, the motor still uses a lot of energy.
Your arms work exactly the same way. You may not being doing any useful work when holding a weight steady, but you muscles still need energy to overcome the effect of gravity.
And in case you’re wondering where the work is being done both both the stationary car and by your arms: it’s the movement *within *the motors that’s doing the work. In the case of a car, the pistons are still moving backwards and forwards, and they are still moving the gas molecules in and out. So work is still being done. In the case of your muscles, the muscle filaments are still whizzing backwards and forwards 4 time/second and dragging the other muscle fibre with them. Work is still being done.
Which would be true, but your muscles are moving weight over distance even if they are resisting a constant load. Just a a car engine is moving weight over distance even if it’s just to prevent a vehicle form rolling downhill.
The muscle fibres are contracting and relaxing several times every second. Every time they relax, the body takes stock of whether the muscle has lengthened infinitesimally, and if it has it instructs the fibre to drag it back by the same amount. So the active fibres are moving the weight over distance. Each individual fibre take an almost immeasurable pat of the weight and moves it an almost immeasurable distance, but the total weight is still moved.
In the same way, the cylinders of a car is still doing work by compressing gases, even if the car as a unit doesn’t move.
So where does the energy go? In the case of the car, it turns to heat (via friction) in the torque converter or the clutch. Obviously, the energy in the muscles is also turned to heat, but what is the actual mechanism involved?
I’m not sure I understand the question. I assume you are talking about the kinetic energy. In the muscle, most of the kinetic energy is converted into potential energy. The motor drags the object into an “elevated” position, which converts the motor’s kinetic energy into potential energy. The object is then allowed to fall downhill a little, releasing the potential energy, whereupon the motor uses kinetic energy to drag it back uphill to to the original position, once more converting kinetic energy into potential. It then is then allowed to fall downhill a little, and so ad infinitum.
If you are asking how the energy leaves the system, it’s mostly through being converted into heat via friction. The fibres rub against each other and the fluids surrounding them. That produces heat which the body disperses to the atmosphere.
This is what I was going to ask. When I raise the cement blocks into the back of the truck, the work is converted in potential energy in the blocks (assume the truck just stays there.)
But if I take one cement block off the truck bed and hold it there, there is no increase to the potential energy of the block.
So, if my muscles expend energy, where does it go? Is there more energy going into heat in my arm?
Guess; Holding blocks for a fixed time in space takes energy. Raising blocks over that same time takes that same energy plus the additional energy that goes into PE of the blocks.
Okay. I guess the question then is why the muscle fibers have to continue whizzing at all. Obviously, if we had locking joints or some such, it would take zero energy to hold the weight.
I guess it’s just a case that has very little evolutionary pressure to optimize, but I still find it curious that muscles don’t have a means for maintaining static loads efficiently.
This is how a starfish defeats a clam. The clam holds its shell closed using a muscle. It requires a continuous supply of energy to hold the shell closed. The starfish has a hydraulic system that tries to pry the shell open. It only needs energy to establish the prying force, after that it just waits. Eventually the muscle fatigues, the clam opens, and the starfish has supper.
It takes energy to produce force, even if that force produces no work. It produces tension. Thus an isometric contraction, keeping the muscle the the same length, produces no work but requires much force.
Note: a muscle can contract without shortening. In fact a muscle can be contracting even while lengthening; those are called “eccentric contraction.” Think slowly lowering that cement block held in front of you down.
This does not conflict with the explanations already given: the individual motor units (made up of multiple sarcomeres) are taking turns contracting (and then relaxing) in order to produce that constant tension. Sarcomeres do not lock after contracting however, they begin to relax (and to rebind ATP) after they have contracted. The process of one motor unit contracting and then beginning to relax again takes less than half a second as might be guessed by the fact that it is literally called a “twitch.” Keeping an enough motor units actively twitching at the same time to keep just enough tension in the muscle to keep the object up, to provide enough force to resist the weight of the object and to prevent the muscle from lengthening, is the goal during an isometric contraction. Providing enough force to produce enough tension to slow the muscle’s lengthening is an eccentric contraction.
It does not require continual energy, though. Once you’ve put the storage element into tension (or compression), no more energy is needed, unless it somehow gets wasted through some other process.
Yes, noticeably more. That’s why you start to sweat when you try to hold a block at arms length
More or less, though it’s nowhere near that simple.
Oh, no, it’s much weirder than that. Your muscles have a near-perfect system for maintaining static loads with no energy input. The only problem is that it only operates when when you are dead. Seriously. That’s why dead bodies go stiff: the muscle fibres lock into position and are able to maintain tension with no energetic input.
And that’s just where it *starts *to get weirder. Muscles have to keep whizzing backwards and forwards because they don’t require energy to contract. They require energy to relax. If they remain contracted they don’t use any energy at all. Which is why dead muscles remain contracted.
All of this is a total pain in the arse, not least because it means that muscles burn vast amounts of energy even when they are relaxed and doing nothing. It’s s system on which there is huge evolutionary pressure to optimise. It’s also a classic example of a system where evolution can’t reach the optimal solution because it started from the wrong position. For animals to evolve muscles that locked under load and required no energy when relaxed, we’d need to stat from scratch, and that can’t happen. We’re locked on our evolutionary path
The problem is, our muscles have evolved from microbial microfibrils. They have the same fibre types, but in their ace the fibres are built up as the needed. Instead of having two opposing muscle fibres that pull against each other, they have a short “motor” fibre that pulls the structural fibre slightly past itself, then clips another unit onto the end of it.
You could think of of like a guy with being handed, one by one, a lot of 4’ lengths of pipe with male and female ends and told told to join them all together. He takes the first length, pushes it along the ground past him so he is holding only the male end, then picks up another length and screws the female in. Then he pushes the entire length past him until he is once again holding only the unjoined male end, takes the next length and screws the female in, them he pushes the entire length past him until he is once again holding only the unjoined male end, takes the next length etc. The guy himself stands still, but the pipe grows infinitely long as it is constructed and pushed past him.
Microbes work the same way. The “motor” unit sits at the end of the structural fibre. It is fed energy, and then sits there “primed” to grab a new piece as soon as it comes within reach and join it on to the structural fibre. By building up a length of structural fibre, the microbe gets rolled or slid in the direction in which it is being built up. It’s like a guy inside a collapsed circus tent pushing the tent around from the inside using the tent poles.
The system works perfectly for this purpose because it’s a push unit, so you want the structural fibre to “lock” as far away from the motor unit as possible. You also want the motor unit primed to grab and join a new section as soon as it becomes available. And if you want the system to stop pushing , then you simply stop providing it with new units of structural fibre and it locks under tension. It’s really elegant and efficient
Our problem is that the first animals co-opted this push system into a pull system. Instead of using the fibres to push the cell in the desired direction, we reversed it used it to pull the end of the cell and make the cells shorter. That’s a more sensible design when you have multiple cells, because it allows adjacent digestive or stinging cells to be brought to bear on a food item, and you can’t do that by pushing.
But from an energetic viewpoint, it’s a complete pig’s breakfast. Because the motor unit is set to “primed” before it starts pushing, when you use it in a pull unit it actually takes energy to relax the unit after it has applied force.
It’s also means that you can’t readily evolve a way to lock the unit in place at no energy cost. The push system “locks” the system instantly by simply refusing to supply any more structural components. But with a pull system, you have to keep the entire fibre system in place all the time. Our muscle fibres don’t get broken down and built up by the millisecond. That means that there is always a new section available for the motor units to grab into, meaning the only way to lock the fibre in place is at full contraction. Since that has little evolutionary advantage, it has never evolved.
It’s more like 1/4 of a second. And they have to relax because because the body can’t control it any other way. If the fibres don’t relax, you have what’s known as a tetanic contraction, which is a fancy way of saying the body has lost control of the fibre and it’s just contracting as hard and fast as it can. The point is that the body has no mechanism for monitoring or controlling the muscle at that stage. If you’ve ever had a cramp then you’ve experienced this on a small scale. It’s not pleasant, but more importantly it’s not useful. The fibre may be producing force, but it’s utterly uncontrollable.
For it to be otherwise, you’d need some sort of system that allows the body to stop the contraction process at the right tension. But we don’t have any such system. Our muscles fibres work on an all-or-none principle: either they contract or they don’t. there’s no such thing as a partial contraction.
And we haven’t evolved any other system because there isn’t any evolutionary pathway from here to there. We’ve so-opted what was a perfectly efficient push system for moving individual fibres within cells, into a pull system for moving entire cells. Needless to say the original control systems don’t work any more, and the systems we’ve evolved to control the jury-rigged system don’t work very well.
Interesting explanation–thanks. But I don’t get this. Sure, I can buy that any given muscle fiber can only contract fully or not at all. But isn’t a given muscle composed of long strings of fibers? Why couldn’t, say, 40% of them along the length be at full contraction, and the rest relaxed? Any given fiber would be all or nothing but the muscle as a whole would be 40% contracted.
I think this is missing the point. The physicist’s definition of work (which does involve force being exerted over distance) is the relevant one here. A coat hanger doesn’t need an energy source to hold my coat up all day, because while it is exerting a force, it’s not exerting it over distance, so no work is done. At a microscopic level, though, your muscles are moving, as others have said.
It takes energy for your muscles to produce force, because they are actually doing work. But in general, no, it does not take energy to produce force.
Actually, I think this does happen. In weightlifting, there’s a phenomenon called “newbie gains” where someone that’s just started working out can increase the weight they’re lifting fairly quickly. Their muscles haven’t gotten stronger, but by training the muscle they’ve begun recruiting more muscle fibers - they’re using more of the muscle they already have.
Not quite what Dr. Strangelove is talking about. You are talking about with newbie gains is neuromuscular adaptations. In terms of strength that means getting as many motor units to fire at the same time as possible; in terms of power (work/time also defined as the scalar product of force and velocity) specific to a particular sport activity it means getting them to fire in a very coordinated sequence. Those neuromuscular adaptations can happen fairly quickly while hypertrophy and endurance changes in the muscle units (increased cross sectional area, changes in architecture, metabolic adaptations, etc.) take a longer period of time. Neither however is locking some percent of fibers in a contracted position.