ExRx.net

Hey, so I just wanted some thoughts on this because I've discussed it with some professionals only to get mixed opinions....
------------------------------------------------------------------
So every movement made requires a "contraction" obviously. This "contraction" is made up of a certain amount of motor units (not to be confused with muscle fibers). Different factors play in to how many motor units are contracted together during movements of all intensities.

Its thought that highly coordinated people (dancers, athletes, etc...) have greater stimulation to a higher percentage of motor units. ....
With each contraction of a motor unit comes expense of ATP....

(Just as a reference the human eye has around 10 motor units, while the leg muscles have thousands...)

So my question is this:
If naturally coordinated people exhibit greater stimulation to MORE motor units, could this lead to an increase in resting and active metabolism???

I don’t think so. A motor unit is a 2 piece package. It is a single motor neuron and all of the muscle fibers it innervates. The eye for example has motor units composed of one motor neuron and 10 or so muscle fibers. Large motor units have a single motor neuron and up to 2,000 or more muscle fibers.

In a muscle, there are many motor units ranging from small (a few muscle fibers) to large (many muscle fibers). For example, the bicep may have thousands of motor units ranging from, say, 100 fibers in the smaller units to 1,000+ muscle fibers in the larger units. In general, the number of muscle fibers per motor unit in a muscle is dictated by the level of fine control required in moving said muscle. The finer the control required (such as the eye) the fewer fibers in a motor unit.

When a motor unit is activated, the motor neuron stimulates all of the fibers connected to it. Well, technically, a motor neuron is not physically connected to the fibers but is instead very close to them. When a motor neuron receives an electrical impulse to begin a contraction, it releases a neurotransmitter called acetylcholine which binds to the receptors on the muscle membrane. There is no “connection” but more of an association or proximity.

Anyway, it is impossible to stimulate a single muscle fiber, but instead, the motor neuron innervates all “connected” fibers at once. The number of motor units activated in a given muscle contraction is directly related to the required force needed to accomplish the task.

Motor unit recruitment is dictated by the “Size Principle” as defined by Elwood Henneman. The size principle states that, when a contraction begins, the smallest motor units are recruited first. As more force is required, more and larger units are recruited. De-recruitment is in the exact opposite direction such that the last (largest) unit activated is the first unit deactivated.

Small motor units are most often composed of slow twitch fibers while larger units are typically composed of fast twitch fibers. The smallest motor units are always called into play first. Think of it as your body’s way of conserving energy and testing the waters. If the force required to do the job is greater than what the first recruited units can handle, the next largest unit is called upon. If more force is still needed, then the next largest unit is called, then the next, then the next, etc, until the appropriate amount of force has been generated to accomplish the task.

The contractile mechanics of the muscle are myosin (thick filament) and actin (thin filament). Myosin and actin filaments make up a sarcomere. Bundles of sarcomeres (about 4,500) make up a myofibril. A single muscle fiber can contain from 5 myofibrils up to 10,000 myofibrils.

During a contraction, myosin binds to actin. If you look to the sliding filament model of muscular contraction, you see that the myosin filament slides over the actin filaments. To make this happen, the myosin filament binds to the actin filament and makes ratchet-like movements as myosin slides incrementally over actin. Once myosin binds to actin, it requires ATP to break the bond so that myosin can continue moving. This occurs in both a contraction and relaxation of a muscle fiber. This means it also takes energy to relax.

In short, it is the fibers themselves (in particular the thousands of sarcomeres within the thousands of myofibrils within the individual fiber) that consume ATP. The motor neuron simply provides a neurotransmitter to start the process. A motor unit is simply a motor neuron coupled with X amount of muscle fiber.

Since a motor unit is a single motor neuron “connected” to many muscle fibers, it is evident that the more motor units activated (thus the more fibers used) the more ATP is consumed resulting in more calories burned.

While muscle fiber does consume ATP (burn calories) for cellular maintenance and repair, increased ATP consumption only happens during muscle contraction. This means that resting metabolism isn’t impacted by motor unit recruitment since you are resting.

So, motor unit recruitment (of skeletal muscle) would not affect resting metabolism (for the most part) since you are resting and not working (contracting your muscle by activating motor units). Resting metabolism consumes calories for body maintenance such as cellular repair, waste removal, breathing, digestion, heart beat, etc.

Interestingly enough, it is the brain that is the biggest user of energy in our bodies. The brain accounts for 20% of total oxygen consumed and 25% of total glucose expenditure. Conversely, it has been shown that a pound of muscle at rest only burns around 6-13 calories per day (a pound of fat burns 2 calories per day). Quite a bit lower than the typical muscle lore of 30-50 calories per day.

As for naturally coordinated people recruiting more motor units at one time, I’ve never heard of that. That theory seems to be invalidated by the Size Principle of motor unit recruitment. Motor unit activation and thus fiber recruitment is based on the amount of force needed to complete the job. The more motor units (and thus fibers) recruited, the more force generated.

If a coordinated person was recruiting more fibers at once (activating more motor units) to do the same job as someone that was recruiting less fibers, then the coordinated individual would be expending too much force on a simple movement making them very jerky and rather uncoordinated.

Look at walking. Let’s say it takes X amount of motor unit recruitment to do the job. If a coordinated individual uses more than that (activates more motor units than required), they would be kicking and swinging their legs madly with the excess force. Quite the opposite of coordinated.

Further, look at typing on the keyboard. It takes X amount of motor units to do so. If you use more than what is required, then you’d be banging and pounding on the keyboard like a baboon. So, to the contrary, I wouldn’t think a coordinated person activates more motor units (and thus more muscle fibers) to do the job.

Thank you for the Ex Phys. lesson.
I know about the all or none principle and I know the Size principle... but both of those don't say anything about some people having the ability to recruit more muscle fibers doing simple activities than others.

Your examples are good, but to the extreme. Your walking/kicking legs example assumes that a person is using dramatically more motor units. I'm just proposing that a person could use a higher percentage of motor units (this doesn't mean 100%).

Work needed is not the only factor in recruitment of motor units, it if was there would be no way to over compensate while lifting weights. Not to mention isometric contractions which stabilize overcompensating actions (a common recruitment tool used in balance through our daily lives).

Ok. It’s just that when you said:

It seemed you might be confused about what a motor unit is as you clearly state a motor unit is not to be confused with muscle fiber. On the contrary, a motor unit IS muscle fiber coupled with a motor neuron.

They actually say everything about it. The Size Principle states that motor units are recruited to meet the level of desired force. That applies to maximal effort weight lifting, explosive speed lifting, sprinting, jogging, etc. The desired level of force is met by motor unit recruitment via the Size Principle. The more force desired, the more motor units recruited. Clearly the Size Principle shows that number of motor units recruited rest on the level of force desired.

Could you please show a study/proposal/theory/abstract/hypothesis where coordinated people may be recruiting more motor units, thus more muscle fibers, thus more force, for a given activity than others. I’ve not heard that before, and would be interested in hearing the theory.

I do know that it has been shown in athletes of various sports that motor unit recruitment differs dramatically. For example, an athlete in Olympic Lifting, which requires immediate and short maximal effort, exhibit a synchronous recruitment pattern where they can activate nearly every motor unit within a muscle for the work to be done.

Conversely, the endurance athlete, such as the marathon runner, exhibits an asynchronous recruitment pattern where the muscle uses some motor units for work while other motor units in the same muscle are recovering. This “back and forth” method of recruitment allows the endurance athlete to avoid fatigue while maintaining the desired level of force.

So, it has been shown in trained athletes, individuals that I would call coordinated, that they exhibit different recruitment or “firing” patterns. But still, these patterns still follow the basic principle of required force. Different firing patterns are used to maintain the desired force, but excessive force (excessive motor unit recruitment) is never used or wasted.

I suppose, in theory, that isometric contractions of antagonist muscles could stabilize the excessive force output of greater than required motor unit recruitment, but that is highly inefficient and actually sounds like a physiological disorder. I’ve never read, nor can I quite understand, why there would be increased spatial recruitment in an individual to perform a task that doesn’t require the increased recruitment.

Increases in spatial recruitment and rate coding are both designed for one thing, and that is to produce a stronger contraction and generate more force (or maintain the desired force). I don’t see why the body would generate more force than is required unless there is some physiological disorder.

But, ultimately, your question was concerning metabolism. And again, even if a coordinated individual does recruit more motor units during an activity, this phenomenon would in no way affect resting metabolism.

I suppose you could say that a person using more motor units has fewer “large” motor units resulting in many smaller motor units (fewer muscle fibers per motor neuron). In this case, a greater number of motor units would be required in order to recruit the same number of muscle fibers used in another individual. This would mean more active motor neurons, but results in the same number of muscle fibers being used at a given time. That seems inefficient, especially when one considers that the ratio of fibers to motor neuron is dictated by the level of fine control needed in a muscle, but still, it’s plausible. However, since the same number of fibers are being use, and only the number of motor neurons have increased, metabolism would remain unaffected.

On the other hand, and in the typical individual, more motor units used in an exercise, which equates to more muscle fiber and greater intensity, will naturally result in a greater expenditure of energy. So one could say that active metabolism is affected by greater motor unit recruitment when said recruitment results in greater muscle fiber use. Yet, this is common knowledge. Increase the intensity, and you increase energy expenditure.

Here is a study that shows exercise type (training) has an effect on motor unit firing patterns. So in the coordinated or trained individual, training type definitely changes the firing pattern, but I've not seen where a person has some sort of inherent physiological make-up that results in greater than needed spatial recruitment.

I've never been the most coordinated person myself, but I always thought coordination was practiced and trained.

Ok obviously there are a lot of factors at play here... when it comes down to it. All I was really asking was:
If a person had more firing in his motor units... wouldn't this expend more energy than another person who fired a lower % of motor units.
Obviously it doesn't affect resting metabolism. If there was a study out there saying this I would not ask it on a forum.