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.