We've all heard it before: In order to maximize motor unit recruitment you need to... (insert your favorite recommendation here). If recruiting a certain number of motor units allows for a certain amount of force production, then recruiting more would allow for more force production. It sounds like a great thing!
While increasing motor unit recruitment is one neural strategy for increasing force production, it isn't the only one, and has very little to do with increases in maximal force production.
Get ready to breakdown the nervous system's role in maximal force production and learn how to manipulate it to improve your strength.
Meet Your Nervous System
In order to understand how the nervous system adapts, we'll have to go through some basic neuroanatomy and physiology. I know anatomy and physiology can be an intimidating (or boring) subject, but try to stick with me.
The first thing you need to understand is that you don't recruit muscle fibers, you recruit motor units. A motor unit is a motor neuron and all the muscle fibers it innervates. All motor neurons are located in the ventral horn (front) of the spinal cord. The axons from these extend out and divide to connect to the individual muscle fibers.
If a motor unit is recruited, all of the muscle fibers that are innervated by that particular motor neuron produce force. This means that, unless a motor unit consists of a motor neuron connected to one muscle fiber (as far as I know, this has never been documented in humans), it's impossible to recruit individual muscle fibers.
The motor unit is the most basic functional unit of the nervous system, so it's important you wrap your mind around that. Got it? Okay, let's move on to some more complex stuff.
Most motor signals originate in the motor cortex. In general, the descending signals originate in the motor cortex, travel down a corticospinal pathway, and synapse (connect) to a motor neuron in the ventral horn of the spinal cord. The motor neuron then relays the message to the muscle fibers, causing them to produce force.
A Quick Side Note on Muscle Contractions
You may notice that I keep saying that muscle fibers produce force and not that they contract. Why? When most people hear "contract," they think shortening. Muscles can produce force while shortening (concentric), lengthening (eccentric), or not changing length (isometric). What few people realize is that during movements where the total muscle length is increasing, the muscle fibers may actually be shortening.
While a discussion on paradoxical movements of contractile and elastic elements of muscle is beyond our focus, I wanted to bring this up for one reason. We don't necessarily know if the contractile elements of muscle are shortening or not during various movements. We do know that, regardless of if the muscle is shortening, lengthening, or not changing length, the muscle fibers are producing force.
Balancing Excitation and Inhibition
And we're back. Now that you have a pretty good idea of the pathway connecting the brain to the muscle, we can move on to discuss some of the sites of neural adaptation.
In general, the nervous system balances excitation (+) and inhibition (-) to achieve a desired result. Increased excitability of corticomotorneurons in the motor cortex and motor neurons in the spinal cord has been documented following training.(1, 2) If a neuron becomes more excitable, any given signal will result in a larger response.
An excited neuron.
This may be a bit confusing. I've found an example with arbitrary units (AU's) usually helps clear this up. Let's say there are 5 AU's that reach a motor neuron in the spinal cord. The motor neuron processes these 5 AU's and sends 5 AU's to the muscle. Fourteen weeks of training later, the excitability of this motor neuron increases. Now, for the same 5 AU's reaching the motor neuron from descending pathways (from the motor cortex), 8 AU's are sent to the muscle. More AU's to the muscle means more force production!
Other possible adaptations involve decreased inhibition from Renshaw cells, Golgi tendon organs (muscle tension receptors), cutaneous and other receptors, and descending influences from supraspinal areas (think: brain). This is when things get a little complex.
The idea of increased force production due to decreased inhibition is somewhat similar to the above example, but we add in a few more characters. If 5 AU's leave the motor cortex on their way to the motor neuron, it's possible that only 3 AU's reach the motor neuron, due to some form of inhibition.
Inhibition that occurs before a signal reaches the motor neuron is referred to as presynaptic inhibition. Decreased inhibition following training may result in 4 AU's reaching the motor neuron. More AU's to the motor neuron typically means more AU's to the muscle. Admittedly, this is an overly simplistic look at inhibition, but it'll do for our purposes.
Still with me?
The reason this idea of excitation and inhibition can get so complex is because of the synaptic organization of the nervous system. Essentially, nothing is as basic as the two examples we've discussed. As an illustration of such complexity, let's take a look at the Renshaw cell, named after, you guessed it, Dr. Renshaw.
A Renshaw cell is a spinal interneuron. When a motor neuron in the spinal cord sends a signal to the muscle, it also sends a signal to a Renshaw cell. The Renshaw cell actually connects back to the motor neuron that excited it, and inhibits it! The motor neuron excites the Renshaw cell; the Renshaw cell inhibits the same motor neuron. This is known as recurrent inhibition, which follows a different neural pathway than presynaptic inhibition.
Why do we have this seemingly ridiculous connection? The presence of the Renshaw cell allows for short latency (rapid) changes in motor neuron signaling. If the motor neuron signals result in too much force production to accomplish a particular task, Renshaw cell inhibition can decrease the signal quantity very quickly, as opposed to having to wait for your brain to process the situation and send down a new signal.
The quality and quantity of a motor neuron signaling is also affected by sensory input from receptors in the skin, joints, muscle-tendon complex (muscle spindles and Golgi tendon organs), and the vestibular system. But we'll leave that for another day.
That's a relatively basic introduction to the nervous system. If maximal force production is the goal, maximal excitation and minimal inhibition should be the chosen strategy.
Grading Muscular Force
Most of you are probably more interested in how the nervous system functions to produce and grade (or control) muscular force. There are five main ways the nervous system does this:
1. Motor unit recruitment
2. Rate coding
3. Motor unit synchronization
4. Doublet firing
5. Alterations in antagonist activity
Motor Unit Recruitment
As we know, it's impossible to recruit individual muscle fibers. Instead, we recruit motor units. Motor units are recruited in a very specific pattern, from smallest to largest (3, 4), based on the size of the motor neuron cell body.(5, 6) This means that the larger motor units have a higher recruitment threshold. It also means that these large motor units can't be recruited unless the smaller motor units are already active and stay active.
Remember, a motor unit consists of a motor neuron and all connected muscle fibers.
Relevant to this issue, I've heard coaches talk about the idea of targeting low-threshold motor units through lower intensity training to achieve hypertrophy of the innervated muscle fibers (supposed slow-twitch muscle fibers). Hopefully someone can explain that concept better to me.
High-threshold motor units aren't recruited unless the task demands higher amounts of force than the low-threshold units are capable of producing. This means that as the high-threshold motor units are recruited, the low-threshold units are active and firing at a maximal rate.
Consequently, high-intensity exercise leads to adaptations in low and high-threshold motor units. Low-intensity training only leads to adaptations in low-threshold units. If maximizing force production through neural adaptations is your goal, low-intensity training seems illogical.
Smaller motor units produce small amounts of force, but are fatigue resistant. Larger motor units produce large amounts of force, but are highly fatigable. Some of you may be reading this and thinking that this idea of low force/low fatigue, high force/high fatigue sounds like the properties of slow-twitch and fast-twitch muscle fibers. A few years ago, I would've agreed with you.
But now, research has shown us that swapping the nerve input to a slow-twitch muscle fiber and a fast-twitch muscle fiber results in the slow-twitch fiber producing high amounts of force and the fast-twitch fiber producing low amounts.(7, 8) Just to upset all the muscle physiologists, it looks like the force production capability of a muscle fiber is primarily dependent on the neural input!
Rate coding simply describes the frequency of motor unit discharge. Once a motor unit is recruited, it fires at an increasingly rapid rate to produce increasing amounts of force. When a motor unit reaches its maximal firing rate, additional motor units are recruited if further increases in force production are needed.
Motor Unit Synchronization
Synchronization is an interesting occurrence that hasn't received enough quality research attention. Basically, motor unit synchronization involves two motor units firing at the same time or at a very short latency (less than five milliseconds). This results in a rapid increase in force production, as the second firing is able to take advantage of increased muscular stiffness created by the first contraction.
One study showed that trained lifters had more synchronization than skilled musicians (dominant and non-dominant hand) and untrained individuals (dominant hand only).(9) It'd be a logical deduction that training results in increased synchronization. While the research in this area is lacking, another study found that 12 weeks of dynamic training didn't increase the amount of motor unit synchronization.(10)
It's possible that some people just have more synchronization and gravitate towards weightlifting because of their improved ability to produce force, but I'm not sold on that argument. The synchronization changes following training begs for further exploration.
Doublet firing involves the same motor unit discharging at a shorter than normal latency. For instance, if a motor unit is firing every 15 milliseconds, and then fires twice within three milliseconds, the two short latency firings would be considered a doublet. Doublet firing also results in rapid increases in force production, as the second firing is able to take advantage of increased muscular stiffness and increased amounts of available calcium resulting from the first firing.
Doublets are usually followed by a longer than normal latency before the next firing (11). Despite this long period with no firing, the increased force production is maintained, even after the normal discharge pattern has resumed. Research has shown increased occurrences of doublet firings in ballistic contractions compared to slow contractions and an increase in doublets following ballistic training. This supports the idea that it's an efficient neural strategy to rapidly increase force production.(10)
Alterations in Antagonist Activity
This idea is pretty straightforward. If you want to perform a biceps curl, you'd want maximal activation of your biceps and minimal activation of your triceps, since triceps activity would somewhat cancel out biceps force production.
Turning to another arbitrary unit example: If your biceps are producing 15 AU's to create elbow flexion and your triceps are producing 5 AU's, the net effect will be 10 AU's of elbow flexion. If we cut the triceps activity down to 2 AU's, the net effect will be 13 AU's of elbow flexion, meaning more weight moved!
There's some research to suggest that training leads to decreased antagonist activity.(12) I'll point out that some antagonist activity may be necessary for joint stability. A great example of this is the muscles around the knee. Quadriceps force production results in an anterior translation of the tibia. While your anterior cruciate ligament (ACL) helps prevent excessive motion in this direction, a certain amount of hamstring activity will help take some of the strain off the ACL and keep your knees healthy.
In the next write-up, I'll go over the key fact that's frequently overlooked in the neural mechanisms behind force production. Hint: It's the reason why max strength isn't improved through recruitment!