A few weeks ago I wrote a post on how to lift heavier weights by utilizing post-activation potentiation (PAP). I eluded to the fact that the nervous system is a huge component in how our strength is manifested. In fact, it has been found that, "When someone starts a strength-training program, there can be, during the first few weeks, a 20% to 40% increase in strength without any noticeable increase in the cross-sectional area of the muscles involved" (Astrand et al. 325). This increase in strength is due to our nervous system, and it can also lead to improvements in strength of more advanced trainees.
Today, I want to cover exactly how the nervous system is able to do this - increase our strength.
It should be a nice physiology refresher for many of you. :)
To start off, let me first define what a motor unit is, which is an important component to this post. A motor unit is made up of a motorneuron, which is located in the spinal cord, and the muscle fibers that it innervates. The motorneuron sends signals to our muscles telling them to contract. A single motorneuron can innervate 100s or even 1000s of muscle fibers (think bigger muscles such as quads, hamstrings, etc.) or very few muscle fibers (smaller muscle groups that require fine coordination such as eye muscles) (Zatsiorsky 60).
So, if our muscles rely on messages from our motorneurons, which are part of our central nervous system (CNS), it is apparent that the CNS is very important in our ability to get strong.
Therefore, if you have any aspirations of getting stronger (which I hope all of you do), it is important to understand how your CNS can help in this.
Our CNS impacts our strength through two factors:
1) Intramuscular Coordination
2) Intermuscular Coordination
Intramuscular coordination involves the recruitment of motor units (how many during an effort), firing rate of motor units (how fast the motor units send signals to the muscle, and therefore, how quickly force can be displayed), and synchronization (motor units firing in a synchronized pattern to allow for more strength expression).
When someone is new to strength training or has had little experience with heavy weights (close to repetition maximums), he or she will recruit less of his or her motorneurons than a person with experience lifting heavy. This inhibition of motorneuron activity is due to inhibitory interneurons. "These interneurons in turn can be inhibited as a result of training. The less the inhibition on the motorneurons, the more motor units can contract at tetanus frequency, eventually becoming synchronized*** (Astrand 326).
It has also been shown that heavy weight training can improve motor unit firing frequency and collaboration of motor units (Astrand 326).
This is why lifting maximal loads is important for improving intramuscular coordination, and therefore, strength - it teaches you to recruit a maximal number of motor units (less CNS inhibition) with a greater firing rate and improved collaboration.
Intermuscular coordination is a little more broad. It explains how our muscle groups work together to produce a movement. I am sure many of you who have tried a new lift often feel weak or uncoordinated, initially. However, after a few weeks you are usually able to use quite a bit more weight than before. This is largely due to improved intermuscular coordination. Your body is becoming more efficient at the movement, and therefore, your ability to display strength is enhanced. Our CNS is key in our learning new behaviors and movements. So if you want to get strong at a particular lift or movement, you need to practice it often (think weightlifters and powerlifters).
This requires A LOT of intermuscular coordination!
So, there you have it. I believe it is important to understand the physiology behind why we, as trainees and/or coaches, program and implement the things we do. Many of us want to get stronger without gaining a lot of muscle mass, or have clients with these aspirations, so you need to understand how strength can be increased through the CNS.
Have a great week everyone!!
***Less inhibition on the motorneurons can also be due to heightened psychological arousal - competition, danger, etc. This is the reason people are able to display crazy levels of strength under extreme circumstances. For example, I am sure you have heard about people lifting cars off of people that are trapped underneath. An increased amount of motor unit activity through disinhibition is how this strength is displayed.
Astrand, Per-Olof & Kaare Rodahl, Hans A. Dahl, Sigmund B. Stromme. Textbook of Work Physiology. Human Kinetics. 2003
Zatsiorsky, Vladimir M. & William J. Kraemer. Science and Practice of Strength Training. Human Kinetics. 2006.