We sometimes perceive our muscles as self-controlling structures that abide by an "all or none" mechanism. This is simply not true. Your muscles are made up of muscle fibers, none of which are thicker than a strand of hair. When your muscle contracts, the entire length of the muscle shortens, however only a small percentage of the muscle fibers are doing the work. So, although all of the fibers are shortening in length, only a specific percentage of the fibers are actually completing the work at any given time.
For the purposes of our explanation, let's imagine a toddler biceps curling a 5lb. dumbbell. Because of the amount of overall strength required to lift the 5lb. dumbbell, the toddler will require quite a large percentage of the muscle fibers in their biceps to contract to lift the dumbbell. An average adult, on the other hand, could curl a 5lb. dumbbell with little to no effort at all, meaning that a lower percentage of their muscle fibers are actually doing the contracting.
You can then begin to think about the 2 (main) types of muscle fibers. Type 1 muscle fibers are known as "slow-twitch" because they don't generate a lot of force when contracting, but can usually contract for great repetition or for longer periods of time. Type 1 could be considered more "endurance" in nature. Type 2 muscle fibers are built for strength and power, and can generate a lot of force. Having said that, Type 2 fibers cannot produce force for too long. There are further sub-sets of Type 2 muscle fibers that can make them more similar to Type 1, but for the sake of this blog, we can stick to just Type 1 and Type 2.
Oh by the way, a couple quick points about Type 1 versus Type 2 muscle fibers. Various muscles in your body will have different ratios of Type 1 vs. Type 2, such as your abdominals which are mostly Type 1, as they are required to be contracted much more frequently throughout the day to keep you upright. Also, your personal ratio of Type 1 vs Type 2 fibers are set by puberty. This is why certain individuals have a greater affinity for muscular strength and growth (more Type 2 fibers) whereas others are more built for endurance (Type 1).
Why is this important? Well, when your muscle fibers fatigue, the fibers that were doing the work stop contracting and other fibers step in to complete the work. There are always at least a small percentage of your muscle fibers resting while others are doing the work. If we think of our dumbbell curl example again, chances are the toddler won't be able to curl the dumbbell as many times as the adult because they are recruiting a higher percentage of muscle fibers for every single repetition, leading to quicker fatigue and failure.
If you are picking up something light, your body will use mostly all Type 1 muscle fibers to meet the demand of the external load. If you are picking up something heavy, then more Type 2 fibers step in to help. Recovery of Type 1 muscle fibers is much quicker than Type 2, this is why picking up something heavy and carrying it may make you a little winded.
Muscle fiber recruitment is orchestrated by the muscle's neurons. One, often overlooked, positive adaptation to resistance training is the improvements in your mind to muscle connection.
Basically, your neurons greatly improve their efficiency at "recruiting" muscle fibers.
So not unlike learning a skill, where your brain must train a synchronized orchestration of your muscles to act in a specific order of events, to say - throw a baseball, the brain must also learn how to actively recruit more muscle fibers in order to generate more force. Although resistance training is great for improving the structural rigidity of your body, it also "teaches" our brains to be more efficient and effective at generating force.
It should be noted that during the first 8 weeks of a resistance training program for beginners, nearly all of the "gains" achieved from resistance training can be attributed to neural adaptations. So, even though one may be experiencing strength gains, this is not due to an increase in muscle size or any measurable improvement in the strength (structural framework) of the muscle itself, instead, it is an improvement in the efficiency of the mind to muscle connection!
Tyler Robbins B.Sc. CSCS
Director of Fitness
Head of CrossFit