Muscle: Structure, Function & Growth

If knowledge is power then after reading this you can consider yourself a muscle god! This is the information you’ll need to push your muscular development and fat burning to its very limits. Throughout our site you’ll find the tools to help you fully understand how the muscle contracts to produce force, how your muscles grow in response to your training and how to manipulate your training to maximize overall muscular development and fat loss. So get ready to jack up your brainpower and blast your physique to a whole new weight class!

Muscle Structure & Function

It’s a little known fact that there are actually three types of muscle in your body: cardiac, smooth or skeletal. For bodybuilding purposes we’ll focus on the latter, skeletal muscle, which comprises the bulk of the body’s muscle. These muscles are responsible for force production, along with movement and posture. So whether you’re putting on a show of raw power in the squat rack, walking on a treadmill or simply taking a breather on a bench between sets, your muscles are constantly working, which is quite remarkable. What’s even more remarkable is exactly how they contract to produce force and how you can improve this.

While the following can be technical, we’re pretty sure we won’t lose you as long as you follow along with the diagram below that accompanies the text. So what are we waiting for? Let’s take a look a the gears behind the machine.

Each and every one of these muscles is comprised of subsections called “fascicles,” which are groups of muscle fibers bundled together. Each muscle fiber is an elongated cell that, in most cases, stretches across the entire length of the muscle. Within each fiber are the sarcoplasm and myofibrils. The sarcoplasm is a jelly-like substance that contains and provides nutrients (e.g., amino acids, creatine, glucose and water) for the myofibrils. The myofibrils contain the working components of muscle that make them contract and relax. Think of the myofibrils as the crank set and chain on a bike and the sarcoplasm as the oil that lubricates them.

Within each myofibril are the thin (actin) and thick (myosin) myofilaments or contractile proteins (illustrated in Diagram 5.2). When the muscle is stimulated to contract, calcium is released via T-tubules and the positive charge affects what are known as troponin and tropomyosin. This results in the myosin and actin strand “clinging” together.

Once bound, the myosin and actin strands slide past each other, thus shortening the myofibril, along with the entire length of the muscle fiber. This produces a contraction.

Motor Unit Recruitment & Force Production

As previously mentioned, muscle fibers are grouped together in fascicles. Each muscle fiber within the fascicle is connected to a motor neuron, which is part of the central nervous system (CNS). Collectively, a motor neuron and all the muscle fibers it stimulates is called a motor unit. When the body needs to produce force (e.g. lifting a weight), the CNS signals the motor unit to contract and produce the force required to move the weight. Depending on the amount of force required, your brain will recruit several motor units, some of which fire at different rates and intensities to complete the task.

Motor Unit Type

Motor units can be classified into two categories, slow (slow twitch) and fast (fast twitch). Each motor unit has a distinctive role in terms of contraction speed, fatigability, force production and growth potential. Slow motor units are small in size, have a limited growth potential and generally contain fewer muscle fibers. They contract slowly and have a low-force production, but are quite resilient to fatigue. Essentially, these motor units are built for endurance (in training terms – more reps). The intensity needed to apply force will influence what type of motor units are recruited. As the intensity needed to apply force increases the body calls on the heavy hitters – the fast motor units.

Fast motor units can be further classified into two subtypes – fast-fatigue resistant and fast-fatigable. Fast-fatigue resistant motor units contract quickly, have a moderate resistance to fatigue (don’t get tired quickly) and a high force production. Functionally, these motor units are used for prolonged anaerobic (without oxygen) activities, such as benching a weight that you can only press for 10 to 15 reps.

Fast-fatigable motor units contract very quickly, fatigue quickly (don’t last for too many reps) and have a very high force production. Basically, these motor units are responsible for maximum strength and power (blasting up the big weights). Additionally, they also have the greatest capacity for growth. However, they’re also the hardest to activate. Go figure! If you want to activate these elusive motor units you generally want to lift weights that are greater than 85 percent of your one-rep max for four to six reps. For instance, if your one-rep max on the bench press is 200 pounds, you’ll need to lift 170 pounds or greater. Additionally, you’ll want to lift the weight in an explosive manner*. (Actually, it takes more motor units to lift a weight in one second than it does in two.) Therefore, in order to produce the force required to move the weight quickly, your body will have to recruit more fast-fatigable motor units, which leads to a greater development in strength, size and power.
*Note: An explosive rep is when you push a weight up as fast as you can.

Right about now you may be saying to yourself, “Why don’t I just focus my training on the fast-fatigable motor units if they have the greatest capacity for size and strength?” Well, there are many reasons why you don’t want to do this and they all have something to do with the way your muscles grow.

Muscle Growth

Muscle grows as an adaptation to physical stress (e.g. weight training). When a sufficient amount of stress is placed on the muscle, the trauma causes small, microscopic tears in the fibers and their surrounding sheaths. These tears actually trigger the release of various local hormones (e.g., Testosterone and Growth Hormone) to begin the healing process.

With an adequate amount of rest and the proper nutrition, these tears are repaired by the body. During this process, the fibers are strengthened and enlarged in order for the muscle to handle such stress in the future. This process of muscle growth is called hypertrophy. However, what many bodybuilders don’t know is that there are actually two very different ways a muscle can grow: myofibrillar hypertrophy and sarcoplasmic expansion.

Myofibrillar hypertrophy is an increase in the density and thickness of the muscle’s contractile proteins (i.e., actin and myosin). To induce this type of hypertrophy, you need to train in low-rep ranges, with heavy weights. As you can see there’s a correlation with myofibrillar hypertrophy and recruiting the aforementioned fast-fatigable motor units. However, as also mentioned earlier, you don’t want to only train within these rep ranges. In doing so, you’d neglect the other factor that can impact muscle growth – sarcoplasmic expansion.

Sarcoplasmic expansion is often referred to as cell volumization, which is an increase in size of the sarcoplasm. If you recall, the sarcoplasm contains nutrients for the myofibrils. Increasing the size of the sarcoplasm can translate into larger muscle fibers, along with an increase in nutrients available for the myofibrils. The result? The myofibrils have access to a greater pool of nutrients necessary to contract longer, recover faster and grow larger. Therefore, if you want to capitalize on the growth-potentiating attributes of sarcoplasmic expansion, you’ll need to train in a higher-rep range than four to six, just like you need to do to stimulate the slow and fast-fatigue resistant motor units.

As you can see, for full development you need to train the entire spectrum of motor units.