How do muscles actually produce force? And how do we get them to produce more?

If you read my previous article on Strength Training and Health (read it here!), you understand that performing strength training a couple of times per week is important. Very important. It will keep chronic illnesses at bay (like heart disease and diabetes), keep you more mobile as you age and actually prevent you from dying prematurely! If this is news to you, stop, and go read my earlier article. 

Knowing the facts is one thing, but if you are anything like me, you want to know the WHY behind things. What are the mechanisms at play here that causing these facts to occur? And not just surface-level explanations (like the lame fitness blogs rampant on the internet), but the meat and potatoes. You may be asking yourself questions like:

How do muscles actually produce force? 

Why does strength training make me stronger?

Everyone knows that if you lift weights, eventually you can lift heavier ones. But almost no one knows what is happening inside the muscle to cause this. Magic? Definitely not. But it isn’t nearly as complicated as you may think, and you don’t need to be a physiologist to grasp the basic concepts. 

Let’s dive in, shall we?

Muscle Anatomy (Greek anatomē, "dissection")

To understand how a muscle contracts to produce muscular force (aka strength, read this article to understand what strength really means!), we must first understand the anatomy of a muscle fibre. As we can see in Figure 1, an individual muscle (e.g., your bicep muscle) consists of a large number of tubes called fascicles, which in turn consists of a large number of muscle fibres, which in turn consists of a large number of myofibrils...which…in...turn...are just long series of these things called Sarcomeres, which are then made up of these 2 myofilaments called Actin and Myosin (this took me an embarrassingly long time to memorize in Anatomy 101). The sarcomere is the actual part of the muscle that does the contracting by pulling Actin and Myosin closer together through a phenomenon called the Sliding Filament Theory. Basically, when a muscle shortens it is these incredibly tiny sarcomeres that are getting shorter, and when a muscle lengthens it is because the sarcomeres get longer.

Figure 1. Muscle Anatomy.

However, the sarcomere, and thus the entire muscle, can’t do anything without some help. This is where the Motor Unit comes in. A motor unit consists of a motor neuron (nerve) and all of the muscle fibres it attaches too (see Figure 2). Large numbers of these motor units work together to coordinate the contraction of a single muscle. When you want to contract your bicep to impress passersby, your brain sends a signal down a specific motor neuron to the motor units of the intended muscle (in this case the bicep), and through a series of very nerdy physiological steps, causes all of the muscle fibres it touches to contract. This is how you express force (aka strength), by contracting sarcomeres via stimulation from your motor units. Now that we understand the basic anatomy and physiology of a muscle contraction, we are better equipped to understand the mechanisms behind how someone becomes stronger through strength training.

Figure 2. The Motor Unit.

Mechanisms of getting stronger!

(1) Hypertrophy

The first way we can produce more force is by increasing muscle mass, also known as Hypertrophy. Contrary to what some would think, your muscles don’t become bigger by increasing the number of muscles fibres you have (termed hyperplasia), but instead by increasing the size of the muscle fibres you already have. The opposite phenomena are when your muscle fibres shrink, known as Atrophy. Consistent resistance training increases muscle protein synthesis, which causes an increase in the number of myofibrils contained in a single muscle fibre, meaning we now have more sarcomeres in each muscle fibre. The muscle fibre will become thicker as the number of sarcomeres in them increases. Over time, this will result in a bigger muscle (read: becoming jacked). However, a larger muscle will only increase the force-generating potential of the muscle, as there are now more sarcomeres that we can use to produce force, but we still need to activate them. It would be like going from a Honda Civic to a Racecar, but keeping the Civic engine...the potential to go fast is there, but you need the proper engine to really go fast.

(2) Intramuscular Coordination

“Intra” is Latin for inside, therefore, Intramuscular coordination refers to the firing pattern of a single muscle. Remember that the Motor Unit is the junction that delivers the signal for the muscle to contract from the brain. By improving how the Motor Unit functions within the muscle, it will allow you to produce higher amounts of force, and therefore, express more strength. There are 3 primary ways we can improve Motor Unit function to become stronger (Cormie, 2011):

Increased Motor Unit Recruitment

As you become more trained through resistance training, you will be able to; (1) recruit more Motor Units at one time (increasing the number of muscle fibres contracting at one time), (2) recruit a greater number of High-Threshold Motor Units (those that attach to more fast-twitch fibres, which are more powerful and produce more force), and (3) you also lower the threshold needed to recruit your Motor Units (you will recruit more powerful Motor Units sooner than if you were less trained). Essentially, you are better able to recruit, and use, a greater percentage of your muscle to produce force as you become more trained. 

Increased Motor Unit Firing Frequency

Firing frequency is the rate at which nerve impulses are transmitted from the brain to the muscle via the motor neurons. An adaptation to strength training is that these nerve impulses fire at faster speeds, which will increase the amount of force generated during a muscular contraction, as well it will increase how fast this force is developed (i.e., explosive power). Basically you can send the signal to produce force at a quicker rate as you become stronger.

Increased Motor Unit Synchronization

Imagine there is a very heavy rock blocking the road, and five people attach 5 different ropes to the rock in the hopes that they can pull it out of the way. However, they all start pulling in different directions, at different times, with differing levels of effort (there is always that bum that slacks off!). Clearly that is an inefficient way to accomplish this task, and will likely result in the rock not budging. Instead, if the 5 people synchronized their performance, and all pulled in the same direction, at the same time, with similar amounts of effort, they stand a greater chance at moving the rock with less overall effort. This is similar to how our Motor Units operate. As you become stronger, your Motor Units will begin to operate together and in unison during a contraction. This allows more force to be produced in a more efficient manner.

(3) Intermuscular Coordination

If “intra” is discussing a single muscle, “inter” muscular coordination refers to the appropriate activation (i.e., the amount and timing of force being produced) of all of the muscles used during the execution of a particular movement. For example, while performing a Barbell Back Squat exercise, the hip, thigh and lower leg musculature needs to coordinate in a systematic way in order to perform a smooth, fluid and efficient movement. If one or more muscles fire too hard, or too early, or too late, then the efficiency of the movement changes. Very similar to our rock on the road example, but on a much larger scale. As you become better at performing various resistance exercises (i.e., a Barbell Back Squat) your body will learn how to coordinate all of the muscles in the most appropriate way to execute the movement properly. This will allow more effective force application, and less wasted energy, ultimately helping to increase the amount of strength you can express.

This was a very high-level overview of the mechanisms that cause us to express force, and the adaptations that occur in the body from consistent strength training. There isn’t just one reason someone gets stronger, but instead a whole host of interconnected reasons. In sum, we become stronger by increasing the size of our muscle fibres (not the number!), improve Motor Unit function, and coordinate all of our muscles more effectively. 

The next segment of this Strength Training series will dive into Programming Principles. We will explore what needs to take place in a program for us to actually get the adaptations we discussed above.

Stay Tuned!