Your body responds to resistance training in a somewhat predictable manner. The reason your muscles increase in size or become stronger relates to your body’s amazing ability to adapt to the overload you place on them. When you weight train, you place controlled stress on the muscles you’re working and increase that stress over time.
The slow and gradual increase in stress you place on a muscle is called progressive overload. The typical way you overload a muscle is by increasing the resistance you use but you can also do it by changing other training variables as well, including a number of sets, number of exercises, types of exercises, number of reps, or how fast or slow you complete a rep.
The Size Principle and Muscle Activation
Muscle fibers are activated and stimulated to contract by a nerve called the alpha motor neuron. Each alpha motor neuron may connect to a single muscle fiber, but more often connects to many muscle fibers. For movements that require fine motor control, like eye movements, an alpha motor neuron connects to only a few muscle fibers, 10 or so, whereas for movements of large muscles, like those in your back or thighs, each alpha motor neuron connects to thousands of muscle fibers.
The alpha motor neuron and the muscle fibers it connects to is called a motor unit. When an alpha motor neuron sends the signal, all the muscle fibers that the alpha neuron controls contract. Now that you know how muscles move – what happens when you place stress on a muscle during a training session? The alpha motor neuron sends the signal to motor units to contract but they do so in an orderly manner. If you place a relatively weak force on a muscle by lifting a light weight, you recruit only a few motor units. As you increase the size of the weight you lift, you progressively recruit more motor units to handle that force.
The type of muscle units you recruit depends on how heavy the weight you’re lifting is. If it’s a light weight, you activate motor units connected to slow-twitch fibers, a fiber type that’s small in size. This fiber type is built mainly for endurance and isn’t capable of generating large amounts of force. That’s okay because you don’t NEED much force since the weight is light. As the weight becomes heavier, your body is forced to recruit motor units with fast-twitch fibers, larger fibers. These are fibers best optimized for force generation. They’re capable of generating large amounts of force but can’t hold that force for very long.
As you can see, you recruit motor units in order or size – smaller, slow-twitch to larger, fast-twitch. When your body doesn’t need to generate a lot of force, when you’re using light resistance, you activate mostly slow-twitch fibers. Only when the weight becomes too heavy for the slow-twitch fibers to handle do the fast-twitch fibers join the party. This orderly recruitment of motor units and muscle fibers is called the size principle. Your body tries to conserve energy and won’t activate the larger muscle fibers unless you need them.
What This Means for Building Muscle Strength and Size
The size principle explains why the best way to get stronger is to lift heavy weights. By working with heavy resistance, you quickly overwhelm your slow-twitch muscle fibers and force your fast-twitch ones into action and these are the fibers that have the most potential for strength gains and growth. In response to the stress of lifting heavy weights, your fast-twitch muscle fibers, over time, lay down new sarcomeres, the component of a muscle fiber that generates force and tension. As a result, you can lift heavier after training for a while.
Not all gains in strength come from the laydown of new sarcomeres in fast-twitch muscle fibers. When you first begin strength training, you may notice you’re a little stronger after only a few weeks BEFORE your muscle fibers have had the time to lay down new sarcomeres. The added strength is coming from your nervous system.
Your nervous system also adapts to strength training by synchronizing the impulses it sends to the motor units. More synchronous firing of the motor units connected to your fast-twitch muscle fibers helps you generate force more quickly and channel it in a given direction. So, you become slightly stronger and more powerful even before your fast-twitch muscles change.
Using the Size Principle to Your Advantage
If your goal is to build strength, use the size principle to focus on your fast-twitch muscle fibers. Since they’re the fibers recruited when your muscles are forced to lift a load heavier than what the slow-twitchers can handle, use a high resistance (80 to 90% of your one-rep max). The heavier the weight, the greater the number of fast-twitch fibers you’ll recruit during a lift. Since fast-twitch muscle fibers lack endurance, you won’t be able to do many reps – but that’s okay. Your goal is to get as many fast-twitch fibers into the game as quickly as you can.
Another way to work your fast-twitch fibers more is to emphasize the eccentric portion of some exercises. Eccentric is when you lengthen a muscle against resistance. An example is the portion of a biceps curl when you bring your arms back down to the starting position. If you slow down the speed of the movement, based on a study published in the Journal of Strength and Conditioning Research, you’ll recruit more fast-twitch fibers.
Yet another way to build up your fast-twitch muscle fibers is to do activities that require speed or power. Sprinting, intense kettlebell workouts, and plyometrics all target fast-twitch muscle fibers. If you’re trying to get strong and defined, moderate-intensity cardio won’t get you there since it activates slow-twitch muscle fibers, at least until you break into a sprint.
In fact, too much moderate-intensity cardio may actually interfere with strength and power gains in a number of ways. That’s why it’s important to structure your workouts around your goals and make sure you’re not sending your body “mixed messages.” Don’t forget – HIIT training, kettlebells, and plyometrics give you the benefits of cardio while activating fast-twitch muscle fibers. There’s more than one way to get a cardiovascular workout.
Finally, using a lighter weight but doing more reps so that you take the muscle to near failure helps you build strength and size. The size principle is at play here too. During the initial reps, you recruit slow-twitch fibers, but as your muscles fatigue, the fast-twitch fibers come to the party. Because you’re able to do more repetitions using a lighter weight, your muscles spend more time under tension. That’s important since time under tension is a factor that impacts muscle growth. With this method, your muscles are kept under tension for a sustained period of time and you’re hitting those fast-twitch muscle fibers toward the end of a set.
The Bottom Line
The size principle explains why you won’t build strength or size by lifting very light weights. You activate the most fast-twitch fibers when you lift heavy, 80 to 90% of your one-rep max. If you use lighter weights, you’ll have to increase the time under tension to recruit the fast-twitch muscle fibers as the muscles you’re working fatigue or use explosive, power movements. Tailor your training accordingly.
References:
Sports Med. 2006;36(2):133-49.
J. of Strength and Conditioning Research Vol 16 (1), 9-13.
OnFitness. March/April 2016. “Strategies for Transforming the Physique”
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Muscle Fiber Composition & How It Changes with Age
How Muscle-Fiber Type Affects Athletic Performance?
Resistance Training: Why You Should Train Using a Variety of Rep Ranges
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