Have you ever looked a raw cut of beef and wondered what all of those lines in it are? The ones that if you accidentally (or purposefully) cut parallel to them when eating that nice flank steak it make the meat incredibly chewy? Unless you're vegan or vegetarian, you might know what I'm talking about. It's not just beef, as you can see it in chicken, pork, or Turkey as well, I only choose steak (and flank steak) because it resembles very closely what the human body looks like at the muscular level.
What you see when you look at a cut of meat, is the muscle fibers that were the source of locomotion for the animal. Our muscle fibers are very similar as adults. They form lines that at some point meet a tendon which attaches it to bone. This may seem like it's kind of mundane, but when we look at producing maximal force and power, recruiting as many muscle fibers as possible becomes important.
Now, it's easy to think that muscle fibers would all contract when we move. But they don't. The body only recruits what it needs in order to perform a given action, thus it does not recruit every muscle fiber in order to conserve energy. If we want to create very forceful actions, however, we need to get as many fibers contracting as possible. Once we get as many of the muscle fibers recruited as possible, they start to adapt to a lesser nervous system stimulus and start to work together more. This is considered to be intramuscular coordination, which leads to bigger increases in strength over time.
But how do we recruit more muscle fibers? Simple:
Fatigue
We often think of fatigue as bad thing, and when considering fatigue levels in preparation for a specific date we should absolutely worry about the role it plays in performance. But in order to create a greater amount of recruitment we have to create moments of fatigue.
When we do movements under fatigue, the initially recruited fibers lose their potential to produce force. This means that in order to continually produce force, the body has to to recruit more fibers. This is what we consider repeated efforts in training. Repeated efforts is often used with submaximal loads and short rest times. This means that working muscle fibers don't get much rest between sets, which ultimately places a new demand for more fibers to be recruited.
Repeated efforts, however, can also mean doing similar exercises in consecutive sessions. My favorite example of this is the Walking Lunge method, which I first learned from Travis Mash's eBook "Squat Everyday". Something I noticed with toying around in my training with daily walking lunges, especially in high volumes of up to 300 yards, was that the first day is hard, is that it had a significant impact on how much muscle contraction I could get from my quads, hamstrings, and glutes. Essentially, by performing a single set of walking lunges upwards of 150 yards at the end of each session, the muscle fibers in your legs that started the action fatigue and more fibers are needed to finish the set. Then do this day after day for a period of time, and you'll start to recruit a lot of fibers.
This can also be done with movements like pushups and pullups done daily for weeks on end. It sounds bizarre to most coaches to say that I know dozens of individuals who would do over 250 pushups a day without accruing shoulder problems, but it's something I witnessed summer in and summer out at a camp I worked at (disclaimer, no one is actually suggesting you do this). But over time, after cranking out set after set of pushups each day, your upper body starts to recruit more muscle fibers and gain better contractions over time. This translates to having more fibers to recruit when developing power and strength.
When we recruit more muscle fibers, we also get to reap one other big benefit: and increase in metabolism. When we recruit more muscle fibers, they need more fuel to fire. If your goal is to lean out or lose weight, recruiting more muscle fibers can beneficially work for you to do so (assuming you maintain caloric intake or are in a deficit). Regardless, however, recruiting muscle fibers is just a simple way to make progress. And in the end, progress and a stronger future is what we're after anyway.