5 Things That Happen When You Start Lifting Heavier Weights ("Bulking" Isn't One Of Them)

According to Kathryn Lepage and Kimberly Tasin, senior personal trainers at Fitness World/BCPTI, the confusion around bulking starts with the terminology itself.

In the Merriam-Webster Dictionary, "bulk" is "a large or corpulent human body." In the fitness space, to "bulk up" simply means to gain muscle mass (aka lean tissue).

Many women associate "bulk up" with looking "bulky" or "too masculine," but they aren't one and the same. What's more, because women have a lot less testosterone compared to men, Lepage and Tasin stress it would take a significant amount of time and dedication to nutrition and fitness to bulk up to that degree. "Resistance training three to four times a week," they say, "will not elicit the amount of stimulus required to be 'too bulky.'"

To understand how different forms of weight training affect the human body, you first have to realize that there are different types of muscle fibers being exerted.

There are two categories of muscle fibers: fast-twitch muscle fibers and slow-twitch muscle fibers1, and they each serve a purpose.

Per the National Academy of Sports Medicine, fast-twitch muscle fibers support quick and powerful movements, while slow-twitch muscle fibers support endurance activities. You can think of fast- and slow-twitch fibers like the tortoise and the hare. Fast-twitch fibers tire out as quickly as they can move, while slow-twitch fibers support well-paced activities and are much more fatigue-resistant. 

According to celebrity trainer and founder of the NWMethod Nicole Winhoffer, you'll see different results if you're lifting fewer repetitions of heavier weights than if you were to do more reps of lighter weights because you're working different muscle fibers and exercising them differently. 

"Lifting heavier requires larger dominant muscle groups (i.e., biceps, triceps, quads, and glutes) to complete more work and recruit more muscle fibers to lift the load," Winhoffer explains. This means that these targeted muscle groups will bulk up. When you lift more reps of lighter weights, you encourage slow-twitch muscle fibers, building a more even tone over time.

Which regimen you stick to will ultimately depend on your preferred result, though for optimal muscle health and performance, Lepage and Tasin recommend doing both.

"Heavier weights and fewer reps will help to create more muscular efficiency and overall strength, while lighter weights and more reps will help with muscular endurance and hypertrophy," they explain. Incorporating both into your routine will encourage progression and help you avoid a plateau.

In a 2022 study performed by the American Heart Association, participants were split into four groups: aerobic exercisers, resistance exercisers, combined aerobic and resistance exercisers, and a control group that did not exercise at all. After one year, results showed that those who committed to regular resistance exercise experienced an increase in sleep duration by 17 minutes, improved sleep efficiency, and fell asleep quicker.