The number of reps you do for a given exercise as well as the total volume of work you do impact strength and muscle development. When training primarily for strength, you can maximize strength by using a heavy resistance and doing a lower number of reps. In contrast, if you’re trying to gain muscle endurance, higher rep ranges using a lighter weight offers benefits. What about increasing muscle size? Strike a balance between resistance and volume. In general, for muscle hypertrophy, you select a weight that’s 70% to 80% of your one-rep max and perform 8 to 10 reps per set with between 3 and 5 sets of each exercise.
Even if your goal is primarily to build strength, you don’t necessarily want to work with high resistance, low rep ranges every time you train. For one, using heavy resistance all the time can lead to soreness and overtraining, and, like every other challenge you throw at your body, it eventually adapts and stops changing. Likewise, if you’re trying to build muscle size, lift heavier than usual on some occasions and go lighter with more reps on others.
Lighter Resistance, Higher Rep Ranges for Hypertrophy?
You might think that using a lighter resistance and doing high rep ranges won’t bring you closer to your goal of gaining muscle size and definition. Don’t be too sure. A study published in Strength and Conditioning Journal in 2014 calls this idea into question.
As you may know, there are two main types of muscle fibers – fast twitch and slow twitch. Fast twitch fibers are ones built for shorts bursts of activity – for strength and power moves like lifting a heavy weight or sprinting. They’re sometimes referred to as type 2B fibers. The downside of fast twitch fibers is they fatigue very quickly.. In contrast, slow twitch muscle fibers, also called type 1 fibers, are smaller in size and contract slowly, but they use aerobic, or oxygen-requiring pathways, for energy, giving them greater resistance to fatigue. You primarily recruit this type of muscle fiber when you do endurance sports like running or cycling.
For completeness, there are also type 2A fibers that lie somewhere between fast twitch and slow twitch – they have more endurance capabilities than type 2B but less than type 1. Type 2A fibers have the capacity to turn into type 2B fibers in response to resistance training. Like type 2B muscles, they become larger, causing the muscle to hypertrophy, in response to resistance training.
The Mysteries of Muscle Fiber Recruitment
How does your body know which muscle fibers to recruit? It depends on the demand placed on the muscle. When you use a heavier weight you can only lift a few times, you call in the “big guns,” the fast twitch fibers that have the strength to handle that load. When you pick up less challenging weights, you tap mostly into slow twitch fibers that aren’t as powerful, yet won’t wear out on you as fast. When you first pick up a weight, slow twitch fibers are recruited followed by recruitment of larger, fast twitch fibers as needed to handle the load.
Fast twitch muscle fibers are the ones we characteristically associate with muscle growth, but research shows slow twitch muscle fibers have some capacity to hypertrophy as well. The key is to use a lower resistance AND lift to muscle failure or near failure.
High Rep Workouts Have Benefits Too
Why bother to use lighter weights and higher reps, especially if you’re trying to boost muscle size? After all, doing a higher number of reps takes more time. For one, introducing a variety of rep ranges into your routine reduces the risk of overtraining and overuse injuries. Secondly, it keeps your muscles “guessing,” which is good for avoiding plateaus.
Finally, there are genetic differences in fiber types. If you’re a person with a high percentage of slow twitch muscle fibers, using less resistance and more reps can help you optimize muscle growth. That doesn’t mean you should do exclusively high rep training, but by using lighter reps and more volume during SOME of your sessions, you’ll activate both slow twitch and fast twitch fibers. Working as many fiber types as possible is advantageous. If you only target fast twitch muscle fibers and you have a higher proportion of slow twitch fibers, you may not maximize your gains.
Light Doesn’t Mean “Easy”
Before picking up a pair of 2-pound weights in hopes of using them to increase the size of your muscles, be aware that “light” lifting should still be challenging. Choose a weight between 40% and 60% of your one-rep max and try to take some of your sets to failure or near failure. You still have to progressively overload slow twitch muscle fibers to get them to grow.
The Bottom Line
Train using a variety of rep ranges so you target fast-twitch AND slow twitch fibers just like we do in our STS 90-day program featured in the video above. Fast twitch muscle fibers offer greater growth potential when you use a heavy resistance, but slow twitch fibers also have some capacity to hypertrophy. You can optimize slow fiber hypertrophy by using a lower resistance and higher reps to near failure.
If you happen to have a higher percentage of slow twitch fibers, you may actually get better results when you focus a greater proportion of your workout on higher reps. Plus, by changing the rep ranges you use, you’ll avoid plateaus and overtraining issues. One way to do this is to periodize your workouts using a traditional or an undulating periodization schedule.
Work those fast twitch muscle fibers with heavy resistance but don’t disregard your slow twitch ones. Research suggests they too have the capacity to become larger and give your muscles more size and definition.
BretContreras.com. “Can Slow-Twitch Fibers Contribute to Overall Muscular Hypertrophy?”
Strength & Conditioning Journal:April 2014 – Volume 36 – Issue 2 – p 20-25 doi: 10.1519/SSC.0000000000000030.
Periodization: Latest Studies and Practical Applications. Christopher C. Frankel and Len Kravitz, Ph.D.
IDEA Health and Fitness Association. “Muscle Hypertrophy”
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