5 Factors That Influence How Quickly You Build Muscle

Your body's ability to build muscle can be affected by many factors. These include your genetics, diet, type of workouts you're doing, hormones, age and even gender. More women are starting to lift weights and are no longer afraid of getting "bulky" from strength training. They're also embracing the many health benefits that come from weight training. 

If your goal is to build more muscle and you're struggling to get there, you'll be glad to know that there are many ways to improve your chances of gaining more muscle and strength. It's important to understand what determines your muscle-building ability.

We chatted with fitness experts and looked at scientific research to explain why muscle growth varies per person and what changes you can make to get those gains. Read on to find out what influences how quickly you build muscle and how you can make them work in your favor. 

Genetics and hormones play a big role

Genetics play an important role in determining your body's ability to put on muscle (and its limitations), partly by influencing your hormonal and muscular make-up. But they're not the end-all, be-all.

Hormones

Anabolic hormones -- consisting of growth hormone, estrogen, insulin and testosterone -- are key for muscle building. Contrary to popular belief, estrogen and testosterone are both important for muscle structure and function. Testosterone is responsible for muscle growth, while estrogen improves muscle mass and strength, as well as growing the collagen of connective tissues, such as your bones, ligaments and tendons. Women typically produce more estrogen and less testosterone than men, which is why men often have an easier time with visible muscle growth. (The same seems to be true for transgender people who take hormone replacement therapy.) 

"Testosterone is an anabolic hormone and 10 times higher in men which can benefit muscle growth goals," explains Ryan Turner, a registered dietitian, certified specialist in sports dietetics and founder of Food is Fuel NYC. Testosterone helps release growth hormones, which stimulate tissue growth, and it connects with nuclear receptors in DNA, which causes protein synthesis (or muscle growth).

Turner points out that as both men and women age, the reduction of both testosterone and estrogen hormones can result in the breakdown of muscle. Other aspects that can diminish your muscles are fluctuating hormones, such as adrenaline, cortisol and glucagon, which prevent them from growing. That's why it's important to monitor your day to day stress, sleep, and diet, since these impact those hormones and in turn affect your ability to progress.

Muscle fibers

Another thing that can influence how well you put on muscle are your fast-twitch and slow-twitch muscle fibers. Skeletal muscles are composed of both of these fibers, which serve different purposes and determine your potential athletic ability. Fast-twitch muscle fibers are large and generate quick bursts of energy that are good for exercises such as sprinting, jumping, powerlifting and strength training. On the other hand, slow-twitch muscle fibers are smaller and intended to help you sustain long periods of cardio such as long distance running, swimming, cycling and any type of endurance training.

We all have fast-twitch and slow-twitch muscle fibers, but genetically some people may be predisposed to have more of one than the other. And fast-twitch fibers are the ones that you need for sizable muscle growth. 

"Different muscle fiber characteristics, Type I and II, slow and fast twitch respectively can both increase in size, but the latter can have more growth potential," explains Turner. 

Research has found that two genes, known as the ACTN3 gene and the ACE gene, heavily influence which muscle fibers we have more of. The ACTN3 gene helps create a protein that is found in fast-twitch muscle fibers, for example, while a genotype known as 577XX can occur across both genes, reducing fast-twitch muscle fibers and increasing slow-twitch fibers. On the other hand, the 577RR genotype is linked to a greater presence of fast-twitch muscle fibers.

Tendon length can also determine how big your muscles get. Tendons vary per person, but it's been found that having shorter tendons allow you to gain bigger muscles, while longer tendons make it harder to do so. 

Still, it's not all about how you're born. "There is an upper limit to a muscle's fiber size; however, don't forget that without proper and consistent training the muscle's true potential won't be realized," Turner warns. So just because your genetics say that you can put on muscle easier, if you don't put in the work, there won't be anything to show for it.

You are what you eat

It's impossible to ignore nutrition when discussing muscle mass. How you eat can make a big difference on how well your body puts on and maintains muscle. "Muscle is a very expensive tissue to maintain," explains Tami Smith, a certified personal trainer and owner and CEO of Fit Healthy Momma. She says you have to be intentional on not only putting it on, but also maintaining it. 

Eating enough calories and protein helps with muscle recovery and growth after a workout. Muscle is made up of protein, and eating adequate protein after strength training is essential to limit muscle protein breakdown and assist with muscle synthesis (growth of new muscle). Turner says that individuals who strength train require more protein than their non-training counterparts. Older adults will require more in general, but even more so if they strength train. Similarly, if you want to put on muscle, you'll need to add more calories to your diet. 

"Well trained individuals consuming an additional 500 to 1,000 calories per day, and untrained individuals, new to strength training, eating up to an additional 2000 calories per day can show positive changes in muscle mass," says Turner. "In my day-to-day work with clients, in many cases 15 to 18 calories per pound has been supportive of client hypertrophy goals." (Muscular hypertrophy refers to growing your muscles through exercise.)

Of the 20 amino acids found in protein, leucine is the most essential to promote muscle growth -- and the body cannot produce it. "Three to four grams of leucine [or 6 to 8 ounces of animal protein] can promote maximal protein synthesis," explains Turner. If you're a vegetarian, you will need to strategically prepare your meals ahead of time to achieve this, because plant based proteins may only provide 25 to 60% of the recommended amount of leucine. 

Some women who are having a hard time building muscle may be self-sabotaging their potential without even realizing it. "Many women are caught in the dieting mindset of always wanting to be smaller and weigh less on the scale, which isn't conducive to building muscle," explains Smith. She says a lot of women are scared to see the scale go up a bit, because adding muscle means you're going to be adding weight. "I have so many clients that weigh more now but look completely different with more muscle on their bodies," Smith says. If you do allow yourself to gain that weight for muscle building, you can change the look and feel of your body, and the number on the scale will become irrelevant.

Turner says simple nutrition strategies such as meal planning, meal scheduling, budgeting and supplementation can be implemented to overcome challenges such as figuring out your food intake. If you aren't sure where to begin, it's a good idea to consult with a sports dietitian who can set you on the right path for your goals.

The power of training (and rest)

Once you have your nutrition in check and understand how your genetics influence your muscle growth, strength training is another key player.

There are two types of muscular hypertrophy, known as myofibrillar hypertrophy and sarcoplasmic hypertrophy. Myofibrillar hypertrophy focuses on building strength, while sarcoplasmic hypertrophy increases the volume of sarcoplasmic fluid within the muscle to make it look bigger (think the "pump" you get after an arm workout). 

Depending on your goals, the way you train will influence whether you get stronger or have more defined muscles. Lifting lighter weights for higher repetitions (ranges from six to 15 reps) will give you a defined look, but often you'll lack strength -- bodybuilders use this method. To achieve strength and up your muscle growth, you'll have to lift heavy weights for fewer repetitions (six or fewer reps) and longer rest periods. Powerlifters use this method. 

Either way, you have to continue to challenge yourself to see continued growth over time. "Using a program that implements some kind of progressive overload to continue to build and challenge your muscles for growth is key," says Smith. This means less cardio, HIIT and circuit-style training and more of a focus on heavy lifting exercises. 

Additionally, making sure you get a proper night's sleep (at least 7 to 9 hours) helps optimize recovery after a tough workout. While you're at rest, your body is putting in the work to repair muscles and regulate your hormones, which as you already know play a big part in muscle building. Lack of sleep not only affects your ability to perform well, but also inhibits your growth hormones.

There are so many benefits to strength training aside from building muscle, such as increasing your metabolic rate, improving your lean body mass which promotes blood sugar control, reducing risk of injury, improving mental health, strengthening bone health and so much more. Aiming to strength train two to three times a week is a good rule of thumb, but if you'd like more guidance, consult with a personal trainer who can create a personalized program that will help you achieve your goals. 

So what's the takeaway?

Muscle-building abilities vary from person to person. That said, it's important to understand the big picture, because it doesn't begin and end with your genetics. You may have a genetic profile similar to that of an Olympic athlete, but if you don't put in the work, you'll never learn your actual potential. Likewise, if you are struggling to grow a certain muscle group with ease, it doesn't mean you won't be able to achieve it with a little extra work.

If stronger or bigger muscles are an important goal for you, dialing in on your daily caloric intake, meeting your protein goals, and adhering to a purposeful strength training program will help improve your chances.