Here's how much muscle you can expect to build every month when you start strength training, according to research

• Men can expect to build 35 to 45 pounds of muscle over their lifetime, and 20 to 25 pounds for women.
• How easily we build muscle depends on a range of factors, personal trainer Mike Matthews said.

How much muscle you build per year — and how quickly — depends on a range of genetic factors including starting body size, hormones and muscle fibers, personal trainer and fitness author Mike Matthews told Insider.

To build muscle as quickly as possible, training and eating right is key. You need to prioritize heavy resistance training with progressive overload, and eat in a calorie surplus with plenty of protein.

According to fitness researcher Lyle McDonald, women in their first year of proper training can expect to build on average one pound of muscle per month, and men can expect two pounds. Muscle growth slows down the longer you've been lifting weights.

Different bodies respond to training differently. If you have more testosterone, which is the main hormonal driver of muscle growth, you will be able to make gains faster.

Similarly, if you have more "type two" muscle fibers — also known as fast-twitch fibers — you'll be able to be more powerful and explosive and find it easier to build muscle.

Men can gain 35 to 45 pounds of muscle in their lifetime

If you're building muscle naturally (ie. without performance enhancing drugs), the average man can expect to build 35 to 45 pounds over the course of his lifetime, and for women the ceiling is about 20 to 25 pounds, Matthews said.

A good indicator of how much muscle a person can build is wrist or ankle circumference — if two people are the same height, the person with the larger wrists and ankles will probably build muscle more easily, Matthews said, citing research by Dr. Casey Butt (which was only carried out on men).

The reason women build less muscle than men is partly because they have less testosterone, but also because they typically have smaller starting body sizes, Matthews said.

A good analogy, according to David Epstein, author of "The Sports Gene," is bookcases — a larger bookcase may only weigh a bit more than a smaller one, but it has the potential to carry more books too.

"The size of our skeleton is correlated to the amount of muscle that we can gain," Matthews said.

We all have the potential to build a similar amount of muscle in relation to our body sizes, but the rate at which we get there can vary.

Your rate of muscle growth will slow over your fitness journey

The average man who is training consistently might take five to seven years to build 45 pounds of muscle, Matthews said. The vast majority of gains will occur at the start.

Different bodies of research offer different suggestions, but this table by fitness researcher Lyle McDonald shows an average muscle-building timescale:

The rate of muscle growth decreases drastically over time, and the older you start, the less muscle you can gain, Matthews said.

He stresses that this table is for people who are resistance training properly and consistently, while also eating well (not just exercising).

What's really happening if you think you can't gain muscle

Some people believe they're "hard gainers," who cannot gain muscle no matter what they try, but Matthews said this is a myth.

You might make slower progress than others, but everyone can build muscle.

People who think they can't gain muscle are usually not eating as much as they think they are, and burning more calories than they realize, Matthews said: "Often these are people who are very active outside of the gym and also have lower than average appetites, as well as having smaller frames."

Ultimately, Matthews believes the only people who should worry about how their genetics might affect their athletic performance and muscle building are professional athletes.

"Most people just want to look good, feel good, be fit, and be healthy, so none of this really matters," he said.