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Two Alternative Models for Predicting Muscle Growth

See my older post How Fast Can You Grow Muscle for more on this topic.

There is a lot of different theories in the bodybuilding industry as to how quickly someone can build muscle. Below are two different theories of how quickly muscle can be built.

The McDonald Theory

This theory was created by fitness writer and author Lyle McDonald who also coaches bodybuilders on nutrition. He came up with the following equation for how fast you can build muscle:

His theory is based on the concept that the longer and harder you train the more difficult it becomes to build additional muscle mass. In theory your body can only support so much muscle depending on your height and bone structure, however his theory emphasizes years of training instead. It amounts to roughly the same thing as after 4 years of training you've probably reached a point where its very difficult to add extra muscle mass - and frankly, unless you are a professional bodybuilder, why would you want to?

Note how in the chart above the estimated 2 pounds per month is only roughly 0.5 lb per week, which is not a very fast pace. Over a year that 2 lbs per month adds up to a solid 24lb of muscle. Certainly other factors may also affect the pace of muscle gain, but the concept is sound.

Also the values in the preceding chart apply to male weightlifters. Lyle estimates that females gain muscle weight at roughly half of these values (e.g. 10-12 pounds in the first year of proper training).

The Alan Aragon Theory

The esteemed Alan Aragon is an exercise physiologist who loves constantly staying on top of the latest exercise and nutrition research. Awhile back he addressed the issue of rates of muscle gain in terms of percentage gain for natural lifters.  Here’s the theoretical model he came up with:

Using this theory a 130 lb teenager who never has lifted weights might gain 1.3 - 1.95 pounds of muscle per month (15-23 pounds per year) in a year with a great weightlifting program and a protein focused meal plan.  After a year, he’s now at around 155lb and might be capable of gaining 0.77 - 1.55 lbs per month (9-18 pounds per year) or he might still be a considered a Beginner since he is still a teenager and still growing...

After another year of proper training and smart eating, he’s now at 170lb and is in the Advanced lifter category. Or is he? He might still, depending on his exact age be considered to be in the Beginner or Intermediate category.

The flaw in Alan Aragon's theory is that it supposes a precise 1 year - 2 year - 3 year approach, almost like years of university or college. Which is a very simplistic approach and doesn't factor in age or the fact the person might still be growing in stature.

Nevertheless lets continue to follow his model...

From here on out, the teenager may only gain 0.5-1 lb per month, at which point the closer he approaches his maximum muscle potential, the slower the rate of muscle growth. To be fair that is why most of the really big guys at the gym have been lifting for a good 5 to 10 years and have peaked at a point where its difficult for them to get any further results.

The Moffat Theory

The following theory was created by Charles Moffat and follows the mathematical formula below using a series of acronyms to represent different factors that effect muscle gain.


H = Height Modifier, above or below average, wherein 100% = 70 inches tall, the average height for a male. The female average is 65 inches tall, but use the male average height for calculating the percentile. Thus if a male or female is 60 inches tall, the percentile to use is 85.7%. If the person is 74 inches tall the percentile is 105.7%.

SW = Shoulder Width (measured from the outer tips of the shoulder blades) above or below average, wherein 100% = average. Average for males is 15.5 inches, for females it is 14.0 inches. Like above use the male average for determining the percentile.

MR = Metabolic Rate, which is based on your age and other health factors. This rate is measured as a percentile wherein 18 to 24 is 100%. Subtract 2% for each year over 24. Thus a 44 year old person has a base Metabolic Rate of 60%. Various health factors such as a history of obesity and other factors may lower this metabolic rate by an additional 10% or more.

DS = Dietary Sufficiency or Insufficiency, wherein 100% = Sufficient protein/nutrients to support lots of muscle gain and a lesser percentage indicates a shortage. If you are not eating properly follow the assumption that this percentile is 50% or worse. 100% would only be possible if you have the help of a nutritionist and are following a strict high protein structured diet, involving daily consumption of 150 mg of whey protein in addition to a balanced meal plan.

G = Gender Modifier, wherein 100% is an Alpha Male. 80% for Average Male or Alpha Female. 60% for Low Testosterone Male or Average Female. 40% for Low Testosterone Female. If in doubt, assume the average for your gender.

XR = Exercise Rate, wherein 100% indicates you are maximizing your muscle gain via 120 minutes of daily weightlifting on a strength gain program. If doing less than that calculate the percentile based on the total number of minutes done weightlifting. eg. 30 minutes of weightlifting indicates an XR of 25%.

TC = Training Category. The year of your weight training. Year 1 - 36; Year 2 - 24; Year 3 - 16; Year 4 - 11; Years 5 to 10 - 8; Year 11+ - 4 or less. Eventually a person would reach a Hypothetical Maximum Weight of bulk muscle and will be unable to add any more.

RMG = Rate of Muscle Growth, for that year. Remember to adjust the TC for each year.

1st Example

Jake is 6' (72 inches) tall, making his H is 102.9%. His shoulder width is 16 inches, making his SW 103.2%. He is 30 years old and has a past history of weight problems, giving him a MR of 80%. He tries to eat healthy, but is actually pretty average in his eating patterns so his DS is 50%. He is an average male and not terribly high in testosterone so his Gender Modifier is 80%. He only does weightlifting for about 20 minutes per day so his XR is 17%. On the plus side he has only been weightlifting a short period of time, so he is in the Year 1 category.

H (102.9%) x SW (103.2%) x MR (80%) x DS (50%) x G (80%) x XR (17%) x TC (36) = 2.08 lbs of muscle per year.

What this tells us is that Jake may be trying a little, but he is being held back a lot by his poor diet and low level of exercise. Lets run the numbers again, but this time lets say that he is taking whey protein and vitamin supplements to boost his DS to 80% and that he doubles his weightlifting regimen to 40 minutes per day.

H (102.9%) x SW (103.2%) x MR (80%) x DS (80%) x G (80%) x XR (33.3%) x TC (36) = 6.52 lbs of muscle per year.

Obviously he is gaining a lot more muscle by improving his diet and how much weightlifting he is doing, but if he maximized his diet as best he could and did weighlifting 120 minutes per day he could improve his results even more dramatically.

H (102.9%) x SW (103.2%) x MR (80%) x DS (100%) x G (80%) x XR (100%) x TC (36) = 24.47 lbs of muscle per year.

The following year, assuming he kept the same routine as above he could add an additional 16.31 lbs of muscle.

2nd Example

Sara is 5'2" (62 inches) tall, making her H 88.6%. She is petite and has small shoulders, 12 inches, making her SW 77.4%. She 26 years old and has no history of health problems, so her MR is 98%. She is a vegetarian and eats healthy, but she doesn't consume a lot of protein so lets assume her DS is 50%. She is an average female so her Gender Modifier is 60%. She is new to weightlifting and is only doing it 30 minutes per day, so her XR is 25% and her TC is 36.

H (88.6) x SW (77.4%) x MR (98%) x DS (50%) x G (60%) x XR (25%) x TC (36) = 1.81 lbs of muscle per year.

So Sara isn't going to be adding much muscle with her weightlifting routine. For her it is more like cardio. Like the 1st Example above, she could also increase her weightlifting regimen and start eating more protein. If she maximized both she could increase her muscle gain to 14.52 lbs in the first year, but maybe she doesn't really want to because she might have different priorities.


The average male's Skeletal Muscle Mass is 42% / the average female's is 36%.

The average male who is medium frame and 5'10" tall has 65.94 lbs of Skeletal Muscle Mass, with an overall weight of 157 lbs

The average female who is medium frame and 5'5" tall has 48.24 lbs of Skeletal Muscle Mass, with an overall weight of 134 lbs.

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