Just a quick couple of questions. I was pondering this process of putting on lean muscle to burn fat and I got thinking;

How long does it take after a weight training session, for your body to turn protein in muscle fibre?

And secondly, at what point does lean mass stop growing (if doing a similar intensity of weight training) and body fat loss is more noticeable..

Also, similar to measuring body fat percentage, is there a way to know the muscle mass percentage?

I'll give it ago:

First, you have three different types of muscle fiber, slow twitch, intermediate twitch, and fast twitch. They each have a different function. You don't grow new muscle fiber, your existing muscle fibers get thicker. The fibers tear during exercise and new cells are signalled to fill in the tears and make the fibers thicker. One problem is that if you train the slow twitch fibers, they may grow but at the expense of the fast twitch fibers and every other combination, e.g. train fast twitch, lose slow twitch. So how fast can you grow muscle? Depends on how you train and which muscles but the muscles grow during rest, not training. Plus, it's not a linear progression, you get diminishing returns. One pound of muscle does not go on as easily 3 years into training as it did in the first 3 weeks.

When does lean mass stop growing? Again, you're not going to get an answer on this. Depends on how you train, which muscle, and genetics. I doubt anyone can give you even a ballpark.

When you measure body fat % you can get two numbers, the total amount of fat on your body and the total amount of everything else including ligaments, organs, tendons, bones and muscles. An adult skeleton compromises 20% of total body weight and your abdominal organs probably compromise 15 pounds. So you could figure out a reasonable weight of muscle by using those numbers. But I don't think there's a direct way to measure.

Another note, when you begin to workout and become more physically fit, your bone density increases, so some of the increases in LBM may be increased bone mass. I have no clue how much increase you can expect and whether or not it would be considered significant in the total increase of LBM.

Splint, which muscle fibers get bigger? Fast or slow twitch? How do you develop each type of fiber?

You can't change the ratio of what types of muscle fiber you're born with. You may be a natural distance runner (more slowtwitch) or a natural sprinter (more fast-twitch). You can increase the relative area that certain muscle fibers occupy. Training for endurance increase the area the slowtwitch fibers occupy, training for strength or sprinting increases the fast twitch fibers but generally one increase is at the expense of the other. This is why no one will ever win a gold medal in both a marathon and a 100m sprint. People generally train to their natural disposition.

Splint/Angie - thanks for the replies, this is fascinating stuff! Are there ways to target certain 'twitch' fibers? Say for example I want to be a better sprinter, what sort of excercises would increaste the fast twitch muscles, or vice versa?

I wonder--how much this happens after age 35, and in particular, for women?

Thinking it at the very least helps to balance out the density loss due to age/hormone changes etc., in this demographic, namely, me, heheh...

Percentage-wise women naturally have more (technically larger, not "more") slow twitch fibers than fast twitch, compared to men. A popular theory says to use this to our advantage: higher reps at lower weight and long cardio sessions. But another way of looking at it is there is more room for growth by developing the fast twitch. That's the approach I'm taking now. I wouldn't necessarily say the opposite holds true for men though.



Splint has the simple answer already: endurance exercise for slow twitch, strength and sprinting for fast twitch. So heavy weight lifting develops the fast twitch fibers, as does HIIT, and powerful, explosive movements. You want to focus on strength and power. Of course, there's always the rule that if you want to get better at something, you have to practice, practice, practice.

If you wanted to be a better sprinter, you'd practice "explosive" movements, like plyometrics, and well, sprinting. But our bodies and muscles don't work alone, many exercises take the entire range of muscles fibers, in differing amounts. So you kind of need to do all sorts of exercises, but focus on what you're training for.


There are ways to tell which kind of fibers you have the most of in certain areas. You figure out your one rep max, then see how many reps you can do at 80% of that. The number of reps you can do tells you what type of muslce fibers you have. But that's what you pay a professional to figure out. Then he trains you according to your natural incllinations.

But your average everyday Jane or Joe can stand to improve in all areas so it's pretty irrelevant.