A lot of people want to recover from anorexia without putting on fat. They just want muscle and to regain weight in pure muscle. The reality is that you can't build much muscle while you're underweight. You will only eat away at your muscles while you exercise while being under-fueled.
Therefore, exercising while anorexic is counterproductive.
The only way to build muscle is to regain lost weight (and lost fat, since fat is vital to body functions) first.
Here's a podcast episode about that: https://tabithafarrar.com/2019/03/dr-g-muscles-in-recovery-and-how-you-can-have-an-eating-disorder-in-any-size-body/
In this podcast episode, Tabitha Farrar talks to Dr. Jennifer Gaudiani about “what happens to the muscular system when a person is in recovery from and eating disorder and/or malnutrition as a result of dietary restriction.”
Notes from that episode:
Carbs are the primary fuel source for the body. When the body doesn't have enough glucose to run itself, especially the brain, the only fuel it has left is muscle mass. This is why the keto diet is bad, for example - it requires the body to break down muscle mass for energy.
Patients often worry that if they eat and rest, they won't gain strength again. That's false - bodies know how much muscle mass they should have. When bodies are rested and nourished, muscle mass will return. You don't need to exercise or lift weights for it.
Diet culture teaches people that their bodies can't be trusted and need to be wrangled into learning how to behave - that's wrong. Just resting and satisfying the body's energy needs will help build muscle.
We're programmed to be cold when we need to rest (think of bears hibernating in the winter). The exhaustion that comes with recovery is biological. People recovering from eating disorders typically feel the exhaustion is numbed, but when they start to nourish themselves more they will feel the exhaustion hit. This is normal. Rest according to it.
Why would your body try to build big muscles if you aren't even able to stand up without getting lightheaded?
The men in Ancel Keys' 1950 starvation studies were unhappy when they gained weight in their stomach area first, before building back muscle. They complained about it, but over time their muscles did come back.
When I was anorexic I also wanted to build muscle, as I realized I was scarily thin. Learning the science helped me realize when I was working out, all I was doing was destroying my bones and existing muscle. Because I wasn’t getting enough calories.
Undereating destroys muscles and when you attempt to strength train or build muscle while too underweight to even have a period, you cannot build muscle.
E.g. Dr. Gaudiani says when you do ab exercises it’s actually tearing apart your muscles without rebuilding them into more muscle. Your body doesn’t have enough energy or calories to build muscle.
You can only start to build more muscle when you eat enough to reach a healthy weight, and have enough minimum viable fat to support your body.
It’s normal during anorexia recovery for the body to store fat first (which is most vital to the starving body right now) than muscle. Trying to build muscle during anorexia recovery is impossible.
Your body will store fat for a good reason - it NEEDS the fat the most.
There is so such thing as gaining weight "too fast."
Or eating "too many calories."
What will make you stronger right now is eating more, which will give you more fat and also some more muscle. When you are finally at a healthy weight you will be able to build more muscle.
Anecdotes about being physically weak while anorexic
One of my anorexic clients loves gardening, but can't pick up shovels or dig holes for herself. She does plenty of outdoor work and laboring, but can't get any stronger. She has no muscle and wants desperately to gain it.
I resonated with her - as an underweight anorexic, I loved doing manual labor but found myself strangely weak. I wondered why I did so much physical activity but wasn't getting any stronger.
Then I read about how my body needed to gain weight and have enough minimum fat first for me to be able to build any sort of muscle. I couldn't leapfrog the process and just start building muscle - I had to eat more fat, and eat more in general, to rebuild my body.
Any amount of weightlifting and bodyweight exercise I did was just eating away at my already-atrophied muscles. I was making myself weaker and weaker, tearing my body apart. Learning this, I stopped all exercise.