Can I skip "back up" in this sentence?
No, you can't skip "back up" in this sentence. "Back up" is referring to rebuilding your muscles to their original strength. If you take out "back up," you longer know that you are trying to regain the muscles you once had. The sentence would just mean that you are trying to gain muscle strength. Even though the meanings seem similar, it's important that you keep "back up" in this sentence.