I do a lot of images of people competing or straining really hard, and while I can do a good expression by mixing prompts and loras, thought I'd try and train a quick one-lora solution to the problem when I don't want to cook up something specific.
It works, but due to the dataset size and nature, will likely cause gender switching on me. Use at a lower rating of 0.3 or so to avoid it. Ratings 0.3-1.0 will get results worth your time, I think.
The main prompt is "extreme effort" but "clenched teeth" gets good results too.