It's fair to say that she was stronger than average, but she increased her strength in the weight room quite a bit. She still throws as much as she did before. The workouts are professionally designed with softball performance in mind. So strength, power and speed are all trained to some degree.
Sometimes the impact from the weight room doesn’t show up immediately. I asked about her overall strength because for some weaker kids they see bigger gains quicker. Strength is the low hanging fruit for those kids. For others who have a better strength base it may not have the immediate effect, but it may show up in other ways or more long term.
I can share one example. My daughter (also 13) has benefited 5mph since strength training. But it is hard to say only strength training. In her case, she is also a year-round swimmer so she has always looks a bit more muscle developed in her shoulders and quads from swimming. She has that broad shoulder swimmer look. When we went to her trainer, he identified that she was disproportionately weak in her hips. She also had some mechanical issues with turning her drive foot as she launched into her pitch. He noted that fixing the hip flexor strength and mobility would help make that change easier. Sure enough after about 6 weeks she has mostly fixed that flaw and seems to be much more explosive. Her strength coach seems to be right because he helped her fix the weakness the existing strength can be more efficiently applied. When we went to her pitching coach she had gained 5 mph.
I think what we have to keep in mind here is that at 13, your dd is probably still growing and densifying, so velocity increases can also be attributed to that. When my dd started weight training around age 16-17, she was already done growing, so velo increase was very minimal (if any) with weight training alone.
