Here are the words that fit in the spaces based on the given prompts:

1. **interested**
2. **fitness**
3. **designed**
4. **development**
5. **active**
6. **knowledge**
7. **gently**
8. **intensity**
9. **impression**
10. **painful**
11. **wise**

So the completed sentences would look like this:

If you're (1) **interested** in getting fit, then what you need is a training programme. Although aimed at improving physical (2) **fitness**, this programme can also be (3) **designed** in such a way that it helps in the (4) **development** of particular athletic skills. There is a range of different (5) **active** to choose from and a growing amount of scientific (6) **knowledge** to explain the effects of each one.

When you begin training, it is important to start (7) **gently**, raising the (8) **intensity** of the programme in a gradual way. Although it is important to work sufficiently hard to make an (9) **impression** on your physical condition, the activities shouldn't be (10) **painful**. It is (11) **wise**, therefore, to ignore warning symptoms such as sharp or persistent pain in particular muscles.