To combine the sentences using compound subjects and verbs, we need to identify the words that are repeated in most of the sentences. In this case, the word "you" is repeated in most of the sentences and can be used as the basis for sentence combining.
Here are the sentences with the identified repeated word "you":
1. They use it to produce the energy they need.
2. Your muscles need a lot of oxygen.
3. You should relax.
4. You should move only as much as necessary.
5. With these techniques, you can train yourself to stay underwater for up to four minutes.
To combine these sentences using compound subjects and verbs, we can use "you" as the subject and combine the verbs:
1. They use it to produce the energy they need, and your muscles need a lot of oxygen.
2. You should relax and move only as much as necessary.
3. With these techniques, you can train yourself to stay underwater for up to four minutes.
By using compound subjects and verbs, we have successfully combined the sentences while maintaining clarity and coherence.