In weight lifting, you would overload certain muscles to strengthen them. In accumulating word power you concentrate on a certain subject matter to make sure that the new words and phrases you need to learn appear often enough for you to learn them. People talk about business English, or academic English or whatever other kind of English. To me there is only one kind of English. To improve your ability to write business English or to write academic papers in English, you just need to overwork that kind of content. You need to select just business papers or academic papers in your field for a period of time. You need to save words and phrases from that kind of content. You need to review these words and phrases. You need to listen to and read this kind of content over and over. You overload in that content type.
It is not normal y recommended in physical training to only lift weights. Running and aerobic exercises are necessary for your heart. In the same way, extensive and pleasurable reading and listening on a variety of subjects wil strengthen your ability to concentrate on your heavy lifting, your focused study of certain content areas. So it is important to have a balanced approach.
In running there is a concept cal ed the "fartlek" run. This comes from the Swedish termsb "fart" which means "speed" and "lek" which means play. So fartlek means speed play. The idea is that when you go jogging you should also do a little sprinting. If you do not, your body gets used to the comfort of running at a steady pace and the training benefit is reduced.
I believe some of the same concepts apply in language learning. I think learners should listen to content read slowly and then read at normal and even high speed. They should listen to different speakers with different accents. It might even be a good idea to hear the same content read by different voices and using different accents.
I certainly notice in my Russian studies that after listening to co ntent that is read very quickly, even though I do not understand so much of it, when I go back to content that is read more slowly, I have an easier time with this slower content than before. I have trained my brain to become more nimble and alert, to become more language fit for Russian.
By the same token, when I start a language, like Russian, I make sure I buy two different beginner books and CDs so that I can cover more or less the same beginner content listening to different people cover more or less the same beginner vocabulary but in different contexts.
Mixing up content, voices, speeds, and accents can help improve our language fitness.
Nevertheless, when it comes to working on pronunciation it is wise to concentrate on one voice which we find pleasant and imitate that person's intonation, pronunciation and tempo. If we want to internalize new phrases and words we also need to listen repetitively to the same content. So the key is to mix it up.
I propose that we offer children comparative language as a subject in grades 1-7. This would consist exclusively of listening to and reading stories. No marking, no output! Children could choose one or two languages a year. They would be able to change every year or stay with the same languages. The purpose would be purely exposure.
Thereafter language would be optional. The students could continue with this exposure approach but would be asked to concentrate on one or two languages. Only then would they be expected to start writing and speaking. But the main thing would remain listening and reading.
If the emphasis were on choice and pleasure, perhaps a larger percentage would end up genuinely motivated to learn. And once they decided to commit to learn they would be more flexible and better prepared for success. In any case what we are doing now in schools does not work for most students.
In strength exercises you need to work the same muscles repetitively in order to get stronger. Language learning is a little like that. Athletes in al sports do repetitive exercises to increase their strength.
I have always found repeating effective. When I start learning a language, I listen to the same content repeatedly. It helps if the content is interesting and the voice pleasant to listen to. When I listen I pick out or focus on different phrases or words each time. I then read that same content repeatedly for fluency in reading. I wil review new words and phrases from that same content. I have saved these words and phrases on a separate list. (This is automated in LingQ). Then I go back to listening and reading the same content again.
This is my strength training in language learning. It can be quite passive. I can listen while walking or jogging or even driving or sitting in a bus. But it is deliberate and is done almost every day during my period of intense study.