These are my opinions. Some are by experience and others are from reading various articles. If you have any reasonable comments on this please let me know.
I have come accross different parents that make me laugh once in a while. It has to with breating in your nose and out your mouth. I have never figured out where that comes from but in my book it is not right. Here is a little trick to see which way brings in more air. Put two fingers in your nose and try and breathe. Can't do it? Put to fingers in your mouth and breathe, you can still get plenty of air. Shouldn't be a problem with this, right?
There are a few things about breathing that comes to mind. I've always read that you should be able to take three steps per breathing cycle. This is for distance running not sprinting. That seems to reason when a person is in shape or is running within their capabilities. Meaning they are not running faster than they are capable and has to stop or slow drasticly because they run out of breath or they are sprinting. There are a few things a distance runner can do to help improve their breathing. Do a long run once a week at a speed you can hold a conversation. Do repeat sessions with speeds faster that race pace. 400, 800, 1200, 1600. What ever distance you run, the repeats should atleast match that distance or longer. The rest periods inbetween should be 2-1 in early season, 1-1 mid season and in late season it should be just long enough for breathing to be back to normal. (For normal in a group I go with the group starts to joke and play around) Ladder is another good workout to do, 400, 800, 1200, 1600, 800, 400. Try to match the 2 400, and 800 times.
Now for beginners, it will always seem that they are out of breath. The main reason for that is because they haven't improved their oxygen carring system. This will happen with patience and persistance. I always start the first time runners out with short slow distances. I try to build them up to longer slow distances. After they have been running for about a month we will have our first race and that will help determine what workouts I will give to improve their speeds.
There is one thing that always comes up with young runners that comes up with their breathing. That is that they live with someone that smokes. They can't understand why their lungs and chest hurts when the first start running. Asthma and allergies are other problems that comes up quite often. I do have several runners that are able to run as long as they have their inhailers.
I have read up a little on the bell test and came to believe that is mainly for the sprinting sports. Soccer was one that was mentioned a lot. I would believe it would work for basketball, field hockey, lacross and any other sport that an athlete would have to do short sprints for a full game.
Track and cross country wouldn't have a need to do it. We have neat things like fartliks and indian runs.
Stretching is one of those things that I don't push a lot until after practice. In my running world I stress warmups and exercises before any racing or practices. I don't tell anyone not to stretch. I believe it works great for the sports that need a wide range of motion in their limbs before playing. Running is not one.
Muscles are a lot like rubberbands. Take a cold rubberband and stretch it quickly and it will most likely break. Warm it up and stretch it to a reasonable length and it will snap back many times. Stretch it to its full length and it will not snap back to its original shape. This is how I approach running practices and races.
This has to be one of the most important things for a runner to do. There are a few things this will do for a runner. It will: increase the heart rate, breathing so it will put more oxygen in the system, body temp to warm up the muscles, use enough energy so the body will start to make more. In my mid season to the end of the season for xc, my runners will do a two mile warm up before the 2 mile race.