Muscle Atrophy is the technical name for loss of muscle mass due to disuse, starvation, or disease.
Muscles get their strength from exercise.
If you exercise everyday, you'll get strength, and muscles.
strength and size
the muscles of the diaphragm and intercostals increase in strength, allowing for greater expansion of the chest cavity.
Exercise and nutrition.
By using your muscle you are strengthening them so all you have to do is use them to increase the strength of the muscles increase the load.
Intercostal muscles increase in strength due to being repeatedly used in exercise, so to strengthen them you will have to do endurance training. The stronger the intercostal muscles the more air you can inhale as they pull your rib cage out giving you a larger thoracic cavity.
The cardiovascular system (heart, lungs, and other big muscle groups) are strengthened during aerobic exercise through continuous, rhythmic movement. When you exercise, your muscles require more blood that is rich in oxygen, which causes your heart to pump more quickly to keep up.
When you excercise, workout or whatever an acid is produced that basically damages your muscles. When your muscles try to recover from this they become larger from the scar tissue formed by the healing process this also explains the pain experienced the day after.
Core exercise is important, because it help strengths the core muscles. Core muscle training will also strengthen other muscles during the training.
increases bone strength and denserty
It is important because both muscle's are a pair, So they should both be exercised in a pair for the equal amount of strength in both muscles.