Microscopic Level:  Repeated muscle contractions cause tears in protein fibers which increase in thickness over time.

Macroscopic Level:  Long story short - Repetition, Repetition, Repetition. Physical activity is non-negotiable! Humans were made to move, plain and simple. Of course, the gym isn't the only way to stay active. There are tons of alternatives - sports, dance, swimming, etc.

Goal Based Training:   The baseline recommendation for body maintenance is at least 150 minutes a week of moderate-intensity physical activity. For muscles, a strategy should always be informed by the desired goal. The general strategies are listed below:

Strength:  The force generated by a single muscle

Power:  The amount of force applied over a period of time

Hypertrophy:  Increase in a size of muscles

Endurance:  The ability to maintain strength of a contraction over time

Microscopic Level:  Long story short - Lung capacity increase which leads to more oxygen intake. Better heart muscle contractions improve blood flow.

Macroscopic Level:   Similar to strength training. Movement! Walk, Run, Jog, Dance, Skip, Rollerskate, etc. Whatever gets the blood flowing

Goal Based Training:   The CDC recommendation is 10,000 steps (approx. 4 miles) per day but even just 1 mile a day really helps! Below gives a more detailed description of running zones based on age and goal.

VO2 Max:  The volume of oxygen your body uses while exercising as hard as you can.

Aerobic Exercise:  Meaning ‘with air’ and refers to the body producing energy with the use of oxygen. This typically involves any exercise that lasts longer than two minutes in duration.

Anaerobic Exercise:  Meaning ‘without air’ and refers to the body producing energy without oxygen. This is typically exercise that is performed at a higher intensity.