When you start a new cardio routine, whether it's attending a spin class for the first time or beginning to train for a road race, it always feels hard. However, if you stick with it, slowly the exercise gets easier, and you're able to ramp up the intensity or duration of that same type of exercise.
Cardio seems to get easier because your body has begun to adapt to aerobic exercise. Regular aerobic training affects every system in your body from your heart to your lungs to your bones.
Read more: Long-Term Effects of Aerobic Exercise
As you progress through a regimen of aerobic exercise, your heart starts to function more efficiently. Your resting heart rate decreases, and your resting stroke volume increases. Your heart becomes more efficient and has a higher functional capacity as it adapts to aerobic training.
Aerobic exercise even affects the heart's size. As it adapts, the heart's muscular walls grow thicker, which allows for a more powerful contraction. Because of this, the left ventricle can stretch and fill more with blood, increasing blood supply to the body.
During aerobic adaptation, your lungs' efficiency improves as well. Respiratory muscles become stronger and allow for larger amounts of air to be inhaled and exhaled with each breath. The diaphragm muscle adapts so that its endurance and strength improves, which means that the diaphragm can consistently handle continuous forceful breathing patterns while exercising. This allows you to keep a normal breathing pattern for longer durations while performing aerobic exercises.
Aerobic exercise improves your maximum oxygen uptake, which measures how well your body utilizes oxygen at the cellular level. When exercise intensity increases, so does your body's necessary demand for oxygen consumption. As you continue with an aerobic exercise training regimen, your body adapts to the oxygen and energy demands required to function at the increased level of physical exertion. Maximum oxygen uptake is measured in milliliters of oxygen absorbed per kilogram of body weight per minute. The more oxygen absorbed per kilogram per minute, the more efficient the cells of your body are at utilizing the oxygen supply.
Your body begins to have muscular and body composition changes as well. Muscular changes aren't as significant in aerobic exercise as they would be if you were incorporating resistance training, but muscular strength and endurance improve in active muscle, and lean tissue percentage in the body increases as you begin to adapt to aerobic training.
For consideration, as your body becomes adapted to your training intensity, you may begin to notice less significant improvements. Continue to increase the intensity of your aerobic exercise — either in duration, exercise type or rate — in order to continuously allow for your body to have positive adaptations to aerobic training.