Gaining strength and improving endurance are often associated with lifting a lot of weights, pushing yourself to new limits, and “feeling the burn”. Often, pushing through pain day in and day out is seen as an advantage for muscle building, fat burning, and strength gaining.
However, muscle growth and gains in strength are actually developed when you’re not exercising. When you exercise, muscles are broken down, and muscles need to be rebuilt during recovery
Recovery days and cycling your workout intensities are also needed to prevent injuries and burn out. Training too many days in a row leads to overreaching, which can increase your chances of injury or illness, but which you can recover from quickly by resting and decreasing training load.