What are muscles?

Muscles are the bands of fibrous tissue that enable your body to move. There are around 700 muscles in a human body, which can be divided into cardiac, visceral and skeletal. The cardiac muscles pump blood around your body, while visceral muscles help organs such as your intestines and stomach to move substances. Your skeletal muscles are the ones that you can control voluntary such as your chest muscles (pectoralis major) and your upper arms (triceps brachii and biceps brachii).
How do muscles work?

For any of your skeletal muscles to grow, they need to undergo what's known as hypertrophy or hyperplasia. Hypertrophy is when any muscle gets larger due to the enlargement of muscle fibres, whereas hyperplasia is when muscle growth occurs as a result of an enlargement of its contractile or non-contractile part.

Each time you exercise a muscle, the fibres are broken down and rebuilt about 24 to 36 hours after the workout. Research has indicated that the body adapts to the stress exerted by each exercise by over-compensation - which involves building more muscle than what existed before. Over time, as you lift more weight and experiment with different exercises and workout routines, your body continually adapts to this stress by building more muscle.
Why beginners can gain a lot of muscle very quickly

If you’ve done any weightlifting or general heavy lifting before, your body is not used to the anabolic stress, which is why it will quickly adapt by building muscle quickly.

For example, deadlift muscles worked include your upper legs, glutes, lower back and forearms. Because the deadlift involves so many muscle groups simultaneously, it's one of the most efficient exercises for building mass and strength fast when you’re a beginner.

Over time, however, your body gets better at performing the same movements, whether it’s the deadlift or a simple bicep curl. That’s why many people on hypertrophy (muscle-building) routines notice that their muscle growth slows down over time, especially after they have been training for a few years. For example, the chest and triceps are the main bench press muscles worked, but as your strength increases and you become better at performing the movement, it will eventually become harder to keep increasing the weight.
What happens when you stop training?

If you take time out from the gym for whatever reason, it’s normal to lose some muscle mass after a few weeks. This can be disheartening, but it’s not the end of the world!

That’s because your body will quickly regain the muscle once you resume your workouts. Why? Because of your muscles' nuclei - which are responsible for protein synthesis. The basic principle is that the more nuclei you have, the easier it is to convert protein into muscle. Fortunately, most of these nuclei survive even after extended periods of inactivity - i.e. when you stop performing muscle-building exercises for several weeks, months, or even years at a time.

In fact, research published in the Journal of Physiology has even found that nuclei survive forever due to the permanent physiological alterations that muscle-building activities have on your body. The European Journal of Applied Physiology also noted that training helps you to remember different movements by improving your muscle coordinations, enabling you to lift more and increase your strength faster - even after you've taken a long break from the gym.