The range of motion is defined as the distance that a lever can move while attached to a fixed point. So, imagine that you are doing bicep curl, your humerus-ulna and radius bones (upper arm) are the lever and your elbow joint are the fixed point. Your bicep muscles are what move those levers to create motion and movement.
Let me share with you why range of motion is so important.
Prevent injuries or the re-occurrence of injuries
The strength of your tendons and ligaments are dictated by the stress that you put on them. Training in the full range of motion will place positive stress on that connective tissue and decrease the risk of injury.
By training with full range of motion, your muscles should be able to produce force even in awkward positions. This is very important, especially to athletes. If you train in the full range of motion, your muscles will be comfortable to producing force even in the far end of mobility.
Burn more calories
When you move using your full range of motions, you create more workload for you muscles. More muscle fibres are activated, and therefore more calories are burned.
Better gains of muscles and strength
When it comes to muscles and strength gain, I am a firm believer that range of motion play a big part. During a workout, we want to engage as many muscle fibres as possible to stimulate growth and increase strength. The best way to do that is move with a good range of motion.