How are Muscles and Tendons injured?

Injuries to muscles and tendons are usually called strains. These occur when you have lifted a weight badly or from a poor sitting posture over a long period. It can also happen by sudden movements like jerking a heavy item or twisting the body especially whilst lifting or carrying.
Muscles do have a protection mechanism that helps protect them from strains. Not so effective with sudden movements, but very effective if you use the back muscles to protect other muscles, ligaments, tendons and discs from injury.