The body is designed to withstand a certain amount of physical force - bumps, jerks and falls, but if the mechanical stress goes beyond the body's limit of adaptability, the effects may become stored as body stress. The causes may be sudden and violent, such as a car accident, a severe fall, or lifting a heavy object incorrectly.
Or, there may be a gradual accumulation of milder mechanical stress, e.g. habitually sitting incorrectly, or doing inappropriate exercises.
We can reduce mechanical stress by improving our posture, by sitting, bending and lifting correctly, and avoiding potentially harmful exercises. Obviously it is helpful to pursue moderate and sensible forms of exercise to strengthen muscles, i.e. a brisk walk for 20 to 30 minutes every day.