Final answer:
The phrase 'use it or lose it' pertains to skeletal muscle, nerves, and bones.
Step-by-step explanation:
The phrase 'use it or lose it' pertains to skeletal muscle, nerves, and bones. In the context of skeletal muscle, if a person doesn't use their muscles regularly, they may experience muscle atrophy, which is a loss of muscle mass and strength. Similarly, if nerves are not stimulated and used, they can become less efficient in transmitting signals. Moreover, bones need regular stress and weight-bearing activity to maintain their strength and density. If bones are not used, they can become weaker and more susceptible to fractures.
Learn more about Effects of inactivity on skeletal muscle, nerves, and bones