What characteristic defines voluntary muscles?

Voluntary muscles are defined by their ability to respond to brain signals, allowing for conscious control of movement. This characteristic is crucial since it enables individuals to perform intentional actions such as walking, lifting, and other movements that require deliberation and coordination. These muscles are primarily skeletal muscles, which are connected to the skeleton and enable body movement.

The distinction between voluntary and involuntary muscles is important in understanding muscle function and control within the body. Involuntary muscles, such as those found in the heart or digestive system, operate without conscious thought and are controlled by the autonomic nervous system, which is not the case for voluntary muscles. Therefore, the response to brain signals underlines the key characteristic of voluntary muscles, making this the correct choice.