What is a common cause of overloading in horses?

The correct answer focuses on the concept of overloading, which occurs when a horse's muscles are subjected to excessive strain or demands that exceed their current level of conditioning. Pushing muscles too hard too soon is a primary cause of overloading because it can lead to muscle fatigue, soreness, or even injury. When the body is not accustomed to the intensity or duration of work being asked, it does not have sufficient time to adapt. This results in the muscles being unable to recover adequately, leading to overuse injuries or other physical issues.

The other choices do not contribute to overloading in the same manner. Riding at a slow pace typically allows for a gradual increase in fitness without putting unnecessary strain on the muscles. Extensive rest periods can actually benefit recovery and help a horse maintain its fitness without risking overload. Providing adequate nutrition supports muscle recovery and strength, helping horses to handle their workloads more effectively rather than contributing to overloading them.