Which of the following is a key benefit of well-conditioned muscles in horses?

The key benefit of well-conditioned muscles in horses is improved strength and explosiveness. This is crucial for performance in various equestrian disciplines, where the horse's ability to exert power and respond quickly can greatly affect its success. Well-conditioned muscles undergo adaptations, such as an increase in muscle fiber size and efficiency, allowing horses to produce more force in their movements. This not only enhances their performance in competitions but also contributes to overall athletic ability and endurance.

In contrast, increased body size doesn't necessarily correlate with muscle condition; rather, it's often about the ratio of muscle to fat and overall fitness. Enhanced flexibility is important for the horse's range of motion but is not primarily a result of muscle conditioning. Lastly, while reduced energy needs might seem beneficial, well-conditioned muscles actually utilize energy more efficiently rather than reducing overall energy requirements. Therefore, the primary emphasis of muscle conditioning is on strength and explosiveness, making the first option the most accurate answer.