Children during middle childhood can run faster and longer. What allows children to achieve these results during this time in the lifespan?

Answer: Lung capacity expansion

Explanation:

Lung capacity is defined as the volume that lungs of a human can consume at different stages of respiration. Lungs can consume good amount of air at stage near the adulthood and then tends to decrease as per aging. Elements that effect the air volume of lungs are age, gender, physical health etc.

According to the question, at the stage of middle childhood, a child is able to run longer distances along with fast speed due to the expansion in lung capacity. They tend to have good volume of lungs to hold air and thus it allows the child to run rapidly without restricting the breathing.