Which sentence is most clearly written?

The first sentence is more clearly written because it maintains a straightforward structure that effectively conveys the relationship between running, stretching, and shin splints. The phrase "Beginning to run before stretching" is presented as a complete thought, allowing the reader to understand that this action is linked to the occurrence of shin splints.

In contrast, the second sentence disrupts the flow with the placement of the word "frequently." By inserting "frequently" after "stretching," it creates ambiguity about what is being modified—making it less clear whether the frequency refers to the action of running, the act of stretching, or the correlation to shin splints. This confusion can divert the reader's attention from the main point of the sentence, which decreases its clarity.

Thus, the first sentence is preferred for its straightforward and unambiguous presentation of the idea, helping the reader grasp the intended message without confusion.