Explain why it is important to assess flexibility both before starting and during a workout regimen

It keeps your muscles stretched, warmed up, and at fullest potential.

Explanation:

Without stretching before and after working out, your muscles can become tight and are weak. It prevents you from experiencing a full extension in your muscles in a deep workout. One more thing (edited) knowing your muscles fullest flexibility can help prevent you from straining a muscle in the future. When working out, knowing what stretches your muscles can take on is important so you don't tear or strain muscles :)