Final answer:
The skeletal muscles are primarily worked when someone is running, as they are responsible for voluntary movements like running. Cardiac muscles are also exercised indirectly due to increased heart activity.
Step-by-step explanation:
When someone is running for exercise, the primary muscle type that is being worked are the skeletal muscles. These muscles are under our voluntary control and are responsible for moving the skeleton, which includes the action of running. As we run, these muscles contract and relax to move the limbs and body, generating force and motion. Additionally, while not directly being worked out in the same way, cardiac muscles also get exercised during running because the heart rate increases to pump more blood and oxygen through the body, though this is more about the cardiovascular benefits of exercise rather than targeting the cardiac muscles for strengthening.
It should be noted that there is no such muscle type as 'elastic muscles' in the body. The options given for the types of muscle tissues in our body are indeed skeletal, smooth, and cardiac, as seen in various biological materials, including Figure 10.2 in the referenced text.