Final answer:
Skeletal muscles adapt to resistance exercise through hypertrophy, increasing in size and strength, while cardiac muscles increase in efficiency with endurance exercise.
Step-by-step explanation:
Regarding the question, "What are the only muscles that are used during exercise that will adapt to stress?" different types of muscle fibers adapt to the stress of exercise in different ways. In exercises like weightlifting, skeletal muscles undergo hypertrophy, where structural proteins are added to muscle fibers, increasing their size and strength when they contract against resistance. For endurance exercises such as running, the heart's cardiac muscles increase in strength and efficiency as they contract faster and pump more blood. Both forms of exercise are necessary to maintain larger and more efficient muscles, with the principle of "use it or lose it" being highly relevant as lack of use leads to atrophy, which is the loss of muscle mass. Endurance exercises primarily affect slow-twitch fibers (SO fibers), enhancing their efficiency through increased mitochondrial production for aerobic metabolism, while resistance exercises cause hypertrophy in power-producing muscles rich in fast-twitch fibers (FG fibers).
Additionally, endurance training in muscles with a higher proportion of SO fibers, like those in postural muscles and endurance athletes, can also lead to an increase in cellular components such as myoglobin. However, it's important to balance exercise with adequate rest to prevent overuse injuries and conditions like delayed onset muscle soreness (DOMS). Moreover, exercise can act as an effective regulator of stress and has been linked to physiological benefits, such as preventing telomere shortening associated with chronic stress.