Final answer:
The soleus muscle is characterized by a higher density of slow-twitch fibers, making it a postural muscle that supports sustained contractions for maintaining posture and standing. (option C)
Step-by-step explanation:
Among the options provided, the soleus is a postural muscle characterized by a relatively greater slow-twitch fiber density. Slow-twitch muscle fibers are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. They fire more slowly than fast-twitch fibers and can go for a long time before they fatigue. Therefore, the soleus, which is a part of the calf muscle group and plays a significant role in maintaining posture and standing, has a high density of slow-twitch fibers. In contrast, the biceps brachii, rectus abdominis, and pectoralis major muscles have higher proportions of fast-twitch fibers because they are involved in rapid, powerful movements rather than sustained contraction.
Understanding the fiber composition of muscles is essential for optimizing training regimens and comprehending their functional roles. In the context of postural stability, the soleus muscle stands out as a key contributor with its predominance of slow-twitch fibers, emphasizing its role in sustaining prolonged activities and supporting an upright stance.