Final answer:
Endurance muscles, such as those in marathon runners, have a higher proportion of slow-twitch muscle fibers, which are dense with capillaries and rich in mitochondria and myoglobin for sustained aerobic activity.
Step-by-step explanation:
The muscle types most likely to have the highest proportion of slow-twitch muscle fibers are those used in endurance activities such as long-distance running or cycling. These fibers are known for their high density of capillaries, richness in mitochondria and myoglobin, and consequently, their ability to sustain aerobic activities over extended periods of time.
For example, the muscles of a professional marathon runner are most likely to contain about 80 percent slow-twitch muscle fibers and 20 percent fast-twitch muscle fibers, enabling them to excel in activities requiring endurance.
Muscles involved in sustained activities and endurance tasks typically have the highest proportion of slow-twitch muscle fibers (Type I fibers). These muscles include postural muscles responsible for maintaining posture against gravity, antigravity muscles in the legs, and those engaged in endurance activities like long-distance running or cycling.
Slow-twitch fibers are characterized by their high resistance to fatigue, relying on oxidative metabolism for energy production. These muscles are well-suited for prolonged, low-intensity contractions, providing the endurance needed for activities requiring continuous effort over extended periods, emphasizing efficient energy utilization for sustained performance.