Final answer:
The true statement about skeletal muscles is that some unicellular organisms use actin and myosin to move. Muscle fibers are categorized by contraction speeds and ATP production, with different fiber types suited for varying physical demands.
Step-by-step explanation:
Choose the true statement about skeletal muscles: C. Some unicellular organisms use actin and myosin to move.
Skeletal muscle fibers are categorized based on contraction speed and ATP production methods. Slow oxidative (SO) fibers contract slowly using aerobic respiration, fast oxidative (FO) fibers contract quickly and predominantly use aerobic respiration but can switch to anaerobic glycolysis, and fast glycolytic (FG) fibers have rapid contractions using primarily anaerobic glycolysis.
Considering this, animals that are adapted for endurance would have a high oxidative capacity rather than glycolytic activity, fast sprinters indeed have a well-developed sarcoplasmic reticulum for rapid calcium release and reuptake, and very fast sprinters typically have high glycolytic enzyme content to support short bursts of high-intensity movement, but not necessarily a high mitochondrial content, as mitochondria are more associated with endurance capacities due to their role in aerobic respiration.
Additionally, actin and myosin, the contractile proteins found in muscle fibers, are also used by some unicellular organisms for movement, demonstrating the versatility of these proteins across different life forms.