This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Muscle Physiology, Subject name: Physiology
In the diagram depicting force velocity relationship of skeletal muscle, the difference between the three curves is mainly due to? 
Select and explain. 
A. Frequency of muscle contraction, B. Muscle mass, C. Hypertrophy, D. Myosin ATPase activity
D: Myosin ATPase activity
Answer: D: Myosin ATPase activity 
Explanation: Ans. (d) Myosin ATPase activity(Ref: Guyton 13th ed p.81)Simply remember, velocity of contraction is maximum when there is no load (weight) to be lifted at all and it decreases whenever the load (weight) increasesThe maximum velocity of shortening depends on myosin ATPase activity of the muscle which is the sole reason behind the difference in these three curves