Answer
If most of the leg's mass is concentrated nearer to the hip joint, then the leg's rotational inertia is smaller which makes it easier to run fast.
Work Step by Step
For the leg, the hip joint is the axis of rotation. If the leg had more mass farther from the hip joint, then the leg would have a larger rotational inertia which would make it harder to run fast. If most of the leg's mass is concentrated nearer to the hip joint, then the leg's rotational inertia is smaller which makes it easier to run fast.
