Muscles are strengthened based on the force placed across the muscle. Higher forces produce greater strengthening. Therefore, exercises performed with muscle activated in a way that allows them to contract at high velocities, necessarily imply that they are also contracting with relatively low force. This is intuitively obvious as you lift a light load compared to a heavy load—the light load can be moved much more quickly. However, these rapid movements would have very small strengthening effects since the muscle forces are so low.