The
efficiency of human muscle has been measured (in the context of rowing and cycling) at 14% to 27%. The efficiency is
defined as the ratio of mechanical work output to the total metabolic cost.
Evolutionarily, specialized forms of skeletal and cardiac muscles predated the divergence of the
vertebrate/arthropod evolutionary line.<7> This indicates that these types of muscle developed in a common ancestor sometime before 700 million years ago (mya). Vertebrate smooth muscle (smooth muscle found in humans) was found to have evolved independently from the skeletal and cardiac muscles.