Vertebrate skeletal muscles act across joints and produce segmental accelerations and therefore animal movement when they contract. Different muscles and different motor units vary in their mechanical contractile properties. Early studies on motor unit recruitment demonstrated orderly recruitment of motor units from the slowest to the fastest during a graded contraction. However, many subsequent studies illustrate conditions when alternative recruitment strategies may exist. Motor unit recruitment during locomotion is thus multifactorial and more complex than typically thought.Different types of motor unit vary in their mechanical properties, including rates of force activation and deactivation, maximum unloaded shortening velocities and the shortening velocities at which maximum mechanical power output and maximum mechanical efficiency occur. In short, it would make mechanical sense to perform fast activities with the faster motor units and slow activities with the slower motor units. However, determining patterns of motor unit recruitment during locomotion has presented experimental challenges.Comparisons between distinct muscles have shown that fast fish swimming and the cat paw shake are activities which employ predominantly the fast and not the slower muscle. Glycogen depletion studies have showed that jumping in the bushbaby uses fast without slow motor units within the vastus lateralis and gastrocnemius muscles. Studies in man show that differential recruitment of the different types of muscle fibre occurs at different times within each running stride. It is suggested that vertebrates may have a strategy of recruiting the motor units that are most mechanically suited for the different locomotor demands. However, we have much to learn about motor recruitment patterns during locomotion.