Revisiting the notion of 'motor unit types'.

Like many areas of biology, the study of motor units and their types has progressed from apparent simplicity to intimations of deep complexities as information has accumulated from different muscles, species, and methodolgical approaches. In my own (not unbiassed) view, the general notion of motor units 'types' has been amply validated by over two decades of work from many laboratories. From the standpoint of function, it still seems valid to think of three basic types in mammalian limb muscles. However, it is clear that the details of how these types are best recognized, and how their mechanical properties are mapped onto muscle fiber biochemistry, are still open questions. The organization of motor units clearly remains an exciting and interesting field.