Now, within the muscle cell we have many of these fibers known as myofibrils. And these myofibrils themselves consist of the Sarcomir units. So Sarcomir units consist of the myosin and the actin protein. Now in our body we have many of these motor units. Now, whenever we're carrying out an activity that basically requires a very small force, we only use several or few of these motor units. But if we're conducting some activity that requires a large force we have to use many of these motor units.