AN EXAMPLE OF HOW OUR MUSCLE FIBERS ARE RECRUITED

When helping someone carry something heavy up a flight of stairs, you would use your hands as grips and let your legs do all the work. On the first step, your legs will start to recruit type IIa fibers. By the 2nd or 3rd step, more type IIa fibers are recruited. Along with these, a number of type IIb fibers are called into play (to maintain fluent motion up the stairs). As your journey continues, more type IIa and type IIb fibers are recruited until by the last step they have all come into play. Your muscle fibers were not firing at maximum speed until the end of the stairs when they neared failure. The faster a muscle fiber fires, the greater the force is. At the beginning, the fibers weren't forced to fire at maximum frequency to overcome the weight, but at the end they had to produce as much force as possible to overcome the weight. This is how recruitment is designed to maintain a certain amount of force.