Actin and myosin are evolutionarily conserved across all animal life, so all muscles have similar force-generating properties. The huge muscles of a weight lifter are evidence that larger muscles can exert more force. Why is that? The summed force produced by individual myofibrils of activated muscle fibers determines the force the whole muscle can exert. This means that larger muscles with more fibers produce greater forces than smaller muscles with fewer fibers. However, the force that a muscle produces also depends on its contraction length and the speed of contraction, as we discuss next.