Based on their activation threshold and the shape of the twitch response to a single action potential, motor units can be categorized into three basic types: slow, fatigue resistant, and fast fatigable. Slow motor units are composed of slow-twitch fibers, do not generate a lot of force, and are extremely resistant to fatigue. Fatigue-resistant motor units are composed of intermediate fibers, can produce moderate force, and are somewhat resistant to fatigue. Fast fatigable motor units, which are composed of fast-twitch fibers, produce high forces, but fatigue rapidly. The plantar flexors consist of the soleus muscle, which works repetitively during walking and running, and the gastrocnemius, or calf muscle, which is more active during jumping and sprinting. The soleus has a predominance of slow motor units, and thus, has a high proportion of slow-twitch fibers. This muscle has high endurance, but is not very powerful. The gastrocnemius has a mixture of slow, fatigue-resistant and fast fatigable motor units, and therefore, has a greater mix of fiber types. While very powerful, this muscle has limited endurance. During maximal voluntary contractions it is the fast fatigable motor units that are difficult to recruit.