The type of muscle used to straighten a joint is called an extensor muscle. The feet, legs, spine, arms and hands all have extensor muscles to aid in movement.
Muscles work in groups to achieve the complex motion of the body's joints. Extending the leg at the hip and knee requires the quadriceps femoris, iliopsoas and sartorius.
The calf muscles, consisting of the gastrocnimius and the soleus, extend the foot. In the upper arm, the triceps brachii extends the lower arm, while the extensor carpi ulnaris and extensor digitarum in the top of the forearm extend the hands and fingers.