Skeletal muscles enable people to move, playing the piano, running and chewing are all activities made possible by skeletal muscles. The muscles in the arm and thigh are among the largest and strongest skeletal muscles in the body. The arm and thigh muscles are some of the many muscles that work in pairs. In each case, one muscle of the pair contracts to bend the joint, the elbow or the knee in the lower part of each limb. The opposite muscle relaxes to straighten the joint. The paired muscle contracts. The quadriceps femoris on the front of the thigh works with the biceps femoris or hamstring in back of the thigh. To bend the knee and raise the leg, the hamstring contracts and the quadriceps relaxes. To straighten the knee and lower the leg, the quadriceps contracts and the hamstring relaxes. The biceps muscle on the front of the ARM works with the triceps muscle on the back of the arm in a similar fashion. When the biceps contracts, the elbow bends and the forearm lifts, the triceps relaxes. At the same time to straighten the elbow and lower the forearm the triceps. And tracks and the biceps relaxes. As muscles contract, they get shorter and thicker.