What is the strongest muscle in the human body?

Source: http://www.loc.gov/rr/scitech/mysteries/muscles.html

There is no one answer for this question since there are different ways to measure strength. There is absolute strength (maximum force), dynamic strength (repeated motions), elastic strength (exert force quickly), and strength endurance (withstand fatigue).

Most sources state that there are over 650 named skeletal muscles in the human body, although some figures go up to as many as 840. The dissension comes from those that count the muscles within a complex muscle. 

The following are muscles that have been deemed the strongest based on various definitions of strength (listed in alphabetical order):

External Muscles of the Eye

The muscles of the eye are constantly moving to readjust the positions of the eye. When the head is in motion, the external muscles are constantly adjusting the position of the eye to maintain a steady fixation point. However, the external muscles of the eye are subject to fatigue. In an hour of reading a book the eyes make nearly 10,000 coordinated movements.

Gluteus Maximus

The gluteus maximus is the largest muscle in the human body. It is large and powerful because it has the job of keeping the trunk of the body in an erect posture. It is the chief antigravity muscle that aids in walking up stairs.

Heart

The hardest working muscle is the heart. It pumps out 2 ounces (71 grams) of blood at every heartbeat. Daily the heart pumps at least 2,500 gallons (9,450 liters) of blood. The heart has the ability to beat over 3 billion times in a person’s life.

Masseter 

The strongest muscle based on its weight is the masseter. With all muscles of the jaw working together it can close the teeth with a force as great as 55 pounds (25 kilograms) on the incisors or 200 pounds (90.7 kilograms) on the molars.

Muscles of the Uterus

The uterus sits in the lower pelvic region. Its muscles are deemed strong because they contract to push a baby through the birth canal. The pituitary gland secretes the hormone oxytocin, which stimulates the contractions.

Soleus

The muscle that can pull with the greatest force is the soleus. It is found below the gastrocnemius (calf muscle). The soleus is very important for walking, running, and dancing. It is considered a very powerful muscle along with calf muscles because it pulls against the force of gravity to keep the body upright.

Tongue

The tongue is a tough worker. It is made up of groups of muscles and like the heart it is always working. It helps in the mixing process of foods. It binds and contorts itself to form letters. The tongue contains linguinal tonsils that filter out germs. Even when a person sleeps, the tongue is constantly pushing saliva down the throat.