Is the heart the strongest muscle in the body

1) "Since three factors affect muscular strength simultaneously and muscles never work individually, it is unrealistic to compare strength in individual muscles, and state that one is the "strongest". Accordingly, no one muscle can be named 'the strongest', but below are several muscles whose strength is noteworthy for different reasons.

In ordinary parlance, muscular "strength" usually refers to the ability to exert a force on an external object—for example, lifting a weight. By this definition, the masseter or jaw muscle is the strongest. The 1992 Guinness Book of Records records the achievement of a bite strength of 4337 N (975 lbf) for 2 seconds. What distinguishes the masseter is not anything special about the muscle itself, but its advantage in working against a much shorter lever arm than other muscles.
If "strength" refers to the force exerted by the muscle itself, e.g., on the place where it inserts into a bone, then the strongest muscles are those with the largest cross-sectional area. This is because the tension exerted by an individual skeletal muscle fiber does not vary much. Each fiber can exert a force on the order of 0.3 micronewton. By this definition, the strongest muscle of the body is usually said to be the quadriceps femoris or the gluteus maximus.
A shorter muscle will be stronger "pound for pound" (i.e., by weight) than a longer muscle. The myometrial layer of the uterus may be the strongest muscle by weight in the human body. At the time when an infant is delivered, the entire human uterus weighs about 1.1 kg (40 oz). During childbirth, the uterus exerts 100 to 400 N (25 to 100 lbf) of downward force with each contraction.
The external muscles of the eye are conspicuously large and strong in relation to the small size and weight of the eyeball. It is frequently said that they are "the strongest muscles for the job they have to do" and are sometimes claimed to be "100 times stronger than they need to be." However, eye movements (particularly saccades used on facial scanning and reading) do require high speed movements, and eye muscles are 'exercised' nightly during Rapid eye movement.
The unexplained statement that "the tongue is the strongest muscle in the body" appears frequently in lists of surprising facts, but it is difficult to find any definition of "strength" that would make this statement true. Note that the tongue consists of sixteen muscles, not one.
The heart has a claim to being the muscle that performs the largest quantity of physical work in the course of a lifetime. Estimates of the power output of the human heart range from 1 to 5 watts. This is much less than the maximum power output of other muscles; for example, the quadriceps can produce over 100 watts, but only for a few minutes. The heart does its work continuously over an entire lifetime without pause, and thus does "outwork" other muscles. An output of one watt continuously for seventy years yields a total work output of two to three gigajoules."
Source and further information:
http://en.wikipedia.org/wiki/Muscle#The_.27strongest.27_human_muscle


2) "The strongest muscle in the body is debatable. Some physiologists believe it's the masseter (used for chewing), while others claim it's the gluteus maximus (buttocks), or the rectus femoris (part of the quadriceps in the thigh). It's not easy to determine partly because muscles don't work alone. For example, you wouldn't be able to isolate the rectus femoris muscle and compare it directly to others because other quadriceps muscles contract along with it. There are biomechanical factors to consider as well like muscle length and the points on the bones where muscles attach called the origins and insertions."
Source:
http://www.medicinenet.com/script/main/art.asp?articlekey=79217

I gave the same answer here:
http://www.answerbag.com/a_view/1573839

By size , The tongue i believe.

With all the physical and emotional stress that it handles, yes it probably is.

I read it was the tongue, which seems true

There has been much debate over this. Some people claim
the heart is "strongest" is because it does more work over one's lifetime than any other muscle, and it's almost impossible to fatigue. Another muscle often mentioned is the tongue, although I don't know why--it works almost constantly (at least in some people), but generates little force and does fatigue. Just try sticking it out and wiggling it up and down for five minutes. You'll see. But the strongest muscle in the body is debatable. Some physiologists believe it's the masseter (on either side of your jaw - used for chewing), while others claim it's the gluteus maximus (buttocks), or the rectus femoris (part of the quadriceps in the thigh). It's not easy to determine partly because muscles don't work alone. For example, you wouldn't be able to isolate the rectus femoris muscle and compare it directly to others because other quadriceps muscles contract along with it. There are biomechanical factors to consider as well like muscle length and the points on the bones where muscles attach called the origins and insertions. I guess it is a matter of opinion, but I think the heart is the most important muscle.

I think I read somewhere that the sphincter is actually the strongest muscle in the human body... if I hadn't read that then I would probably have said the heart was strongest though, I remember cutting one up in biology class once and its walls were really thick with all the muscle.