What is the hardest working muscle in the body?

The heart, on any reasonable measure - around 100,000 contractions a day, some 2.5 billion in a lifetime, moving roughly 7,500 litres of blood every twenty-four hours, and never resting for more than a fraction of a second between beats.

It manages that because cardiac muscle is built differently from the muscle in your arm. Its cells are packed with mitochondria - up to a third of the cell by volume - and it is almost entirely aerobic, which means it cannot work without oxygen for even a short period. Skeletal muscle can run an oxygen debt and pay it back later; heart muscle cannot, which is precisely why a blocked coronary artery kills tissue within minutes.

There are rival claims worth acknowledging. The muscles that move the eye make the most movements - several hundred thousand a day, including during sleep. The jaw muscles produce the greatest force for their size. And the soleus in the calf holds you upright against gravity all day without you noticing. Hardest working depends on whether you mean force, frequency or endurance, and the heart wins only the third convincingly.