Question

Why do the thigh muscles get tired soon, but not the muscles of the heart?

Answer 1

Hint:There are three types of muscles in our bodies: striated skeletal muscle, striated cardiac muscle, and smooth involuntary muscles. Of these three, the skeletal or voluntary muscles are prone to tiredness.

Complete answer:
The thigh muscle is a striated skeletal muscle. It experiences tiredness or asthenia. This is simply the exhaustion that we feel when we have been using the muscle for a long period of time or to carry out some sudden intense work. The main reason for tiredness is the depletion of available energy for the muscle to function. Muscles obtain energy from stored ATP, but cells do not store large quantities of ATP.

When a muscle starts working at a higher level, it needs to quickly obtain ATP from glycogen, creatine phosphate, and aerobic respiration. So first stored ATP, then creatine phosphate, then glycogen, and then finally aerobic metabolism take over. However, as the muscles continue to work, aerobic metabolism may not be quick enough to use oxygen to produce ATP, and lactic acid accumulates as a by-product as the activity becomes more intense. Both lead to the feeling of tiredness.

Cardiac muscle, on the other hand, is out of our voluntary control. The heart has an immense blood supply to get sufficient quantities of oxygen, and cardiac muscles have much higher numbers of mitochondria to convert glucose into energy/ATP.

Note:Cardiac fatigue can occur as a normal part of aging as the heart gets bigger and stiffer. Blood supply to the heart is not as efficient, and also the heart's ability to supply the rest of the body. But that is not the same as tiredness.