The build-up of lactic acid in our muscles while running (less oxygen conditions) causes cramps.

During heavy exercise, the demand for energy is high but the supply of oxygen to produce energy is limited. Therefore, anaerobic respiration takes places in the muscles cells to fulfil the demand for energy. This anaerobic breakdown of glucose leads to the formation of lactic acid in muscles. The accumulation of lactic acid in muscles leads to muscle cramps. HenceDuring heavy exercise, we get cramps in the legs due to the accumulation of Lactic acid. So, the correct answer is 'Lactic acid'.

**Explanation:**

*Lactic acid* is the correct answer for the accumulation that causes painful cramps in leg muscles after running for a long time. Here's an explanation of why lactic acid is responsible for these cramps:

**1. What is lactic acid?**
Lactic acid is a byproduct of anaerobic respiration, which occurs when the body doesn't have enough oxygen to produce energy aerobically. During intense exercise or when the body is under stress, the muscles use up oxygen faster than it can be supplied. As a result, the body switches to anaerobic respiration, where glucose is broken down without the use of oxygen to produce energy.

**2. Formation of lactic acid:**
During anaerobic respiration, glucose is converted into ATP (adenosine triphosphate) and lactic acid. The ATP provides energy for the muscles to contract, while lactic acid is produced as a waste product. The accumulation of lactic acid in the muscles leads to an increased acidity level in the muscle fibers.

**3. Muscle cramps:**
When there is a high concentration of lactic acid in the muscles, it can interfere with muscle contractions and cause painful cramps. The increased acidity disrupts the normal functioning of the muscle fibers, leading to a decreased ability to contract and relax properly. This can result in muscle spasms, tightness, and pain.

**4. Recovery and removal of lactic acid:**
After exercise, the body works to remove the lactic acid and restore a normal pH level in the muscles. This process is known as the "lactic acid shuttle." The liver and other tissues convert lactic acid back into glucose, which can be used for energy or stored as glycogen. The removal of lactic acid helps to alleviate the cramps and discomfort experienced after exercise.

**5. Prevention and treatment:**
To prevent and alleviate muscle cramps caused by lactic acid accumulation, it is important to properly warm up and stretch before exercise, maintain adequate hydration, and gradually increase exercise intensity to allow the body to adapt. Additionally, cooling down and stretching after exercise can help remove lactic acid and reduce the likelihood of cramps.

In conclusion, the accumulation of lactic acid in the muscles during anaerobic respiration is responsible for the painful cramps experienced after running for a long time. Understanding the role of lactic acid can help individuals take preventive measures and manage muscle cramps effectively.