Which of the following statements about the mechanism of muscle contraction are correct ?
(i) Acetylcholine is released when the neural signal reaches the motor end plate.
(ii) Muscle contraction is initiated by a signal sent by CNS via sensory neuron.
(iii) During muscle contraction, isotropic band gets elongated. (iv) Repeated activation of the muscles can lead to lactic acid accumulation.

Answer

To determine which statements about the mechanism of muscle contraction are correct, let's analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement (i): Acetylcholine is released when the neural signal reaches the motor end plate.**
- **Explanation:** Acetylcholine is indeed the chief neurotransmitter involved in muscle contraction. When a nerve impulse arrives at the terminal of a motor neuron, acetylcholine is released into the neuromuscular junction (also known as the motor end plate). This initiates the process of muscle contraction.
- **Conclusion:** This statement is **correct**.

2. **Statement (ii): Muscle contraction is initiated by a signal sent by CNS via sensory neuron.**
- **Explanation:** Muscle contraction is initiated by a signal sent from the central nervous system (CNS) through motor neurons, not sensory neurons. Sensory neurons transmit impulses from receptors to the nervous system, but they do not initiate muscle contraction.
- **Conclusion:** This statement is **incorrect**.

3. **Statement (iii): During muscle contraction, isotropic band gets elongated.**
- **Explanation:** The isotropic band, also known as the I band, actually shortens during muscle contraction. This is due to the sliding filament model where actin and myosin filaments slide past each other, causing the I bands to decrease in length, while the A bands remain the same.
- **Conclusion:** This statement is **incorrect**.

4. **Statement (iv): Repeated activation of the muscles can lead to lactic acid accumulation.**
- **Explanation:** During intense exercise, when oxygen levels are insufficient, the body produces lactic acid as a byproduct of anaerobic respiration. This accumulation can lead to muscle fatigue and soreness.
- **Conclusion:** This statement is **correct**.

### Final Answer:
The correct statements about the mechanism of muscle contraction are:
- Statement (i) is correct.
- Statement (iv) is correct.