Which of the following is not a cause of sports injury during training?

The correct option is Proper Warm-Up.

To understand why this is the correct answer, let us analyze what causes sports injuries during training and how each option relates to injury prevention or causation:
1. Sudden increase of load: When an athlete rapidly increases the intensity, duration, or frequency of training without giving the body adequate time to adapt, it causes excessive stress on muscles, joints, and tendons, leading to overuse injuries.
2. Lack of proper sports facilities: Training in poorly maintained environments, using sub-standard equipment, or playing on uneven surfaces significantly increases the risk of accidental slips, trips, falls, and impact-related injuries.
3. Poor exercise technique: Performing movements with incorrect posture or biomechanics puts unnatural stress on specific anatomical structures, making injuries highly likely over time.
4. Proper Warm-Up: A proper warm-up prepares the cardiovascular system, increases blood flow to active muscles, elevates core body temperature, and improves joint flexibility and range of motion. Consequently, it is a crucial preventative measure that reduces the risk of sports injuries rather than causing them.

Therefore, among the given choices, a proper warm-up is the only factor that actively prevents sports injuries rather than causing them.