1, this is a fitness ball for tennis practice. The bag for tying the tennis ball belongs to the high elastic belt, which has a strong telescopic function, reducing the back-and-forth work of the practitioner to get the ball back and forth.

2. If the strap no longer sends the ball back to the athlete, it is elastic loss and lacks telescopic function, so this strap must be updated, otherwise it will inevitably run back and forth to increase the amount of exercise.