Viable sperm are important for fertilization, but current methods may miss healthy sperm that don't move. Scientists tested a new method called hypo-osmotic swelling to check sperm viability. They found that this method accurately identifies viable sperm that don't swim. By using this technique, they could detect healthy sperm with high accuracy, especially when looking at tail coiling. However, the accuracy of the test depends on how long the sperm are exposed to the hypo-osmotic solution.