When the tension becomes great enough, contraction suddenly ceases and the muscle relaxes, this phenomenon is known as:

Up to a point, the harder a muscle is stretched, the stronger is the reflex contraction. However, when the tension becomes great enough, contraction suddenly ceases and the muscle relaxes. This relaxation in response to strong stretch is called the inverse stretch reflex or autogenic inhibition. Ref: Ganong's Review of Medical Physiology 23rd edition, Chapter 9.