Plantar flexion resulting from contraction of the calf muscles after a sharp blow to the Achilles tendon, activating the SI spinal reflex. The variations and their significance correspond closely to those of the knee jerk. It is exaggerated in upper motor neuron disease and diminished or absent in lower motor neuron disease.
The calf muscles contracting and the foot pointing downward (plantar flexion), resulting from a sharp strike to the Achilles tendon.