INVOLUNTARY plantar flexion of the toes which continues for many seconds after withdrawal of an evoking stimulus is occasionally observed in patients with diffuse brain lesions or focal lesions involving the frontal lobe. This response has been designated by various authors as the "grasp reflex of the foot,"1 the "tonic foot response,"2 or the "tonic innervation phenomenon in the foot"3 emphasizing the involuntary quality, prolonged duration, and flexor direction of the abnormal motor synergy.
The site and modality of stimulation which is adequate to evoke the tonic foot response is still unclear. Brain and Curran1 stated that the appropriate stimulus is light pressure on the distal plantar surface of the foot or toes. Goldstein2 reported that stationary tactile plantar stimulation, particularly if intense or painful, was adequate to induce the flexor synergy; and in one of his seven cases, stimulation of the contralateral plantar
Cohen L, Iannone A. The Tonic Foot Response. Arch Neurol. 1967;17(4):419–428. doi:10.1001/archneur.1967.00470280085009
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