Abnormal flexion of the fetlock joint during weight bearing to the point of contact between the dorsal surface of the fetlock and the ground. The cause of this may be reduced function (paralysis) of nerves (specifically the N. radialis in the forelimb and the N. peroneus in the hindlimb), or proprioceptive deficits (reduced awareness of one's body position). Horses with a flexural deformity involving contraction of the superficial flexor tendon have extrmely upright fetlocks and are predisposed to this disorder.