MIDDLETON, K.M.: Influence of hallucal reversion on terrestrial locomotion in birds
The reversed or opposable avian hallux (digit I) has been the subject of few anatomical or functional studies. Primitively, the theropod hallux was short and did not normally reach the substrate during locomotion, while the reversed hallux of birds represents a subsequent elongation and rotation of digit I. It has been proposed that the presence of a reversed hallux in a terrestrial theropod would hinder over-ground locomotion and, therefore, would have evolved exclusively in an arboreal form. I recorded foot force output and center of pressure variation in the European Starling (Sturnus vulgaris). The hallux of the European Starling is large – approximately equal to the length of digit III – and is in contact with the substrate over the full length of the digit. During most of the stride cycle, the center of pressure is located near the base of digit III, and the hallux contributes significantly to the ground force of the foot. A contrasting outcome would be expected in a bird with a reduced hallux that has less contact with the ground. Possible tradeoffs of hallucal reversion include a larger potential base of support and better stability and increased mechanical advantage at the tarsometatarsal joint contrasted with altered locomotor kinematics.