On the hindlimb biomechanics of the avian take-off leap

Although extant land birds take to the air by leaping, generating the initial take-off velocity primarily from the hindlimbs, the detailed musculoskeletal mechanics remain largely unknown. We therefore simulated in silico the take-off leap of the zebra finch, Taeniopygia guttata, a model species of passerine, a class of bird which includes over half of all extant bird species. A 3D computational musculoskeletal model of the zebra finch hindlimb, comprising of 43 musculotendon units was developed and driven with previously published take-off ground reaction forces and kinematics. Using inverse dynamics, the external moments at the ankle, knee, and hip joints were calculated and contrasted to the cumulative muscle capability to balance these moments. Mean peak external flexion moments at the hip and ankle were 0.55 bodyweight times leg length (BWL) each whilst peak knee extension moments were about half that value (0.29 BWL). Muscles had the capacity to generate 146%, 230%, and 212 % of the mean peak external moments at the hip, knee, and ankle, respectively. Similarities in hindlimb morphology and external loading across passerine species suggest that the effective take-off strategy employed by the zebra finch may be shared across the passerine clade and therefore half of all birds.