Leg morphology and locomotion in birds: requirements for force and speed during ankle flexion.

Muscle force production and speed of movement of a bone are not only highly dependent on muscle properties but also on the biomechanical arrangements of the musculoskeletal systems. The aim of this study was to investigate whether the leverages of a leg flexion system alone could be used to trace adaptations to different locomotion patterns by different groups of birds. We focused on ankle flexion, and measured the length (tmt) of the tarsometatarsus, representing the out-force lever arm, and the distance (d) between the ankle joint and the tendon insertion of the flexor muscle of the tarsometatarsus, representing the in-force lever arm.