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. By the use of residuals from regressions, tmt and d were made independent of body mass, and d independent of tmt, forming indices of the lever arms, d(index) and tmt(index). The investigation included 67 bird species divided into six groups according to differences in their hind limb movements and requirements of force and speed. These were birds that walk/run/hop (WH), climb (C) or hang (H), birds of prey (BOP), fast swimmers (FS) and slow swimmers (SS). Predictions for each group correlating their requirements for force and speed are made, based on biomechanical and ecological factors, and the lengths of the moment arms are calculated. The results show that the means for the groups could largely be separated from the norm (i.e. zero), and in many cases the predictions are fulfilled. d is significantly larger than average in species affected by strong forces, for example, gravity (BOP and C), but shorter in species affected only by drag (WH, FS and SS). No differences associated with drag due to differences in medium density were seen. Furthermore, the tarsometatarsus is longer than average only in the BOP species, and shorter in the SS species. Discriminant analysis reveals that using our predictions there is a 53.7% chance of placing a species in the correct group, compared with the 17% chance expected if the species are randomly placed in a group.