Skeletal kinematics reveal the complex interactions of degrees of freedom at joints

Measuring range of motion (ROM) is a common technique for investigating the relationship between morphology and function at joints in both extant and extinct taxa. ROM studies often present results as tables or graphs of minimum and maximum joint angles; however, joint motion is a complex three‐dimensional interaction among multiple degrees of freedom. Using X‐ray Reconstruction of Moving Morphology, I present results from experiments that directly measure three rotational degree of freedom kinematics in birds. Every pose achieved at a joint is plotted in angle‐angle‐angle space for the three possible degrees of freedom, producing a ROM envelope. The results demonstrate that degrees of freedom interact in complex ways that cannot be summarized as mere minima and maxima. Two and three degree of freedom joints produce irregular two‐dimensional surfaces and three‐dimensional volumes. Plotting in vivo poses within these ROM envelopes demonstrates that the ROM of certain degrees of freedom may be overestimated, but that living animals were willing to push joints farther than the researchers in other cases. These results demonstrate that soft tissues and bone interact in complex ways to guide joint motion. The NSF, the W.M. Keck Foundation, and the Bushnell Research and Education Fund supported this work.