Constrained and flexible features of rhythmical hindlimb movements in chicks: kinematic profiles of walking, swimming and airstepping.

Although studies of walking have generated many of the principles for motor control, walking is but one of the many behaviors that an animal produces. This study investigates the relationships among the kinematic profiles of three behaviors in chicks: walking, swimming and airstepping. In addition to describing features of the hindlimb movements, such as retraction and protraction, we also examined the intra- and interjoint coordination patterns at the hip, knee and ankle. By using multiple levels of analysis, we identified some features that are common to all three behaviors, and therefore appear to be constrained, as well as some dissimilar or flexible features. Specifically, we show that resistance differentially affects the hip, knee and ankle joints. Our results also show that each joint plays a distinctive role in these behaviors. For example, we suggest that the hip stabilizes the hindlimb and regulates the rhythmicity of its movements. We also show that movement at the knee consistently precedes movement at the other joints in each behavior. Finally, differences in ankle movements are the key features that discriminate one behavior from another. Continuity among prenatal, perinatal and postnatal behaviors in chicks is discussed.