Fig. 1. The spatiotemporal dynamics of the swimming gait of C. elegans. (A) Dark-field video image of a young adult worm swimming in viscous fluid (0.5% w/w methylcellulose in NGM buffer) with its undulations within the focal plane of the microscope (also see Movie in supplementary material). Throughout this study, we used young adult worms of this size. In each video frame, custom-written machine-vision software fits a curve to the body centerline (blue line) and calculates the radius of curvature at each point along the body centerline (red line). We define a body coordinate that varies from l=0 at the head to l=L at the tail. (B) Contour plot of the spatiotemporal dynamics of about six cycles of the forward-swimming gait represented as the curvature measured at each point along the body centerline over time. Values of curvature are scaled by color, with positive (negative) curvature indicating bend in the ventral (dorsal) direction. We show the body coordinate as the fractional distance along the body length (l/L) and display data corresponding to 0.1-0.9 to avoid showing the hyperflexible movements of the worm nose and whiplike tail. We measure temporal frequency by quantifying the time period () between undulations. We measure the wave speed by quantifying the propagation of curvature down the body centerline. (C) To characterize the force and energetics of the swimming gait, we directly measured the velocity of each point along the body centerline throughout each undulation cycle. Here, we show the measurements of lateral speed, the direction orthogonal to the body centerline, at three different points along the body centerline through about six cycles of the forward-swimming gait. Positive (negative) speed indicates movement towards the ventral (dorsal) direction.