The Journal of Experimental Biology 3742 © 2014. Published by The Company of Biologists Ltd | The Journal of Experimental Biology (2014) 217, 3742-3747 doi:10.1242/jeb.108506 ABSTRACT A controlled landing, where an animal does not crash or topple, requires enough stability to allow muscles to effectively dissipate mechanical energy. Toads (Rhinella marina) are exemplary models for understanding the mechanics and motor control of landing given their ability to land consistently during bouts of continuous hopping. Previous studies in anurans have shown that ground reaction forces (GRFs) during landing are significantly higher compared with takeoff and can potentially impart large torques about the center of mass (COM), destabilizing the body at impact. We predict that in order to minimize such torques, toads will align their COM with the GRF vector during the aerial phase in anticipation of impact. We combined high-speed videography and force-plate ergometry to quantify torques at the COM and relate the magnitude of torques to limb posture at impact. We show that modulation of hindlimb posture can shift the position of the COM by about 20% of snout–vent length. Rapid hindlimb flexion during the aerial phase of a hop moved the COM anteriorly and reduced torque by aligning the COM with the GRF vector. We found that the addition of extrinsic loads did not significantly alter landing behavior but did change the torques experienced at impact. We conclude that anticipatory hindlimb flexion during the aerial phase of a hop is a critical feature of a mechanically stable landing that allows toads to quickly string together multiple, continuous hops. KEY WORDS: Landing, Ground reaction force, COM, Stability, Hopping, Pitching moment INTRODUCTION Landing after a hop or jump is a common locomotor task among terrestrial organisms. These locomotor events can pose a significant biomechanical challenge given the large ground reaction forces (GRFs) associated with such impacts (Nauwelaerts and Aerts, 2006; Ericksen et al., 2013). Large GRFs can pose a potential danger to the bones, ligaments or muscles of the limbs, which are tasked with rapidly dissipating mechanical energy and decelerating the body (Aerts et al., 2013). Strategies used to ensure a safe landing often involve anticipatory motor control strategies (Santello, 2005) and modulation of limb posture prior to impact (Wikstrom et al., 2008). Anticipatory muscle recruitment, which is associated with landings across a diverse array of taxa (Dyhre-Poulsen and Laursen, 1984; Santello and McDonagh, 1998; Prochazka et al., 1977; Gillis et al., 2010), is thought to increase joint stiffness and allow for more effective energy dissipation by muscles (Horita et al., 2002; Santello, 2005). Furthermore, proper alignment of the limb at impact can RESEARCH ARTICLE Department of Ecology and Evolutionary Biology, University of California, Irvine, Irvine, CA 92697, USA. *Author for correspondence (eazizi@uci.edu) Received 22 May 2014; Accepted 16 August 2014 minimize the stresses experienced by skeletal elements and ligaments during landing (Wikstrom et al., 2008; Norcross et al., 2013). In addition to possible injury, large GRFs associated with impact have the potential to destabilize the body during landing. These large GRFs can produce significant torques if they act with even a modest moment arm relative to the center of mass (COM). The excessive torques experienced during landing can produce a pitching or rolling moment about the COM, which can cause an organism to stumble or crash. One strategy to improve stability during landing is to use movements of the torso and the forelimbs to better align the COM with the GRF vector (Bates et al., 2013). Such changes in posture can stabilize the body by either reducing or quickly counteracting torques experienced at the COM (Blackburn and Padua, 2008). Thus, a safe and stable landing often involves an intricate interplay of limb and body configuration. Although most anurans are considered highly specialized jumpers or hoppers, toads (Family: Bufonidae) are particularly effective at landing. In most anurans, the movements associated with the takeoff phase of a hop or jump are largely conserved (Zug, 1985). In contrast, the mechanics of landing tend to vary significantly among different anuran radiations. For example, species from basal radiations tend to land rather inelegantly, performing what amounts to a ‘belly flop’ during each landing (Essner et al., 2010). This pattern has led to the hypothesis that a finely tuned landing behavior may be a derived characteristic among certain radiations (Essner et al., 2010). Landing is also rather inconsistent in semi-aquatic species (Family: Ranidae), where the chest, torso or legs frequently contact the substrate during landing (Nauwelaerts and Aerts, 2006; Essner et al., 2010). However, landing behavior in toads is characterized by a controlled deceleration accomplished solely with the forelimbs (Gillis et al., 2010; Akella and Gillis, 2011). During landings, toads appear balanced and stable, providing sufficient time for the muscle of the forelimbs to dissipate energy and slow the body before gently bringing the hindlimbs toward the ground (Azizi and Abbott, 2013; Azizi, 2014). Similar to landings in other organisms, stable and well- coordinated landings in toads are thought be associated with anticipatory motor strategies prior to impact. The intensity of activation in the muscles of the forelimbs has been shown to be tuned to the perceived magnitude of the impending impact (Gillis et al., 2010). This tuning of forelimb activity can help to stiffen joints in anticipation of large impacts (Gillis et al., 2010) and also shift where landing muscles operate on the force–length curve (Azizi and Abbott, 2013). Anticipatory strategies may not be limited to the forelimbs but could also include changes in body angle or postural changes in the hindlimbs. In this study, we used cane toads, Rhinella marina (Linnaeus 1758), as a model system to examine how hindlimb posture during landing affects the stability of the whole body. Previous studies have shown that the hindlimbs can make up more than 30% of body mass Reduce torques and stick the landing: limb posture during landing in toads Emanuel Azizi*, Neil P. Larson, Emily M. Abbott and Nicole Danos