Investigating woodpecker drumming using the center of percussion

Woodpeckers are known for their distinctive drumming behavior used to forage for food and attract mates. During drumming, woodpeckers endure decelerations up to 1200 g without developing apparent brain injuries. Because of this extraordinary capability, woodpecker anatomy has served as an inspiration for structures that reduce forces during impacts, with potential applications in helmet design and vehicle safety. Previous studies theorized that impact energy from drumming is primarily absorbed by anatomical features in the woodpecker's head such as the tongue, hyoid bone, spongy bone, or lower beak. However, this explanation was challenged by a recent study reporting that shock-absorbing anatomy in the head would decrease drumming efficiency to an unrealistic threshold. In light of these conflicting theories, the exact methods used by woodpeckers to mitigate the adverse effects of drumming remain unclear. This work investigates the dynamics of woodpecker drumming using a principle called the center of percussion, which relates the location of an applied force (called the center of percussion, CoP) to a corresponding location of zero reaction force (called the center of rotation, CoR). We hypothesized that woodpecker anatomy exploits the relationship between the CoP and CoR to reduce reaction forces at critical anatomical locations, mitigating the adverse effects of drumming. We apply this hypothesis to woodpeckers by using two simplified rigid body models of the woodpecker's anatomy and performing parameter sweeps to investigate the location of zero reaction force when the applied force is at the woodpecker's beak. Results indicate zero reaction force in the lower body near the joint connecting the woodpecker's femur for a body-head model and zero reaction force near a joint in the lower neck for a neck-head model. The absence of reaction forces at these locations may provide critical insight into woodpecker dynamics and future investigation into strategies for reducing impact forces.