If you've ever tried to catch a lizard you'll know they move like greased lightening. More like the Usain Bolt of the animal world than Haile Gebrselassie, they swiftly dodge predators and pounce on prey. Eric McElroy, from the College of Charleston, USA, explains that he is intrigued by how muscles power movement and says, ‘Most of the thoughts on how lizards move are based on studies of steady speed movement, but what we realised was that burst activity is another aspect of the animal's behaviour. The first step to understanding what their muscles are doing during that burst is to understand the kinematics of them moving.’ Teaming up with undergraduate Kristen Archambeau, McElroy began filming lizards in 3-D as they took off from a standing start to begin to understand how they power fast take-offs (p. 442).
Having built a 3 m long racetrack, McElroy was ready to start filming Sceloporous woodi lizards when Lance McBrayer delivered 10 of the animals from Florida. Painting white markers on the lizards' hindlimb joints, McElroy carefully covered each animal with a bag to transfer it to the racetrack before gently removing the bag and spooking the animal with a loud clap. Filming the first three strides of each frantic dash at 300 frames s–1 from the side and above, McElroy and Archambeau then painstakingly reconstructed the three-dimensional course of each limb joint to find out how the lizards sprang into action.
Analysing the animals' stride patterns, McElroy and McBrayer realised that instead of pushing off with one foot, the lizards started about half of their escape dashes by pushing off with two. ‘Kinematically that first step looks like a jump’, says McElroy. And as they analysed the lizards' movements further, they realised that each stride was very different. ‘One of the key differences is that the hip, the knee and the ankle seem to really change what they are doing across all three strides. For example, the knee and ankle sweep through huge angles during the first stride and then by the third stride they seem to stiffen and not sweep through very large angles’, McElroy recalls.
The animals' accelerations also varied dramatically from stride to stride, peaking during the first stride and falling to almost zero by the third stride. ‘The first stride is pure acceleration’, says McElroy, and adds that the animals are likely approaching their top speed by the time they hit the fourth and fifth strides and they generate the most power during the first and second strides.
However, when the team searched for correlations between particular aspects of the lizards' movements and their acceleration patterns, they found few. ‘We were surprised that the kinematics that we recorded were in general poor predictors of acceleration performance. Our working hypothesis entering this study was that the angular kinematics at the joint should predict acceleration because it is the angular kinematics that presumably reflect what a muscle is doing, and yet the joint kinematics that we recorded had relatively poor predictive power.’
Having measured the fleeing animals' movements as they powered off from a standing start, McElroy is keen to build a computational model of the lizards and their movements. Ultimately, he hopes to use this model to calculate the forces pulling the joints and to understand how muscles in animals with radically different musculature function to move their limbs in similar ways.