It is easy to appreciate an antelope's grace as it bounds across the savannah, but how about an ostrich? At first glance they seem to be unlikely runners, with their huge egg-shaped torsos and skinny legs. However, these birds have evolved to run and manoeuvre at speed to shake off predators. Devin Jindrich of Arizona State University and his colleagues report that it is their shape and behaviour that allow running ostriches to change direction so effortlessly, improving their chances of escape (p. 1378).

`We want to get at what makes them graceful,' explains Jindrich. While movement in one direction has been well modelled mathematically, the same models cannot easily be applied to variations in movement such as stops, starts, or changes in direction. Jindrich has developed his own mathematical model to describe such changes, so that he can understand how the effects of stability and manoeuvrability constrain organism design. Initially tested on cockroaches and humans, Jindrich wanted to test his model on a high-performance two-legged runner. Ostriches are ideal since they evolved as runners long before humans and have a completely different body shape. Alan Wilson, Nicola Smith and Karin Jespers at the Royal Veterinary College were already studying straight-line running in ostriches, and invited Jindrich to collaborate with them.

The team trained ostriches to run along a track and over a plate that measures the force as the foot hits the ground. They recorded the ostriches' body position using motion capture as they ran in a straight line, or around obstructions. An obstruction on the running track immediately after the plate caused the ostriches to change direction while stepping on the plate. They either turned to the left with a crossover step – stepping with the left leg and crossing over the right – or took a side step with the right leg to bypass the obstruction.

To make a successful turn, a runner needs to move in the intended direction without over- or under-rotating. Jindrich calculated that the ostrich's egg-shaped, horizontally orientated body has a higher inertia than the more vertical human body shape. As objects with a higher moment of inertia are more difficult to rotate, Jindrich predicted that ostriches were less likely to over-rotate than humans. Indeed he found that while humans decelerate to prevent over-rotation, on average ostriches generate fewer deceleration forces. In individual cases the birds generated both acceleration and deceleration forces to control their body orientation, but these are reduced because of their body shape with its higher inertia.

To find out if the ostriches were using twisting forces, or torques, in turning, the team used markers placed near the leg joints to measure the torques produced by the leg muscles. They found that as the leg hits the ground, the angle of the leg is very close to the angle of the force. This reduces the torque and produces similar forces to those recorded during straight running. So rather than twisting at the joints, the torque is maintained and ostriches change direction by simply rolling their body into the turn.

It is this combination of body shape and behaviour that allows running ostriches to change direction so gracefully. Exactly how the muscles generate stabilising forces while manoeuvring will be the focus of future work, along with neural control of the muscles. In the future Jindrich aims to apply his findings to design engineering solutions for patients with spinal cord damage. While he may not get patients running, just regaining some ability to manoeuvre would be a huge achievement.