Karl Zelik wants to help people. He wants to build better prosthetic limbs for amputees and help surgeons improve the care offered to children with cerebral palsy. But before he can realise this mission, Zelik needs to understand the way that people move naturally and the clever energy-saving mechanisms that our bodies use. To do this, he analyses the movement of our limbs and the forces they generate to calculate how various muscles and joints perform work during locomotion (inverse dynamics). However, Zelik noticed recently that the sums weren't adding up. When reanalysing data collected earlier on walkers in Art Kuo's lab at the University of Michigan, USA, Zelik realised that the inverse dynamics approach couldn't account for 25% of the energy changes during a stride cycle. Zelik adds, ‘We could not identify which muscles, tendons, joints or segments were responsible for generating this movement’. Deciding that the conventional inverse dynamics approach was too simplistic, Zelik and his colleagues Kota Takahashi and Greg Sawicki, both from North Carolina State University and the University of North Carolina at Chapel Hill, USA, decided to build a new, more realistic model of our body movements to see if the trio could account for the missing energy and to find out where it was coming from.
Recruiting ten healthy students to walk at speeds ranging from a leisurely saunter to a brisk march on a treadmill, Zelik measured the forces that each individual exerted on the ground as their feet pushed down while using a sophisticated 3D motion capture system to record each movement in fine detail. Then, based on the force measurements, Zelik, Takahashi and Sawicki calculated the total kinetic and potential energy of each walker's body over individual stride cycles. Finally, having analysed the walkers’ limb and body motions from the 3D reconstructions of their movements, the trio built a series of models with increasing complexity – ranging from the simplest, which accounted only for the rotational movements about each limb joint (three degrees of freedom model); to a model that included the contribution of the work done by the swing limbs and the muscles and tendons in the foot (three degrees of freedom plus foot); and the most complex model, which incorporated sliding motions and tissue compression in the hip, knee, ankle and foot (six degrees of freedom) – to calculate how much work each joint and body segment contributed to their movements.
Comparing the total energy values that had been calculated for the whole body with the mechanical work calculated from each of the models based on the body segment motions, Zelik and his colleagues were delighted to see how well their most sophisticated model agreed with the whole body energy calculations. ‘It all added up properly,’ says Zelik, adding, ‘that positive mechanical work performed by the body's joints and segments actually summed up to explain the change in energy on and about the body's center-of-mass was a surprise’. And when the trio analysed the individual contributions of different body segments and joints to walking, they realised that the hip muscles make a more significant contribution than had been previously recognised.
‘This study is the first to demonstrate that we could directly link joint- and segment-level work sources with the observed energy changes of the body during locomotion’, says Zelik, who is now keen to learn more about how changes in our bodies, such as weight gain, affect walking. He adds, ‘This new approach has implications for assistive technology design, musculoskeletal walking simulations and potentially clinical treatment’.