The Impact of Medio-lateral Trunk Movement on Mechanical Energy Generation and Distribution during Gait

A typical gait pattern of a healthy individual is characterized by coordinated kinematics of body segments that minimize the energy cost of transport. This is achieved by a well-coordinated sequence of energy generation, transmission, and absorption by muscles. Knowing the energy exchange between the body segments can explain the mechanisms that govern the dynamics of walking. In the case of disability, injury or disease, these mechanisms are often altered. In some disorders, such as cerebral palsy, the contribution of the ankle plantar flexors to power generation decreases that may lead to an increase in medio-lateral trunk movements. However, the direct cause or effect between increased sway and decreased contribution of the ankle is unknown. We asked how power generation and the energy balance of all segments were affected by the lateral movement of the trunk? To answer this question we evaluated the impact of voluntarily increased medio-lateral trunk movements on the mechanical energy generation and transfer during gait using a human musculoskeletal model.11 healthy subjects walked on an instrumented walkway and were asked to walk normally and with exaggerated medio-lateral trunk sway while measuring kinematic and kinetic data using motion capture cameras and force plates. To determine the effect on the energy generation and transfer, three estimates of total body power were computed using an inverse dynamics model consisting of 13 segments and 12 joints. The contribution of individual joints and body segments was determined by calculating the positive work done over the gait cycle.Increased trunk movement led to an increase in total positive work of about 8%. Work during the Rebound phase (at ~15-30% of the gait cycle) increased while work during the Push-off phase (at ~45-65% of the gait cycle) decreased. The power contribution of the ankle decreased, while the knee, hip and -most significantly- the lumbosacral joint generated more power. Segment analysis revealed that the contribution of the individual limbs to the total energy generation remained the same, while the head-arms-trunk (HAT) contributed more. Both parts of the push-off work, Pre- and Post-strike work decreased. Therefore, the voluntarily induced trunk movements did not impact the division of push-off work into Pre- and Post-strike work.Our analyses reveal that the human musculoskeletal system is able to redistribute the power generation of the joints during different walking styles. With increased medio-lateral trunk movements, the human musculoskeletal system compensates for the increase in upper body energy with a decrease in push-off by the ankle. While these different walking styles make use of different muscles, they accomplish the same objective: moving the body forward. The increase in total positive (and negative) work suggests that walking with increased trunk movement is a less efficient mechanism of moving the body forward.