Locomotor pattern and mechanical exchanges during collective load transport.

While the locomotor behavior of humans walking alone, loaded or unloaded, has been extensively studied, the locomotor behavior of humans transporting a load collectively is very poorly documented in the biomechanics literature. Yet, collective carriage is a task commonly performed in sport (CrossFit), military and health care (carriage of an injured person) activities and is a task that raises growing interest in robotics (Cobots). The primary aim of our research was to test the hypothesis that the mechanical cost of locomotion is comparable when two individuals are transporting an object collectively and when they are walking alone. To test this, the movements of ten pairs of individuals walking side by side, separately or while transporting collectively an object, were recorded with a three-dimensional motion analysis system (Vicon©). Our results show a similar pattern in the periodic displacement of the center of mass and in mechanical costs, between individuals walking alone and individuals carrying a load collectively. Moreover, a better pendulum-like behavior was found in the sagittal plane and in 3D for the pairs of individuals carrying an object, which suggests that the saving in mechanical exchanges is higher when two individuals are carrying an object collectively than when they are walking alone. The values of the parameters measured in our experiment could be used as a benchmark for the implementation of collective carriage tasks in robotics.