Responses to External Perturbations in Selected Human Motor Tasks - A Systematic Review and Analysis.

Balance is critical for human posture control when standing upright and during cyclic locomotor tasks such as walking or running, as well as for acyclic tasks such as gait initiation or complex sport movements. In the course of evolution, Homo sapiens developed an upright posture for bipedalism, thus freeing the upper limbs (arms) to interact with objects. Human upright stance is characterized by two straight legs and the center of mass (COM) is located above the hip, thus maximizing potential energy (due to high COM position) enabling great maneuverability for fast re-orientation of the body axis and re-direction of movement direction in space. Nevertheless, bipedalism is mechanically much more challenging (e.g. regarding stability) compared to other body morphologies in legged animals such as a quadrupedal leg configuration. This evolutionary innovation does not only provide benefits (such as those mentioned above), but also makes control functions difficult, which might involve instability of the whole mechanical system with segments arranged like an upside-down chain. To achieve the stable upright human stance and to prevent collapse, it is fundamental to continuously balance all segments above the feet by introducing appropriate joint torques and continuously adjusting the orientation of the ground reaction forces (GRF). Nevertheless, human motor control during tasks such as standing and walking provides stability in the case of external perturbations. Thus, humans are able to respond to external perturbations, such as changing ground level, different ground properties as well as pushes and pulls at different body regions, in order to keep their balance and maintain an upright posture. Given the current biomechanical understanding of human balance and posture control, it is still not well understood which neuro-muscular control mechanisms contribute to maintaining balance in response to external perturbations. In particular, it is not clear, how the contributions to recover from perturbations are organized at different levels, e.g. muscle mechanical response, spinal reflexes, and higher control contributions (e.g. from cortical areas in the brain). The present work focuses on improving this understanding by investigating human movement and posture control in response to different external perturbations. This thesis describes how healthy humans respond to unexpected external perturbations and identifies underlying neuro-muscular mechanisms enabling the motor system to cope with such challenges during locomotion and upright standing through passive and active strategies (e.g. tendon and muscles response, changed muscle activation). The first part of the thesis presents previous research results in a systematic review thus providing insights into how leg function responds to external perturbations in selected motion tasks. It is shown that humans adjust their movements not only to the environmental context (e.g. when walking on even ground vs. slopes or climbing stairs) but also dependent on the state of their motion (i.e. current phase of gait, COM position) and in relation to the type of perturbation like changes in ground or external forces. In the following part of the thesis, human standing experiments were designed to address the ability to cope with external perturbations with respect to axial and rotational leg function. Axial leg function described forces and displacements along the leg axis, pointing from the contact point of the foot to the COM. In contrast, rotational leg function described corresponding forces and displacements perpendicular to leg axis in sagittal plane. As predicted by biomechanical leg models, the results show that biarticular muscles strongly contribute to the redirection of the GRF in order to maintain an upright posture. In a second experimental study on human hopping, the ability to adapt leg stiffness (representing the axial leg function) in order to maintain cyclic movements in response to vertical ground level perturbations was investigated. The findings demonstrate a robust leg function, reflecting the ability of the human neuro-muscular system to cope with unexpected perturbations such as moving ground during hopping. An increase in leg stiffness was found in response to an upward surface acceleration. This thesis describes and analyses the ability of the human body to respond to external perturbations leading to robust movement patterns. By translating the observed coping strategies into biomechanical and motor control models, new approaches for the design and control of legged systems (e.g. for assistive and rehabilitation devices) can be derived.