The contribution of a central pattern generator in a reflex-based neuromuscular model

Abstract

Although the concept of central pattern generators (CPGs) controlling locomotion in vertebrates is widely accepted, the presence of specialized CPGs in human locomotion is still a matter of debate. An interesting numerical model developed in the 90s’ demonstrated the important role CPGs could play in human locomotion, both in terms of stability against perturbations, and in terms of speed control. Recently, a reflex-based neuro-musculo-skeletal model has been proposed, showing a level of stability to perturbations similar to the previous model, without any CPG components. Although exhibiting striking similarities with human gaits, the lack of CPG makes the control of speed/step length in the model difficult. In this paper, we hypothesize that a CPG component will offer a meaningful way of controlling the locomotion speed. After introducing the CPG component in the reflex model, and taking advantage of the resulting properties, a simple model for gait modulation is presented. The results highlight the advantages of a CPG as feedforward component in terms of gait modulation.

Effects of a neuromuscular controller on a powered ankle exoskeleton during human walking

“…Although the spinal architecture responsible for locomotion proposed by Geyer is neuro-physiologically incomplete, the dynamics of the produced gaits are in striking agreement with physiological observations of healthy human walking. This agreement is observed at the muscle level, where similar patterns to those recorded in humans with surface EMG are observed [5] at normal walking speeds.…”

“…The virtual gastrocnemius, a bi-articular muscle, was omitted to avoid a shared control issue, as torques would have been created at both the ankle and absent knee joint. Both the tibialis anterior and soleus control loops were used in pure reflex mode, without the CPG component, as previous studies showed the CPG component is more critical for proximal joints, in particular, the hip joint [5]. The parameters used for the control were the same as presented in [10] except for the soleus muscle resting length, which was decreased to 0.3 cm to account for the difference in morphology between the model and the subjects.…”

“…Ground sensors are used to detect whether the limb is on stance or swing phase. Then, depending on whether the limb is in contact with ground or in swing, different reflex rules are generated, stance reflexes in green and swing reflexes in blue (Table 1 in [5] shows the different reflex loops acting depending on the gait phases). Here only the soleus and tibialis muscle modules are used.…”

“…The approach relies on the use of neuromuscular models (NMM) [10], [5] -muscle-tendon models used in neuromechanical simulation of human gait -as the basis for the design of the exoskeleton controller. The novelty of the control paradigm is to base the controller on functional models of dynamic elements in the human leg (i.e.…”

“…A consequence is that the exact same controller (with same parameters) can be used under a variety of conditions, such as different speeds and different terrains [5].…”