A Neuromuscular Model of Human Locomotion Combines Spinal Reflex Circuits with Voluntary Movements

Abstract

Existing models of human walking use low-level reflexes or neural oscillators to generate movement. While appropriate to generate the stable, rhythmic movement patterns of steady-state walking, these models lack the ability to change their movement patterns or spontaneously generate new movements in the specific, goal-directed way characteristic of voluntary movements. Here we present a neuromuscular model of human locomotion that bridges this gap and combines the ability to execute goal directed movements with the generation of stable, rhythmic movement patterns that are required for robust locomotion. The model represents goals for voluntary movements of the swing leg on the task level of swing leg joint kinematics. Smooth movements plans towards the goal configuration are generated on the task level and transformed into descending motor commands that execute the planned movements, using internal models. The movement goals and plans are updated in real time based on sensory feedback and task constraints. On the spinal level, the descending commands during the swing phase are integrated with a generic stretch reflex for each muscle. Stance leg control solely relies on dedicated spinal reflex pathways. Spinal reflexes stimulate Hill-type muscles that actuate a biomechanical model with eight internal joints and six free-body degrees of freedom. The model is able to generate voluntary, goal-directed reaching movements with the swing leg and combine multiple movements in a rhythmic sequence. During walking, the swing leg is moved in a goal-directed manner to a target that is updated in real-time based on sensory feedback to maintain upright balance, while the stance leg is stabilized by low-level reflexes and a behavioral organization switching between swing and stance control for each leg. With this combination of reflexive stance leg and voluntary, goal-directed control of the swing leg, the model controller generates rhythmic, stable walking patterns in which the swing leg movement can be flexibly updated in real-time to step over or around obstacles.

High-level motor planning allows flexible walking at different gait patterns in a neuromechanical model

“…Freely choosing a gait pattern requires the ability to flexibly modify the kinematic trajectory of the swing leg. In [30], we presented a neuromuscular modelling approach to generate flexible swing leg movements that we followed and expanded in this study. The model generates a motor plan to bring a kinematic task variable from its current state to a goal state in a desired time, using a minimal jerk trajectory.…”

“…We represent the movement goal as desired joint angles for hip flexion, knee flexion and ankle flexion. As in [30], we subdivide the swing phase into an early and a late swing phase and define a set of target joint angles for the end of each sub-phase. Specifically for the ankle flexion, we define only one target joint angle for the entire swing phase as the task variable, since ankle flexion does not change its movement direction during the swing phase in normal human walking [23].…”

“…For details about the implementation of movement goals, minimal jerk trajectories and balance control please refer to [30].…”

“…This parameter is constant and does not require movement planning or the modulation by an internal model. Trunk balance in the lateral direction is controlled as in [30].…”

“…Experimental studies of human walking suggest a duality of steps as both (1) part of rhythmic movement patterns of the whole body and (2) reaching movements with the foot [10]. We previously presented a model that attempts to bridge this gap by integrating high-level, voluntary movement planning for the swing leg with low-level, habitual control [30]. The model was able to avoid obstacles, vary movement speed and walking direction and perform goal directed movements with the swing leg by executing a motor plan to reach a kinematic goal, without re-optimizing model parameters.…”