Balance-corrective responses to unexpected perturbations at the arms during treadmill walking

Activation of ankle muscles following rapid displacement of a light touch contact during treadmill walking

The first exposure of a rapid displacement of a light touch reference induces an inappropriate balance corrective response during standing in a proportion of participants that is extinguished with repeated exposures. We hypothesized that if the spatial touch reference was critical to performing of a task the evoked response would be more consistently expressed across participants and observed with repeated exposures to the disturbance. To test this, 20 participants received either forward (N = 10) or backward right-touch displacements at right-heel strike during motorized treadmill walking without visual feedback. Electromyographic recordings from four arm, four leg and one neck muscle were sampled along with joint kinematic and step cycle data. Rapid displacement of the touch surface elicited responses in all 20 participants. However, the frequency of first trial responses was not different from what was observed during standing. In contrast, responses were observed in all participants with subsequent trials. None of the participants tripped or stumbled as a result of the touch perturbations; however, the step cycle duration was consistently shorter following the first forward-touch displacement. A post-experiment questionnaire revealed that many participants often perceived the touch plate displacement as a disturbance to the treadmill belt speed, suggesting the disturbance was occasionally misinterpreted. The activation of ankle muscles following the unexpected slip of a touch reference during walking suggests that tactile information from the finger is a relevant sensory cue for the regulation and control of stepping and stability.

Rapid and flexible whole body postural responses are evoked from perturbations to the upper limb during goal-directed reaching

An important aspect of motor control is the ability to perform tasks with the upper limbs while maintaining whole body balance. However, little is known about the coordination of upper limb voluntary and whole body postural control after mechanical disturbances that require both upper limb motor corrections to attain a behavioral goal and lower limb motor responses to maintain whole body balance. The present study identified the temporal organization of muscle responses and center of pressure (COP) changes following mechanical perturbations during reaching. Our results demonstrate that muscle responses in the upper limb are evoked first (∼50 ms), with lower limb muscle activity occurring immediately after, in as little as ∼60 ms after perturbation. Hand motion was immediately altered by the load, while COP changes occurred after ∼100 ms, when lower limb muscle activity was already present. Our secondary findings showed that both muscle activity and COP changes were influenced by behavioral context (by altering target shape, circle vs. rectangle). Voluntary and postural actions initially directed the hand toward the center of both target types, but after the perturbation upper limb and postural responses redirected the hand toward different spatial locations along the rectangle. Muscle activity was increased for both upper and lower limbs when correcting to the circle vs. the rectangle, and these differences emerged as early as the long-latency epoch (∼75-120 ms). Our results demonstrate that postural responses are rapidly and flexibly altered to consider the behavioral goal of the upper limb.NEW & NOTEWORTHY The present work establishes that, when reaching to a target while standing, perturbations applied to the upper limb elicit a rapid response in lower limb muscles. Unlike voluntary movements, postural responses do not occur before corrections of the upper limb. We show the first evidence that corrective postural adjustments are modulated by upper limb behavioral context (target shape). Importantly, this indicates that postural responses take into account upper limb feedback for online control.

Control of Leg Movements Driven by EMG Activity of Shoulder Muscles

During human walking, there exists a functional neural coupling between arms and legs, and between cervical and lumbosacral pattern generators. Here, we present a novel approach for associating the electromyographic (EMG) activity from upper limb muscles with leg kinematics. Our methodology takes advantage of the high involvement of shoulder muscles in most locomotor-related movements and of the natural co-ordination between arms and legs. Nine healthy subjects were asked to walk at different constant and variable speeds (3–5 km/h), while EMG activity of shoulder (deltoid) muscles and the kinematics of walking were recorded. To ensure a high level of EMG activity in deltoid, the subjects performed slightly larger arm swinging than they usually do. The temporal structure of the burst-like EMG activity was used to predict the spatiotemporal kinematic pattern of the forthcoming step. A comparison of actual and predicted stride leg kinematics showed a high degree of correspondence (r > 0.9). This algorithm has been also implemented in pilot experiments for controlling avatar walking in a virtual reality setup and an exoskeleton during over-ground stepping. The proposed approach may have important implications for the design of human–machine interfaces and neuroprosthetic technologies such as those of assistive lower limb exoskeletons.