Emily A. Keshner

Neck muscle activation patterns in humans during isometric head stabilization

SummaryA musculoskeletal system with more muscles than there are motions could be programmed in alternative ways to produce a single movement. In this case, the muscles would have the potential to be maximally responsive in multiple directions rather than responding preferentially in a single direction. To determine the response patterns of muscles in the head-neck motor system, the simultaneous activation of four of the 23 neck muscles acting on the head was recorded with both surface and intramuscular electrodes. Fifteen human subjects were tested during an isometric head stabilization task. When the EMG response patterns were plotted, each muscle demonstrated a preferred direction of activation. This preferred activation direction was consistent in all of the subjects for three of the muscles tested. The fourth muscle, splenius, was preferentially activated during neck flexion in half of the subjects and during neck extension in the other half. Increasing the force parameters of the task suggested a linear relationship between force and the EMG output in the preferred response directions. Responses in the nonpreferred directions were produced by a nonlinear change in EMG activation of the muscle. This finding could have implications for theories of how reciprocal activation and cocontraction patterns of response are elicited. Results from this study, that the CNS programs neck muscles to respond in specific orientations rather than generating an infinite variety of muscle patterns, are in agreement with our findings in the cat.

Virtual reality and physical rehabilitation: a new toy or a new research and rehabilitation tool?

Virtual reality (VR) technology is rapidly becoming a popular application for physical rehabilitation and motor control research. But questions remain about whether this technology really extends our ability to influence the nervous system or whether moving within a virtual environment just motivates the individual to perform. I served as guest editor of this months issue of the Journal of NeuroEngineering and Rehabilitation (JNER) for a group of papers on augmented and virtual reality in rehabilitation. These papers demonstrate a variety of approaches taken for applying VR technology to physical rehabilitation. The papers by Kenyon et al. and Sparto et al. address critical questions about how this technology can be applied to physical rehabilitation and research. The papers by Sveistrup and Viau et al. explore whether action within a virtual environment is equivalent to motor performance within the physical environment. Finally, papers by Riva et al. and Weiss et al. discuss the important characteristics of a virtual environment that will be most effective for obtaining changes in the motor system.

Emergence of Virtual Reality as a Tool for Upper Limb Rehabilitation: Incorporation of Motor Control and Motor Learning Principles

The primary focus of rehabilitation for individuals with loss of upper limb movement as a result of acquired brain injury is the relearning of specific motor skills and daily tasks. This relearning is essential because the loss of upper limb movement often results in a reduced quality of life. Although rehabilitation strives to take advantage of neuroplastic processes during recovery, results of traditional approaches to upper limb rehabilitation have not entirely met this goal. In contrast, enriched training tasks, simulated with a wide range of low- to high-end virtual reality–based simulations, can be used to provide meaningful, repetitive practice together with salient feedback, thereby maximizing neuroplastic processes via motor learning and motor recovery. Such enriched virtual environments have the potential to optimize motor learning by manipulating practice conditions that explicitly engage motivational, cognitive, motor control, and sensory feedback–based learning mechanisms. The objectives of this article are to review motor control and motor learning principles, to discuss how they can be exploited by virtual reality training environments, and to provide evidence concerning current applications for upper limb motor recovery. The limitations of the current technologies with respect to their effectiveness and transfer of learning to daily life tasks also are discussed.

Using immersive technology for postural research and rehabilitation

Posture has traditionally been examined by isolating individual control pathways to determine their specific contributions. However, if these pathways are responsive to functional contexts, then their responses may differ when the system is receiving simultaneous inputs from multiple pathways. Thus, we may never fully understand how the central nervous system (CNS) organizes behaviors in the real world from studies conducted in the minimized environment of the laboratory. The consequence of this is that when findings from the laboratory are applied to therapeutic intervention, the intervention may not be appropriate for all circumstances and will not fully meet the needs of the patient. We have united an immersive dynamic virtual environment with motion of a posture platform to record the biomechanical and physiological responses to combined visual, vestibular, and proprioceptive inputs. The virtual environment possesses content, contrast, and texture so that we can examine postural responses as they might occur in a complex, real-world environment. In this paper we specifically describe the factors guiding our choices of virtual technology and present data from young adults, elderly adults, and an individual with bilateral labyrinthine loss to demonstrate how multimodal inputs influence their postural response organization. Significant implications for future experimental and rehabilitation protocols are also discussed.

Vertebral orientations and muscle activation patterns during controlled head movements in cats

The focus of these experiments was to determine the relationships between head movement, neck muscle activation patterns, and the positions and movements of the cervical vertebrae. One standing cat and one prone cat were trained to produce voluntary sinusoidal movements of the head in the sagittal plane. Video-opaque markers were placed on the cervical vertebrae, and intramuscular patch electrodes implanted in four muscles of the head and neck. Cinefluoroscopic images of cervical vertebral motion and electromyographic responses were simultaneously recorded. Analysis of the spinal movement revealed that the two cats used different strategies to keep their heads aligned with the tracker. In the standing cat, vertebral motion described a more circular arc, compared to a forward diagonal in the prone cat. Intervertebral motion was limited, but more acute angles appeared between the vertebrae of the prone lying than of the standing animal. Data revealed that the central nervous system could control several axes of motion to keep the cervical spine matched to the moving stimulus. Phase relations between the sinusoidal motion of the vertebral column, peak activation of the neck muscles, and that of the stimulus were examined, and several different control strategies were observed both between and within animals. The results suggest that the central nervous system engages in multiple strategies of musculo-skeletal coordination to achieve a single movement outcome.

Patterns of neck muscle activation in cats during reflex and voluntary head movements

SummaryWhen the head rotates, vestibulocollic reflexes counteract the rotation by causing contraction of the neck muscles that pull against the imposed motion. With voluntary head rotations, these same muscles contract and assist the movement of the head. The purpose of this study was to determine if an infinite variety of muscle activation patterns are available to generate a particular head movement, or if the CNS selects a consistent and unique muscle pattern for the same head movement whether performed in a voluntary or reflex mode. The relationship of neck muscle activity to reflex and voluntary head movements was examined by recording intramuscular EMG activity from six neck muscles in three alert cats during sinusoidal head rotations about 24 vertical and horizontal axes. The cats were trained to voluntarily follow a water spout with their heads. Vestibulocollic reflex (VCR) responses were recorded in the same cats by rotating them in an equivalent set of planes with the head stabilized to the trunk so that only the vestibular labyrinths were stimulated. Gain and phase of the EMG responses were calculated, and data analyzed to determine the directions of rotation for which specific muscles produced their greatest EMG output. Each muscle exhibited preferential activation for a unique direction of rotation, and weak responses during rotations orthogonal to that preferred direction. The direction of maximal activation could differ for reflex and voluntary responses. Also, the best excitation of the muscle was not always in the direction that would produce a maximum mechanical advantage for the muscle based on its line of pull. The results of this study suggest that a unique pattern of activity is selected for VCR and tracking responses in any one animal. Patterns for the two behaviors differ, indicating that the CNS can generate movements in the same direction using different muscle patterns.

Considerations for the future development of virtual technology as a rehabilitation tool

BackgroundVirtual environments (VE) are a powerful tool for various forms of rehabilitation. Coupling VE with high-speed networking [Tele-Immersion] that approaches speeds of 100 Gb/sec can greatly expand its influence in rehabilitation. Accordingly, these new networks will permit various peripherals attached to computers on this network to be connected and to act as fast as if connected to a local PC. This innovation may soon allow the development of previously unheard of networked rehabilitation systems. Rapid advances in this technology need to be coupled with an understanding of how human behavior is affected when immersed in the VE.MethodsThis paper will discuss various forms of VE that are currently available for rehabilitation. The characteristic of these new networks and examine how such networks might be used for extending the rehabilitation clinic to remote areas will be explained. In addition, we will present data from an immersive dynamic virtual environment united with motion of a posture platform to record biomechanical and physiological responses to combined visual, vestibular, and proprioceptive inputs. A 6 degree-of-freedom force plate provides measurements of moments exerted on the base of support. Kinematic data from the head, trunk, and lower limb was collected using 3-D video motion analysis.ResultsOur data suggest that when there is a confluence of meaningful inputs, neither vision, vestibular, or proprioceptive inputs are suppressed in healthy adults; the postural response is modulated by all existing sensory signals in a non-additive fashion. Individual perception of the sensory structure appears to be a significant component of the response to these protocols and underlies much of the observed response variability.ConclusionThe ability to provide new technology for rehabilitation services is emerging as an important option for clinicians and patients. The use of data mining software would help analyze the incoming data to provide both the patient and the therapist with evaluation of the current treatment and modifications needed for future therapies. Quantification of individual perceptual styles in the VE will support development of individualized treatment programs. The virtual environment can be a valuable tool for therapeutic interventions that require adaptation to complex, multimodal environments.

Modulating active stiffness affects head stabilizing strategies in young and elderly adults during trunk rotations in the vertical plane.

Healthy young and elderly adults were asked to actively modulate neck muscle stiffness during random rotations of the trunk in the vertical plane. Angular velocity of head with respect to trunk and myoelectric activity of semispinalis capitis and sternocleidomastoid muscles were recorded. A MANOVA was performed on group, condition, and frequency variables. A gain and phase drop at 2.15 Hz in young adults indicated neural (i.e. reflex) damping of system mechanics. In the elderly, a steady rise in gain and drop in phase (P<0.0002) was indicative of a second order underdamped system. Even when instructed to not intervene elderly subjects exhibited cocontraction. Ineffective reflex mechanisms may underlie the emergence of this strategy.

Self versus Environment Motion in Postural Control

To stabilize our position in space we use visual information as well as non-visual physical motion cues. However, visual cues can be ambiguous: visually perceived motion may be caused by self-movement, movement of the environment, or both. The nervous system must combine the ambiguous visual cues with noisy physical motion cues to resolve this ambiguity and control our body posture. Here we have developed a Bayesian model that formalizes how the nervous system could solve this problem. In this model, the nervous system combines the sensory cues to estimate the movement of the body. We analytically demonstrate that, as long as visual stimulation is fast in comparison to the uncertainty in our perception of body movement, the optimal strategy is to weight visually perceived movement velocities proportional to a power law. We find that this model accounts for the nonlinear influence of experimentally induced visual motion on human postural behavior both in our data and in previously published results.

Influence of visual scene velocity on segmental kinematics during stance

We investigated how the velocity of anterior-posterior movement of a visual surround affected segmental kinematics during stance. Ten healthy young adults were exposed to sinusoidal oscillation of an immersive virtual scene at five peak velocities ranging from 1.2 to 188 cm/s at each of four frequencies: 0.05, 0.1, 0.2 and 0.55 Hz. Root mean square (RMS) values of head, trunk, thigh and shank angular displacements were calculated. RMS values of head-neck, hip, knee and ankle joint angles were also calculated. RMS values of head, trunk, thigh and shank displacements exhibited significant increases at a scene velocity of 188 cm/s when compared with lower scene velocities. RMS values of hip, knee and ankle joint angles exhibited significant increases at scene velocities of 125 and 188 cm/s when compared with lower scene velocities. These results suggest that visual cues continued to drive postural adjustments even during high velocity movement of the virtual scene. Significant increases in the RMS values of the lower-limb joint angles suggest that as visually-induced postural instability increased, the body was primarily controlled as a multi-segmental structure instead of a single-link inverted pendulum, with the knee playing a key role in postural stabilization.