Frequency-dependent behavior of paretic and non-paretic leg force during standing post stroke.

Maintaining upright posture in quiet standing is an important skill that is often disrupted by stroke. Despite extensive study of human standing, current understanding is incomplete regarding the muscle coordination strategies that produce the ground-on-foot force (F) that regulates translational and rotational accelerations of the body. Even less is understood about how stroke disrupts that coordination. Humans produce sagittal plane variations in the location (center of pressure, xCP) and orientation (Fx/Fz) of F that, along with the force of gravity, produce sagittal plane body motions. As F changes during quiet standing there is a strong correlation between the xCP and Fx/Fz time-varying signals within narrow frequency bands. The slope of the correlation varies systematically with frequency in non-disabled populations, is sensitive to changes in both environmental and neuromuscular control factors, and emerges from the interaction of body mechanics and neural control. This study characterized the xCP versus Fx/Fz relationship as frequency-dependent Intersection Point (IP) heights for the paretic and non-paretic legs of individuals with history of a stroke (n = 12) as well as in both legs of non-disabled controls (n = 22) to reveal distinguishing motor coordination patterns. No inter-leg difference of IP height was present in the control group. The paretic leg IP height was lower than the non-paretic, and differences from control legs were in opposite directions. These results quantify disrupted coordination that may characterize the paretic leg balance deficit and non-paretic leg compensatory behavior, providing a means of monitoring balance impairment and a target for therapeutic interventions.

Modulation of sagittal-plane center of pressure and force vector direction in human standing on sloped surfaces.

The multi-joint coordination responsible for maintaining upright posture in the standing human manifests in the pattern of variation of the support-surface force (F). Assessment of both the translational and rotational kinematics in the sagittal-plane requires understanding the critical relationship between the direction and location of F. Prior work demonstrated that band-pass filtered F direction and center-of-pressure (CoP) covary in time such that the F vector lines-of-action pass near a fixed point called an intersection point (IP). The height of that IP (IPz) varies systematically with the frequency of the pass band. From F measurements in able-bodied humans (n = 17) standing on various pitched surfaces, the present study also found the emergent property of an IP, with IPz located above the center of mass (CoM) at frequencies <1.75 Hz and below the CoM for higher frequencies. This property aids in maintaining upright posture for various perturbation modes within a single control structure. From purely mechanical effects, standing on a pitched surface should not change IPz, however these measurements of F show that IPz is generally closer to CoM height. This characterization of quiet standing provides simple means of assessing the complex multi-joint coordination of standing and relates directly to the physical demands of controlling the translational and rotational aspects of body posture.

Frequency-dependent contributions of sagittal-plane foot force to upright human standing.

Quiet standing is a mechanically unstable postural objective that humans typically perform with ease. Control of upright posture requires stabilization of both translational and rotational degrees-of-freedom that is accomplished by neuro-muscular coordination. This coordination produces a force at the ground-foot interface (F) that is quantified by magnitude, direction (θF), and point of application (center-of-pressure, CP). Previous research has shown that the nervous system controls muscle activation such that CP motion occurs at both slow and fast time scales. However, it is unknown how θF varies with respect to CP and how that relationship varies across time scales. We present a novel method for assessing the frequency-dependent relative variation in θF and CP. The center-of-pressure (CP) and direction of the ground-on-foot force (F) in the sagittal-plane during quiet standing were decomposed into 0.2 Hz-width frequency bands within 0.4-8.0 Hz. The relation between the direction and CP was approximately linear with a slope positively related to frequency. These frequency-dependent features of F have critical implications for understanding balance strategy because the translational and rotational acceleration effects of F were coupled, but with opposite phasing at high versus low frequencies. Such results suggest a system tuned for one stability mode at low frequencies and another mode at higher frequencies. This frequency-wise approach to examining the translational and rotational effects of humans' preferred F may be useful for establishing balance rehabilitation metrics, directing study of the underlying neural mechanisms responsible for the observed coordination, and for setting a biometric standard to inform biomimetic prosthetics and robotics.

Standing Balance on Unsteady Surfaces in Children on the Autism Spectrum: The Effects of IQ.

BACKGROUND: Postural stability difficulties are commonly reported in people on the autism spectrum. However, it is unclear whether unsteady surfaces may exacerbate postural stability difficulties in children and adolescents with autism spectrum disorder (ASD). Understanding balance on unsteady surfaces is important because uneven surfaces are commonly encountered in daily life. METHODS: Twenty-one youth on the autism spectrum and 16 youth with typical development (ages 6-16 years, IQ ≥ 79) stood on both a fixed and unsteady (tiltable) platform, and center of pressure was measured. RESULTS: The group with ASD exhibited differentially more postural sway on the unsteady surface compared to the group with typical development. However, there was substantial variability within the ASD group. Follow-up analyses suggested that much of the variability in postural sway in the ASD group was accounted for by IQ. CONCLUSIONS: Clinically, these findings suggest that not all individuals with ASD struggle more with postural stability on unsteady surfaces. Instead children and adolescents with ASD and below-average IQ may have particular difficulty on unsteady surfaces and may require accommodations. Further, these findings lay the groundwork for future research to investigate the underlying mechanisms of poorer balance across the autism spectrum.

Development of KIINCE: A kinetic feedback-based robotic environment for study of neuromuscular coordination and rehabilitation of human standing and walking.

INTRODUCTION: The objective of this article is to introduce the robotic platform KIINCE and its emphasis on the potential of kinetic objectives for studying and training human walking and standing. The device is motivated by the need to characterize and train lower limb muscle coordination to address balance deficits in impaired walking and standing. METHODS: The device measures the forces between the user and his or her environment, particularly the force of the ground on the feet (F) that reflects lower limb joint torque coordination. In an environment that allows for exploration of the user's capabilities, various forms of real-time feedback guide neural training to produce F appropriate for remaining upright. Control of the foot plate motion is configurable and may be user driven or prescribed. Design choices are motivated from theory of motor control and learning as well as empirical observations of F during walking and standing. RESULTS: Preliminary studies of impaired individuals demonstrate the feasibility and potential utility of patient interaction with kinetic immersive interface for neuromuscular coordination enhancement. CONCLUSION: Applications include study and rehabilitation of standing and walking after injury, amputation, and neurological insult, with an initial focus on stroke discussed here.

Post-Stroke Walking Behaviors Consistent with Altered Ground Reaction Force Direction Control Advise New Approaches to Research and Therapy.

Recovery of walking after stroke requires an understanding of how motor control deficits lead to gait impairment. Traditional therapy focuses on removing specific observable gait behaviors that deviate from unimpaired walking; however, those behaviors may be effective compensations for underlying problematic motor control deficits rather than direct effects of the stroke. Neurological deficits caused by stroke are not well understood, and thus, efficient interventions for gait rehabilitation likely remain unrealized. Our laboratory has previously characterized a post-stroke control deficit that yields a specific difference in direction of the ground reaction force (F, limb endpoint force) exerted with the hemiplegic limb of study participants pushing on both stationary and moving pedals while seated. That task was not dependent on F to retain upright posture, and thus, the task did not constrain F direction. Rather, the F direction was the product of neural preference. It is not known if this specific muscle coordination deficit causes the observed walking deviations, but if present during walking, the deficit would prevent upright posture unless counteracted by compensatory behaviors. Compensations are presented that mechanically counteract the F misdirection to allow upright posture. Those compensations are similar to behaviors observed in stroke patients. Based on that alignment between predictions of this theory and clinical observations, we theorize that post-stroke gait results from the attempt to compensate for the underlying F misdirection deficit. Limb endpoint force direction has been shown to be trainable in the paretic upper limb, making it a feasible goal in the lower limb. If this F misdirection theory is valid, these ideas have tremendous promise for advancing the field of post-stroke gait rehabilitation.

Ankle torque control that shifts the center of pressure from heel to toe contributes non-zero sagittal plane angular momentum during human walking.

A principle objective of human walking is controlling angular motion of the body as a whole to remain upright. The force of the ground on each foot (F) reflects that control, and recent studies show that in the sagittal plane F exhibits a specific coordination between F direction and center-of-pressure (CP) that is conducive to remaining upright. Typical walking involves the CP shifting relative to the body due to two factors: posterior motion of the foot with respect to the hip (stepping) and motion of the CP relative to the foot (foot roll-over). Recent research has also shown how adjusting ankle torque alone to shift CP relative to the foot systematically alters the direction of F, and thus, could play a key role in upright posture and the F measured during walking. This study explores how the CP shifts due to stepping and foot roll-over contribute to the observed F and its role in maintaining upright posture. Experimental walking kinetics and kinematics were combined with a mechanical model of the human to show that variation in F that was not attributable to foot roll-over had systematic correlation between direction and CP that could be described by an intersection point located near the center-of-mass. The findings characterize a component of walking motor control, describe how typical foot roll-over contributes to postural control, and provide a rationale for the increased fall risk observed in individuals with atypical ankle muscle function.

Mechanical interaction of center of pressure and force direction in the upright human.

Humans maintain upright bipedal posture by producing appropriate force against the environment through the interaction of neural controlled muscle force with the mechanics of the skeletal system. Characterizing these mechanics facilitates understanding of the neural control. We used a mechanical model of an upright human to analyze how the mechanical linkage aspects of the human body affect the force between the feet and the ground (F). Key parameters of F that directly regulate upright body posture are the direction of F (θ(F)) and its point of application (x(CP), anterior-posterior position of the center of pressure). Instantaneous analysis of the equations of motion demonstrated that θ(F) varied systematically with x(CP) such that the F vectors intersected at a point called the Posture-specific force Intersection point or PI (Π). The Π was located above the center of mass when the hip and knee joints were modeled as rigid and was located near the knee when the hip and knee torques were held constant. Limb posture and the knee torque affected the location of Π. This Π behavior quantifies the purely mechanical effect of anterior-posterior center of pressure shifts on the direction of F, which has consequences for the control of whole body posture.

Force direction pattern stabilizes sagittal plane mechanics of human walking.

The neural control and mechanics of human bipedalism are inadequately understood. The variable at the interface of neural control and body mechanics that is key to upright posture during human walking is the force of the ground on the foot (ground reaction force, F). We present a model that predicts sagittal plane F direction as passing through a divergent point (DP) fixed in a reference frame attached to the person. Four reference frames were tested to identify which provided the simplest and most accurate description of F direction. For all reference frames, the DP model predicted nearly all the observed variation in F direction and whole body angular momentum during single leg stance. The reference frame with vertical orientation and with origin on the pelvis provided the best combination of accuracy and simplicity. The DP was located higher than the CM and the predicted F produced a pattern of torque about the CM that caused body pitch oscillations that disrupted upright posture. Despite those oscillations, that torque was evidence of a stability mechanism that may be a critical component enabling humans to remain upright while walking and performing other tasks.

Task-dependent organization of pinch grip forces.

The organization of thumb and index finger forces in a pinch formation was investigated under conditions where kinetic constraints on interdigit force coupling were removed. Two visually guided isometric force tasks at submaximal levels were used to characterize the spatial and temporal aspects of interdigit force coupling. Task 1 provided an initial characterization of interdigit force coordination when the force relationship between the digits was not specified. Task 2 probed the extent to which a preferred coordination of the thumb and index finger could be decoupled, both temporally and with respect to force magnitude, by specifying the coordination between the digit forces. Digit forces were measured using a pinch apparatus that was instrumented to record the magnitude and direction of the thumb (F(t)) and index finger (F(i)) forces, independently. Two apparatus conditions allowed further examination of interdigit force coordination when the relationship between digit forces was mechanically constrained (pivot condition), and when the relationship between digit forces was not constrained, allowing the neuromotor system to select a preferred pattern of interdigit coordination (fixed condition). Sixteen right-handed adults exerted a pinch force against the apparatus to match a single-cycle sine wave that varied between 15 and 35% of each participant's maximal voluntary pinch force. The target was presented with positive or negative target sense, to vary the order of force level and direction of force change across the trials. When the mechanical constraints allowed selection of a preferred coordination pattern, F(t) = F(i) was a robust result. In contrast, when the coordination between the digit forces was specified by the requirement to simultaneously produce and control independent thumb and index finger forces while acting on a stable object, subjects were able to produce forces that markedly deviated from the F(t) = F(i) coordination. The organization of pinch is characterized by a preferred, tight coupling of digit forces, which can be modified based on task demands.

Afferent contributions to digit force coupling and force level variation during performance of non-lift pinch.

Afferent contributions to the coordination of thumb and index finger forces during non-lift pinch were studied using an anesthetization case study design. Two subjects, one performing with and without digital anesthetization and one with intact sensation, produced dynamic pinch forces against a stable object, with and without visual feedback. Error corrections were less frequent post-anesthetization, and the cross correlation between digit forces was lower when sensation was removed. However, this decrease in cross correlation between digit forces seemed to reflect a loss in the magnitude of tightly coupled error corrections when sensation was removed, rather than more frequent deviations of force magnitude between the digit forces. Force-time output without visual feedback lacked these error corrections, and the correlation between digit forces remained high, irrespective of sensory status. Additionally, with vision occluded, the time rate of force change did not vary in a gradual manner as would be expected from a neural representation of a sinusoidal target, but was instead marked by sudden abrupt reversals of force rate of change, invariant of somatosensory status. The coupling of digit forces and rates of force change during non-lift pinch appear to be controlled primarily with feedforward mechanisms, where the lack of proprioceptive feedback does not seem to disrupt this coupling.

Direction of foot force for pushes against a fixed pedal: role of effort level.

Control of the force exerted by the foot on the ground is critical to human locomotion. During running on a treadmill and pushing against a fixed pedal, humans increased foot force in a linear manner in sagittal plane force space. However, for pushes against a moving pedal, force output was linear for some participants but slightly curved for others. A primary difference between the static and dynamic pedaling studies was that the dynamic study required participants to push with varying peak effort levels, whereas a constant peak effort level was used for the fixed pedal pushes. The present study evaluated the possibility that force direction varied with level of effort. Seated humans pushed against a fixed pedal to a series of force magnitude targets. The force direction varied systematically with effort level consistent with the force path curvature observed for dynamic pedaling.

The control of foot force during pushing efforts against a moving pedal.

The muscle component of the force applied to a bicycle pedal (foot force) by seated humans provided insight into the organization of the motor system. Healthy adults ( n=11) pedaled a stationary cycle ergometer while attempting to match peak foot force magnitude to visually presented force targets (200, 250,., 650 N). Pedaling cadence was maintained at 60 rpm by a motor. Measurements of the foot force, pedal angle, and crank angle were recorded. The experimental design and data analysis allowed the isolation of the muscle component of the foot force from the contributions due to gravity and inertia. A graphical representation of the muscle component of the foot force (force path) was created for each of several crank angles throughout the extension phase of the pedaling cycle. The force paths showed several highly conserved characteristics across participants and crank angles. Each force path occupied a narrow range in force space despite the ability of the participants to produce force in a wide region of force space. Three control strategies were observed in the geometry of the force paths. Eighty five percent of the force paths were linear for six of the participants, and 79% of the force paths had second-order curvature for the other five participants. The curvature was concave to the posterior for four of the participants and concave to the anterior for one participant. The linear force paths were consistent with the previously reported linear nature of the force paths for pushes against a quasi-static pedal. The observation of simple force path geometry for two tasks with dissimilar dynamic characteristics suggests that this aspect of foot force control may be common to a range of lower limb tasks and may reflect a mechanism by which the nervous system organizes the control of foot force.

Direction of foot force for pushes against a fixed pedal: variation with pedal position.

The force that healthy humans generated against a fixed pedal was measured and compared with that predicted by four models. The participants (n = 11) were seated on a stationary bicycle and performed brief pushing efforts against an instrumented pedal with the crank fixed. Pushes were performed to 10 force magnitude targets and at 12 crank angles. The increasing magnitude portion of the sagittal-plane force path for each push effort was fitted with a line to determine the direction of the muscle component of the foot force. Those directions varied systematically with the position of the pedal (crank angle) such that the force path lines intersected a common region superior and slightly anterior to the hip. The ability of four models to predict force path direction was tested. All four models captured the general variation of direction with pedal position. Two of the models provided the best performance. One was a musculoskeletal model consisting of nine muscles with parameters adjusted to provide the best possible fit. The other model was derived from (a) observations that the lines-of-action of the muscle component of foot force tended to intersect in a common region near the hip, and (b) the corresponding need for foot force to intersect the center-of-mass during walking. Thus, this model predicted force direction at each pedal position as that of a line intersecting the pedal pivot and a common point located near the hip (divergent point). The results suggest that the control strategy employed in this seated pushing task reflects the extensive experience of the leg in directing force appropriately to maintain upright posture and that relative muscle strengths have adapted to that pattern of typical activation.