Stepping Responses of Young and Old Adults to Postural Disturbances: Kinematics

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111171/1/j.1532-5415.1994.tb04972.x.pd

Subthreshold white noise vibration alters trembling sway in older adults

Background Somatosensory deficit is a significant contributor to falls in older adults. Stochastic resonance has shown promise in recent studies of somatosensation-based balance disorders, improving many measures of stability both inside and outside of the clinic. However, our understanding of this effect from a physiological perspective is poorly understood. Therefore, the primary goal of this study is to explore the influence of subthreshold vibratory stimulation on sway under the rambling-trembling framework. Methods 10 Healthy older adults (60–65 years) volunteered to participate in this study. Each participant underwent two randomized testing sessions on separate days, one experimental and one placebo. During each session, the participants' baseline sway was captured during one 90-s quiet standing trial. Their sensation threshold was then captured using a custom vibratory mat and 4–2-1 vibration perception threshold test. Finally, participants completed another 90-s quiet standing trial while the vibratory mat vibrated at 90% of their measured threshold (if experimental) or with the mat off (if placebo). While they completed these trials, an AMTI force plate collected force and moment data in the anteroposterior (AP) and mediolateral (ML), from which the center of pressure (COP), rambling (RM), and trembling (TR) time series were calculated. From each of these time series, range, variability (root-mean-square), and predictability (sample entropy) were extracted. One-tailed paired t-tests were used to compare baseline and during-vibration measures. Results No significant differences were found during the placebo session. For the experimental session, significant increases were found in AP TR range, ML TR RMS, AP COP predictability, and AP & ML TR predictability. The TR time series was particularly sensitive to vibration, suggesting a strong influence on peripheral/spinal mechanisms of postural control. Significance Though it is unclear whether observed effects are indicative of “improvements” or not, it does suggest that there was a measurable effect of subthreshold vibration on sway. This knowledge should be utilized in future studies of stochastic resonance, potentially acting as a mode of customization, tailoring vibration location, duration, magnitude, and frequency content to achieve the desired effect. One day, this work may aid in our ability to treat somatosensation-based balance deficits, ultimately reducing the incidence and severity of falls in older adults

A model for studies of the deformable rib cage

An earlier model for the study of rib cage mechanics was modified so that rib deformity in scoliosis could be better represented. The rigid ribs of that model were replaced by five-segment deformable rits. Literature data on cadaver rib mechanical behavior were used to assign stiffnesses to the new individual model ribs so that experimental and model rib deflections agreed. Shear and tension/compression stiffnesses had little effect on individual rib deformation, but bending stiffnesses had a major effect. Level-to-level differences in mechanical behavior could be explained almost exclusively by level to level differences in the rib shape. The model ribs were then assembled into a whole rib cage. Computer simulations of whole rib cage behaviors, both in vivo and in vitro, showed a reasonable agreement with the measured behaviors.The model was used to study rib cage mechanics in two scolioses, one with a 43[deg] and the other with a 70[deg] Cobb angle. Scoliotic rib cage deformities were quantified by parameters measuring the rib cage lateral offset, rib cage axial rotation, rib cage volume and rib distortion. Rib distortion was quantified both in best-fit and simulated computer tomography (CT) scan planes. Model rib distortion was much smaller in best-fit planes than in CT planes. The total rib cage volume changed little in the presence of the scolioses, but it became asymmetrically distributed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30087/1/0000458.pd

Muscle Synergies Obtained from Comprehensive Mapping of the Cortical Forelimb Representation Using Stimulus Triggered Averaging of EMG Activity

This work is licensed under a Creative Commons Attribution 4.0 International License.Neuromuscular control of voluntary movement may be simplified using muscle synergies similar to those found using non-negative matrix factorization. We recently identified synergies in electromyography (EMG) recordings associated with both voluntary movement and movement evoked by high-frequency long-duration intracortical microstimulation applied to the forelimb representation of the primary motor cortex (M1). The goal of this study was to use stimulus-triggered averaging (StTA) of EMG activity to investigate the synergy profiles and weighting coefficients associated with poststimulus facilitation, as synergies may be hard-wired into elemental cortical output modules and revealed by StTA. We applied StTA at low (LOW, ∼15 μA) and high intensities (HIGH, ∼110 μA) to 247 cortical locations of the M1 forelimb region in two male rhesus macaques while recording the EMG of 24 forelimb muscles. Our results show that 10–11 synergies accounted for 90% of the variation in poststimulus EMG facilitation peaks from the LOW-intensity StTA dataset while only 4–5 synergies were needed for the HIGH-intensity dataset. Synergies were similar across monkeys and current intensities. Most synergy profiles strongly activated only one or two muscles; all joints were represented and most, but not all, joint directions of motion were represented. Cortical maps of the synergy weighting coefficients suggest only a weak organization. StTA of M1 resulted in highly diverse muscle activations, suggestive of the limiting condition of requiring a synergy for each muscle to account for the patterns observed

A novel device to measure power grip forces in squirrel monkeys

Understanding the neural bases for grip force behaviors in both normal and neurologically impaired animals is imperative prior to improving treatments and therapeutic approaches. The present paper describes a novel device for the assessment of power grip forces in squirrel monkeys. The control of grasping and object manipulation represents a vital aspect of daily living by allowing the performance of a wide variety of complex hand movements. However, following neurological injury such as stroke, these grasping behaviors are often severely affected, resulting in persistent impairments in strength, grip force modulation and kinematic hand control. While there is a significant clinical focus on rehabilitative strategies to address these issues, there exists the need for translational animal models. In the study presented here, we describe a simple grip force device designed for use in nonhuman primates, which provides detailed quantitative information regarding distal grip force dynamics. Adult squirrel monkeys were trained to exceed a specific grip force threshold, which was rewarded with a food pellet. One of these subjects then received an infarct of the M1 hand representation area. Results suggest that the device provides detailed and reliable information on grip behaviors in healthy monkeys and can detect deficits in grip dynamics in monkeys with cortical lesions (significantly longer release times). Understanding the physiological and neuroanatomical aspects of grasping function following neurological injury may lead to more effective rehabilitative interventions

Fall arrest biomechanics: Sway and stepping responses in healthy young and old adults.

Unintentional falls in the elderly are a serious problem. Despite this, few studies have quantified age differences in the biomechanical response to impending falls. The biomechanics of human responses to small, medium and large fall-provoking disturbances were investigated through the use of experimental observations and an inverse dynamics model. Twelve healthy Young and twelve healthy Old adult female subjects were tested. The subjects' responses to sudden backwards pulls at the waist level were measured in terms of body segment kinematics and foot/floor reactions. Few age differences were found for small disturbance sway responses in the joint torques, joint and segment rotation excursions, peak angular momenta and kinetic energy. The disturbance magnitude provoking a step response did not differ between the two groups. At a medium level disturbance approximately one half of the subjects used sway responses and one half used step responses to regain balance. The biomechanical requirements for a step and a sway response were compared at this disturbance level. Comparing the step with the sway response, joint rotation excursions in the stepped knee and hip and the stance hip were significantly larger, peak joint torques in the stepped leg knee flexor and hip extensor and the stance leg hip extensor were significantly larger, the step leg ankle dorsiflexor and knee extensor were significantly smaller and the peak angular momenta and peak kinetic energy were significantly larger and occurred significantly later. At large disturbance levels, all subjects used a step response to regain their balance. Significant age differences were found in the step strategy: the Old used primarily a multiple step strategy, the Young used primarily a single step strategy. Age group differences were found in the step kinematics: the Old, compared to the Young, took steps shorter in length, lower in height, earlier in their response, and they landed earlier in their response. Few age differences were found in the joint rotations excursions and joint torques, which were modest compared to literature values of the available joint range of motion and maximum voluntary strengths.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105568/1/9135641.pdfDescription of 9135641.pdf : Restricted to UM users only