Effect of age on stiffness modulation during postural maintenance of the arm

Abstract-The ability to modify the mechanical impedance of our limbs allows us to perform a variety of motor control tasks while interacting with the environment in a stable manner. Prior work has shown that young, healthy people are capable of modulating arm stiffness via selective muscle co-contraction to account for external disturbances in various directions. Increased age detrimentally affects control of movement and stability, although the neural mechanisms underlying these deficits are not entirely understood. In this study, younger and older subjects performed a static postural maintenance task with two types of directional force perturbations. Older individuals showed significantly less stiffness modification between the two perturbation conditions compared to the younger individuals, indicating less optimal modulation of arm impedance. This impairment should be considered during motor control evaluation in older populations, whether it be activities of daily living or skill assessment

EMG feedback tasks reduce reflexive stiffness during force and position perturbations

Force and position perturbations are widely applied to identify muscular and reflexive contributions to posture maintenance of the arm. Both task instruction (force vs. position) and the inherently linked perturbation type (i.e., force perturbations-position task and position perturbations-force tasks) affect these contributions and their mutual balance. The goal of this study is to explore the modulation of muscular and reflexive contributions in shoulder muscles using EMG biofeedback. The EMG biofeedback provides a harmonized task instruction to facilitate the investigation of perturbation type effects irrespective of task instruction. External continuous force and position perturbations with a bandwidth of 0.5–20 Hz were applied at the hand while subjects maintained prescribed constant levels of muscular co-activation using visual feedback of an EMG biofeedback signal. Joint admittance and reflexive impedance were identified in the frequency domain, and parametric identification separated intrinsic muscular and reflexive feedback properties. In tests with EMG biofeedback, perturbation type (position and force) had no effect on joint admittance and reflexive impedance, indicating task as the dominant factor. A reduction in muscular and reflexive stiffness was observed when performing the EMG biofeedback task relative to the position task. Reflexive position feedback was effectively suppressed during the equivalent EMG biofeedback task, while velocity and acceleration feedback were both decreased by approximately 37%. This indicates that force perturbations with position tasks are a more effective paradigm to investigate complete dynamic motor control of the arm, while EMG tasks tend to reduce the reflexive contribution

Use of Self-Selected Postures to Regulate Multi-Joint Stiffness During Unconstrained Tasks

The human motor system is highly redundant, having more kinematic degrees of freedom than necessary to complete a given task. Understanding how kinematic redundancies are utilized in different tasks remains a fundamental question in motor control. One possibility is that they can be used to tune the mechanical properties of a limb to the specific requirements of a task. For example, many tasks such as tool usage compromise arm stability along specific directions. These tasks only can be completed if the nervous system adapts the mechanical properties of the arm such that the arm, coupled to the tool, remains stable. The purpose of this study was to determine if posture selection is a critical component of endpoint stiffness regulation during unconstrained tasks.Three-dimensional (3D) estimates of endpoint stiffness were used to quantify limb mechanics. Most previous studies examining endpoint stiffness adaptation were completed in 2D using constrained postures to maintain a non-redundant mapping between joint angles and hand location. Our hypothesis was that during unconstrained conditions, subjects would select arm postures that matched endpoint stiffness to the functional requirements of the task. The hypothesis was tested during endpoint tracking tasks in which subjects interacted with unstable haptic environments, simulated using a 3D robotic manipulator. We found that arm posture had a significant effect on endpoint tracking accuracy and that subjects selected postures that improved tracking performance. For environments in which arm posture had a large effect on tracking accuracy, the self-selected postures oriented the direction of maximal endpoint stiffness towards the direction of the unstable haptic environment.These results demonstrate how changes in arm posture can have a dramatic effect on task performance and suggest that postural selection is a fundamental mechanism by which kinematic redundancies can be exploited to regulate arm stiffness in unconstrained tasks

Vibration as an exercise modality: how it may work, and what its potential might be

Whilst exposure to vibration is traditionally regarded as perilous, recent research has focussed on potential benefits. Here, the physical principles of forced oscillations are discussed in relation to vibration as an exercise modality. Acute physiological responses to isolated tendon and muscle vibration and to whole body vibration exercise are reviewed, as well as the training effects upon the musculature, bone mineral density and posture. Possible applications in sports and medicine are discussed. Evidence suggests that acute vibration exercise seems to elicit a specific warm-up effect, and that vibration training seems to improve muscle power, although the potential benefits over traditional forms of resistive exercise are still unclear. Vibration training also seems to improve balance in sub-populations prone to fall, such as frail elderly people. Moreover, literature suggests that vibration is beneficial to reduce chronic lower back pain and other types of pain. Other future indications are perceivable

Neuromechanical Tuning for Arm Motor Control

Movement is a fundamental behavior that allows us to interact with the external world. Its importance to human health is most evident when it becomes impaired due to disease or injury. Physical and occupational rehabilitation remains the most common treatment for these types of disorders. Although therapeutic interventions may improve motor function, residual deficits are common for many pathologies, such as stroke. The development of novel therapeutics is dependent upon a better understanding of the underlying mechanisms that govern movement. Movement of the human body adheres to the principles of classic Newtonian mechanics. However, due to the inherent complexity of the body and the highly variable repertoire of environmental contexts in which it operates, the musculoskeletal system presents a challenging control problem and the onus is on the central nervous system to reliably solve this problem. The neural motor system is comprised of numerous efferent and afferent pathways with a hierarchical organization which create a complex arrangement of feedforward and feedback circuits. However, the strategy that the neural motor system employs to reliably control these complex mechanics is still unknown. This dissertation will investigate the neural control of mechanics employing a “bottom-up” approach. It is organized into three research chapters with an additional introductory chapter and a chapter addressing final conclusions. Chapter 1 provides a brief description of the anatomical and physiological principles of the human motor system and the challenges and strategies that may be employed to control it. Chapter 2 describes a computational study where we developed a musculoskeletal model of the upper limb to investigate the complex mechanical interactions due to muscle geometry. Muscle lengths and moment arms contribute to force and torque generation, but the inherent redundancy of these actuators create a high-dimensional control problem. By characterizing these relationships, we found mechanical coupling of muscle lengths which the nervous system could exploit. Chapter 3 describes a study of muscle spindle contribution to muscle coactivation using a computational model of primary afferent activity. We investigated whether these afferents could contribute to motoneuron recruitment during voluntary reaching tasks in humans and found that afferent activity was orthogonal to that of muscle activity. Chapter 4 describes a study of the role of the descending corticospinal tract in the compensation of limb dynamics during arm reaching movements. We found evidence that corticospinal excitability is modulated in proportion to muscle activity and that the coefficients of proportionality vary in the course of these movements. Finally, further questions and future directions for this work are discussed in the Chapter 5

A Comparison of Biomechanical Variables, Neuromuscular Control and Strength during Controlled and Unexpected Falls on the Outstretched Hands in Young and Older Women

Purpose: This thesis evaluated the age differences in biomechanics and muscle activity during controlled and unexpected descents simulating a fall on the outstretched hands (FOOSH) in women. Laboratory simulation using two different protocols investigated this common mechanism of injury in older and younger women. The primary purpose of the controlled descent (FOOSH 1) was to examine the differences between young and older women to control the post-impact phase of a forward fall descent at three body angles. The primary purpose of the unexpected descent (FOOSH 2) was to examine biomechanical and muscle activity age differences in pre-impact, impact and post-impact phases of a simulated FOOSH. Methods: FOOSH 1 was a cross sectional study comparing twenty healthy young (mean 24.8±3.4 yrs.) and 18 healthy older (68.4±5.7 yrs.) women performing controlled descents on outstretched arms at three body lean angles (60, 45, and 30° from horizontal) and a muscle strength test of the non-dominant UE [isometric (ISO) concentric (CON) and eccentric (ECC)] using an isokinetic dynamometer. FOOSH 2, also a cross sectional design, evaluated twenty young (mean age 22.9 yrs., SD±3.7) and 16 older (mean age 68.1yrs., SD ±5.0) women performing five trials of unexpected FOOSHs at a body lean angle of 60° from horizontal with the same muscle strength testing protocol. A three-dimensional motion capture system (VICON Nexus, VICON, Centennial, CO) and force plate apparatus (OR6-7, AMTI, Watertown, MA) was used to determine the biomechanical measures of peak energy absorption, maximum vertical force, maximum elbow angle and maximum elbow joint extensor moment. Additional biomechanical measures of FOOSH 2 included: elbow angle and elbow angular velocity at impact, elbow joint stiffness, end elbow angle, and impulse. Surface EMG detected muscle activity of six muscle sites: anterior deltoid (AntDEL), pectoralis major (PM), triceps brachii (long head) (TRI), biceps brachii (BB), external oblique (EO) and internal oblique/transversus abdominus (IO/TrA). Results: In FOOSH 1 and FOOSH 2, older women demonstrated decreased CON elbow extensor strength compared with younger women. During FOOSH 1, at all angles, the older women had increased BB activity and decreased EO activity. In FOOSH 2 older women had significantly less IO/TrA activity prior to impact than younger women. The women differed in landing strategy in that younger women had significantly greater elbow joint angle and velocity at impact. Older women demonstrated diminished capacity to absorb energy in both the controlled (30°) and unexpected descent. Significance of findings: This is the first study to investigate biomechanical and muscle activation age differences for a simulated controlled and unexpected forward descent in women. Older women demonstrate differences that could potentially increase their risk of injury during a forward fall. The results of these studies could help clinicians develop fall injury prevention protocols by considering the neuromuscular and biomechanical factors that are important to control a forward descent. The findings suggest that UE and trunk muscle strengthening may be important components to include in a fall injury prevention training program. The modulation of energy absorption capabilities by altering elbow velocity and increasing elbow flexion angles at impact may also be an injury prevention tactic to be adopted

The influence of artificially increased hip and trunk stiffness on balance control in man

Lightweight corsets were used to produce mid-body stiffening, rendering the hip and trunk joints practically inflexible. To examine the effect of this artificially increased stiffness on balance control, we perturbed the upright stance of young subjects (20-34years of age) while they wore one of two types of corset or no corset at all. One type, the "half-corset”, only increased hip stiffness, and the other, the "full-corset”, increased stiffness of the hips and trunk. The perturbations consisted of combined roll and pitch rotations of the support surface (7.5deg, 60deg/s) in one of six different directions. Outcome measures were biomechanical responses of the legs, trunk, arms and head, and electromyographic (EMG) responses from leg, trunk, and upper arm muscles. With the full-corset, a decrease in forward stabilising trunk pitch rotation compared to the no-corset condition occurred for backward pitch tilts of the support surface. In contrast, the half-corset condition yielded increased forward trunk motion. Trunk backward pitch motion after forwards support-surface perturbations was the same for all corset conditions. Ankle torques and lower leg angle changes in the pitch direction were decreased for both corset conditions for forward pitch tilts of the support-surface but unaltered for backward tilts. Changes in trunk roll motion with increased stiffness were profound. After onset of a roll support-surface perturbation, the trunk rolled in the opposite direction to the support-surface tilt for the no-corset and half-corset conditions, but in the same direction as the tilt for the full-corset condition. Initial head roll angular accelerations (at 100ms) were larger for the full-corset condition but in the same direction (opposite platform tilt) for all conditions. Arm roll movements were initially in the same direction as trunk movements, and were followed by large compensatory arm movements only for the full-corset condition. Leg muscle (soleus, peroneus longus, but not tibialis anterior) balance-correcting responses were reduced for roll and pitch tilts under both corset conditions. Responses in paraspinals were also reduced. These results indicate that young healthy normals cannot rapidly modify movement strategies sufficiently to account for changes in link flexibility following increases in hip and trunk stiffness. The changes in leg and trunk muscle responses failed to achieve a normal roll or pitch trunk end position at 700ms (except for forward tilt rotations), even though head accelerations and trunk joint proprioception seemed to provide information on changed trunk movement profiles over the first 300ms following the perturbation. The major adaptation to stiffness involved increased use of arm movements to regain stability. The major differences in trunk motion for the no-corset, half-corset and full-corset conditions support the concept of a multi-link pendulum with different control dynamics in the pitch and roll planes as a model of human stance. Stiffening of the hip and trunk increases the likelihood of a loss of balance laterally and/or backwards. Thus, these results may have implications for the elderly and others, with and without disease states, who stiffen for a variety of reason

Role of The Cortex in Visuomotor Control of Arm Stability

Whereas numerous motor control theories describe the control of arm trajectory during reach, the control of stabilization in a constant arm position (i.e., visuomotor control of arm posture) is less clear. Three potential mechanisms have been proposed for visuomotor control of arm posture: 1) increased impedance of the arm through co-contraction of antagonistic muscles, 2) corrective muscle activity via spinal/supraspinal reflex circuits, and/or 3) intermittent voluntary corrections to errors in position. We examined the cortical mechanisms of visuomotor control of arm posture and tested the hypothesis that cortical error networks contribute to arm stabilization. We collected electroencephalography (EEG) data from 10 young healthy participants across four experimental planar movement tasks. We examined brain activity associated with intermittent voluntary corrections of position error and antagonist co-contraction during stabilization. EEG beta-band (13–26 Hz) power fluctuations were used as indicators of brain activity, and coherence between EEG electrodes was used as a measure of functional connectivity between brain regions. Cortical activity in the sensory, motor, and visual areas during arm stabilization was similar to activity during volitional arm movements and was larger than activity during co-contraction of the arm. However, cortical connectivity between the sensorimotor and visual regions was higher during arm stabilization compared with volitional arm movements and co-contraction of the arm. The difference in cortical activity and connectivity between tasks might be attributed to an underlying visuomotor error network used to update motor commands for visuomotor control of arm posture

Biomechanical and neurophysiological mechanisms related to postural control and efficiency of movement: A review

Understanding postural control requires considering various mechanisms underlying a person's ability to stand, to walk, and to interact with the environment safely and efficiently. The purpose of this paper is to summarize the functional relation between biomechanical and neurophysiological perspectives related to postural control in both standing and walking based on movement efficiency. Evidence related to the biomechanical and neurophysiological mechanisms is explored as well as the role of proprioceptive input on postural and movement control.info:eu-repo/semantics/publishedVersio