Ankle bracing and the neuromuscular factors influencing joint stiffness.

Context: Health care professionals commonly prescribe external stabilization to decrease the incidence and severity of ankle sprains. The mechanism for this decrease is not clearly understood. Examining the effects of ankle bracing on biomechanical stability and influencing factors may provide important information regarding the neuromuscular effects of bracing.

Dynamic joint stiffness of the ankle during walking: gender-related differences.

Objectives: To characterize and compare dynamic joint stiffness (DJS) of the ankle in the sagittal plane during natural cadence walking in both genders.

Design: Observation, cross-sectional and matched pairs.

Participants: Twenty-one males (mean age=27+/-4.

Linear time delay methods and stability analyses of the human spine. Effects of neuromuscular reflex response.

Linear stability methods were applied to a biomechanical model of the human musculoskeletal spine to investigate effects of reflex gain and reflex delay on stability. Equations of motion represented a dynamic 18 degrees-of-freedom rigid-body model with time-delayed reflexes. Optimal muscle activation levels were identified by minimizing metabolic power with the constraints of equilibrium and stability with zero reflex time delay.

The viscoelastic standard nonlinear solid model: predicting the response of the lumbar intervertebral disk to low-frequency vibrations.

Due to the mathematical complexity of current musculoskeletal spine models, there is a need for computationally efficient models of the intervertebral disk (IVD). The aim of this study is to develop a mathematical model that will adequately describe the motion of the IVD under axial cyclic loading as well as maintain computational efficiency for use in future musculoskeletal spine models. Several studies have successfully modeled the creep characteristics of the IVD using the three-parameter viscoelastic standard linear solid (SLS) model.

Process stationarity and reliability of trunk postural stability.

Background: Empirical assessments of torso stability can be estimated from postural variability and nonlinear analyses of seated balance tasks. However, processing methods require sufficient signal duration and test-retest experiments require the assessment must be reliable. Our goal was to characterize the reliability and establish the trial duration for torso stability assessment.

Effects of trunk exertion force and direction on postural control of the trunk during unstable sitting.

Background: Pushing and pulling exertions have been implicated as risk factors of low-back disorders. In an attempt to investigate the mechanisms by which pushing and pulling influence risk for low-back disorders, the goal of this study was to investigate the effects of trunk exertion force and exertion direction on postural control of the trunk during unstable sitting.

Methods: Seat movements were recorded while subjects maintained a seated posture on a wobbly chair against different exertion forces (0N, 40N, and 80N) and exertion directions (trunk flexion and extension).

Effects of seated whole-body vibration on postural control of the trunk during unstable seated balance.

Background: Low back disorders and their prevention is of great importance for companies and their employees. Whole-body vibration is thought to be a risk factor for low back disorders, but the neuromuscular, biomechanical, and/or physiological mechanisms responsible for this increased risk are unclear. The purpose of this study was to measure the acute effect of seated whole-body vibration on the postural control of the trunk during unstable seated balance.

Dynamic stability differences in fall-prone and healthy adults.

Typical stability assessments characterize performance in standing balance despite the fact that most falls occur during dynamic activities such as walking. The objective of this study was to identify dynamic stability differences between fall-prone elderly individuals, healthy age-matched adults, and young adults. Three-dimensional video-motion analysis kinematic data were recorded for 35 contiguous steps while subjects walked on a treadmill at three speeds.

Active trunk stiffness during voluntary isometric flexion and extension exertions.

Objective: Compare muscle activity and trunk stiffness during isometric trunk flexion and extension exertions.

Background: Elastic stiffness of the torso musculature is considered the primary stabilizing mechanism of the spine. Therefore, stiffness of the trunk during voluntary exertions provides insight into the stabilizing control of pushing and pulling tasks.

Role of reflex gain and reflex delay in spinal stability--a dynamic simulation.

The goal of this study was to investigate the role of reflex and reflex time delay in muscle recruitment and spinal stability. A dynamic biomechanical model of the musculoskeletal spine with reflex response was implemented to investigate the relationship between reflex gain, co-contraction, and stability in the spine. The first aim of the study was to investigate how reflex gain affected co-contraction predicted in the model.

Fatigue, vertical leg stiffness, and stiffness control strategies in males and females.

Context: Fatigue appears to influence musculoskeletal injury rates during athletic activities, but whether males and females respond differently to fatigue is unknown.

Objective: To determine the influence of fatigue on vertical leg stiffness (K (VERT)) and muscle activation and joint movement strategies and whether healthy males and females respond similarly to fatigue.

Design: Repeated-measures design with all data collected during a single laboratory session.

Effect of augmented plantarflexion power on preferred walking speed and economy in young and older adults.

With age, loss of skeletal muscle mass (sarcopenia) results in decreased muscle strength and power. Decreased strength and power, in turn, are closely linked with declines in physical function. Preferred walking speed, a marker of physical function, is slower in older adults compared to young adults.

Role of reflex dynamics in spinal stability: intrinsic muscle stiffness alone is insufficient for stability.

Spinal stability is related to both the intrinsic stiffness of active muscle as well as neuromuscular reflex response. However, existing analyses of spinal stability ignore the role of the reflex response, focusing solely on the intrinsic muscle stiffness associated with voluntary activation patterns in the torso musculature. The goal of this study was to empirically characterize the role of reflex components of spinal stability during voluntary trunk extension exertions.

Passive mechanical properties of maturing extensor digitorum longus are not affected by lack of dystrophin.

Mechanical weakness of skeletal muscle is thought to contribute to onset and early progression of Duchenne muscular dystrophy, but this has not been systematically assessed. The purpose of this study was to determine in mice: (1) whether the passive mechanical properties of maturing dystrophic (mdx) muscles were different from control; and (2) if different, the time during maturation when these properties change. Prior to and following the overt onset of the dystrophic process (14-35 days), control and dystrophic extensor digitorum longus (EDL) muscles were subjected to two passive stretch protocols in vitro (5% strain at instantaneous and 1.

Stability of dynamic trunk movement.

Study Design: Nonlinear systems analyses of trunk kinematics were performed to estimate control of dynamic stability during repetitive flexion and extension movements.

Objective: Determine whether movement pace and movement direction of dynamic trunk flexion and extension influence control of local dynamic stability.

Summary Of Background Data: Spinal stability has been previously characterized in static, but not in dynamic movements.

The influence of gait speed on local dynamic stability of walking.

The focus of this study was to examine the role of walking velocity in stability during normal gait. Local dynamic stability was quantified through the use of maximum finite-time Lyapunov exponents, lambda(Max). These quantify the rate of attenuation of kinematic variability of joint angle data recorded as subjects walked on a motorized treadmill at 20%, 40%, 60%, and 80% of the Froude velocity.

Disturbed paraspinal reflex following prolonged flexion-relaxation and recovery.

Study Design: Repeated measures experimental study of the effect of flexion-relaxation, recovery, and gender on paraspinal reflex dynamics.

Objective: To determine the effect of prolonged flexion-relaxation and recovery time on reflex behavior in human subjects.

Summary Of Background Data: Prolonged spinal flexion has been shown to disturb the paraspinal reflex activity in both animals and human beings.

Validity and reliability of a new in vivo ankle stiffness measurement device.

This investigation was designed to test the validity and reliability of a new measure of inversion/eversion ankle stiffness on a unique medial/lateral swaying cradle device utilizing a test/retest with comparison to a known standard. Ankle stiffness is essential to maintaining joint stability. Most ankle injuries occur via an inversion mechanism.

Low-back biomechanics and static stability during isometric pushing.

Pushing and pulling tasks are increasingly prevalent in industrial workplaces. Few studies have investigated low-back biomechanical risk factors associated with pushing, and we are aware of none that has quantified spinal stability during pushing exertions. Data recorded from 11 healthy participants performing isometric pushing exertions demonstrated that trunk posture, vector force direction of the applied load, and trunk moment were influenced (p < .

Effect of lumbar extensor fatigue on paraspinal muscle reflexes.

Low back disorders are a frequent medical problem. Altered neuromuscular control of the spine has been associated with low back pain, and may contribute to its occurrence. The purpose of this study was to investigate the effect of lumbar extensor fatigue on reflex delay and amplitude in the paraspinal muscles.

Repeatability of surface EMG during gait in children.

Although mean amplitude and ON-OFF timing of muscle recruitment and electromyography (EMG) activation during gait is achieved by an age of six to eight years in normally developing children, recruitment dynamics illustrated by the shape of the EMG waveform may require continued developmental practice to achieve a stable pattern. Previous analyses have quantified the repeatability of the EMG waveform in adult subjects, but EMG variability for a pediatric population may be significantly different. The goal of this study was to quantify intra-session and inter-session variability in the phasic EMG waveform patterns from the lower limb muscles during self-selected speeds of walking in healthy-normal children for comparison with adult variability in gait EMG.

Co-contraction recruitment and spinal load during isometric trunk flexion and extension.

Background: Pushing and pulling tasks account for 20% of occupational low-back injury claims. Primary torso muscle groups recruited during pushing tasks include rectus abdominis and the external obliques. However, analyses suggest that antagonistic co-contraction of the paraspinal muscles is necessary to stabilize the spine during flexion exertions.

Active trunk stiffness increases with co-contraction.

Trunk dynamics, including stiffness, mass and damping were quantified during trunk extension exertions with and without voluntary recruitment of antagonistic co-contraction. The objective of this study was to empirically evaluate the influence of co-activation on trunk stiffness. Muscle activity associated with voluntary co-contraction has been shown to increase joint stiffness in the ankle and elbow.

Trunk stiffness and dynamics during active extension exertions.

Spinal stability is related to the recruitment and control of active muscle stiffness. Stochastic system identification techniques were used to calculate the effective stiffness and dynamics of the trunk during active trunk extension exertions. Twenty-one healthy adult subjects (10 males, 11 females) wore a harness with a cable attached to a servomotor such that isotonic flexion preloads of 100, 135, and 170 N were applied at the T10 level of the trunk.

Females recruit quadriceps faster than males at multiple knee flexion angles following a weight-bearing rotary perturbation.

Objective: To compare the effect of knee angle on muscle response times and neuromuscular recruitment patterns between sexes following a perturbation in single leg stance at 10 degrees, 20 degrees, and 30 degrees. We hypothesized that response times would be faster at lesser knee flexion angles and that females would recruit their quadriceps faster than males at all angles.

Design: A repeated-measures design.