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. From this data, we estimated the vector from the center-of-mass to the center of pressure at each foot-strike. Dynamic stability of walking was computed by methods of Poincare analyses of these vectors. Results revealed that the fall-prone group demonstrated poorer dynamic stability than the healthy elderly and young adult groups. Stability was not influenced by walking velocity, indicating that group differences in walking speed could not fully explain the differences in stability. This pilot study supports the need for future investigations using larger population samples to study fall-prone individuals using nonlinear dynamic analyses of movement kinematics.

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. METHODS: Kinetic and kinematic data were recorded while subjects maintained a seated posture on a wobbly seat pan. Stability was evaluated from dynamic variability and nonlinear stability analyses. Process stationarity of the measured signals characterized the minimum necessary trial duration. Intra-class correlations measured within-session and between-session reliability. FINDINGS: Trial duration necessary to achieve process stationarity was 30.2 s. Shorter time to stationarity was observed with measures that included multi-dimensional movement behavior. Summary statistics of movement variability demonstrated moderate intra-session reliability, intra-class correlation=0.64 (range 0.38-0.87). Inter-session reliability for movement variance was moderate, intra-class correlation=0.42 (range 0.22-0.64). Nonlinear stability measures typically performed better than estimates of variability with inter-session reliability as high as intra-class correlation=0.83. Process stationarity and reliability were improved in more difficult balance conditions. INTERPRETATION: To adequately capture torso dynamics during the stability assessment the trial duration should be at least 30 s. Moderate to excellent test-retest reliability can be achieved in intra-session analyses, but more repeated measurements are required for inter-session comparisons. Stability diffusion exponents, H(S), and the Lyapunov exponents provide excellent measures for intra-session analyses, while H(S) provides excellent inter-session comparisons of torso stability.

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). Postural control of the trunk was assessed from kinematic variability (root-mean-squared amplitude and 95% ellipse area) and non-linear stability analyses (stability diffusion exponent and maximum finite-time Lyapunov exponent). FINDINGS: Kinematic variability and non-linear stability estimates increased as exertion force increased including root-mean-squared amplitude (P<0.001), 95% ellipse area (P<0.001), stability diffusion exponent (P=0.042), and maximum finite-time Lyapunov exponent (P<0.001). A subset of measures indicated postural control of the trunk was poorer during flexion exertions compared to extension exertions including root-mean-squared amplitude (P<0.001), 95% ellipse area (P=0.046), and maximum finite-time Lyapunov exponent (P=0.002). INTERPRETATION: Trunk exertion force and exertion direction affect postural control of the trunk. This study may aid in understanding how pushing and pulling exertions can potentially contribute to low-back disorders.

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. METHODS: Twenty-one healthy subjects (age: 23 years (SD 4 years)) were tested on a wobble chair designed to measure trunk postural control. Measurements of kinematic variance and non-linear stability control were based on seat angle before and after 30 min of seated whole-body vibration (bandwidth=2-20 Hz, root-mean-squared amplitude=1.15m/s(2)). FINDINGS: All measures of kinematic variance of unstable seated balance increased (P<0.05) after vibration including: ellipse area (35.5%), root-mean-squared radial lean angle (17.9%), and path length (12.2%). Measures of non-linear stability control also increased (P<0.05) including Lyapunov exponent (8.78%), stability diffusion analysis (1.95%), and Hurst rescaled range analysis (5.2%). INTERPRETATION: Whole-body vibration impaired postural control of the trunk as evidenced by the increase in kinematic variance and non-linear stability control measures during unstable sitting. These findings imply an impairment in spinal stability and a mechanism by which vibration may increase low back injury risk. Future work should investigate the effects of whole-body vibration on the anatomical and neuromuscular components that contribute to spinal stability.

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. Pseudorandom position perturbations of the torso and associated driving forces were recorded in 11 healthy adults. Nonlinear systems-identification analyses of the measured data provided an estimate of total systems dynamics that explained 81% of the movement variability. Proportional intrinsic response was less than zero in more than 60% of the trials, e.g. mean value of P(INT) during the 20% maximum voluntary exertion trunk extension exertions -415+/-354N/m. The negative value indicated that the intrinsic muscle stiffness was not sufficient to stabilize the spine without reflex response. Reflexes accounted for 42% of the total stabilizing trunk stiffness. Both intrinsic and reflex components of stiffness increased significantly with trunk extension effort. Results reveal that reflex dynamics are a necessary component in the stabilizing control of spinal stability.

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. It was found that reflexes allowed the model to stabilize with less antagonistic co-contraction and hence lower metabolic power than when limited to intrinsic stiffness alone. In fact, without reflexes there was no feasible recruitment pattern that could maintain spinal stability when the torso was loaded with 200N external load. Reflex delay is manifest in the paraspinal muscles and represents the time from a perturbation to the onset of reflex activation. The second aim of the study was to investigate the relationship between reflex delay and the maximum tolerable reflex gain. The maximum acceptable upper bound on reflex gain decreased logarithmically with reflex delay. Thus, increased reflex delay and reduced reflex gain requires greater antagonistic co-contraction to maintain spinal stability. Results of this study may help understanding of how patients with retarded reflex delay utilize reflex for stability, and may explain why some patients preferentially recruit more intrinsic stiffness than healthy subjects.

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. To date, very little information is available regarding stiffness of the evertor muscles in the prevention of excessive inversion joint rotation. Transient oscillation data representing inversion/eversion stiffness was obtained in a bipedal weight-bearing stance with an upright posture. Using commercially available springs with stiffness of 4.80N/cm the measured value recorded by the cradle was 4.87N/cm. Mean active stiffness values of the ankle were 35.70Nm/cm (SD 9.45). The trial-to-trial reliability ICC (2,1) coefficient was 0.96 with an SEM of 2.05Nm/rad, and the day-to-day reliability ICC (2,k) coefficient was 0.93 and an SEM of 3.00Nm/rad. The results demonstrate that inversion/eversion ankle stiffness measures on this device are a valid, repeatable and consistent measure. This is relevant because the ability to accurately quantify inversion/eversion ankle stiffness will improve our understanding of biomechanical stability and factors that influence it. It will also enable identification of ankle injury risk factors that will lead to more efficient rehabilitation programs and injury prevention strategies.

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. A monotonic trend between lambda(Max) and walking velocity was observed with smaller lambda(Max) at slower walking velocities. Smaller lambda(Max) indicates more stable walking dynamics. This trend was evident whether stride duration variability remained or was removed by time normalizing the data. This suggests that slower walking velocities lead to increases in stability. These results may reveal more detailed information on the behavior of the neuro-controller than variability-based analyses alone.

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. Research suggests that older adults may walk slower as a consequence of decreased plantarflexor power at push-off. In this study, we hypothesized that providing additional plantarflexion (PF) power during push-off would (1) increase preferred walking speed, and (2) reduce metabolic cost of transport (MCOT), in young and older adults. PF power was augmented using powered ankle-foot orthoses (PAFOs). The PAFOs, which use pneumatic actuators to provide an additional PF moment, were based on a design by Ferris et al. [Ferris DP, Czerniecki JM, Hannaford B. An ankle-foot orthosis powered by artificial pneumatic muscles. J Appl Biomech 2005;21:189-97.]. Nine young (23.3+/-1.6 years) and seven older (74.6+/-6.6 years) adults participated. For the young adults, eight out of nine increased their preferred walking speed when push-off power was augmented (1.18+/-0.16 to 1.25+/-0.16m/s, p=0.03). A similar, but non-significant, trend in preferred walking speed was observed for the older adults. With augmented push-off power, MCOT for young adults decreased from 0.395+/-0.057 to 0.343+/-0.047 (p=0.008); indicating that the neuromuscular system was able to adapt to use external energy to reduce metabolic cost. Only three older adults were tested but MCOT values showed a similar trend. Augmenting PF power increases gait speed and reduces MCOT in young adults. Older adults may need a longer period to take advantage of additional push-off power.

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. Although biomechanical models assume co-active recruitment causes increase trunk stiffness it has never been empirically demonstrated. Small trunk displacements invoked by pseudorandom force disturbances during trunk extension exertions were recorded from 17 subjects at two co-contraction conditions (minimal and maximal voluntary co-contraction recruitment). EMG data were recorded from eight trunk muscles as a baseline measure of co-activation. Increased EMG activity confirms that muscle recruitment patterns were different between the two co-contraction conditions. Trunk stiffness was determined from analyses of impulse response functions (IRFs) of trunk dynamics wherein the kinematics were represented as a second-order behavior. Trunk stiffness increased 37.8% (p < 0.004) from minimal to maximal co-activation. Results support the assumption used in published models of spine biomechanics that recruitment of trunk muscle co-contraction increases trunk stiffness thereby supporting conclusions from those models that co-contraction may contribute to spinal stability.

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. Ten healthy males (20-22 years of age) were subjected to an anteriorly-directed perturbation applied at the inferior margin of the scapulae while standing quietly before and after a lumbar extensor fatiguing protocol. The fatiguing protocol consisted of multiple sets of back extensions and intermittent isometric maximum voluntary contraction on a Roman chair for 14 min until 60% of unfatigued lumbar extensor MVC was reached. Reflexes were recorded from the paraspinal muscles at the level of L4. Results indicated the mean reflex delay was 60+/-18 ms and was not affected by fatigue (p=0.278). Reflex amplitude increased 36+/-32% with fatigue (p=0.017). The increase in reflex amplitude may reflect an attempt to compensate for losses in muscle force capacity with fatigue in order to maintain sufficient spinal stability. However, additional studies are necessary to investigate the mechanisms of this fatigue-related change in paraspinal reflex.

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. SETTING: Laboratory. PATIENTS OR OTHER PARTICIPANTS: Physically active males (n = 11) and females (n = 10). INTERVENTION(S): Subjects performed hopping protocols at 2 frequencies before and after fatigue, which was induced by repeated squatting at submaximal loads. MAIN OUTCOME MEASURE(S): We measured K (VERT) with a forceplate and peak muscle activity of the quadriceps, hamstrings, gastrocnemius, soleus, and anterior tibialis muscles with surface electromyography. Sagittal-plane kinematics at the knee and ankle were recorded with an electrogoniometer. RESULTS: After fatigue, K (VERT) was unchanged for all subjects. However, both males and females demonstrated reduced peak hamstrings ( P = .002) and anterior tibialis ( P = .001) activation, coupled with increased gastrocnemius ( P = .005) and soleus ( P = .001) peak activity, as well as increased quadriceps-hamstrings ( P = .005) and gastrocnemius/soleus-anterior tibialis coactivation ratios ( P = .03) after fatigue. Overall, females demonstrated greater quadriceps-hamstrings coactivation ratios than males, regardless of the fatigue condition ( P = .026). Only females showed increased knee flexion at initial contact after fatigue during hopping ( P = .03). CONCLUSIONS: Although K (VERT) was unaffected, the peak muscle activation and joint movement strategies used to modulate K (VERT) were affected after fatigue. Once fatigued, both males and females used an ankle-dominant strategy, with greater reliance on the ankle musculature and less on the knee musculature. Also, once fatigued, all subjects used an antagonist inhibition strategy by minimizing antagonist coactivation. Overall, females used a more quadriceps-dominant strategy than males, showing greater quadriceps activity and a larger quadriceps-hamstrings coactivation ratio. Changes in muscle activation and coactivation ratios because of fatigue and sex are suggested to alter knee joint stability and increase anterior cruciate ligament injury risk.

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. A pseudorandom stochastic force sequence (bandwidth 0-10 Hz, amplitude +/-30 N) was superimposed on the preload causing small amplitude trunk movements. Nonparametric impulse response functions of trunk dynamics were computed and revealed that the system exhibited underdamped second-order behavior. Second-order trunk dynamics were determined by calculating the best least-squares fit to the IRF. The quality of the model was quantified by comparing estimated and observed displacement variance accounted for (VAF), and quality of the second-order fits was calculated as a percentage and referred to as fit accuracy. Mean VAF and fit accuracy were 87.8 +/- 4.0% and 96.0 +/- 4.3%, respectively, indicating that the model accurately represented active trunk kinematic response. The accuracy of the kinematic representation was not influenced by preload or gender. Mean effective stiffness was 2.78 +/- 0.96 N/mm and increased significantly with preload (p < 0.001), but did not vary with gender (p = 0.425). Mean effective damping was 314 +/- 72 Ns/m and effective trunk mass was 37.0 +/- 9.3 kg. We conclude that stochastic system identification techniques should be used to calculate effective trunk stiffness and dynamics.

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. The variance ratio quantifies the repeatability of the integrated EMG waveform shape in a group of normally-developing children. Results reveal that between-session EMG waveform variability were similar in adult and pediatric populations, but within-session variability for the children was approximately twice the published value for adults. Clinical implications of this pediatric EMG variability suggest cautious interpretation of data from limited trial samples or inter-session changes in performance of gait data.

Gender differences in leg stiffness and stiffness recruitment strategy during two-legged hopping.

The authors compared leg stiffness (K(VERT)), muscle activation, and joint movement patterns between 11 men and 10 women during hopping. Physically active and healthy men and women performed continuous 2-legged hopping at their preferred rate and at 3.0 Hz. Compared with men, women demonstrated decreased K(VERT); however, after the authors normalized for body mass, gender differences in K(VERT) were eliminated. In comparison with men, women also demonstrated increased quadriceps and soleus activity, as well as greater quadriceps-to-hamstrings coactivation ratios. There were no significant gender differences for joint movement patterns (p>.05). The relationship between the observed gender differences in muscle recruitment and the increased risk of anterior cruciate ligament injury in women requires further study.

Effects of static flexion-relaxation on paraspinal reflex behavior.

BACKGROUND: Static trunk flexion working postures and disturbed trunk muscle reflexes are related to increased risk of low-back pain. Animal studies conclude that these factors may be related; passive tissue strain in spinal ligaments causes subsequent short-term changes in reflex. Although studies have documented changes in the myoelectric onset angle of flexion-relaxation following prolonged static flexion and cyclic flexion we could find no published evidence related to the human reflex response of the trunk extensor muscles following a period of static flexion-relaxation loading. METHODS: Eighteen subjects maintained static lumbar flexion for 15 min. Paraspinal muscle reflexes were elicited both before and after the flexion-relaxation protocol using pseudorandom stochastic force disturbances while recording EMG. Reflex gain was computed from the peak value of the impulse response function relating input force perturbation to EMG response using time-domain deconvolution analyses. FINDINGS: Reflexes showed a trend toward increased gain after the period of flexion-relaxation (P < 0.055) and were increased with trunk extension exertion (P < 0.021). Significant gender differences in reflex gain were observed (P < 0.01). INTERPRETATIONS: Occupational activities requiring extended periods of trunk flexion contribute to changes in reflex behavior of the paraspinal muscles. Results suggest potential mechanisms by which flexed posture work may contribute to low-back pain. Significant gender differences indicate risk analyses should consider personal factors when considering neuromuscular behavior.

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. The study quantified co-contraction and spinal load differences during isometric flexion and extension exertions. The goal was to provide insight into the mechanisms requiring greater co-contraction during trunk flexion exertions compared to extension exertions. METHODS: Electromyographic (EMG) signals were recorded from the trunk muscles of healthy volunteers during isometric trunk flexion and extension exertions. A biomechanical model was implemented to estimate total muscle force from the measured EMG and trunk moment data. A similar model estimated the muscle forces necessary to achieve equilibrium while minimizing the sum of squared muscle forces. The difference in these forces represented co-contraction. Spinal load attributed to co-contraction was computed. RESULTS: Average co-contraction during flexion exertions was approximately twice the value of co-contraction during extension, i.e. 28% and 13% of total muscle forces respectively. Co-contraction accounted for up to 47% of the total spinal load during flexion exertions. Consequently, spinal compression during the flexion tasks was nearly 50% greater than during extension exertions despite similar levels of trunk moment. INTERPRETATION: Co-contraction must be considered when evaluating spinal load during pushing exertions. Results underscore the need to consider neuromuscular control of spinal stability when evaluating the biomechanical risks.

A new approach to assessing function in elderly people.

Current functional assessment methods and measures of elderly people are limited in their ability to detect small decrements in function or in discriminating between different patterns of functional loss. Nor do they directly assess function in the patient's usual environment. Recent technological advances have led to the development of small, wearable microelectronic devices that detect motion, velocity and acceleration. These devices can be used to develop new tools for more precise monitoring, assessment, and prediction of function by characterizing the 'electronic signatures' of successful or unsuccessful task-specific performance, and to allow for continuous assessment in a home environment. This presentation will summarize current efforts to translate new technologies into a clinical and research tool for improved assessment, monitoring, and prediction of function among older individuals.

Gender differences in active musculoskeletal stiffness. Part I. Quantification in controlled measurements of knee joint dynamics.

Active females demonstrate increased risk for musculoskeletal injuries relative to equivalently-trained males. Although gender differences in factors such as passive laxity, skeletal geometry and kinematics have been examined, the effect of gender on active muscle stiffness has not been reported. Stiffness of the active quadriceps and hamstrings musculature were recorded during isometric knee flexion and extension exertions from twelve male and eleven female subjects. A second-order biomechanical model of joint dynamics was used to quantify stiffness from the transient motion response to an angular perturbation of the lower-leg. Female subjects demonstrated reduced active stiffness relative to male subjects at all torque levels, with levels 56-73% of the males. Effective stiffness increased linearly with the torque load, with stiffness increasing at a rate of 3.3 Nm/rad per unit of knee moment in knee flexion exertions (hamstrings) and 6.6 Nm/rad per unit of knee moment extension exertions (quadriceps). To account for gender differences in applied moment associated with leg mass, regressions analyses were completed that demonstrated a gender difference in the slope of stiffness-versus-knee moment relation. Further research is necessary to identify the cause of the observed biomechanical difference and implications for controlling injury.

Knee ligament behavior following a controlled loading protocol does not differ by menstrual cycle day.

BACKGROUND: Females experience a disproportionate number of anterior cruciate ligament injuries compared to males. Increased estradiol concentration has been suggested to alter ligament properties and strength. Determining whether the knee responds differently to an external load at various hormonal levels may be helpful in further explaining the gender disparity. METHODS: Estradiol, progesterone and testosterone were quantified at menses, near ovulation and at the mid-luteal phase. With one knee serving as the control limb and the other as the experimental limb, displacement at 134N and stiffness between 90 and 134N were recorded with a knee ligament arthrometer on both knees before and after a loading protocol. The protocol consisted of three, 3-min, posterior to anterior normalized loads directed to the posterior calf with a ligament testing device. FINDINGS: The loading protocol produced a measurable increase in displacement but not stiffness. Neither displacement nor stiffness measures however were affected by day of the menstrual cycle. No consistent relationships between hormonal concentrations and displacement or stiffness were evident. INTERPRETATION: Following a controlled, static external load, displacement and stiffness were not affected differently by day of the menstrual cycle.