Effects of stimulated hip extension moment and position on upper-limb support forces during FNS-induced standing--a technical note.

This study explores the effects of active hip extension moment produced by electrical stimulation on the support forces the arms must exert through an assistive device during quiet erect standing with functional neuromuscular stimulation (FNS) in individuals with spinal cord injuries (SCI). A static sagittal plane biomechanical model of human standing was developed to predict the effects of stimulated hip extension moment and sagittal plane hip angle on the arm support necessary to maintain an upright posture. Two individuals with complete thoracic SCI were then tested while they stood with continuous stimulation to the knee and trunk extensors. The steady-state active extension moment exerted at the hip was varied by activating different combinations of hip extensor muscles with continuous stimulation while steady-state support forces applied to the arms and feet during standing were measured. The steady-state support forces imposed on the arms during quiet standing decrease with increased stimulated hip extension moment and are highly dependent upon hip flexion angle, as predicted by the biomechanical simulations. Experimentally, the combination of gluteus maximus and semimembranosus stimulation produced three times more steady-state hip extension moment than did stimulation of the gluteus maximus and adductor magnus. This resulted in a ten-fold decrease in body weight supported on the arms. More vertical postures (smaller hip flexion angles) improve the effectiveness of the hip extensor muscles in reducing the support forces placed on the arms. A single Newton-meter of stimulated hip extension moment with the hips fixed at 5 degrees of flexion results in almost five times the reduction in arm support forces as with the hips at 20 degrees. To minimize the forces applied by the arms on an assistive device for support while standing with FNS, these preliminary results suggest that (1) efforts should be made to assume the most erect postures possible and (2) muscles and stimulation paradigms that maximize active hip extension moment should be chosen.     

An algorithm for estimation of shoulder muscle forces for clinical use

BACKGROUND. The shoulder joint represents an indeterminate mechanical system, making it difficult to predict individual muscle forces required to equilibrate a given arbitrary external force. Although considerable work has been published on this matter, no model exhibits the adaptability required for the analysis involving different positions of the humerus and for any external load. An algorithm involving decision-making loops is developed to predict forces exerted by muscles that cross the shoulder joint in equilibrating a given external force acting in an arbitrary direction, with the humerus in any one of 12 selected positions. METHODS. Muscle lever arms and directions of action collected from a full-size epoxy model of the shoulder joint are used together with the external force as input. The algorithm selects an appropriate group of muscles and step by step attributes small force increments to withstand the external moment while aiming at minimising the forces involved. Each muscle force increment is stored after every loop and eventually summed up. Stability of the glenohumeral joint is the final determining factor. FINDINGS. Six worked-out examples show interesting features of probable muscular activity. Muscle segmentation is of paramount importance for spatial control. Although stability can be achieved by increasing the overall rotator cuff activity (co-contraction), this is rarely necessary. INTERPRETATION. The strategy of force sharing among the muscles opens up the possibility to examine the outcome of muscle deficiencies and to investigate causes of joint instability as encountered in clinical practice. Further validation of the model is still needed, but certain clinical observations can be explained.     

人体动态和静态地面反力检测系统的研制

目的：研制一种集成检测动态行走和静态站立的地面反力分析系统。方法：设计三 维测力台检测地面反力，开发数据采集、计算和分析的专用计算机软件。结果：利用该系统不仅可以检测行走过程中三个方向的地面反力，而且可以检测站立时压力中心曲线，计算其动摇长度、面积等特征值。结论：在测试常规行走的三维测力台上增加测试平衡能力的功能，可以丰富设备的功能，提高设备的利用率。     

单拐和四脚拐对偏瘫患者站立位平衡能力的影响

目的 研究脑卒中后偏瘫患者使用单拐或四脚拐对其站立位平衡能力的影响。方法 ２６名脑卒中后偏瘫患者作为受试者,分别对其独立站立,使用单拐站立及使用四脚拐站立三种状态进行平衡能力定量测定。结果 无论使用单拐或者使用四脚拐均能明显减少重心移动占据面积,缩短重心移动路径总长,降低平均摆动速度,减少Ｘ轴及Ｙ轴最大摆幅（Ｐ〈０．００１）,使用四脚拐与使用单拐相比能够使上述参数进一步减小（Ｐ〈０．００１）。结论     

Twelve-week biomechanical ankle platform system training on postural stability and ankle proprioception in subjects with unilateral functional ankle instability

BACKGROUND: Functional ankle instability is defined as the subjective sensation of giving way or feeling joint instability after repeated episodes of ankle sprain. The purpose of this study was to examine the effects of 12-week biomechanical ankle platform system training on static postural stability and ankle reposition sense in subjects with unilateral functional ankle instability. METHODS: Twelve university students (4 females and 8 males) with unilateral functional ankle instability volunteered as subjects. The active and passive reposition senses were assessed using an isokinetic dynamometer. The mean radius of the center of pressure excursion was recorded during single-leg standing with a force platform. A 12-week training program and a progression test for controlling the platform in certain directions and advancing to next training level was given to each subject. Repeated-measures 2-way analyses of variance were conducted to determine differences in postural stability and ankle proprioception between each limb before and after the training period. FINDINGS: The mean radius of center of pressure on unilateral standing and the absolute error from pre-selected ankle angle in the functional ankle instability limb were significantly reduced after 12 weeks of training. INTERPRETATION: These improvements in postural stability appear to reflect improved neuromuscular ability along with enhanced functional joint stability, as ankle proprioception also demonstrated the same positive improvements after training.     

Head and neck extension more than 30° may disturb standing balance in healthy older adults

Although there is clinical evidence of postural instability at extreme angles of head extension, the effects of lower angles on balance have been not investigated. This study aimed to investigate the effects of different head and neck extension angles on standing balance in older adults, and to determine the critical angle of instability. Twenty-eight healthy older adults were tested at 0°, 20°, 30° and 40° head and neck extension. The center of pressure (COP) parameters were recorded with a force plate. Significant differences were observed between 30° and 40° compared to 0° in anteroposterior and mediolateral COP displacement and total COP velocity, and between 40° and 0° in mediolateral COP velocity. Head and neck extension at 30° was the critical angle associated with the appearance of instability, and this value should be considered in the ergonomic design of work and living spaces, exercise programming and daily activities in older adults.     

Patterned Interference Radiation Force for Transcranial Neuromodulation

Compared with the conventional method of transcranial focused ultrasound stimulation using a single transducer or a focused beam, the compression and tensile forces are generated from the high-pressure gradient of a standing wave that can generate increased stimulation. We experimentally verified a neuromodulation system using patterned interference radiation force (PIRF) and propose a method for obtaining the magnitude of the radiation force, which is considered the main factor influencing ultrasound neuromodulation. The radiation forces generated using a single focused transducer and a standing wave created via two focused transducers were compared using simulations. Radiation force was calculated based on the relationship between the acoustic pressure, radiation force and time-averaged second-order pressure obtained using an acoustic streaming simulation. The presence of the radiation force was verified by measuring the time-averaged second-order pressure generated due to the radiation force, by using a glass tube.     

The hip joint: structure, stability, and stress; a review.

A review of the structure of the hip joint as related to its stability is presented. The forces exerted at the hip and along the proximal end of the femur are discussed in relation to the anatomical position of bone and to normal stresses incurred during standing and gait. Clinical considerations of selected hip abnormalities include a review of changes in normal mechanical forces found in coxa valga, coxa vava, arthritis, and fracture.     

Spinal loads during position changes

Background

Recommendations exist how patients should change from one body position to another in order to keep the spinal loads low. However, until now it is not clear whether the loads are in fact lower if the patients follow these recommendations. The aim was to measure the loads while changing the body position.

Methods

Telemeterized vertebral body replacements have been inserted into 5 patients who had a severe compression fracture of a lumbar vertebral body. The acting loads were measured during a changing of the body position while lying and when moving from lying to sitting, from sitting to standing and vice versa.

Findings

When the lying patients changed their position according to the physiotherapist's recommendations, the resultant force was nearly as high as it was during relaxed standing. Otherwise, the force was nearly twice as high. Changing from a lateral lying position to sitting and vice versa caused forces of about 180% of those seen for standing when the recommendations were heeded. Without instructions, the loads were about 70% higher. Use of a trapeze bar mounted to the bed did not increase the loads. Rising from a chair with the arms hanging down laterally led to average resultant forces of 380% related to standing. Placing the hands on armrests reduced this value to 180%.

Interpretation

High forces may act on the spine when changing from one body position to another. These loads can be minimized when following the physiotherapist's instructions and when supporting the upper body by the arms.     

Effect of Achilles tendon loading on plantar fascia tension in the standing foot

BACKGROUND: The plantar fascia, which is one of the major arch-supporting structures of the human foot, sustains high tensions during weight-bearing. A positive correlation between Achilles tendon loading and plantar fascia tension has been reported. Excessive stretching and tightness of the Achilles tendon are thought to be the risk factors of plantar fasciitis but their biomechanical effects on the plantar fascia have not been fully addressed. METHODS: A three-dimensional finite element model of the human foot and ankle, incorporating geometrical and material nonlinearity, was employed to investigate the loading response of the plantar fascia in the standing foot with different magnitudes of Achilles tendon loading. FINDINGS: With the total ground reaction forces of one foot maintained at 350 N to represent half body weight, an increase in Achilles tendon load from (0-700 N) resulted in a general increase in total force and peak plantar pressure at the forefoot of up to about 250%. There was a lateral and anterior shift of the centre of pressure and a reduction in the arch height with an increasing Achilles tendon load as a result of the plantar flexion moment on the calcaneus. From the finite element predictions of simulated balanced standing, Achilles tendon forces of 75% of the total weight on the foot (350 N) were found to provide the closest match of the measured centre of pressure of the subject during balanced standing. Both the weight on the foot and Achilles tendon loading resulted in an increase in tension of the plantar fascia with the latter showing a two-times larger straining effect. INTERPRETATION: Increasing tension on the Achilles tendon is coupled with an increasing strain on the plantar fascia. Overstretching of the Achilles tendon resulting from intense muscle contraction and passive stretching of tight Achilles tendon are plausible mechanical factors for overstraining of the plantar fascia.     

Stepping stability: effects of sensory perturbation

Background

Good balance depends on accurate and adequate information from the senses. One way to substitute missing sensory information for balance is with biofeedback technology. We previously reported that audio-biofeedback (ABF) has beneficial effects in subjects with profound vestibular loss, since it significantly reduces body sway in quiet standing tasks.

Methods

In this paper, we present the effects of a portable prototype of an ABF system on healthy subjects' upright stance postural stability, in conditions of limited and unreliable sensory information. Stabilogram diffusion analysis, combined with traditional center of pressure analysis and surface electromyography, were applied to the analysis of quiet standing tasks on a Temper foam surface with eyes closed.

Results

These analyses provided new evidence that ABF may be used to treat postural instability. In fact, the results of the stabilogram diffusion analysis suggest that ABF increased the amount of feedback control exerted by the brain for maintaining balance. The resulting increase in postural stability was not at the expense of leg muscular activity, which remained almost unchanged.

Conclusion

Examination of the SDA and the EMG activity supported the hypothesis that ABF does not induce an increased stiffness (and hence more co-activation) in leg muscles, but rather helps the brain to actively change to a more feedback-based control activity over standing posture.     

Changes in ground reaction force during jump landing in subjects with functional instability of the ankle joint

OBJECTIVE: To identify changes in ground reaction force during jump landing in subjects with functional instability of the ankle joint. DESIGN: Comparison of ground reaction force during jump landing between subjects with functional instability and healthy controls. BACKGROUND: We have recently demonstrated significantly altered patterns of ankle and knee movement immediately pre- and post-impact in subjects with functional instability compared to healthy controls. We now examine the changes in timing and magnitude of forces sustained by the unstable ankle during jump landing. METHODS: Fourteen subjects with unstable ankles and 10 age, sex and activity matched controls performed five single leg jumps onto a force platform whilst ground reaction forces were sampled. Timing and magnitudes of forces during the first 150 ms following impact were analysed and compared between groups. RESULTS: Lateral and anterior force peaks occurred significantly earlier in subjects with functional instability. Significant differences were seen between groups' time-averaged vertical, frontal and sagittal components of ground reaction force. These ranged from 5% (frontal force) to 100% (vertical force) of body mass. These changes occur immediately post-impact and too early for reflex correction/modification. CONCLUSIONS: The disordered force patterns observed in subjects with functional instability are likely to result in repeated injury due to significant increase in stress on ankle joint structures during jump landing. We suggest that they are most likely to result from deficits in feed-forward motor control. RELEVANCE: These results identify the potentially injurious nature of the changes in the forces applied to the unstable ankle joint during jump landing. The timing of these changes suggests that they are caused by a motor control deficit. Treatment approaches aimed at retraining feed-forward control of ankle joint movement could succeed in restoring more normal patterns of force absorption and reduce the occurrence of repeated micro-trauma to ankle structures.     

Sensitivity of plantar cutaneous sensation and postural stability

BACKGROUND: Impaired plantar cutaneous sensation is often seen clinically and can lead to postural instability. It is not clear if the severity of such sensory loss would be associated with postural stability, and if such an association would be affected by sensory redundancy and task difficulty. The purpose of this study was to investigate the association of the degree of somatosensory loss and postural stability by experimentally induced somatosensory loss in healthy young adults. METHODS: Twenty-one healthy young adults performed four quiet standing tasks (normal or narrow base, and eyes open or closed) on a force platform under three somatosensory conditions induced by ischemic blocking of afferent conduction below the ankle, normal, partial loss and complete loss. Differences in the standing center of pressure motion between the three sensory loss conditions were compared. FINDINGS: There was a significant trend of greater center of pressure motion with increasing severity of sensory loss in all task conditions. The differences in the center of pressure motion between partial and total loss conditions were significant only in conditions where vision was removed and the support surface was narrow. INTERPRETATION: Increased severity of experimentally induced loss of plantar cutaneous sensitivity was associated with greater postural sway. Such an association was affected by the availability of visual input and the size of the support surface. Clinically for patients with somatosensory impairments of the foot, postural stability should be given special attention.     

Effect of externally applied skin surface forces on tissue vasculature

The effect of surface forces on the morphologic pattern of skin capillaries was examined on the anterior surface of the forearm of healthy volunteers. A rig was designed to permit the application of uniaxial forces between two pads attached to the skin surface. A grid was printed onto the skin surface, and mechanical data are presented in terms of force intensity versus strain. During force application the skin capillaries were monitored using vital capillary microscopy. Mean values of force intensity and strain of 1.33N/mm and 10%, respectively, were sufficient to produce virtual obliteration of the blood flow to the capillaries. A simplified finite element model of the tissue was also employed to illustrate the effect of surface forces at different depths within the tissue. This study emphasizes the potentially damaging effects of surface forces on the integrity of the skin vasculature. If these forces are maintained or repeated, cell necrosis and eventual tissue damage would result.     

Elbow moment and forces at the hands during swing-through axillary crutch gait

We investigated swing-through axillary crutch gait (nonweight bearing on the left lower extremity) to determine the effects of gait speed, crutch length, and handle position on the forces exerted at the hands and on the moments exerted about the elbow joints. Ten healthy subjects, skilled in swing-through crutch gait, walked at three speeds using fitted crutches, at a fixed speed with four different crutch lengths, and at a fixed speed with four different handle positions. We collected ground reaction forces that exerted simultaneously on the right crutch and motion data with a force plate and three high-speed movie cameras. A biomechanical model was developed to calculate the forces exerted at the right hand and the moments exerted about the right elbow joint. Changing gait speed from slow to the normal gait of the subject showed statistically significant effects (p less than .05) on the forces at the hand. When we changed crutch heights for the subjects, we found no significant effects on the forces at the subjects' hands. Changing handle position significantly affected the moment at the elbow. Increasing the elbow-flexion angle above 30 degrees by raising the crutch handle 1 to 2 in resulted in a 100 percent increase in elbow-extension moment. We found a correlation of .82 between actual average elbow-flexion angle and elbow-extension moment. Changing gait speed or crutch length did not affect elbow moment.     

The effect of two different arm positions on body weight distribution when rising from sitting to standing in stroke patients

Acommon goal in the rehabilitation of persons with stroke is to improve their abilityto bear weighton the affected lower extremity.With respect to arm positions, rising from a sitting to standing position can be trained in different ways. For example, the hands can be clasped with the arms stretched forward or the arms can be free to move. The purpose of this studywas to examine howthese twodifferent arm positions effect body weight distribution and estimation of perceived exertion when rising from sitting to standing in stroke patients. Ten stroke patients (23-63 years) and ten age and sex-matched controls participated in the study. Two force measuring platforms were used to estimate ground reaction forces and body weight distribution. Perceived exertion was rated on a scale developed by Borg. No differences in body weight distribution or perceived exertion were found between the two ways of rising from sitting to standing in either the patient group or the control group. The results of this studysuggest that to improve stroke patients' symmetryof bodyweight distribution, it is not helpful to train the use of specific arm positions.     

Three-dimensional analysis of the talocrural and subtalar joints in single-limb stance

A quasi-static three-dimensional model of the rear foot was developed to enable calculation of joint motion and net torque with respect to talocrural and subtalar joint axes. The functional behaviour of these joints was analysed during single-limb stance.

Six female soccer players with unilateral functional instability of the ankle joint were examined. Recordings were performed both with eyes open and eyes closed. The rear-foot model was implemented into an optoelectronic motion analysis system together with a force plate.

The subtalar torque precedes the angular movement by 0.1–0.2 s, indicating active postural corrections. Up to a certain limit of subtalar joint inversion, the external everting torque increases. If the inversion is further increased an external inverting torque is obtained. With eyes closed the amplitudes increased for joint motions, torques, centre of pressure trajectories, shear forces, and vertical forces. All events found with external inverting torques occurred when standing on the injured foot with the eyes closed.     

Between-limb synchronization for control of standing balance in individuals with stroke

Background

During standing, forces and moments exerted at the feet serve to maintain stability in the face of constant centre-of-mass movement. These actions are temporally synchronized in healthy individuals. Stroke is typically a unilateral injury resulting in increased sensori-motor impairment in the contra-lesional compared with the ipsi-lesional lower-limb, which could lead to reduced between-limb synchronization for control of standing balance. The purpose of this study is to investigate between-limb synchronization of standing balance control in individuals with stroke; a potentially important index of control of upright stability.

Methods

Twenty healthy controls and 33 individuals with unilateral stroke were assessed. Stability was assessed during a 30-second quiet standing trial by measuring data from two force plates (one per foot). Limb-specific centre of pressure was calculated. Between-limb synchronization was defined as the coefficient of the correlation between the left and right foot for both the antero-posterior and medio-lateral centre of pressure time series. Synchronization, weight-bearing symmetry, and root mean square of the total centre of pressure excursion were compared between controls and stroke participants.

Findings

Stroke participants swayed more, were more asymmetric, and had less between-limb synchronization than healthy controls. Among individuals with stroke, reduced between-limb synchronization was related to increased postural sway in the medio-lateral direction and increased weight-bearing asymmetry.

Interpretation

Individuals with stroke have reduced temporal synchronization of centre of pressure fluctuations under the feet when controlling quiet standing. The clinical significance of reduced synchronization remains to be determined, although it appears linked to increased medio-lateral sway and weight-bearing asymmetry.     

The effects of simulated knee-flexion contractures on standing balance

The effects of simulated unilateral and bilateral knee-flexion contractures on standing balance were studied by testing 15 normal subjects on a Kistler force platform. Postural sway (mediolateral and anteroposterior travel) and the mean position of the center of pressure (as a percentage of the distance between the midlines of the feet and from heels to toes) were determined from 20 s of data. Unilateral and bilateral knee-flexion contractures of 15 degrees and 30 degrees were simulated for each subject by means of an adjustable line from the subject's waist to the sole of each foot. Paired t tests were used to compare balance parameters while standing with the simulated contractures with those during relaxed standing. Mediolateral travel increased by a mean difference of 3.6 cm with a 30 degree unilateral contracture (P less than 0.01) and by 5.0 cm with 30 degrees bilateral contractures (P less than 0.01). Anteroposterior travel increased by 4.7 cm (P less than 0.05) and 8.8 cm (P = 0.08) with 15 degrees and 30 degrees bilateral contractures, respectively. With a unilateral contracture of 30 degrees, the center of pressure shifted 15.6% (P less than 0.0005) toward the unflexed side, changes that were not eliminated by correction of the induced leg-length discrepancy. The center of pressure moved anteriorly by 8.3% with 30 degrees bilateral contractures (P less than 0.001). The results provide insight into how knee-flexion contractures alter standing balance, and underline the importance of preventing and treating this common disorder.     

Effects of Holding an External Load on the Standing Balance of Older and Younger Adults With and Without Chronic Low Back Pain

Objective

The purpose of this study was to assess the effect of holding an external load on the standing balance of younger and older adults with and without chronic low back pain (CLBP).