The spectral content of postural sway during quiet stance: Influences of age, vision and somatosensory inputs

Maintenance of human upright stance requires the acquisition and integration of sensory inputs. Conventional measures of sway have had success in identifying age- and some disease-related changes, but remain unable to address the complexities and dynamics associated with postural control. We investigated the effects of vision, surface compliance, age, and gender on the spectral content of center of pressure (COP) time series. Sixteen healthy young (age 18-24) and older participants (age 55-65) performed trials of quiet, upright stance under different vision (eyes open vs. closed) and surface (hard vs. compliant) conditions. Spectral analyses were conducted to describe COP mean normalized power in discretized bands. Effects of the two sensory modalities and age were distinct in the antero-posterior and medio-lateral directions, and a reorganization of spectral content was evident with increasing task difficulty (eyes open vs. closed and hard vs. compliant surface) and among older adults. These results indicate that vision and surface compliance are predominantly associated with responses from musculature associated with antero-posterior and medio-lateral directions of sway, respectively. Finally, distinguishing between the contributions of different afferent systems to the postural control system using the spectral content of sway bi-directionally may help in diagnosing individuals with balance impairments.     

Effect of six weeks of dura disc and mini-trampoline balance training on postural sway in athletes with functional ankle instability

Lateral ankle sprain (LAS) is one of the most common injuries incurred during sporting activities, and effective rehabilitation programs for this condition are challenging to develop. The purpose of this research was to compare the effect of 6 weeks of balance training on either a mini-trampoline or a dura disc on postural sway and to determine if the mini-trampoline or the dura disc is more effective in improving postural sway. Twenty subjects (11 men, 9 women) with a mean age of 25.4 +/- 4.2 years were randomly allocated into a control group, a dura disc training (DT) group, or a mini-trampoline (MT) group. Subjects completed 6 weeks of balance training. Postural sway was measured by subjects performing a single limb stance on a force plate. The disbursement of the center of pressure was obtained from the force plate in the medial-lateral and the anterior-posterior sway path and was subsequently used for pretest and posttest analysis. After the 6-week training intervention, there was a significant (p < 0.05) difference in postural sway between pre- and posttesting for both the MT (pretest = 56.8 +/- 20.5 mm, posttest = 33.3 +/- 8.5 mm) and DT (pretest = 41.3 +/- 2.6 mm, posttest = 27.2 +/- 4.8 mm) groups. There was no significant (p > 0.05) difference detected for improvements between the MT and DT groups. These results indicate that not only is the mini-trampoline an effective tool for improving balance after LAS, but it is equally as effective as the dura disc.     

Light touch for balance: influence of a time-varying external driving signal

Sensory information about body sway is used to drive corrective muscle action to keep the body's centre of mass located over the base of support provided by the feet. Loss of vision, by closing the eyes, usually results in increased sway as indexed by fluctuations (i.e. standard deviation, s.d.) in the velocity of a marker at C7 on the neck, s.d. dC7. Variability in the rate of change of centre of pressure (s.d. dCoP), which indexes corrective muscle action, also increases during upright standing with eyes closed. Light touch contact by the tip of one finger with an environmental surface can reduce s.d. dC7 and s.d. dCoP as effectively as opening the eyes. We review studies of light touch and balance and then describe a novel paradigm for studying the nature of somatosensory information contributing to effects of light touch balance. We show that 'light tight touch' contact by the index finger held in the thimble of a haptic device results in increased anteroposterior (AP) sway with entraining by either simple or complex AP sinusoidal oscillations of the haptic device. Moreover, sway is also increased when the haptic device plays back the pre-recorded AP sway path of another person. Cross-correlations between hand and C7 motion reveal a 176 ms lead for the hand and we conclude that light tight touch affords an efficient route for somatosensory feedback support for balance. Furthermore, we suggest that the paradigm has potential to contribute to the understanding of interpersonal postural coordination with light touch in future research.     

Natural sway frequencies and damping ratios of trees: influence of crown structure

Natural frequency and damping ratio were measured for nine plantation-grown Douglas-fir (Pseudotsuga menziesii Mirb. Franco) trees from the Oregon Coast Range under different levels of crown removal. Natural frequency of trees, in both their unpruned and completely de-branched states, was linearly related to the ratio of diameter at breast height to total tree height squared (i.e., DBH/H²), as expected from the theory governing the oscillation of a cantilever beam. Pruning resulted in an increase in natural frequency; however, at least 80% of the crown mass needed to be removed before this increase was noticeable. A single equation was developed that enabled the natural frequency of a tree of given size and pruning intensity to be predicted. Damping ratios of unpruned trees varied considerably from 8% to almost critical, while those for completely de-branched trees ranged from 1% to 8%. Two different trends in damping ratio were observed during pruning. Some trees exhibited an increase in damping ratio with initial crown removal, followed by a sharp decrease when the uppermost portion of the crown was removed. Others showed little or no change in damping ratio followed by a sharp reduction upon removal of the uppermost portion of the crown. Damping was mainly due to aerodynamic drag and preventing interference with neighbouring trees had little effect. Theoretical analysis using the finite element method indicated that changes in natural frequency as a result of pruning are not due to changes in damping ratio, but rather changes in mass distribution. This analysis also suggested that treating branches as lumped masses rather than individual cantilevers attached to the main stem may not be appropriate.     

Age-related changes in force and power associated with balance of women in quiet bilateral stance on a firm surface

Quantitative assessment of the fluctuations of the body centre of mass (CM) while in a stationary bilateral stance on a firm surface is an important criterion of the functional state of human motor-vestibular and sensory apparatus. From analysis of the literature we conclude that more objective characteristics of human balance in quiet standing may be the amount of energy used to maintain the CM in a constant position. Further analysis of the references showed that these characteristics have not been investigated in neurological practice.In this study, the displacement of CM in participants standing in a normal anatomical position was analysed. Forty-five healthy women in three age groups: 18–24, 45–55 and over 60 years participated in the experiments, which consisted of recording changes in partial body weight on the force platform (under one leg) in situations with opened and closed eyes. The specific power of oscillation of body sway and force of lateral swing of CM were calculated. Results indicated that the maximum specific power of oscillation and force of lateral swing were observed in the group of women older than 60 years, especially in the absence of vision. Minimum values occurred in the group of 18–24 years.We also found a considerable variability in all indices in all age groups. This indicates that the stability of the vertical posture in humans depends also on the individual biological characteristics of the central nervous and muscular systems.     

Balancing on a narrow ridge: biomechanics and control.

The balance of standing humans is usually explained by the inverted pendulum model. The subject invokes a horizontal ground-reaction force in this model and controls it by changing the location of the centre of pressure under the foot or feet. In experiments I showed that humans are able to stand on a ridge of only a few millimetres wide on one foot for a few minutes. In the present paper I investigate whether the inverted pendulum model is able to explain this achievement. I found that the centre of mass of the subjects sways beyond the surface of support, rendering the inverted pendulum model inadequate. Using inverse simulations of the dynamics of the human body, I found that hip-joint moments of the stance leg are used to vary the horizontal component of the ground-reaction force. This force brings the centre of mass back over the surface of support. The subjects generate moments of force at the hip-joint of the swing leg, at the shoulder-joints and at the neck. These moments work in conjunction with a hip strategy of the stance leg to limit the angular acceleration of the head-arms-trunk complex. The synchrony of the variation in moments suggests that subjects use a motor programme rather than long latency reflexes.     

Use of a cane for recovery from backward balance loss during treadmill walking

Purpose. To study whether a cane improved balance recovery after perturbation during walking.

Method. This study was a crossover comparison comparing the effect of walking with and without a cane for balance recovery after perturbation during treadmill walking. Five normal young volunteers participated. The velocity and acceleration of a marker sited on the seventh cerebral vertebra (C7) and vertical hand motion were measured by a motion analysis system.

Result. When using a cane, C7 backward velocity increased by approximately 15% (413 SD 95?mm/s with cane vs. 358 SD 88?mm/s without). In addition, C7 backward acceleration increased by approximately 23% (3.2 SD 0.7?m/s² with cane vs. 2.6 SD 0.8?m/s² without) and the vertical motion of the right hand decreased (187 SD 98?mm with cane vs. 372 SD 260?mm without). Additionally, no subject was able to use a cane to broaden their base of support.

Conclusions. The ability to limit trunk extension is crucial for preventing falls. Therefore, using a cane jeopardizes recovery from backward balance loss. The results encourage further research on the risk of a cane on balance recovery for the elderly population and habitual cane users.     

Effect of ageing and vision on limb load asymmetry during quiet stance

Although the identification and characterization of limb load asymmetries during quiet standing has not received much research attention, they may greatly extend our understanding of the upright stance stability control. It seems that the limb load asymmetry factor may serve as a veridical measure of postural stability and thus it can be used for early diagnostic of the age-related decline in balance control. The effects of ageing and of vision on limb load asymmetry (LLA) during quiet stance were studied in 43 healthy subjects (22 elderly, mean age 72.3+/-4.0 yr, and 21 young, mean age 23.9+/-4.8 yr). Postural sway and body weight distribution were recorded while the subject was standing on two adjacent force platforms during two 120 s trials: one trial was performed with the eyes open (EO), while the other trial was with the eyes closed (EC). The results indicate that LLA was greater in the old adults when compared with the young control subjects. The LLA values were correlated with the postural sway magnitudes especially in the anteroposterior direction. Eyes closure which destabilized posture resulted in a significant increase of body weight distribution asymmetry in the elderly but not in the young persons. The limb load difference between EO and EC conditions showed a significantly greater effect of vision on LLA in the elderly compared to the young subjects. The observed differences in the LLA may be attributed to the decline of postural stability control in the elderly. Ageing results in the progressive decline of postural control and usually the nervous system requires more time to complete a balance recovery action. To compensate for such a deficiency, different compensatory strategies are developed. One of them, as evidenced in our study, is preparatory limb unload strategy (a stance asymmetry strategy) which could significantly shorten reaction time in balance recovery.     

Minimal step length necessary for recovery of forward balance loss with a single step

Although the boundary conditions necessary to trigger a step in reaction to a forward balance loss have been predicted in previous research, the relationship between minimal step length needed for balance recovery with this single step and the center of mass (COM) motion state (i.e., its position and velocity) remains unknown. The purpose of this paper was to present a theoretical framework within which the minimal step length needed for balance recovery can be estimated. We therefore developed a simplified four-segment sagittal model of human body stepping for balance recovery. The work-energy principle of the Newtonian mechanics was employed in the simulation to determine the amount of excess mechanical energy that can be absorbed as a function of step length and the corresponding eccentric joint work that can be generated in a single step. We found that an increase in initial forward velocity and a greater forward shift of the COM require a corresponding increase in the minimal step length needed for balance recovery. Furthermore, the minimal step length is also a function of the muscle strength at the ankle: the lower the muscle strength, the greater the minimal step length required. Our theoretical framework reduces the complexity associated with previous studies relying on forward dynamics and iterative optimization processes. This method may also be applied to study aspects of balance control such as the prevention of balance loss in the posterior or mediolateral direction.     

Effect of a zinc-deficient diet on nitrogen balance in rats

The effects of a zinc-deficient diet (1.5 mg/kg) on nitrogen balance in rats, fed ad libitum during 30 days, was tested. Three nitrogen balances, each of 5 days, were done on the 4th, 15th and 25th days. A pair-fed group, with a supplemented diet at 80 mg/kg of zinc, was used as control. No significant differences (P less than or equal to 0.05) in any nitrogen balances for True Digestibility, Operative Biological Value and body weight were found. Nevertheless a trend was observed in all studied variables, indicating that the proteins of the control diet were better utilized than those of the zinc-deficient ones. The variation of the Biological Value of the proteins in the zinc-deficient group along the experimental period was similar to the control group.     

Mitotic stability of yeast chromosomes: a colony color assay that measures nondisjunction and chromosome loss

A colony color assay that measures chromosome stability is described and is used to study several parameters affecting the mitotic maintenance of yeast chromosomes, including ARS function, CEN function, and chromosome size. A cloned ochre-suppressing form of a tRNA gene, SUP11, serves as a marker on natural and in vitro-constructed chromosomes. In diploid strains homozygous for an ochre mutation in ade2, cells carrying no copies of the SUP11 gene are red, those carrying one copy are pink, and those carrying two or more copies are white. Thus, the degree of red sectoring in colonies reflects the frequency of mitotic chromosome loss. The assay also distinguishes between chromosome loss (1:0 segregation) and nondisjunction (2:0 segregation). The most dramatic effect on improving mitotic stability is caused by increasing chromosome size. Circular chromosomes increase in stability through a size range up to approximately 100 kb, but do not continue to be stabilized above this value. However, linear chromosomes continue to increase in mitotic stability throughout the size range tested (up to 137 kb). It is possible that the mitotic stability of linear chromosomes is proportional to chromosome length, up to a plateau value that has not yet been reached in our synthetic constructions.     

A shoe sole-based apparatus and method for randomly perturbing the stance phase of gait: test-retest reliability in young adults

Walking on an irregular surface is associated with an elevated risk for a fall at any age. Yet, relatively little is known about how a human responds to an unexpected underfoot perturbation during gait. This is partly due to the difficulty of generating an intermittent but repeatable, unexpected, underfoot perturbation whose size and location are precisely known. So we developed a shoe sole-embedded apparatus for randomly perturbing the stance phase of gait. Medial and lateral flaps were concealed in the soles of pairs of sandals, along with their actuators. Either flap could be deployed within 400ms in the parasagittal plane under a swing foot; this altered the resulting sagittal and frontal plane orientations of the foot during the next stance phase, whereafter the flap was retracted following toe-off for the rest of that gait trial. We tested six healthy young subjects by randomly presenting a single medial or lateral perturbation in 12 of 30 gait trials. Traditional step kinematic measures were used to evaluate the test-retest reliability of the response to the stimulus at two different walking speeds in 60 randomized trials conducted 1 week apart. The method was effective in systematically inducing an alteration in gait, reproducible across visits, as evidenced by acceptable intraclass correlation coefficients for step width, time and length. We conclude that the apparatus and method has potential for measuring the ability of humans to reject one or more unexpected underfoot perturbations during gait.     

Modeling individual-specific human optic nerve head biomechanics. Part I: IOP-induced deformations and influence of geometry

Glaucoma, the second most common cause of blindness worldwide, is an ocular disease characterized by progressive loss of retinal ganglion cell (RGC) axons. Biomechanical factors are thought to play a central role in RGC loss, but the specific mechanism underlying this disease remains unknown. Our goal was to characterize the biomechanical environment in the optic nerve head (ONH)—the region where RGC damage occurs—in human eyes. Post mortem human eyes were imaged, fixed at either 5 or 50 mmHg pressure and processed histologically to acquire serial sections through the ONH. Three-dimensional models of the ONH region were reconstructed from these sections and embedded in a generic scleral shell to create a model of an entire eye. We used finite element simulations to quantify the effects of an acute change in intraocular pressure from 5 to 50 mmHg on the ONH biomechanical environment. Computed strains varied substantially within the ONH, with the pre-laminar neural tissue and the lamina cribrosa showing the greatest strains. The mode of strain having the largest magnitude was third principal strain (compression), reaching 12–15% in both the lamina cribrosa and the pre-laminar neural tissue. Shear strains were also substantial. The distribution of strains in all ONH tissues was remarkably similar between eyes. Inter-individual variations in ONH geometry (anatomy) have only modest effects on ONH biomechanics, and may not explain inter-individual susceptibility to elevated intraocular pressure. Consistent with previous results using generic ONH models, the displacements of the vitreo-retinal interface and the anterior surface of the lamina cribrosa can differ substantially, suggesting that currently available optical imaging methods do not provide information of the acute deformations within ONH tissues. Predicted strains within ONH tissues are potentially biologically significant and support the hypothesis that biomechanical factors contribute to the initial insult that leads to RGC loss in glaucoma. Ian A. Sigal now a post-doctoral research fellow at Ocular Biomechanics Laboratory, Devers Eye Institute, Legacy Health Research. Portland, OR, USA. (isigal@deverseye.org).     

Occupational biomechanics of the neck: a review and recommendations.

Musculoskeletal disorders of the neck are becoming a major concern in industry. Several studies reported the association of neck pain, discomfort, and symptoms with different occupations in industry. Thus, the main objective of this study is to review and evaluate biomechanical techniques used in analyzing the occupational factors leading to neck pain and disorders. Recommendations for future research are also discussed.     

Air resistance and its influence on the biomechanics and energetics of sprinting at sea level and at altitude

Following an examination of the processes by which chemical energy is converted into useful work during running, a mathematical model of the energetics of sprinting is constructed. This is used in conjunction with a careful analysis of Olympic records, in particular those obtained in the 1968 Games at Mexico City, to determine the magnitude of the rate of working against air resistance during running. It is established that times in the 100 m, 200 m and 400 m events at the Mexico Olympics were approximately 1.7% lower than they would otherwise have been if the races had been run at sea level. This information is used to deduce that the external work done per unit time against air resistance is about 7.5-9% of the total power output of a sprinter, running at maximum speed at sea level. These figures compare well with the value of 7.8% obtained independently by Davies (J. appl. Physiol 48, 702-709, 1980). The analysis provides evidence that a linear relation exists between running speed and the rate of degradation of mechanical energy into thermal energy up to the highest sprinting speeds attainable. The maximum power generated by a sprinter is approximately 3 kW.     

A hybrid of bees algorithm and flux balance analysis with OptKnock as a platform for in silico optimization of microbial strains

Microbial strain optimization focuses on improving technological properties of the strain of microorganisms. However, the complexities of the metabolic networks, which lead to data ambiguity, often cause genetic modification on the desirable phenotypes difficult to predict. Furthermore, vast number of reactions in cellular metabolism lead to the combinatorial problem in obtaining optimal gene deletion strategy. Consequently, the computation time increases exponentially with the increase in the size of the problem. Hence, we propose an extension of a hybrid of Bees Algorithm and Flux Balance Analysis (BAFBA) by integrating OptKnock into BAFBA to validate the result. This paper presents a number of computational experiments to test on the performance and capability of BAFBA. Escherichia coli, Bacillus subtilis and Clostridium thermocellum are the model organisms in this paper. Also included is the identification of potential reactions to improve the production of succinic acid, lactic acid and ethanol, plus the discussion on the changes in the flux distribution of the predicted mutants. BAFBA shows potential in suggesting the non-intuitive gene knockout strategies and a low variability among the several runs. The results show that BAFBA is suitable, reliable and applicable in predicting optimal gene knockout strategy.     

Diseased leaf lyophilization: a method for long-term prevention of loss of virulence in phytopathogenic bacteria

A technique for long-term preservation of phytopathogenic bacteria is described. It is based on selective multiplication of the bacterial pathogen in host tissue, disinfection of leaf surfaces to reduce contamination, lyophilization of leaves, and storage under dry conditions at - 80°C. With this technique, the pathogenicity of large numbers of the pathogens Pseudomonas syringae pv. tomato and Xanthomonas campestris pv. vesicatoria was maintained for 4 years.