The effects of early stages of aging on postural sway: A multiple domain balance assessment using a force platform

Technical advancements in instrumentation and analytical methods have improved the ability of assessing balance control. This study investigated the effects of early stages of aging on postural sway using traditional and contemporary postural indices from different domains. Eleven healthy young adults and fourteen healthy non-faller older adults performed two postural tasks: (a) functional limits of stability and (b) unperturbed bipedal stance for 120 s. Postural indices from spatial, temporal, frequency, and structural domains were extracted from the body’s center of pressure (COP) signals and its Rambling and Trembling components. Results revealed a preservation of functional limits of upright stability in older adults accompanied by larger, faster, and shakier body sway in both anterior-posterior and medio-lateral directions; increased medio-lateral sway frequency; increased irregularity of body sway pattern in time in both directions; and increased area, variability, velocity, and jerkiness of both rambling and trembling components of the COP displacement in the anterior-posterior direction (p < 0.02). Such changes might be interpreted as compensatory adjustments to the age-related decline of sensory, neural, and motor functions. In conclusion, balance assessment using postural indices from different domains extracted from the COP displacement was able to capture subtle effects of the natural process of aging on the mechanisms of postural control. Our findings suggest the use of such indices as potential markers for postural instability and fall risk in older adults.     

Bi-lateral reactive force patterns in postural sway activity of normal subjects

Postural activity of normal subjects while standing still, as measured via the reactive foot-ground reactive forces, was investigated separately for each of the supporting legs, to provide detailed information on the individual activity of each side. Such information is not accessible if the overall reactive forces on the whole body are measured instead. Twenty-three normal adult subjects (average age 41 years) took part in the measuring tests, which were made on two collaterally installed force platforms. From the force traces obtained, the following parameters were determined: timings and amplitudes of the waveforms, separately for each foot and sequence of the force vectors on both feet and in relation to each other. Weight-bearing imbalance was defined in the vertical direction to express the difference between the average forces supported by each of the legs. Two parameters were defined by combining the force results obtained on each of the legs in the horizontal plane: sway total activity, representing the resultant of the added vectornorms in the anteroposterior and mediolateral directions, respectively; and asymmetry, representing the resultant of the subtracted above vector norms. The results indicated that, although the force traces were synchronous to each other, different vectorial force patterns were found, indicating different levels of stabilizing activities on each of the legs. This was also demonstrated by the fact that sway total activity was found considerably higher than the net reactive forces acting on the whole body during sway. Hence, in treating the external forces involved in the stabilization and regulation of human posture, the activity of each of the legs should be taken into account, rather than the resultant forces acting on the body as a whole. The obtained results may thus serve as input force vectors in a multi-segmental model of biped standing.This research was supported by the Technion V.P.R. Fund — M. Mandel Biomedical Engineering Research Fund     

A cycle ergometer mounted on a standard force platform for three-dimensional pedal forces measurement during cycling

This report describes a new method allowing to measure the three-dimensional forces applied on right and left pedals during cycling. This method is based on a cycle ergometer mounted on a force platform. By recording the forces applied on the force platform and applying the fundamental mechanical equations, it was possible to calculate the instantaneous three-dimensional forces applied on pedals. It was validated by static and dynamic tests. The accuracy of the present system was -7.61 N, -3.37 N and -2.81 N, respectively, for the vertical, the horizontal and the lateral direction when applying a mono-directional force and -4.52 N when applying combined forces. In pedaling condition, the orientation and magnitude of the pedal forces were comparable to the literature. Moreover, this method did not modify the mechanical properties of the pedals and offered the possibility for pedal force measurement with materials often accessible in laboratories. Measurements obtained showed that this method has an interesting potential for biomechanical analyses in cycling.     

A novel force transducer for the measurement of grip force.

A new strain gauge transducer has been developed to measure functional grip forces. The gripping area is a cylinder of diameter 30 mm and length 150 mm and simulates the handle of a number of devices, allowing a range of activities to be studied. The device measures radial forces divided into six components and forces of up to 250 N per segment can be measured with an accuracy of +/- 1%. The device therefore gives information about the magnitude and distribution of force around the cylinder during gripping, and has been shown to be a valuable research tool in a study of four different types of grip, providing valuable input data for biomechanical models.     

Low-frequency force steadiness practice in plantar flexor muscle reduces postural sway during quiet standing

The purpose of this study was to assess the effect of low-frequency force steadiness practice in the plantar flexor muscles on postural sway during quiet standing. Healthy young 21 men (21±1 yrs) were randomly assigned to a practice group (n=14) and a nonexercising control group (n=7). Practice groups were divided by frequency of practice: 7 participants practiced once a week, and the other 7 twice a week, for 4 weeks. Steadiness practice required practice group to 5 sets of 60-s contraction at levels corresponding to 10% and 20% maximal voluntary contraction (MVC) in the plantar flexor muscles. The 4-week-long practice period reduced the force fluctuations (assessed as the standard deviation (SD) of the outputted force during steady isometric plantar flexion) and postural sway (assessed as SD of the center of mass velocity during quiet standing). However, these practice effects were not significantly affected by the practice frequencies (1 vs. 2 sessions per week) examined in this study. Further, a linear regression analysis revealed the association between prepractice postural sway and the relative change in postural sway by the practice (r=-0.904) in the practice group. These results suggest that the steadiness practice in plantar flexor muscles improves postural stability during quiet standing, even though the practice is low-frequency (once a week) and low-intensity (within 20% MVC). These practice effects are dependent on prepractice postural stability. Further, the present results have provided the functional significance of force fluctuation in lower limb muscles.     

Cell traction force and measurement methods

Cell traction forces (CTFs) are crucial to many biological processes such as inflammation, wound healing, angiogenesis, and metastasis. CTFs are generated by actomyosin interactions and actin polymerization and regulated by intracellular proteins such as alpha-smooth muscle actin (α-SMA) and soluble factors such as transforming growth factor-β (TGF-β). Once transmitted to the extracellular matrix (ECM) through stress fibers via focal adhesions, which are assemblies of ECM proteins, transmembrane receptors, and cytoplasmic structural and signaling proteins (e.g., integrins), CTFs direct many cellular functions, including cell migration, ECM organization, and mechanical signal generation. Various methods have been developed over the years to measure CTFs of both populations of cells and of single cells. At present, cell traction force microscopy (CTFM) is among the most efficient and reliable method for determining CTF field of an entire cell spreading on a two-dimensional (2D) substrate surface. There are currently three CTFM methods, each of which is unique in both how displacement field is extracted from images and how CTFs are subsequently estimated. A detailed review and comparison of these methods are presented. Future research should improve CTFM methods such that they can automatically track dynamic CTFs, thereby providing new insights into cell motility in response to altered biological conditions. In addition, research effort should be devoted to developing novel experimental and theoretical methods for determining CTFs in three-dimensional (3D) matrix, which better reflects physiological conditions than 2D substrate used in current CTFM methods.     

A force sensor and peak-reading recorder for measurement of cervical dilatation force

Earlier dilatation force-sensing transducers, when subjected to side loads, suffered frictional losses which affected their accuracy. This new instrument incorporates a thermal-writing chart recorder and a digital readout of the peak force during dilatation of the cervix.     

Development of glutathione-coupled cantilever for the single-molecule force measurement by scanning force microscopy

The accuracy and the fidelity of a single-molecule force measurement largely rely on how the molecule of interest is attached to the solid substrate surface (bead, cantilever, cover glass and etc.). A site-specific attachment of a protein without affecting its structure and enzymatic function has been a major concern. Here, we established a glutathione-coupled cantilever to which any glutathione S-transferase (GST)-fused proteins can be attached in a desired direction. The rupture force between glutathione and GST was approximately 100 pN on average. By using this cantilever, we succeeded in measuring the interaction force between importin alpha and importin beta.     

Compensation for inertial and gravity effects in a moving force platform

Force plates for human movement analysis provide accurate measurements when mounted rigidly on an inertial reference frame. Large measurement errors occur, however, when the force plate is accelerated, or tilted relative to gravity. This prohibits the use of force plates in human perturbation studies with controlled surface movements, or in conditions where the foundation is moving or not sufficiently rigid. Here we present a linear model to predict the inertial and gravitational artifacts using accelerometer signals. The model is first calibrated with data collected from random movements of the unloaded system and then used to compensate for the errors in another trial. The method was tested experimentally on an instrumented force treadmill capable of dynamic mediolateral translation and sagittal pitch. The compensation was evaluated in five experimental conditions, including platform motions induced by actuators, by motor vibration, and by human ground reaction forces. In the test that included all sources of platform motion, the root-mean-square (RMS) errors were 39.0 N and 15.3 N m in force and moment, before compensation, and 1.6 N and 1.1 N m, after compensation. A sensitivity analysis was performed to determine the effect on estimating joint moments during human gait. Joint moment errors in hip, knee, and ankle were initially 53.80 N m, 32.69 N m, and 19.10 N m, and reduced to 1.67 N m, 1.37 N m, and 1.13 N m with our method. It was concluded that the compensation method can reduce the inertial and gravitational artifacts to an acceptable level for human gait analysis.     

A method of bite force measurement in primates

A bite force transducer consisting of two differential strain beams with four strain gages in a full bridge configuration was modified for measuring occlusal forces in rhesus monkeys. A procedure of muscle stimulation (20-50 V, 60 Hz, and 0.8 ms duration) produced maximal unilateral masticatory muscle contraction when stimulating electrodes were placed in the masseter muscle. Tests of this procedure revealed reproducible results and a potential for use in studies of the force of isometric contraction of the masticatory muscles in normal and experimentally altered macaques and other primates.     

Direct measurement of the lamellipodial protrusive force in a migrating cell

There has been a great deal of interest in the mechanism of lamellipodial protrusion (Pollard, T., and G. Borisy. 2003. Cell. 112:453-465). However, one of this mechanism's endpoints, the force of protrusion, has never been directly measured. We place an atomic force microscopy cantilever in the path of a migrating keratocyte. The deflection of the cantilever, which occurs over a period of approximately 10 s, provides a direct measure of the force exerted by the lamellipodial leading edge. Stall forces are consistent with approximately 100 polymerizing actin filaments per micrometer of the leading edge, each working as an elastic Brownian ratchet and generating a force of several piconewtons. However, the force-velocity curves obtained from this measurement, in which velocity drops sharply under very small loads, is not sensitive to low loading forces, and finally stalls rapidly at large loads, are not consistent with current theoretical models for the actin polymerization force. Rather, the curves indicate that the protrusive force generation is a complex multiphase process involving actin and adhesion dynamics.     

Atomic force microscopy measurement of leukocyte-endothelial interaction

Leukocyte adhesion to vascular endothelium is a key initiating step in the pathogenesis of many inflammatory diseases. In this study, we present real-time force measurements of the interaction between monocytic human promyelocytic leukemia cells (HL-60) cells and a monolayer of human umbilical vein endothelial cells (HUVECs) by using atomic force microscopy (AFM). The detachment of HL-60-HUVEC conjugates involved a series of rupture events with force transitions of 40-100 pN. The integrated force of these rupture events provided a quantitative measure of the adhesion strength on a whole cell level. The AFM measurements revealed that HL-60 adhesion is heightened in the borders formed by adjacent HUVECs. The average force and mechanical work required to detach a single HL-60 from the borders of a tumor necrosis factor-alpha-activated HUVEC layer were twice as high as those of the HUVEC bodies. HL-60 adhesion to the monolayer was significantly reduced by a monoclonal antibody against beta1-integrins and partially inhibited by antibodies against selectins ICAM-1 and VCAM-1 but was not affected by anti-alphaVbeta3. Interestingly, adhesion was also inhibited in a dose-dependent manner (IC50 approximately 100 nM) by a cyclic arginine-glycine-aspartic acid (cRGD) peptide. This effect was mediated via interfering with the VLA-4-VCAM-1 binding. In parallel measurements, transmigration of HL-60 cells across a confluent HUVEC monolayer was inhibited by the cRGD peptide and by both anti-beta1 and anti-alphaVbeta3 antibodies. In conclusion, these data demonstrate the role played by beta1-integrins in leukocyte-endothelial adhesion and transmigration and the role played by alphaVbeta3 in transmigration, thus underscoring the high efficacy of cRGD peptide in blocking both the adhesion and transmigration of monocytes.     

A non-invasive method of tendon force measurement

The ability to measure the forces exerted in vivo on tendons and, consequently, the forces produced by muscles on tendons, offers a unique opportunity to investigate questions in disciplines as varied as physiology, biomechanics, orthopaedics and neuroscience. Until now, tendon loads could be assessed directly only by means of invasive sensors implanted within or attached to these collagenous structures. This study shows that the forces acting on tendons can be measured, in a non-invasive way, from the analysis of the propagation of an acoustic wave. Using the equine superficial digital flexor tendon as a model, it is demonstrated that the velocity of an ultrasonic wave propagating along the main axis of a tendon increases with the force applied to this tendon. Furthermore, we show that this velocity measurement can be performed even in the presence of skin overlying the tendon. To validate this measurement technique in vivo, the ultrasonic velocity plots obtained in the Achilles tendon at the walk were compared to the loads plots reported by other authors using invasive transducers.     

An estimation of center of gravity from force platform data

A general, dynamic relationship between the data obtained from a force platform, center of gravity of the body on the platform and the time rate of change of moment of momentum of the body about its center of gravity was derived from principles of dynamics for a system of particles. The derived equations are useful for processing and interpreting the force platform data. Displacement and path of center of gravity of human body during standing on one foot and level walking were estimated by using the derived equations. An estimation of the time rate of change of moment of momentum of the body was also obtained. A biomechanical interpretation of point of application of the resultant of ground reactions was presented.     

Insertion of microneedles into skin: measurement and prediction of insertion force and needle fracture force

As a hybrid between a hypodermic needle and transdermal patch, we have used microfabrication technology to make arrays of micron-scale needles that transport drugs and other compounds across the skin without causing pain. However, not all microneedle geometries are able to insert into skin at reasonable forces and without breaking. In this study, we experimentally measured and theoretically modeled two critical mechanical events associated with microneedles: the force required to insert microneedles into living skin and the force needles can withstand before fracturing. Over the range of microneedle geometries investigated, insertion force was found to vary linearly with the interfacial area of the needle tip. Measured insertion forces ranged from approximately 0.1-3N, which is sufficiently low to permit insertion by hand. The force required to fracture microneedles was found to increase with increasing wall thickness, wall angle, and possibly tip radius, in agreement with finite element simulations and a thin shell analytical model. For almost all geometries considered, the margin of safety, or the ratio of fracture force to insertion force, was much greater than one and was found to increase with increasing wall thickness and decreasing tip radius. Together, these results provide the ability to predict insertion and fracture forces, which facilitates rational design of microneedles with robust mechanical properties.     

Magnetic tweezers: micromanipulation and force measurement at the molecular level

Cantilevers and optical tweezers are widely used for micromanipulating cells or biomolecules for measuring their mechanical properties. However, they do not allow easy rotary motion and can sometimes damage the handled material. We present here a system of magnetic tweezers that overcomes those drawbacks while retaining most of the previous dynamometers properties. Electromagnets are coupled to a microscope-based particle tracking system through a digital feedback loop. Magnetic beads are first trapped in a potential well of stiffness approximately 10(-7) N/m. Thus, they can be manipulated in three dimensions at a speed of approximately 10 microm/s and rotated along the optical axis at a frequency of 10 Hz. In addition, our apparatus can work as a dynamometer relying on either usual calibration against the viscous drag or complete calibration using Brownian fluctuations. By stretching a DNA molecule between a magnetic particle and a glass surface, we applied and measured vertical forces ranging from 50 fN to 20 pN. Similarly, nearly horizontal forces up to 5 pN were obtained. From those experiments, we conclude that magnetic tweezers represent a low-cost and biocompatible setup that could become a suitable alternative to the other available micromanipulators.     

Rapid affinity measurement of protein-protein interactions in a microfluidic platform

A rapid screening method has been developed to determine binding affinities for protein-ligand interactions using the Gyrolab workstation, a commercial microfluidic platform developed to accurately and precisely quantify proteins in solution. This method was particularly suited for assessing the high-affinity interactions that have become typical of therapeutic antibody-antigen systems. Five different commercially available antibodies that bind digoxin and a digoxin-bovine serum albumin (BSA) conjugate with high affinity were rigorously evaluated by this method and by the more conventional kinetic exclusion assay (KinExA) method. Binding parameter values obtained using Gyrolab were similar to those recovered from KinExA. However, the total experimental time for 20 binding affinity titrations, with each titration covering 12 data points in duplicate, took approximately 4h by the Gyrolab method, which reduced the experimental duration by more than 10-fold when compared with the KinExA method. This rapid binding analysis method has significant applications in the screening and affinity ranking selection of antibodies from a very large pool of candidates spanning a wide range of binding affinities from the low pM to μM range.     

Atomic force microscopic measurement of the interdomain angle in symmetric Holliday junctions

The Holliday junction is a key intermediate in genetic recombination. It consists of four DNA strands that associate by base pairing to produce four double helices flanking a junction point. In the presence of multivalent cations, the four helices, in turn, stack in pairs to form two double-helical domains. The angle between these domains has been shown in a number of solution studies to be approximately 60 degrees in junctions flanked by asymmetric sequences. However, the recently determined crystal structure of a symmetric junction [Eichman, B. F., Vargason, J. M., Mooers, B. H. M., and Ho, P. S. (2000) Proc. Natl. Acad. Sci. U.S.A. 97, 3971-3976] finds an angle closer to 40 degrees, possibly because of sequence effects. From the crystal structure alone, one cannot exclude the possibility that this unusual angle is a consequence of crystal packing effects. We have formed two-dimensional (2D) periodic arrays of DNA parallelograms with the same junction-flanking sequence used to produce the crystals; these parallelograms are free to adopt their preferred interdomain angle. Atomic force microscopy can be used to establish the interdomain angle in this system. We find that the angle in this junction is 43 degrees, in good agreement with the results of crystallography. We have used hydroxyl radical autofootprinting to establish that the branch point is at the same migratory position seen in the crystals.