Does medio-lateral motion occur in the normal knee? An in-vitro study in passive motion

Medio-lateral translation during knee flexion continues to raise controversy. Small population sizes, small joint flexion ranges, less-reliable measurement techniques and disparate experimental conditions led to inconsistent reports in the past. To study this subject with more accurate and reliable measurements, we carried out femur and tibia tracking in 22 intact cadaver knees during passive joint motion using a state-of-the-art surgical navigation system. Trackers with active light-emitting diodes were fixed onto the femur and tibia, and an instrumented pointer was used to digitize a number of anatomical landmarks. International recommendations were adopted for anatomical-based reference frame definitions and joint kinematic analysis. For the first time, knee joint translations were reported in both the femoral and tibial reference frames, and over a flexion/extension arc as large as 140°. During flexion, in the femoral reference frame, the center of the tibial plateau moved 4.8 ± 2.8mm medially when averaged over the specimens. In the tibial frame, the knee center moved 13.3 ± 5.7 mm laterally. The relative femoral-to-tibial medio-lateral translation was, on average over the specimens, nearly 20% of the width of the tibial plateau, and can be as large as 35%. Medio-lateral translation occurs in the natural normal knee joint.     

Orientation by helical motion—II. Changing the direction of the axis of motion

We analyse the helical motion of organisms, concentrating on the means by which organisms change the direction in space of the axis of the helical trajectory, which is the net direction of motion. We demonstrate that the direction of the axis is determined largely by the direction of the organism's rotational velocity. Changes in direction of the rotational velocity, with respect to the organism's body, change the direction in space of the axis of the helical trajectory. Conversely, changes in direction of the translational velocity, with respect to the body of the organism, have little effect on the direction in space of the axis of the trajectory. Because the axis of helical motion is the net direction of motion, it is likely that organisms that move in helices change direction by pointing their rotational velocity, not their translational velocity, in a new direction.     

Can fabric sensors monitor breast motion?

To establish whether conducting polymer-coated fabric sensors could be used to monitor breast motion, vertical breast motion of two large breasted women (C+ bra cup) was simultaneously monitored using an OPTOTRAK 3020 motion analysis system (200 Hz) and polymer-coated fabric sensors linked to a custom-made Bluetooth telemetry system (100 Hz) as the subjects walked and ran on a treadmill (7-10 km h(-1)). Sensor strain, change in resistance and vertical breast displacement relative to trunk movement were output for analysis. It was concluded that, although polymer-coated fabric sensors may exhibit a small response lag due to textile geometry changes, they were able to accurately and reliably represent changes in the amplitude of vertical breast displacement during treadmill gait.     

Vision: can colour contribute to motion?

Whether colour patterns that have no luminance variation can evoke the perception of visual motion has long been a controversial issue. Recent studies using new and old techniques have now provided compelling evidence that colour can indeed contribute to motion perception.     

β2-Adrenergic ion-channel coupled receptors as conformational motion detectors

Ion Channel-Coupled Receptors (ICCRs) are artificial proteins comprised of a G protein-coupled receptor and a fused ion channel, engineered to couple channel gating to ligand binding. These novel biological objects have potential use in drug screening and functional characterization, in addition to providing new tools in the synthetic biology repertoire as synthetic K(+)-selective ligand-gated channels. The ICCR concept was previously validated with fusion proteins between the K(+) channel Kir6.2 and muscarinic M(2) or dopaminergic D(2) receptors. Here, we extend the concept to the distinct, longer β(2)-adrenergic receptor which, unlike M(2) and D(2) receptors, displayed barely detectable surface expression in our Xenopus oocyte expression system and did not couple to Kir6.2 when unmodified. Here, we show that a Kir6.2-binding protein, the N-terminal transmembrane domain of the sulfonylurea receptor, can greatly increase plasma membrane expression of β(2) constructs. We then demonstrate how engineering of both receptor and channel can produce β(2)-Kir6.2 ICCRs. Specifically, removal of 62-72 residues from the cytoplasmic C-terminus of the receptor was required to enable coupling, suggesting that ligand-dependent conformational changes do not efficiently propagate to the distal C-terminus. Characterization of the β(2) ICCRs demonstrated that full and partial agonists had the same coupling efficacy, that an inverse agonist had no effect and that the stabilizing mutation E122 W reduced agonist-induced coupling efficacy without affecting affinity. Because the ICCRs are expected to report motions of the receptor C-terminus, these results provide novel insights into the conformational dynamics of the β(2) receptor.     

Fluorescence studies of the restricted motion of tryptophan in α-cobratoxin

The fluorescence lifetime and rotational correlation time of the single tryptophan residue in -cobratoxin have been measured between pH 2 and 10. The fluorescence decays are non-exponential and give lifetimes that are shorter than normally observed in small proteins (0.3 ns and 1 ns). This emission is consistent with a model in which the tryptophan residue is in slightly different environments in the protein. Fluorescence anisotropy decays show that the tryptophan residue is almost completely immobilised by neighbouring groups in the protein. The range of the wobbling motion is slightly pH dependent and limited to between 5° and 10°.     

A video-based technique for measuring ankle—subtalar motion during stance

This study was designed to demonstrate that an accurate and reliable technique for measuring the rotational motion between rigid bodies can be developed from a video-based data collection system. Tests on the accuracy of the rotational measurements demonstrated that for each of five trials, under three test conditions, mean errors of less than 0.4° and standard deviations of less than 0.2° were found for all three joint axes. The reliability of the technique for evaluating ankle-subtalar motion during stance was also demonstrated, and the technique was then applied to evaluate ankle-subtalar motion during the stance phase of gait. The rotational motion of the combined ankle-subtalar joint was characterized by complexity and inter-subject variability. Two simplified models previously used to describe ankle-subtalar motion were tested using the current data and were proven to be inadequate for describing ankle-subtalar motion during stance.     

Component and intrinsic motion integrate in ‘dancing bar’ illusion

We introduce a new illusion that contradicts common assumptions in the field of visual motion perception. When an unoccluded bar moves at certain speeds and oscillates at certain frequencies, the perceived direction of the bar is not predicted by its intrinsic terminators but is biased to move in the direction orthogonal to its orientation. It appears that the veridical terminator motions are integrated with spurious component motion signals, generating an at times complex pattern of motion around an apparently closed loop path. In the absence of oscillation the effect does not occur. Several factors, including optimal angle, speed, and oscillating distance of the bar, are quantified and possible mechanisms are discussed. In a model, we suggest that the effect arises because of the failure to inhibit spurious component motion signals arising from contours that are nearly oriented along the direction of true motion. Electronic Supplementary Material The online version of this article (doi:) material, which is available to authorized users.     

Self-recognition of avatar motion: how do I know it's me?

When motion is isolated from form cues and viewed from third-person perspectives, individuals are able to recognize their own whole body movements better than those of friends. Because we rarely see our own bodies in motion from third-person viewpoints, this self-recognition advantage may indicate a contribution to perception from the motor system. Our first experiment provides evidence that recognition of self-produced and friends' motion dissociate, with only the latter showing sensitivity to orientation. Through the use of selectively disrupted avatar motion, our second experiment shows that self-recognition of facial motion is mediated by knowledge of the local temporal characteristics of one's own actions. Specifically, inverted self-recognition was unaffected by disruption of feature configurations and trajectories, but eliminated by temporal distortion. While actors lack third-person visual experience of their actions, they have a lifetime of proprioceptive, somatosensory, vestibular and first-person-visual experience. These sources of contingent feedback may provide actors with knowledge about the temporal properties of their actions, potentially supporting recognition of characteristic rhythmic variation when viewing self-produced motion. In contrast, the ability to recognize the motion signatures of familiar others may be dependent on configural topographic cues.     

How do painters represent motion in garments? Graphic invariants across centuries

Western visual art has radically changed throughout the centuries: different techniques, interest in the representation of reality, and use of graphic signs. Indeed, only a few pictorial cues have retained the same meaning and use. These kinds of graphic invariants may play a key role not only in a comparative study of art history, but also for discovering underlying common perceptual mechanisms. Here the aim is to show that western painters use the same graphic solutions to represent motion in garments, across countries and centuries. A pilot experiment, using 160 paintings representative of all main western European art movements from the thirteenth to the twentieth century, shows that different artists represented the motion of garments with the same orientation, curvature and convergence of lines. Experiment 1 demonstrates, with a smaller sample of paintings (16, i.e. two per century) that the relationship between orientation, curvature and convergence of lines is a good predictor of perceived motion. Experiment 2 shows how the same garments, isolated from the context of the paintings, still give different dynamic impressions according to the same rules. Finally, Experiment 3 confirmed the same results, whilst patterns previously used are simplified to their geometrical structure. These results call for an underlying perceptual mechanism that specifically recognizes orientation, curvature and parallelism levels as cues of motion in a static pattern.     

Motion vision: are 'speed lines' used in human visual motion?

Motion analysis poses problems for any visual system, not least because of the ambiguities inherent in motion signals. Recent studies suggest that the human motion system may exploit 'motion streaks' - analogous to the cartoonist's speed lines - to help resolve the direction of ambiguous motion.     

Natural fluctuations of an electropore show fractional Lévy stable motion

Until now a stable long-lived electronanopore could be generated in a lipid membrane only under current-clamp conditions, and stochastic properties of a single nanopore have been studied by the chronopotentiometry. The current-clamp experiment introduces negative feedback, which could be responsible for the electropore fluctuations and observed 1/fB power spectrum. A new electroporation method, chronoamperometry after current clamp (CACC), prevents irreversible rupture of the membrane and eliminates the feedback by clamping the voltage after previous electroporation. The experiments show that the electropore size can also be stabilized under constant potential. The electropore fluctuations do not need feedback to appear. The fluctuations are self-similar with a short memory. CACC provides an effective tool for studying the natural dynamics of an electropore in various environments, which was tested with Na+ and Al3+ ions. Comparison between chronopotentiometry and CACC reveals that the feedback mainly shortens the memory of the stochastic fluctuations. Statistical analysis shows that the conductance fluctuations can be approximately modeled as a fractional Lévy stable motion for a small hydrophilic electropore, which tends to fractional Brownian motion when the electropore increases its size. A hypothesis is presented that this transition reflects a more regular shape of big nanopores.     

AMP-activated protein kinase β-subunit requires internal motion for optimal carbohydrate binding

AMP-activated protein kinase interacts with oligosaccharides and glycogen through the carbohydrate-binding module (CBM) containing the β-subunit, for which there are two isoforms (β(1) and β(2)). Muscle-specific β(2)-CBM, either as an isolated domain or in the intact enzyme, binds carbohydrates more tightly than the ubiquitous β(1)-CBM. Although residues that contact carbohydrate are strictly conserved, an additional threonine in a loop of β(2)-CBM is concurrent with an increase in flexibility in β(2)-CBM, which may account for the affinity differences between the two isoforms. In contrast to β(1)-CBM, unbound β(2)-CBM showed microsecond-to-millisecond motion at the base of a β-hairpin that contains residues that make critical contacts with carbohydrate. Upon binding to carbohydrate, similar microsecond-to-millisecond motion was observed in this β-hairpin and the loop that contains the threonine insertion. Deletion of the threonine from β(2)-CBM resulted in reduced carbohydrate affinity. Although motion was retained in the unbound state, a significant loss of motion was observed in the bound state of the β(2)-CBM mutant. Insertion of a threonine into the background of β(1)-CBM resulted in increased ligand affinity and flexibility in these loops when bound to carbohydrate. However, these mutations indicate that the additional threonine is not solely responsible for the differences in carbohydrate affinity and protein dynamics. Nevertheless, these results suggest that altered protein dynamics may contribute to differences in the ligand affinity of the two naturally occurring CBM isoforms.     

A moving–resting process with an embedded Brownian motion for animal movements

Animal movements are of great importance in studying home ranges, migration routes, resource selection, and social interactions. The Global Positioning System provides relatively continuous animal tracking over time and long distances. Nevertheless, the continuous trajectory of an animal’s movement is usually only observed at discrete time points. Brownian bridge models have been used to model movement of an animal between two observed locations within a reasonably short time interval. Assuming that animals are in perpetual motion, these models ignore inactivity such as resting or sleeping. Using the latest developments in applied probability, we propose a moving–resting process model where an animal is assumed to alternate between a moving state, during which it moves in a Brownian motion, and a resting state, during which it does not move. Theoretical properties of the process are studied as a first step towards more realistic models for animal movements. Analytic expressions are derived for the distribution of one increment and two consecutive increments, and are validated with simulations. The induced bridge model conditioning on the starting and end points is used to compute an animal’s probability of occurrence in an observation area during the time of observation, which has wide applications in wildlife behavior research.     

Is torso soft tissue motion really an artefact within breast biomechanics research?

For rigid body POSE estimation, any relative movement of the tracking markers on a segment is often referred to as an artefact; however this may be an important part of the signal within breast biomechanics. This study aimed to quantify differences in breast range of motion when calculated relative to the torso segment using either direct or segment optimised POSE estimation algorithms. Markers on the torso and right nipple were tracked using infrared cameras (200 Hz) during five running gait cycles in three breast support conditions (no bra, everyday bra and sports bra). Multiplanar breast range of motion was calculated relative to the torso segment using two POSE estimation algorithms. First, the torso segment was defined using direct POSE estimation (direct). Second, while standing stationary in the anatomical position; the positional data of the torso markers were used to construct the torso using segment optimised POSE estimation (optimised). The torso segment length defined using direct POSE estimation changed significantly by 3.4 cm compared to that of the segment optimisation POSE estimation in the no bra condition. Subsequently, superioinferior breast range of motion was significantly greater (p<0.017) when calculated using direct POSE estimation, within each of the three breast support conditions. Segment optimisation POSE estimation is recommended to minimise any differences in breast motion associated with intra segment deformation between physical activity types. However, either algorithm is recommended when evaluating different breast support garments, as a correctly fitted bra does not cause the torso markers to move relative to each other.     

Ubiquitin in motion: structural studies of the ubiquitin-conjugating enzyme∼ubiquitin conjugate

Ubiquitination of proteins provides a powerful and versatile post-translational signal in the eukaryotic cell. The formation of a thioester bond between ubiquitin (Ub) and the active site of a ubiquitin-conjugating enzyme (E2) is critical for the transfer of Ub to substrates. Assembly of a functional ubiquitin ligase (E3) complex poised for Ub transfer involves recognition and binding of an E2～Ub conjugate. Therefore, full characterization of the structure and dynamics of E2～Ub conjugates is required for further mechanistic understanding of Ub transfer reactions. Here we present characterization of the dynamic behavior of E2～Ub conjugates of two human enzymes, UbcH5c～Ub and Ubc13～Ub, in solution as determined by nuclear magnetic resonance and small-angle X-ray scattering. Within each conjugate, Ub retains great flexibility with respect to the E2, indicative of highly dynamic species that adopt manifold orientations. The population distribution of Ub conformations is dictated by the identity of the E2: the UbcH5c～Ub conjugate populates an array of extended conformations, and the population of Ubc13～Ub conjugates favors a closed conformation in which the hydrophobic surface of Ub faces helix 2 of Ubc13. We propose that the varied conformations adopted by Ub represent available binding modes of the E2～Ub species and thus provide insight into the diverse E2～Ub protein interactome, particularly with regard to interaction with Ub ligases.     

Elastic behavior of a red blood cell with the membrane’s nonuniform natural state: equilibrium shape,motion transition under shear flow,and elongation during tank-treading motion

Direct numerical simulations of the mechanics of a single red blood cell (RBC) were performed by considering the nonuniform natural state of the elastic membrane. A RBC was modeled as an incompressible viscous fluid encapsulated by an elastic membrane. The in-plane shear and area dilatation deformations of the membrane were modeled by Skalak constitutive equation, while out-of-plane bending deformation was formulated by the spring model. The natural state of the membrane with respect to in-plane shear deformation was modeled as a sphere ( \(\alpha =0\) ), biconcave disk shape ( \(\alpha =1\) ) and their intermediate shapes ( \(0<\alpha <1\) ) with the nonuniformity parameter \(\alpha \) , while the natural state with respect to out-of-plane bending deformation was modeled as a flat plane. According to the numerical simulations, at an experimentally measured in-plane shear modulus of \(2.5\times 10^{-6}\,\hbox {N}/\hbox {m}\) and an out-of-plane bending rigidity of \(2.0\times 10^{-19}\,\hbox {N}\cdot \hbox {m}\) of the cell membrane, the following results were obtained. (i) The RBC shape at equilibrium was biconcave discoid for \(\alpha >0.22\) and cupped otherwise; (ii) the experimentally measured fluid shear stress at the transition between tumbling and tank-treading motions under shear flow was reproduced for \(0.05<\alpha <0.34\) ; (iii) the elongation deformation of the RBC during tank-treading motion from the simulation was consistent with that from in vitro experiments, irrespective of the \(\alpha \) value. Based on our RBC modeling, the three phenomena (i), (ii), and (iii) were mechanically consistent for \(0.22<\alpha <0.34\) . The condition \(0.05<\alpha <0.22\) precludes a biconcave discoid shape at equilibrium (i); however, it gives appropriate fluid shear stress at the motion transition under shear flow (ii), suggesting that a combined effect of \(\alpha \) and the natural state with respect to out-of-plane bending deformation is necessary for understanding details of the RBC mechanics at equilibrium. Our numerical results demonstrate that moderate nonuniformity in a membrane’s natural state with respect to in-plane shear deformation plays a key role in RBC mechanics.