Inertial vestibular coding of motion: concepts and evidence

Central processing of inertial sensory information about head attitude and motion in space is crucial for motor control. Vestibular signals are coded relative to a non-inertial system, the head, that is virtually continuously in motion. Evidence for transformation of vestibular signals from head-fixed sensory coordinates to gravity-centered coordinates have been provided by studies of the vestibulo-ocular reflex. The underlying central processing depends on otolith afferent information that needs to be resolved in terms of head translation related inertial forces and head attitude dependent pull of gravity. Theoretical solutions have been suggested, but experimental evidence is still scarce. It appears, along these lines, that gaze control systems are intimately linked to motor control of head attitude and posture.     

Recent approaches in the analysis of weightlessness effects on arthropod development

The concept of Biological Development refers to the extremely complex process by which every biological organism reproduces starting from a huge single cell, the fertilized egg. It includes all aspects of cellular and intercellular structure and function. In spite of many recent advances, especially at the molecular and genetical level, we are still far from fully understanding the details and mechanisms at work in developmental systems. It is even unclear what physical mechanisms are used by the different molecular components resulting in the emergence of these higher levels of organization. Newman and Comper, have extensively discussed the "generic" physical forces potentially involved in pattern formation, arguing that among others, gravitational effects could be involved in the production of cytoplasmic, tissue and extracellular matrix components rearrangements playing a role in morphogenesis. Although plagued with the problem of being a very weak force, specially at the tiny dimensions of cells, gravity is one of the "generic" physical forces that have been continuously operating on biological organisms during evolution. Few scientists would argue against the idea that at least in the early times of evolution, gravity could have been involved in shaping the spatial inhomogeneities behind the initial phases of development.     

Motor control and learning in altered dynamic environments

Dynamic perturbations of reaching movements are an important technique for studying motor learning and adaptation. Adaptation to non-contacting, velocity-dependent inertial Coriolis forces generated by arm movements during passive body rotation is very rapid, and when complete the Coriolis forces are no longer sensed. Adaptation to velocity-dependent forces delivered by a robotic manipulandum takes longer and the perturbations continue to be perceived even when adaptation is complete. These differences reflect adaptive self-calibration of motor control versus learning the behavior of an external object or 'tool'. Velocity-dependent inertial Coriolis forces also arise in everyday behavior during voluntary turn and reach movements but because of anticipatory feedforward motor compensations do not affect movement accuracy despite being larger than the velocity-dependent forces typically used in experimental studies. Progress has been made in understanding: the common features that determine adaptive responses to velocity-dependent perturbations of jaw and limb movements; the transfer of adaptation to mechanical perturbations across different contact sites on a limb; and the parcellation and separate representation of the static and dynamic components of multiforce perturbations.     

Inertial force as a possible factor in mitosis

Some previous studies of cell division have suggested that chromosome movements in mitosis involve two distinct forces: one which pulls chromosomes poleward by means of attached fibers, and another which tends to push chromosome arms away from the pole. The latter force may also be a factor in non-chromosomal spindle transport, by which objects other than chromosomes are transported toward or away from spindle poles. Based on a survey of previous literature, this paper makes a prima facie case for describing this latter force as "inertial", since in some respects it can be simulated by centrifugation. A theoretical analysis demonstrates that an inertial force could arise in the spindle from postulated high-frequency, small-amplitude oscillations, which could be caused by changes in coherently processing electron spin alignments at the spindle poles. Some possible experimental approaches to the problem are briefly outlined.     

Behavioral responses to short periods of lowered gravitational force in blind goldfish

1. The movements of blind goldfish flown in an aircraft through vertical flight patterns show a consistent correlation with the varyingg loads as recorded by ag meter.
2. Decreasing theg load in all tested cases caused the fish to rapidly dive downwards after an approximate delay of 0.5–1.5 sec. The opposite reaction, a tilt and movement upwards, was observed in transitions from a lower to a higherg load, though the latter reaction was not as pronounced.
3. Because any potential influence of visual cues, swimming bladder reflexes, and changing barometric pressure was excluded by experimental precautions, we concluded that the response of the fish to varyingg loads were vestibular reflexes probably resulting from displacements of the otoliths during vertical accelerations.
4. It is concluded from data in the literature and from the present results, that the behavioral responses resulting from static or dynamic forces are counterdirected toward the otolith displacements caused by these forces. Consequently, the behavioral, responses are counterdirected toward the gravitational pull on the animal, but must have the same direction as mechanical forces, which accelerate the whole organism and displace the otolith by inertia. It is concluded therefore, that the otolith reflexes have adaptive value only in a gravitational or centrifugal field, but are unsuitable to subserve any stabilizing function in the case of inertial forces during weightlessness.

     

Muscle activity reduces soft-tissue resonance at heel-strike during walking

Muscle activity has previously been suggested to minimize soft-tissue resonance which occurs at heel-strike during walking and running. If this concept were true then the greatest vibration damping would occur when the input force was closest to the resonant frequency of the soft-tissues at heel-strike. However, this idea has not been tested. The purpose of this study was to test whether muscle activity in the lower extremity is used to damp soft-tissue resonance which occurs at heel-strike during walking. Hard and soft shoe conditions were tested in a randomized block design. Ground reaction forces, soft-tissue accelerations and myoelectric activity were measured during walking for 40 subjects. Soft-tissue mass was estimated from anthropologic measurements, allowing inertial forces in the soft-tissues to be calculated. The force transfer from the ground to the tissues was compared with changes in the muscle activity. The soft condition resulted in relative frequencies (input/tissue) to be closer to resonance for the main soft-tissue groups. However, no increase in force transmission was observed. Therefore, the vibration damping in the tissues must have increased. This increase concurred with increases in the muscle activity for the biceps femoris and lateral gastrocnemius. The evidence supports the proposal that muscle activity damps soft-tissue resonance at heel-strike. Muscles generate forces which act across the joints and, therefore, shoe design may be used to modify muscle activity and thus joint loading during walking and running.     

Inertial shear forces and the use of centrifuges in gravity research. What is the proper control?

Centrifuges are used for 1 x g controls in space flight microgravity experiments and in ground based research. Using centrifugation as a tool to generate an Earth like acceleration introduces unwanted inertial shear forces to the sample. Depending on the centrifuge and the geometry of the experiment hardware used these shear forces contribute significantly to the total force acting on the cells or tissues. The inertial shear force artifact should be dealt with for future experiment hardware development for Shuttle and the International Space Station (ISS) as well as for the interpretation of previous space-flight and on-ground research data.     

DNA bending by charged peptides: electrophoretic and spectroscopic analyses

We are testing the idea that placement of fixed charges near one face of the DNA double helix can induce DNA bending by a purely electrostatic mechanism. If stretching forces between DNA phosphates are significant, fixed charges should induce DNA bending by asymmetrically modulating these forces. We have previously tested this hypothesis by adding charged residues to small bZIP DNA binding peptides and monitoring DNA bending using electrophoretic phasing assays. Our results were consistent with an electrostatic model of DNA bending in predicted directions. We now confirm these observations with fluorescence resonance energy transfer (FRET). Using a "U"-shaped DNA probe, we report that DNA bending by charged bZIP peptides is readily detected by FRET. We further show that charged bZIP peptides cause DNA bending rather than DNA twisting.     

Dynamic interactions between limb segments during planar arm movement

Movement of multiple segment limbs requires generation of appropriate joint torques which include terms arising from dynamic interactions among the moving segments as well as from such external forces as gravity. The interaction torques, arising from inertial, centripetal, and Coriolis forces, are not present for single joint movements. The significance of the individual interaction forces during reaching movements in a horizontal plane involving only the shoulder and elbow joints has been assessed for different movement paths and movement speeds. Trajectory formation strategies which simplify the dynamics computation are presented.     

Force-sharing among the primary cat ankle muscles

Force-sharing measurements among the cat ankle extensors have been used for the past 20 years to elucidate the mechanisms underlying normal movement control and to study the force-sharing among synergistic muscles. Despite these efforts, many questions have eluded satisfactory explanation. Specifically, there has been great debate to what degree the soleus muscle is activated during low level locomotion, and whether the force-sharing among the ankle extensors is primarily caused by central or peripheral factors. We have measured the forces and corresponding electromyo-graphical activities in the soleus, gastrocnemius, plantaris, and tibialis anterior in more than 20 cats and for a variety of movements. Based on these measurements, and additional information obtained on the structural and functional properties of the cat ankle muscles, we conclude that the peak soleus forces are virtually maximal for all speeds of locomotion, despite an apparent submaximal level of activation at slow speeds. Furthermore, we found that the force-sharing among the soleus and gastrocnemius encompasses the entire possible range; from large soleus forces and zero gastrocnemius forces during still standing, to large gastrocnemius forces and no soleus forces during a high-frequency paw shake. Finally, we support the idea that shifts in force-sharing from one muscle to the other are primarily caused by central factors; the peripheral factors, although appealing when considering the large differences in size, morphology, and fibre type distribution, are likely only of secondary importance.     

Adaptive control for insect leg position: controller properties depend on substrate compliance

This paper concentrates on the system that controls the femur-tibia joint in the legs of the stick insect, Carausius morosus. Earlier investigations have shown that this joint is subject to a mixture of proportional and differential control whereby the differential part plays a prominent role. Experiments presented here suggest another interpretation: single legs of a stick insect were systematically perturbed using devices of different compliance and compensatory forces and movements monitored. When the compliance is high (soft spring), forces are generated that return the leg close to its original position. When the compliance is low (stiff spring), larger forces are generated but sustained changes in position occur that are proportional to the force that is applied. Selective ablation of leg sense organs showed that the leg did not maintain its position after elimination of afferents of the femoral chordotonal organ. Ablation of leg campaniform sensilla had no effect. These data support the idea that different control strategies are used, depending upon substrate compliance. In particular, what we and other authors have called a differential controller, is now considered as an integral controller that intelligently gives up when the correlation between motor output and movement of the leg is low.We would like to dedicate this article to Prof. Dr. Ulrich Bässler. Starting in the 1960s, his seminal work stimulated a long series of fruitful studies that, even today, reveal exciting insights into motor control.     

The hemodynamic and embolizing forces acting on thrombi--II. The effect of pulsatile blood flow

A previous analysis (Basmadjian, J. Biomechanics 17, 287-298, 1984) of the embolizing forces acting on thrombi in steady Poiseuille flow has been extended to pulsatile blood flow conditions in the major blood vessels. We show that for incipient and small compact thrombi up to 0.1 mm height, the maximum embolizing stresses can be calculated from the corresponding 'quasi-steady' viscous drag forces and measured maximum wall shear. Their magnitude is from 5 to 30 times (tau w)Max, the maximum wall shear stress during the cardiac cycle in the absence of thrombi. For larger thrombi, inertial and 'history' effects have to be taken into account, leading to embolizing stresses in excess of 100 Pa (1000 dyn cm-2).     

Centrifuges and inertial shear forces

Centrifuges are often used in biological studies for 1 x g control samples in space flight microgravity experiments as well as in ground based research. Using centrifugation as a tool to generate an Earth like acceleration introduces unwanted inertial shear forces to the sample. Depending on the centrifuge and the geometry of the experiment hardware used these shear forces may contribute as much as 99% to the total force acting on the cells or tissues. The inertial shear force artifact should be dealt with for future experiment hardware development for Shuttle and the International Space Station (ISS) as well as for the interpretation of previous spaceflight and on-ground research data.     

Dynamically and statically determined low back moments during lifting

Assessment of the effects of lifting on the low back has most frequently been done with the aid of static models. Many lifting movements appear to have substantial inertial components. It was of interest, therefore, to determine the size of the difference between statically and dynamically calculated lumbar moments during a demanding but not unusual manual lift observed in a metal fabrication industry.

The results of several trials by four young men showed that the dynamic model resulted in peak L4 L5 moments 19% higher on average, with a maximum difference of 52%, than those determined from the static model. The technique adopted in the lift could minimize the difference. When the inertial forces of the load itself and the load weight were incorporated into an otherwise static model (quasi-dynamic) then the resulting L4/L5 moments exceeded those of the fully dynamic model by 25%.

In many industrial tasks static analyses may severely underestimate the demands of dynamic lifts. These results show that a reasonably inexpensive approach in lifting task analysis is to measure the dynamic forces of the load on the hands and to use these in an otherwise static model. This results in a conservative assessment of the injury risk of lifts at least of the type reported in this study.     

A finite element model of the lower limb during stance phase of gait cycle including the muscle forces

Background

Results of finite element (FE) analyses can give insight into musculoskeletal diseases if physiological boundary conditions, which include the muscle forces during specific activities of daily life, are considered in the FE modelling. So far, many simplifications of the boundary conditions are currently made. This study presents an approach for FE modelling of the lower limb for which muscle forces were included.

Methods

The stance phase of normal gait was simulated. Muscle forces were calculated using a musculoskeletal rigid body (RB) model of the human body, and were subsequently applied to a FE model of the lower limb. It was shown that the inertial forces are negligible during the stance phase of normal gait. The contact surfaces between the parts within the knee were modelled as bonded. Weak springs were attached to the distal tibia for numerical reasons.

Results

Hip joint reaction forces from the RB model and those from the FE model were similar in magnitude with relative differences less than 16%. The forces of the weak spring were negligible compared to the applied muscle forces. The maximal strain was 0.23% in the proximal region of the femoral diaphysis and 1.7% in the contact zone between the tibia and the fibula.

Conclusions

The presented approach based on FE modelling by including muscle forces from inverse dynamic analysis of musculoskeletal RB model can be used to perform analyses of the lower limb with very realistic boundary conditions. In the present form, this model can be used to better understand the loading, stresses and strains of bones in the knee area and hence to analyse osteotomy fixation devices.

     

Effects of force detecting sense organs on muscle synergies are correlated with their response properties

Sense organs that monitor forces in legs can contribute to activation of muscles as synergist groups. Previous studies in cockroaches and stick insects showed that campaniform sensilla, receptors that encode forces via exoskeletal strains, enhance muscle synergies in substrate grip. However synergist activation was mediated by different groups of receptors in cockroaches (trochanteral sensilla) and stick insects (femoral sensilla). The factors underlying the differential effects are unclear as the responses of femoral campaniform sensilla have not previously been characterized. The present study characterized the structure and response properties (via extracellular recording) of the femoral sensilla in both insects. The cockroach trochantero-femoral (TrF) joint is mobile and the joint membrane acts as an elastic antagonist to the reductor muscle. Cockroach femoral campaniform sensilla show weak discharges to forces in the coxo-trochanteral (CTr) joint plane (in which forces are generated by coxal muscles) but instead encode forces directed posteriorly (TrF joint plane). In stick insects, the TrF joint is fused and femoral campaniform sensilla discharge both to forces directed posteriorly and forces in the CTr joint plane. These findings support the idea that receptors that enhance synergies encode forces in the plane of action of leg muscles used in support and propulsion.     

Parametrization of direct and soft steric-undulatory forces between DNA double helical polyelectrolytes in solutions of several different anions and cations.

Directly measured forces between DNA helices in ordered arrays have been reduced to simple force coefficients and mathematical expressions for the interactions between pairs of molecules. The tabulated force parameters and mathematical expressions can be applied to parallel molecules or, by transformation, to skewed molecules of variable separation and mutual angle. This "toolbox" of intermolecular forces is intended for use in modelling molecular interactions, assembly, and conformation. The coefficients characterizing both the exponential hydration and the electrostatic interactions depend strongly on the univalent counterion species in solution, but are only weakly sensitive to anion type and temperature (from 5 to 50 degrees C). Interaction coefficients for the exponentially varying hydration force seen at spacings less than 10 to 15 A between surfaces are extracted directly from pressure versus interaxial distance curves. Electrostatic interactions are only observed at larger spacings and are always coupled with configurational fluctuation forces that result in observed exponential decay lengths that are twice the expected Debye-Huckel length. The extraction of electrostatic force parameters relies on a theoretical expression describing steric forces of molecules "colliding" through soft exponentially varying direct interactions.     

A comparison between a new model and current models for estimating trunk segment inertial parameters

Modeling of the body segments to estimate segment inertial parameters is required in the kinetic analysis of human motion. A new geometric model for the trunk has been developed that uses various cross-sectional shapes to estimate segment volume and adopts a non-uniform density function that is gender-specific. The goal of this study was to test the accuracy of the new model for estimating the trunk's inertial parameters by comparing it to the more current models used in biomechanical research. Trunk inertial parameters estimated from dual X-ray absorptiometry (DXA) were used as the standard. Twenty-five female and 24 male college-aged participants were recruited for the study. Comparisons of the new model to the accepted models were accomplished by determining the error between the models’ trunk inertial estimates and that from DXA. Results showed that the new model was more accurate across all inertial estimates than the other models. The new model had errors within 6.0% for both genders, whereas the other models had higher average errors ranging from 10% to over 50% and were much more inconsistent between the genders. In addition, there was little consistency in the level of accuracy for the other models when estimating the different inertial parameters. These results suggest that the new model provides more accurate and consistent trunk inertial estimates than the other models for both female and male college-aged individuals. However, similar studies need to be performed using other populations, such as elderly or individuals from a distinct morphology (e.g. obese). In addition, the effect of using different models on the outcome of kinetic parameters, such as joint moments and forces needs to be assessed.     

The development of contact force construction in the dynamic-contact task of cycling [corrected

Purposeful movement requires that an individual produce appropriate joint torques to accelerate segments, and when environmental contact is involved, to develop task-appropriate contact forces. Developmental research has been confined largely to the mastery of unconstrained movement skills (pointing, kicking). The purpose of this study was to study the developmental progression that characterizes the interaction of muscular and non-muscular forces in tasks constrained by contact with the environment. Seven younger children (YC, 6-8 years), 7 older children (OC, 9-11 years) and 7 adults (AD) pedaled an ergometer (80 rpm) at an anthropometrically scaled cycling power. Resultant forces measured at the pedal's surface were decomposed into muscle, inertia and gravity components. Muscle pedal forces were further examined in terms of the underlying lower extremity joint torques and kinematic weights that constitute the muscular component of the pedal force. Data showed children applied muscle forces to the pedal in a significantly different manner compared to adults, and that this was due to the children's lower segmental mass and inertia. The children adjusted the contribution of the proximal joint muscle torques to compensate for reduced contributions to the resultant pedal force by gravitational and inertial components. These data show that smaller segmental mass and inertia limit younger children's ability to construct the dynamic-contact task of cycling in an adult-like form. On the basis of these results, however, the children's response was not "immature". Rather, the results show a task-appropriate adaptation to lower segmental mass and inertia.     

A procedure for estimating the relevant forces in the human knee using a four-bar mechanism

Knee injuries, especially those that affect the cruciate and lateral ligaments, are one of the most serious and frequent pathologies that affect the lower human extremity. Hence, the aim of this study is to develop a dynamic model for the lower extremity capable of estimating forces, forces in the cruciate and collateral ligaments and those normal to the articular cartilage, generated in the knee. The proposed model considers a four-bar mechanism in the knee, a spherical joint in the pelvis and a revolute one in the ankle. The four-bar mechanism is obtained by a synthesis process. The dynamic model includes the inertial properties of the femur, tibia, patella and the foot, the ground reaction force and the most important muscles in the knee. Muscle forces are estimated using an optimisation technique. Results from the application of the model on a real human task are presented.