Force,mass and acceleration

Force, mass and acceleration are everyday words but often used inaccurately. Force is a physical influence, which when applied to an object causes it to accelerate in the direction from which it was applied. Mass is the amount of matter in an object and is expressed in kilograms. Acceleration is the rate of change of velocity of an object in the same straight line of the unbalanced force. When forces become balanced, there is no net force and therefore no movement. Newton’s second law links these three terms and concerns the effect that an unbalanced force has on the motion of an object. It states that the rate of change of velocity of an object is directly proportional to the force applied and takes place in the direction of the force. It is summarized by the equation: Force (N) = mass (kg) × acceleration (m/s²). Thus, an object of constant mass accelerates in proportion to the force applied. Gravity is the variable force of attraction between any two objects. All matter possesses gravitational ‘pull’ towards other matter. The amount of gravity between two objects is dependent on their mass and the distance between their centres. The word ‘weight’ in its proper context refers to the downward vertical force exerted on an object as a result of the earth’s gravity. An object with greater mass is therefore subjected to a greater gravitational force (i.e. it has greater weight).     

Effect of short-distance spondylodesis of the thoracolumbar transition on neighboring facet joints. A biomechanical study

This study was performed to investigate the range of motion and the forces on the facet joints that are neighboured to spondylodesis on thoracolumbar spine. We used a special spine testing device for a continuous application of pure moments in each direction. For measuring the ranges of motion we used a magnetic tracking device for measuring forces on facet joints we chose a direct measuring system of quartz crystal and prepared for investigation of the spine. The biomechanical testing was done on 18 human spinal specimen. We investigated the range of motion and the forces on facet joints in T11/12 and L2/3 segment with a maximal loading of 8 Nm in each direction (flexion, extension, lateral bending and rotation). This was done before and after double level dorsal instrumentation T12-L2 with an internal fixateur. Statistical analysis was performed using the paired t-test and the Wilcoxon test (p < 0.05). After double level instrumentation there were significant larger ranges of motion in flexion and extension and significant larger forces on facet joints in left lateral bending in the T11/12 segment. No significant differences were found in the L2/3 segment. Our findings could be an indication for changing in joints loading. This could be an explanation for early degenerative changes in spinal segments adjacent to spondylodesis. The results confirm the demand of short segment instrumentation and early remove of implants to keep influence as low as possible.     

Similar motion of a hand-held object may trigger nonsimilar grip force adjustments.

The tight coupling between load (L) and grip (G) forces during voluntary manipulation of a hand-held object is well established. The current study is to examine grip-load force coupling when motion of the hand with an object was either self-generated (voluntary) or externally generated. Subjects performed similar cyclic movements of different loads at various frequencies with three types of manipulations: 1) voluntary oscillation, 2) oscillating the right arm via the pulley system by the left leg (self-driven oscillation), and 3) oscillating the arm via the pulley system by another person (other-driven oscillation). During the self-generated movements: 1) the grip forces were larger and 2) grip-load force modulation was more pronounced than in the externally generated movements. The G-L adjustments are not completely determined by the mechanics of object motion; nonmechanical factors related to movement performance, for instance perceptual factors, may affect the G-L coupling.     

GLOBAL ASYMPTOTIC STABILIZATION OF THE SPINNING TOP

We consider the problem of controlling a top to the sleeping motion using two different actuation schemes. For a fixed-base top two actuators are assumed to provide forces at the centre of mass in inertially fixed directions, while for a cart-mounted top two actuators are assumed to apply forces to the cart in inertially fixed directions. The controller for the cart-mounted top is obtained from the controller designed for the fixed-base top using d'Alembert's principle. Both controllers are proved to be globally asymptotically stabilizing. For the uncontrolled fixed-base top, necessary and sufficient conditions for Lyapunov stability of the sleeping motion are derived. For the case in which there is only one force actuator, locally asymptotically stabilizing control laws that drive the fixed-base top to the sleeping motion are also obtained.     

Precision grip function after free toe transfer in children with hypoplastic digits.

Although toe-to-hand transfer has a defined role in the management of congenital hand deformities, it remains unclear how well children integrate the transferred digits into physiological grasping. We analysed fingertip forces in the precision grip of 13 patients when lifting a test object more than three years after free toe transfer for absent or hypoplastic digits. Clinically, most patients showed normal sensibility of transferred digits, but active motion and pinch strength were limited as compared to the normal hand. For the control of fingertip forces, two key features of the normal two-digit opposition grip were seen in all operated hands: adaptation of grip force to object weight and parallel coordination of lift and grip forces. These physiological grasping strategies developed independently of the patients' age at the time of operation, which ranged from one to 13 years. In four patients, we observed increased tangential load forces with the operated hand due to misalignments in the application of fingertips on the grasp surfaces. Such forces lead to increased grip force requirements on both fingers that may overload transferred digits with limited motor function. The need for optimal alignment of the grip axis during toe-transfer surgery is emphasised.     

A commentary from the pioneers on the innovation of the relative motion concept: History,biologic considerations,and anatomic rationale

The relative motion concept is simply recognition of the normal functional anatomic relationships that allow powerful extrinsic muscles, the extensor digitorum communis (EDC) and flexor digitorum profundus (FDP), to vary forces on individual finger joints and function in response to the relative position of adjacent metacarpophalangeal joints (MCPJs) in the hand, one to another. First identified as a cause of complications after surgery, a better understanding now allows us to harness these forces by way of differential metacarpophalangeal joint (MCPJ) positioning using an orthosis. This can reduce undesirable tension and allow immediate controlled active motion while permitting functional use of the hand. Tissue gliding with active motion prevents restrictive scarring, maintains joint mobility and avoids unnecessary limitations and stiffness on normal neighboring structures. The historical development of this concept is shared with explanation of the anatomic and biologic rationale for this approach. Acute and chronic hand conditions that may benefit from better understanding of relative motion are numerous and growing.     

A new halo-vest: rationale, design and biomechanical comparison to standard halo-vest designs

The traditional halo-vest rigidly grips the cranium, but not the torso. Unexpectedly large motion and forces in the cervical spine have been shown by others to be present during halo-vest wear. In an effort to reduce these motions and forces, an experimental vest has been designed. Motion of the vest on the thorax has been measured on four normal volunteers, for each of nine load types, for each of seven commercially available vests as well as the experimental vest. Despite its lighter weight and less cumbersome structure, the experimental vest has the lowest mobility score of all the vests tested.     

Altered loading in the injured knee after ACL rupture

Articular loading is an important factor in the joint degenerative process for individuals with anterior cruciate ligament (ACL) rupture. Evaluation of loading for a population that exhibits neuromuscular compensation for injury requires an approach which can incorporate individual muscle activation strategies in its estimation of muscle forces. The purpose of this study was to evaluate knee joint contact forces for patients with ACL deficiency using an EMG‐driven modeling approach to estimate muscle forces. Thirty athletes with acute, unilateral ACL rupture underwent gait analysis after resolving range of motion, effusion, pain, and obvious gait impairments. Electromyography was recorded bilaterally from 14 lower extremity muscles and input to a musculoskeletal model for estimation of muscle forces and joint contact forces. Gait mechanics were consistent with previous reports for individuals with ACL‐deficiency. Our major finding was that joint loading was altered in the injured limb after acute ACL injury; patients walked with decreased contact force on their injured knee compared to their uninjured knee. Both medial and lateral compartment forces were reduced without a significant change in the distribution of tibiofemoral load between compartments. This is the first study to estimate medial and lateral compartment contact forces in patients with acute ACL rupture using an approach which is sensitive to individual muscle activation patterns. Further work is needed to determine whether this early decreased loading of the injured limb is involved in the development of osteoarthritis in these patients. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 31: 458–464, 2013     

A dynamic object-processing network: metric shape discrimination of dynamic objects by activation of occipitotemporal, parietal, and frontal cortices

Shape perception is important for object recognition. However, behavioral studies have shown that rigid motion also contributes directly to the recognition process, in addition to providing visual cues to shape. Using psychophysics and functional brain imaging, we investigated the neural mechanisms involved in shape and motion processing for dynamic object recognition. Observers discriminated between pairs of rotating novel objects in which the 3-dimensional shape difference between the pair was systematically varied in metric steps. In addition, the objects rotated in either the same or the different direction to determine the effect of task-irrelevant motion on behavior and neural activity. We found that observers' shape discrimination performance increased systematically with shape differences, as did the hemodynamic responses of occipitotemporal, parietal, and frontal regions. Furthermore, responses in occipital regions were only correlated with observers' perceived shape differences. We also found different effects of object motion on shape discrimination across observers, which were reflected in responses of the superior temporal sulcus. These results suggest a network of regions that are involved in the discrimination of metric shape differences for dynamic object recognition.     

Biomechanics of the ankle joint. A perspective on total ankle replacement

The biomechanics of the ankle present a unique set of challenges for arthroplasty surgery. Its biomechanics are not simple. Although the ankle joint may seem like a hinge, it is not in a line perpendicular to the tibia. The axis of rotation does not stay constant during range of motion, despite a relative congruency of this joint. Allowing for rotational forces must be accomplished, while maintaining the stability of the joint and its components. Success of the arthroplasty depends on how successful designs can dissipate these rotational forces, while maintaining the stability of the joint. It is not yet clear from the biomechanical analysis of the normal ankle joint that this dissipation of forces has been accomplished successfully in modern implants, although early results in the semiconstrained designs are encouraging. Careful assessment of long-term follow-up will determine how close the present designs are to mimicking the unique requirements of the arthritic foot and ankle. Further work on the biomechanics of these replacements would be beneficial.     

Cortical area MSTd combines visual cues to represent 3-D self-movement

As arboreal primates move through the jungle, they are immersed in visual motion that they must distinguish from the movement of predators and prey. We recorded dorsal medial superior temporal (MSTd) cortical neuronal responses to visual motion stimuli simulating self-movement and object motion. MSTd neurons encode the heading of simulated self-movement in three-dimensional (3-D) space. 3-D heading responses can be evoked either by the large patterns of visual motion in optic flow or by the visual object motion seen when an observer passes an earth-fixed landmark. Responses to naturalistically combined optic flow and object motion depend on their relative directions: an object moving as part of the optic flow field has little effect on neuronal responses. In contrast, an object moving separately from the optic flow field has large effects, decreasing the amplitude of the population response and shifting the population's heading estimate to match the direction of object motion as the object moves toward central vision. These effects parallel those seen in human heading perception with minimal effects of objects moving with the optic flow and substantial effects of objects violating the optic flow. We conclude that MSTd can contribute to navigation by supporting 3-D heading estimation, potentially switching from optic flow to object cues when a moving object passes in front of the observer.     

The lateral occipital complex subserves the perceptual persistence of motion-defined groupings

How are the bits and pieces of retinal information assembled and integrated to form the coherent objects that we see? One long-established principle is that elements that move as a group are linked together. For instance a fragmented line-drawing of an object, placed on a background of randomly distributed short lines, can be impossible to see. But if the object moves relative to the background, its shape is instantly recognized. Even after the motion stops, the percept of the object persists briefly before it fades into the background of random lines. Where in the brain does the percept of the object persist? Using functional brain imaging, we found that such moving line-drawings activated both motion-sensitive areas (medial temporal complex, MT+) and object-sensitive areas (lateral occipital complex, LOC). However, after the motion stopped only the LOC maintained its activity while the percept endured. Evidently a percept assembled by motion-sensitive areas like MT+ can be stored, at least briefly, in the LOC.     

The role of spinal flexion and extension in changing nerve root compression in disc herniations

Changes in nerve root compression forces with spinal motion were measured on six freshly frozen adult cadaver spine specimens. A model was devised to represent a herniated disc at the L4-5 level. This was done using an anterior approach placing a compression-measuring device through the disc at the L4-5 level and against the L5 root. An accelerometer was used to monitor the range of motion of the spine. Because the compression device was held in a static position, the only variable was the tautness of the nerve root across the tip of the device. By simultaneously monitoring motion and force delivered at the tip of the compression meter placed at the nerve root, we were able to quantitate nerve root tension forces across the tip of the measuring device in relation to spinal motion. The force was measured with controls as well as in flexion and extension. In addition, the force was measured as traction was applied to the L5 root. The amount of compressive force and tension in the nerve root increased with flexion of the spine and decreased with extension of the spine. In conclusion, flexion of the lumbar spine increased the compressive force on the L5 root and extension decreased the compressive force on the L5 root.     

Optimizing femoral component rotation in total knee arthroplasty.

Femoral component rotation is important in total knee arthroplasty to optimize patellofemoral and tibiofemoral kinematics. More recently, the epicondylar axis has been cited as the definitive landmark for femoral component rotation. However, there are few studies to support the validity of this rotational landmark and its effect on the patellofemoral and tibiofemoral articulations. In the current study, a total knee arthroplasty was done in 11 knees from cadavers. The knees were tested with various femoral component rotations from 5 degrees internal rotation to 5 degrees external rotation referenced to the epicondylar axis and to the posterior femoral condyles. Each knee acted as its own internal control. The knees were actively ranged from 0 degrees to 100 degrees by a force on the quadriceps tendon in an Oxford knee simulator. Three-dimensional kinematics of all three components were measured whereas a multiaxial transducer imbedded in the patella measured patellofemoral forces. Femoral component rotation parallel to the epicondylar axis resulted in the most normal patellar tracking and minimized patellofemoral shear forces early in flexion. This optimal rotation also minimized tibiofemoral wear motions. These beneficial effects of femoral rotation were less reproducibly related to the posterior condyles. Rotating the femoral component either internal or external to the epicondylar axis worsened knee function by increasing tibiofemoral wear motion and significantly worsening patellar tracking with increased shear forces early in flexion. Based on the current study, the femoral component should be rotationally aligned parallel to the epicondylar axis to avoid patellofemoral and tibiofemoral complications.     

Giovanni Alfonso Borelli--the father of biomechanics

Giovanni Alfonso Borelli is often described as the father of biomechanics. He was born in Naples in 1608. His De Motu Animalium, published in 1680, extended to biology the rigorous analytical methods developed by Galileo in the field of mechanics. Borelli calculated the forces required for equilibrium in various joints of the human body well before Newton published The Laws of Motion Borelli was the first to understand that the levers of the musculoskeletal system magnify motion rather than force, so that muscles must produce much larger forces than those resisting the motion. Borelli died in Rome on December 31, 1679, but his impressive body of original work helped inspire a great number of future scientists, microscopists, and inventors. The highest honor bestowed by the American Society of Biomechanics is the Giovanni Borelli Award.     

Leakpoint pressures in female stress urinary incontinence

Patient selection is critical to achieving good results in the surgical management of stress urinary incontinence. The evaluation of urethral function in these women is of great importance, since the choice of operative technique often depends on the ability of the urethra to generate adequate resistance to the explusive forces of increased abdominal pressure. The Valsalva leakpoint pressure (VLPP) has been described as an easily performed, reproducible and accurate urodynamic test to assess the patient for the presence of intrinsic sphincter deficiency (ISD). Critical review of the VLPP demonstrates its reproducibility and correlation with other measures of ISD. However, more work needs to be done to identify the truly critical values of VLPP that would help in selecting the most appropriate procedure in surgery for stress incontinence.     

In vitro spinal biomechanics. Experimental methods and apparatus.

The functional units making up the spinal column allow fully six degrees of freedom, but with range of motion constraints due to the associated ligamentous and bony structures. Research into the behavior of the spine requires complex testing methodologies, consequently. Reproducing physiologic behavior in the spine requires the application of suitable forces or displacements through loading frames that do not interfere with the its intrinsic behavior. Monitoring of the outcomes can be done on the basis of force, length or pressure measurements, or combinations thereof. An exploration of testing methodologies and apparatus is presented. The applicability and related overheads, due to data acquisition or analysis, of such methods are discussed. This will provide a means for the researcher to assess the hardware requirements to attain the objectives of their project.     

Direct Numerical Simulation of Multiple Particles Sedimentation at an Intermediate Reynolds Number

In this work the previously developed Lattice Boltzmann-Direct Forcing/ Fictitious Domain (LB-DF/FD) method is adopted to simulate the sedimentation of eight circular particles under gravity at an intermediate Reynolds number of about 248. The particle clustering and the resulting Drafting-Kissing-Tumbling (DKT) motion which takes place for the first time are explored. The effects of initial particle-particle gap on the DKT motion are found significant. In addition, the trajectories of particles are presented under different initial particle-particle gaps, which display totally three kinds of falling patterns provided that no DKT motion takes place, i.e. the concave-down shape, the shape of letter "M" and "in-line" shape. Furthermore, the lateral and vertical hydrodynamic forces on the particles are investigated. It has been found that the value of Strouhal number for all particles is the same which is about 0.157 when initial particle-particle gap is relatively large. The wall effects on falling patterns and particle expansions are examined in the final.     

Shoulder-muscle strength and range of motion following surgical repair of full-thickness rotator-cuff tears

Postoperative measurements of the range of motion and muscle strength of the shoulder and ratings of pain and the ability to perform daily activities were made in fifty-eight patients (sixty-three shoulders) who had a repair of a full-thickness rotator-cuff tear. Postoperatively, the patients had an average of 126 degrees of active flexion of the shoulder and an average of 130 degrees of active abduction. Passive motion averaged 21 degrees more than active motion. The strength of the abductor muscles of the shoulder averaged approximately 86 per cent of normal. Most patients reported marked relief of pain and rated themselves as having mild or no deficits in their ability to perform daily activities. The length of the cuff tear significantly affected the functional results. Short tears (less than 2.5 centimeters) were associated with greater strength and range of motion than were long tears. Fifteen of the nineteen patients who were unable to work preoperatively because of the shoulder returned to work after surgery, but not necessarily to the same type of work that they had done before the onset of the problem with the shoulder.     

Processing shape,motion and three-dimensional shape-from-motion in the human cortex

Shape and motion are complementary visual features and each appears to be processed in unique cortical areas. However, object motion is a powerful cue for the perception of three-dimensional (3-D) shape, implying that the two types of information--motion and form--are well integrated. We conducted a series of fMRI experiments aimed at identifying the brain regions involved in inferring 3-D shape from motion cues. For each subject, we identified regions in occipital-temporal cortex that were activated when perceiving: (i) motion in unstructured random-dot patterns, (ii) 2-D and 3-D line drawing shapes, and (iii) 3-D shapes defined by motion cues (shape-from-motion, SFM). We found closely adjacent areas in the lateral occipital region activated by random motion and line-drawing shapes. In addition, we found that the SFM stimuli produced a greater MRI signal in only one of the areas identified with the random motion and line-drawing stimuli: the superior lateral occipital (SLO) region. High-resolution analysis showed that SFM objects and line drawings were processed in separate but adjacent sub-regions in SLO, suggesting that SLO codes object shape but retains topographic segregation based on shape cues. Expanding the analysis to the entire cortex identified a parietal area that had overlapping activation for both SFM and line drawings and increased MRI signal for 3-D versus 2-D shapes, suggesting this area is important for processing shape information.