Effect of load position on muscle forces, internal loads and stability of the human spine in upright postures

A novel kinematics-based approach coupled with a non-linear finite element model was used to investigate the effect of changes in the load position and posture on muscle activity, internal loads and stability margin of the human spine in upright standing postures. In addition to 397 N gravity, external loads of 195 and 380 N were considered at different lever arms and heights. Muscle forces, internal loads and stability margin substantially increased as loads displaced anteriorly away from the body. Under same load magnitude and location, adopting a kyphotic posture as compared with a lordotic one increased muscle forces, internal loads and stability margin. An increase in the height of a load held at a fixed lever arm substantially diminished system stability thus requiring additional muscle activations to maintain the same margin of stability. Results suggest the importance of the load position and lumbar posture in spinal biomechanics during various manual material handling operations.     

Effect of changes in orientation and position of external loads on trunk muscle activity and kinematics in upright standing

Forces at different heights and orientations are often carried by hands while performing occupational tasks. Trunk muscle activity and spinal loads are likely dependent on not only moments but also the orientation and height of these forces. Here, we measured trunk kinematics and select superficial muscle activity of 12 asymptomatic subjects while supporting forces in hands in upright standing. Magnitude of forces in 5 orientations (−25°, 0°, 25°, 50° and 90°) and 2 heights (20 cm and 40 cm) were adjusted to generate flexion moments of 15, 30 and 45 N m at the L5-S1 disc centre. External forces were of much greater magnitude when applied at lower elevation or oriented upward at 25°. Spinal kinematics remained nearly unchanged in various tasks.Changes in orientation and elevation of external forces substantially influenced the recorded EMG, despite similar trunk posture and identical moments at the L5-S1. Greater EMG activity was overall recorded under larger forces albeit constant moment. Increases in the external moment at the L5-S1 substantially increased EMG in extensor muscles (p < 0.001) but had little effect on abdominals; e.g., mean longissimus EMG for all orientations increased by 38% and 75% as the moment level altered from 15 N m to 30 N m and to 45 N m while that in the rectus abdominus increased only by 2% and 4%, respectively. Under 45 N m moment and as the load orientation altered from 90° to 50°, 25°, 0° and −25°, mean EMG dropped by 3%, 12%, 12% and 1% in back muscles and by 17%, 17%, 19% and 13% in abdominals, respectively. As the load elevation increased from 20 cm to 40 cm, mean EMG under maximum moment decreased by 21% in back muscles and by 17% in abdominals.Due to the lack of EMG recording of deep lumbar muscles, changes in relative shear/compression components and different net moments at cranial discs despite identical moments at the caudal L5-S1 disc, complementary model studies are essential for a better comprehension of neuromuscular strategies in response to alterations in load height and orientation.     

Computational stability of human knee joint at early stance in Gait: Effects of muscle coactivity and anterior cruciate ligament deficiency

As one of the most complex and vulnerable structures of body, the human knee joint should maintain dynamic equilibrium and stability in occupational and recreational activities. The evaluation of its stability and factors affecting it is vital in performance evaluation/enhancement, injury prevention and treatment managements. Knee stability often manifests itself by pain, hypermobility and giving-way sensations and is usually assessed by the passive joint laxity tests. Mechanical stability of both the human knee joint and the lower extremity at early stance periods of gait (0% and 5%) were quantified here for the first time using a hybrid musculoskeletal model of the lower extremity. The roles of muscle coactivity, simulated by setting minimum muscle activation at 0–10% levels and ACL deficiency, simulated by reducing ACL resistance by up to 85%, on the stability margin as well as joint biomechanics (contact/muscle/ligament forces) were investigated. Dynamic stability was analyzed using both linear buckling and perturbation approaches at the final deformed configurations in gait. The knee joint was much more stable at 0% stance than at 5% due to smaller ground reaction and contact forces. Muscle coactivity, when at lower intensities (<3% of its maximum active force), increased dynamic stability margin. Greater minimum activation levels, however, acted as an ineffective strategy to enhance stability. Coactivation also substantially increased muscle forces, joint loads and ACL force and hence the risk of further injury and degeneration. A deficiency in ACL decreases total ACL force (by 31% at 85% reduced stiffness) and the stability margin of the knee joint at the heel strike. It also markedly diminishes forces in lateral hamstrings (by up to 39%) and contact forces on the lateral plateau (by up to 17%). Current work emphasizes the need for quantification of the lower extremity stability margin in gait.     

A computationally efficient strategy to estimate muscle forces in a finite element musculoskeletal model of the lower limb

Concurrent multiscale simulation strategies are required in computational biomechanics to study the interdependence between body scales. However, detailed finite element models rarely include muscle recruitment due to the computational burden of both the finite element method and the optimization strategies widely used to estimate muscle forces. The aim of this study was twofold: first, to develop a computationally efficient muscle force prediction strategy based on proportional-integral-derivative (PID) controllers to track gait and chair rise experimental joint motion with a finite element musculoskeletal model of the lower limb, including a deformable knee representation with 12 degrees of freedom; and, second, to demonstrate that the inclusion of joint-level deformability affects muscle force estimation by using two different knee models and comparing muscle forces between the two solutions. The PID control strategy tracked experimental hip, knee, and ankle flexion/extension with root mean square errors below 1°, and estimated muscle, contact and ligament forces in good agreement with previous results and electromyography signals. Differences up to 11% and 20% in the vasti and biceps femoris forces, respectively, were observed between the two knee models, which might be attributed to a combination of differing joint contact geometry, ligament behavior, joint kinematics, and muscle moment arms. The tracking strategy developed in this study addressed the inevitable tradeoff between computational cost and model detail in musculoskeletal simulations and can be used with finite element musculoskeletal models to efficiently estimate the interdependence between muscle forces and tissue deformation.     

Effect of intervertebral translational flexibilities on estimations of trunk muscle forces,kinematics, loads,and stability

Due to the complexity of the human spinal motion segments, the intervertebral joints are often simulated in the musculoskeletal trunk models as pivots thus allowing no translational degrees of freedom (DOFs). This work aims to investigate, for the first time, the effect of such widely used assumption on trunk muscle forces, spinal loads, kinematics, and stability during a number of static activities. To address this, the shear deformable beam elements used in our nonlinear finite element (OFE) musculoskeletal model of the trunk were either substantially stiffened in translational directions (SFE model) or replaced by hinge joints interconnected through rotational springs (HFE model). Results indicated that ignoring intervertebral translational DOFs had in general low to moderate impact on model predictions. Compared with the OFE model, the SFE and HFE models predicted generally larger L4–L5 and L5–S1 compression and shear loads, especially for tasks with greater trunk angles; differences reached ～15% for the L4–L5 compression, ～36% for the L4–L5 shear and ～18% for the L5–S1 shear loads. Such differences increased, as location of the hinge joints in the HFE model moved from the mid-disc height to either the lower or upper endplates. Stability analyses of these models for some select activities revealed small changes in predicted margin of stability. Model studies dealing exclusively with the estimation of spinal loads and/or stability may, hence with small loss of accuracy, neglect intervertebral translational DOFs at smaller trunk flexion angles for the sake of computational simplicity.     

Partitioning of knee joint internal forces in gait is dictated by the knee adduction angle and not by the knee adduction moment

Medial knee osteoarthritis is a debilitating disease. Surgical and conservative interventions are performed to manage its progression via reduction of load on the medial compartment or equivalently its surrogate measure, the external adduction moment. However, some studies have questioned a correlation between the medial load and adduction moment. Using a musculoskeletal model of the lower extremity driven by kinematics–kinetics of asymptomatic subjects at gait midstance, we aim here to quantify the relative effects of changes in the knee adduction angle versus changes in the adduction moment on the joint response and medial/lateral load partitioning. The reference adduction rotation of 1.6° is altered by ±1.5° to 3.1° and 0.1° or the knee reference adduction moment of 17 N m is varied by ±50% to 25.5 N m and 8.5 N m. Quadriceps, hamstrings and tibiofemoral contact forces substantially increased as adduction angle dropped and diminished as it increased. The medial/lateral ratio of contact forces slightly altered by changes in the adduction moment but a larger adduction rotation hugely increased this ratio from 8.8 to a 90 while in contrast a smaller adduction rotation yielded a more uniform distribution. If the aim in an intervention is to diminish the medial contact force and medial/lateral load ratio, a drop of 1.5° in adduction angle is much more effective (causing respectively 12% and 80% decreases) than a reduction of 50% in the adduction moment (causing respectively 4% and 13% decreases). Substantial role of changes in adduction angle is due to the associated alterations in joint nonlinear passive resistance. These findings explain the poor correlation between knee adduction moment and tibiofemoral compartment loading during gait suggesting that the internal load partitioning is dictated by the joint adduction angle.     

Effects of lumbo-pelvic rhythm on trunk muscle forces and disc loads during forward flexion: A combined musculoskeletal and finite element simulation study

Previous in-vivo studies suggest that the ratio of total lumbar rotation over pelvic rotation (lumbo-pelvic rhythm) during trunk sagittal movement is essential to evaluate spinal loads and discriminate between low back pain and asymptomatic population. Similarly, there is also evidence that the lumbo-pelvic rhythm is key for evaluation of realistic muscle and joint reaction forces and moments predicted by various computational musculoskeletal models. This study investigated the effects of three lumbo-pelvic rhythms defined based on in-vivo measurements on the spinal response during moderate forward flexion (60°) using a combined approach of musculoskeletal modeling of the upper body and finite element model of the lumbosacral spine. The muscle forces and joint loads predicted by the musculoskeletal model, together with the gravitational forces, were applied to the finite element model to compute the disc force and moment, intradiscal pressure, annular fibers strain, and load-sharing. The results revealed that a rhythm with high pelvic rotation and low lumbar flexion involves more global muscles and increases the role of the disc in resisting spinal loads, while its counterpart, with low pelvic rotation, recruits more local muscles and engages the ligaments to lower the disc loads. On the other hand, a normal rhythm that has balanced pelvic and lumbar rotations yields almost equal disc and ligament load-sharing and results in more balanced synergy between global and local muscles. The lumbo-pelvic rhythm has less effect on the intradiscal pressure and annular fibers strain. This work demonstrated that the spinal response during forward flexion is highly dependent on the lumbo-pelvic rhythm. It is therefore, essential to adapt this parameter instead of using the default values in musculoskeletal models for accurate prediction of muscle forces and joint reaction forces and moments. The findings provided by this work are expected to improve knowledge of spinal response during forward flexion, and are clinically relevant towards low back pain treatment and disc injury prevention.     

Effect of stable and unstable load carriage on walking gait variability,dynamic stability and muscle activity of older adults

Load carriage perturbs the neuromuscular system, which can be impaired due to ageing. The ability to counteract perturbations is an indicator of neuromuscular function but if the response is insufficient the risk of falls will increase. However, it is unknown how load carriage affects older adults. Fourteen older adults (65 ± 6 years) attended a single visit during which they performed 4 min of walking in 3 conditions, unloaded, stable backpack load and unstable backpack load. During each walking trial, 3-dimensional kinematics of the lower limb and trunk movements and electromyographic activity of 6 lower limb muscles were recorded. The local dynamic stability (local divergence exponents), joint angle variability and spatio-temporal variability were determined along with muscle activation magnitudes. Medio-lateral dynamic stability was lower (p = 0.018) and step width (p = 0.019) and step width variability (p = 0.015) were greater in unstable load walking and step width variability was greater in stable load walking (p = 0.009) compared to unloaded walking. However, there was no effect on joint angle variability. Unstable load carriage increased activity of the Rectus Femoris (p = 0.001) and Soleus (p = 0.043) and stable load carriage increased Rectus Femoris activity (p = 0.006). These results suggest that loaded walking alters the gait of older adults and that unstable load carriage reduces dynamic stability compared to unloaded walking. This can potentially increase the risk of falls, but also offers the potential to use unstable loads as part of fall prevention programmes.     

A quantitative analysis of elastic,entropic, electrostatic,and osmotic forces within relaxed skinned muscle fibers

The elastic behavior of mechanically skinned skeletal muscle fibers in relaxing solution is modelled using equations developed by Flory (1953) for the elasticity of non-biological polymers. Mechanically, the relaxed skinned fiber is considered to be a semi-crystalline network of inextensible polymer chains, which are periodically cross-linked and which are bathed in an aqueous medium. We consider (1) configurational elastic forces in the network, (2) entropic forces due to mixing of polymer and water, (3) electrostatic forces due to fixed charges on the muscle proteins and mobile charges in the bathing solution, and (4) compressive forces due to large colloids in the bathing solution. Van der Waals forces are not considered since calculations show that they are probably negligible under our conditions. We derive an expression which relates known quantities (ionic strength, osmotic compressive pressure, and fiber width), experimentally estimated quantities (fixed charge density and volume fraction of muscle proteins), and derived quantities (concentration of cross-links and a parameter reflecting the interaction energy between protein and water).The model was tested by comparison with observed changes in skinned fiber width under a variety of experimental conditions which included changes in osmotic compressive pressure, pH, sarcomere length, and ionic strength. Over a wide range of compressive pressure (0–36 atm) the theory predicted the nonlinear relation between fiber width and logarithm of pressure. The direction and magnitude of the decrease in width when pH was decreased to 4 could be modelled asssuming the fixed charge density on the protein network was 0.34 moles of electrons per liter protein, a value in accordance with the estimates of others. The relation between width and sarcomere length over the complete range of compressive pressures could be modelled with the assumption that the number of cross-links increases somewhat with sarcomere length. Changes of width with ionic strength were modelled assuming that increasing salt concentration both increased the electrostatic shielding of fixed charges and decreased the number of cross-links. The decrease of fiber width in 1% glutaraldehyde was modelled by assuming that the concentration of crosslinks increased by some 10%. The theory predicted the order of magnitude but not the detailed shape of the passive tension-length relation which may indicate that, as with non-biological polymers, the theory does not adequately describe the behavior of semi-crystalline networks at high degrees of deformation.In summary, the theory provides a semiquantitative approach to an understanding of the nature and relative magnitudes of the forces underlying the mechanical behavior of relaxed skinned fibers. It indicates, for instance, that when fibers are returned to near their in vivo size with 3% PVP, the forces in order of their importance are: ¦ elastic forces ¦ ¦ entropic forces > ¦ electrostatic forces ¦ ¦ osmotic compressive forces ¦.     

The effect of axial load on the sagittal plane curvature of the upright human spine in vivo

Determining the effect of load carriage on the human spine in vivo is important for determining spinal forces and establishing potential mechanisms of back injury. Previous studies have suggested that the natural curvature of the spine straightens under load, but are based on modelling and external measurements from the surface of the back. In the current study, an upright positional MRI scanner was used to acquire sagittal images of the lumbar and lower thoracic spine of 24 subjects. The subjects were imaged in standing whilst supporting 0, 8 and 16kg of load which was applied axially across the shoulders using an apron. An active shape model of the vertebral bodies from T10 to S1 was created and used to characterise the effect of load. The results from the shape model showed that the behaviour of the average-shaped spine was to straighten slightly. However, the shape model also showed that the effect of load exhibited systematic variation between individuals. Those who had a smaller than average curvature before loading straightened under load, whereas those who had a greater than average curvature before loading showed an increase in curvature under load. The variation in behaviour of differently shaped spines may have further implications for the effects of load in lifting manoeuvres and in understanding the aetiology of back pain.     

Biomechanics of changes in ACL and PCL material properties or prestrains in flexion under muscle force-implications in ligament reconstruction

The effects of changes in cruciate ligament material and prestrain on knee joint biomechanics following ligament reconstruction surgery by a tendon are not adequately known. A 3D nonlinear finite element model of the entire knee joint was used to investigate the joint response at different flexion angles under a quadriceps force while varying ACL and PCL initial strains or material properties. The ACL and PCL forces as well as tibiofemoral contact forces/areas substantially increased with greater ACL or PCL initial strains or stiffness. The patellofemoral contact force slightly increased whereas the tibial extensor moment slightly decreased with tenser or stiffer ACL. Reverse trends were predicted with slacker ACL. Results confirm the hypotheses that changes in the prestrain of one cruciate ligament substantially influence the force in the other cruciate ligament and the entire joint and that the use of the patellar tendon (PT) as a replacement for cruciate ligaments markedly alters the joint biomechanics with trends similar to those predicted when increasing prestrains. Forces in both ACL and PCL ligaments increased as one of them became tenser or stiffer and diminished as it became slacker. These results have important consequences in joint biomechanics following ligament injuries or replacement and tend to recommend the use of grafts with smaller prestrains (i.e. slacker than intact) when using the PT as the replacement material with stiffness greater than that of replaced ligament itself.     

Effect of screw position on load transfer in lumbar pedicle screws: a non-idealized finite element analysis

Angled screw insertion has been advocated to enhance fixation strength during posterior spine fixation. Stresses on a pedicle screw and surrounding vertebral bone with different screw angles were studied by finite element analysis during simulated multidirectional loading. Correlations between screw-specific vertebral geometric parameters and stresses were studied. Angulations in both the sagittal and axial planes affected stresses on the cortical and cancellous bones and the screw. Pedicle screws pointing laterally (vs. straight or medially) in the axial plane during superior screw angulation may be advantageous in terms of reducing the risk of both screw loosening and screw breakage.     

Effect of unilateral load carriage on postures and gait symmetry in ground reaction force during walking

Unilateral load carriage is more hazardous to the musculoskeletal system than bilateral load. The purpose of this study was to examine the effect of such asymmetric carriage on postures and gait symmetry in ground reaction force (GRF) during walking. Kinematics and GRF of 19 adults were recorded while they walked under five load conditions: no load, dumbbell (10 and 20% body weight) held in right and left hand, respectively. After loading, the trunk bent towards the loaded or unloaded side in right- and left-hand trials and under different load weight conditions. The amplitude of trunk bend increased with load, accompanied by decreased stride width, progressively inclined legs towards unloaded side and higher level of asymmetry in medial/lateral GRF (GRFm/l) and free vertical moment GRF (GRFm). The findings indicate the postural adjustment is likely related to the characteristics of load and the task experience and handedness of subject and the unilateral load increases the gait asymmetry in GRFm/l and GRFm.     

Influence of muscle activity on the forces in the femur: An in vivo study

Experiments were performed on two patients with custom-made instrumented massive proximal femoral prostheses implanted after tumour resection. In vivo axial forces transmitted along the prostheses were telemetered during level walking, single- and double-leg stance, and isometric exercises of the hip muscles. These activities varied the lever arms available to the external loads: minimum for double-leg stance and maximum for hip isometric exercises. Kinematic, force plate, EMG and telemetered force data were recorded simultaneously. The force magnification ration (FMR; the ratio of the telemetered axial force to the external force) was calculated. The FMRs ranged from 1.3 (during double-leg stance) to 29.8 (during abductors test), indicating that a major part of the axial force in the long bones is a response to muscle activity, the strength of which depends on the lever arms available to the external loads. From these results, it was shown that the bulk of the bending moment along limbs is transmitted by a combination of tensile forces in muscles and compressive forces in bones, so moments transmitted by the bones are smaller than the limb moments. It was concluded that appropriate simulation of muscle forces is important in experimental or theoretical studies of load transmission along bones.     

Effect of low frequency fatigue on human muscle strength and fatigability during subsequent stimulated activity

Fatiguing contractions of the adductor pollicis muscle were produced by intermittent supramaximal stimulation of the ulnar nerve in a set frequency pattern, in six normal subjects. At the end of an initial fatiguing contraction series, low frequency fatigue (LFF) had been induced and persisted at 15 min of recovery. Stimulated fatiguing activity was then repeated in an identical fashion to the initial series. At high frequencies, declines in force were similar for both series. At low frequencies, declines in force were greater during the second series despite similar changes in compound muscle action potential amplitude. This confirmation that LFF persists during subsequent stimulated activity, and reduces low but not high frequency fatigue resistance, suggests that the impaired endurance of fatigued muscle during voluntary activity primarily results from peripheral changes at low frequency. These findings also have implications for therapeutic electrical stimulation of muscle.     

Effect of eccentric exercise on position sense at the human forearm in different postures.

This is a study of the ability of blindfolded human subjects to match the position of their forearms before and after eccentric exercise. The hypothesis tested was that the sense of effort contributed to forearm position sense. The fall in force after the exercise was predicted to alter the relationship between effort and force and thereby induce position errors. In the arms-in-front posture, subjects had their unsupported reference arm set to one of two angles from the horizontal, 30 or 60 degrees , and they matched its position by voluntary placement of their other arm. Matching errors were compared with a task where the arms were counterweighted, so could be moved in the vertical plane with minimal effort, and where the arms were moved in the horizontal plane. In these latter two tasks, the intention was to test whether removal of an effort sensation from holding the arm against gravity influenced matching performance. It was found that, although absolute errors for counterweighted and horizontal matching were no larger than for unsupported matching, their standard deviations, 6.1 and 6.8 degrees , respectively, were significantly greater than for unsupported matching (4.6 degrees ), indicating more erratic matching. The eccentric exercise led, the next day, to a fall in maximum voluntary muscle torque of >or=15%. This was accompanied by a significant increase in matching errors for the unsupported matching task from 2.7 +/- 0.5 to 0.8 +/- 0.7 degrees but not for counterweighted (1.4 +/- 0.2 to -0.2 degrees +/- 1.1 degrees ) or horizontal matching (-1.3 +/- 0.7 degrees to -1.8 +/- 0.7 degrees ). This, it is postulated, is because the reduced voluntary torque after exercise was accompanied by a greater effort required to support the arms, leading to larger matching errors. However, effort is only able to provide positional information for unsupported matching where gravity plays a role. In gravity-neutral tasks like counterweighted or horizontal matching, a change in the effort-force relationship after exercise leaves matching accuracy unaffected.     

The influence of ligament modelling strategies on the predictive capability of finite element models of the human knee joint

In finite element (FE) models knee ligaments can represented either by a group of one-dimensional springs, or by three-dimensional continuum elements based on segmentations. Continuum models closer approximate the anatomy, and facilitate ligament wrapping, while spring models are computationally less expensive. The mechanical properties of ligaments can be based on literature, or adjusted specifically for the subject. In the current study we investigated the effect of ligament modelling strategy on the predictive capability of FE models of the human knee joint. The effect of literature-based versus specimen-specific optimized material parameters was evaluated. Experiments were performed on three human cadaver knees, which were modelled in FE models with ligaments represented either using springs, or using continuum representations. In spring representation collateral ligaments were each modelled with three and cruciate ligaments with two single-element bundles. Stiffness parameters and pre-strains were optimized based on laxity tests for both approaches. Validation experiments were conducted to evaluate the outcomes of the FE models.Models (both spring and continuum) with subject-specific properties improved the predicted kinematics and contact outcome parameters. Models incorporating literature-based parameters, and particularly the spring models (with the representations implemented in this study), led to relatively high errors in kinematics and contact pressures. Using a continuum modelling approach resulted in more accurate contact outcome variables than the spring representation with two (cruciate ligaments) and three (collateral ligaments) single-element-bundle representations. However, when the prediction of joint kinematics is of main interest, spring ligament models provide a faster option with acceptable outcome.     

Effect of instruction,surface stability,and load intensity on trunk muscle activity

The aim of this study was to assess the effect of verbal instruction, surface stability, and load intensity on trunk muscle activity levels during the free weight squat exercise. Twelve trained males performed a free weight squat under four conditions: (1) standing on stable ground lifting 50% of their 1-repetition maximum (RM), (2) standing on a BOSU balance trainer lifting 50% of their 1-RM, (3) standing on stable ground lifting 75% of their 1-RM, and (4) receiving verbal instructions to activate the trunk muscles followed by lifting 50% of their 1-RM. Surface EMG activity from muscles rectus abdominis (RA), external oblique (EO), transversus abdominis/internal oblique (TA/IO), and erector spinae (ES) were recorded for each condition and normalized for comparisons. Muscles RA, EO, and TA/IO displayed greater peak activity (39–167%) during squats with instructions compared to the other squat conditions (P = 0.04–0.007). Peak EMG activity of muscle ES was greater for the 75% 1-RM condition than squats with instructions or lifting 50% of 1-RM (P = 0.04–0.02). The results indicate that if the goal is to enhance EMG activity of the abdominal muscles during a multi-joint squat exercise then verbal instructions may be more effective than increasing load intensity or lifting on an unstable surface. However, in light of other research, conscious co-activation of the trunk muscles during the squat exercise may lead to spinal instability and hazardous compression forces in the lumbar spine.     

Effect of recording electrode position along a muscle fibre on surface potential power spectrum

Extracellular action potentials produced by a muscle fibre of finite length were calculated for recordings at the skin surface. The sensitivity of power spectra to variations in propagation velocity (ν) and intracellular action potential (IAP) duration (T_in) was studied theoretically. The magnitude and distribution of the spectral power of muscle fibre potentials depend on the electrode longitudinal position. The relative shifts of the spectra in dB induced by variation in ν or T_in hardly depend on the longitudinal position of the electrode. A variation in ν affects only the power spectrum positive slope and the initial part of the high-frequency roll-off and a variation in T_in affects only the remaining part of the high-frequency roll-off. The total spectral amplitude is practically non-sensitive to variations in the wavelength, b = ν.T_in. The total power is sensitive to variations in ν, T_in as well as in b, and its relative changes depend on the electrode longitudinal position. The whole power spectrum is shifted along the frequency axis and mode (F_max), median (F_med) and mean (F_mean) frequencies have practically equal percentage changes only when ν and T_in vary jointly in such a way that the product ν.T_in keeps unchanged.     

The effect of static muscle forces on the fracture strength of the intact distal radius in vitro in response to simulated forward fall impacts

The distal radius fracture (DRF) is a particularly dominant injury of the wrist, commonly resulting from a forward fall on an outstretched hand. In an attempt to reduce the prevalence, costs, and potential long-term pain/deformities associated with this injury, in vivo and in vitro investigations have sought to classify the kinematics and kinetics of DRFs. In vivo forward fall work has identified a preparatory muscle contraction that occurs in the upper extremity prior to peak impact force. The present investigation constitutes the first attempt to systematically determine the effect of static muscle forces on the fracture threshold of the distal radius in vitro. Paired human cadaveric forearm specimens were divided into two groups, one that had no muscle forces applied (i.e., right arms) and the other that had muscle forces applied to ECU, ECRL, FCU and FCR (i.e., left arms), with magnitudes based on peak muscle forces and in vivo lower bound forward fall activation patterns. The specimens were secured in a custom-built pneumatic impact loading device and subjected to incremental impacts at pre-fracture (25 J) and fracture (150  J) levels. Similar fracture forces (6565 (866) N and 8665 (5133) N), impulses (47 (6) Ns and 57 (30) Ns), and energies (152 (38) J and 144 (45) J) were observed for both groups of specimens (p>0.05). Accordingly, it is suggested that, at the magnitudes presently simulated, muscle forces have little effect on the way the distal radius responds to forward fall initiated impact loading.