Knee joint passive stiffness and moment in sagittal and frontal planes markedly increase with compression

Knee joints are subject to large compression forces in daily activities. Due to artefact moments and instability under large compression loads, biomechanical studies impose additional constraints to circumvent the compression position–dependency in response. To quantify the effect of compression on passive knee moment resistance and stiffness, two validated finite element models of the tibiofemoral (TF) joint, one refined with depth-dependent fibril-reinforced cartilage and the other less refined with homogeneous isotropic cartilage, are used. The unconstrained TF joint response in sagittal and frontal planes is investigated at different flexion angles (0°, 15°, 30° and 45°) up to 1800 N compression preloads. The compression is applied at a novel joint mechanical balance point (MBP) identified as a point at which the compression does not cause any coupled rotations in sagittal and frontal planes. The MBP of the unconstrained joint is located at the lateral plateau in small compressions and shifts medially towards the inter-compartmental area at larger compression forces. The compression force substantially increases the joint moment-bearing capacities and instantaneous angular rigidities in both frontal and sagittal planes. The varus–valgus laxities diminish with compression preloads despite concomitant substantial reductions in collateral ligament forces. While the angular rigidity would enhance the joint stability, the augmented passive moment resistance under compression preloads plays a role in supporting external moments and should as such be considered in the knee joint musculoskeletal models.     

The relationship between external knee moments and muscle co-activation in subjects with medial knee osteoarthritis

PurposeExternal knee moments are reliable to measure knee load but it does not take into account muscle activity. Considering that muscle co-activation increases compressive forces at the knee joint, identifying relationships between muscle co-activations and knee joint load would complement the investigation of the knee loading in subjects with knee osteoarthritis. The purpose of this study was to identify relationships between muscle co-activation and external knee moments during walking in subjects with medial knee osteoarthritis.Methods19 controls (11 males, aged 56.6 ± 5, and BMI 25.2 ± 3.3) and 25 subjects with medial knee osteoarthritis (12 males, aged 57.3 ± 5.3, and BMI 28.2 ± 4) were included in this study. Knee adduction and flexion moments, and co-activation (ratios and sums of quadriceps, hamstring, and gastrocnemius) were assessed during walking and compared between groups. The relationship between knee moments and co-activation was investigated in both groups.FindingsSubjects with knee osteoarthritis presented a moderate and strong correlation between co-activation (ratios and sums) and knee moments.InterpretationMuscle co-activation should be used to measure the contribution of quadriceps, hamstring, and gastrocnemius on knee loading. This information would cooperate to develop a more comprehensive approach of knee loading in this population.     

Effect of an additional manual motor task on the maintenance of equilibrium in the frontal and sagittal planes in standing subjects

The postural oscillations of a standing subject during an additional manual motor task consisting in holding a movable ball in the center of a flat box were studied. The movements of the center of pressure (CP) in the frontal and sagittal planes were studied when subjects were standing on a stable rigid support and on a movable unstable support. The effect of the additional motor task on the movement of the CP depended on the stability of the support. When the additional task was performed, the sagittal movements of the CP increased in the case a movable support and did not increase when the support was stable. The additional task decreased the frontal movements of CP in the case of a stable support, and it did not change the frontal movements of CP when the support was unstable. Thus, the performance of an additional motor task led to a reduction of the efficiency of the postural control system in maintaining equilibrium on an unstable support. This decrease may be due to a greater cortical influence on the posture control system in subjects standing on a movable support in comparison with this influence in the case of a stable support.     

Total trunk muscle force and spinal compression are lower in asymmetric moments as compared to pure extension moments.

The aim of the present study was to test the assumption that asymmetric trunk loading requires a higher total muscle force and consequently entails a higher compression forces on the spine as compared to symmetric loading. When the trunk musculature is modelled in sufficient detail, optimisation shows that there is no mechanical necessity for an increase in total muscle force (or compression force) with task asymmetry. A physiologically based optimisation does also not predict an increase in total muscle force or spinal loading with asymmetry. EMG data on 14 trunk muscles collected in eight subjects showed antagonistic coactivity to be present in both conditions. However, estimates of total muscle force based on the EMG were lower when producing an asymmetric moment. In conclusion, producing an asymmetric moment appears to cause slightly lower forces on the lumbosacral joint as compared to a symmetric moment. Only lateral shear forces increase with asymmetry but these remain well below failure levels.     

Changes in intra-abdominal pressure, trunk muscle activation and force during isokinetic lifting and lowering

Intra-abdominal pressure (IAP), force and electromyographic (EMG) activity from the abdominal (intra-muscular) and trunk extensor (surface) muscles were measured in seven male subjects during maximal and sub-maximal sagittal lifting and lowering with straight arms and legs. An isokinetic dynamometer was used to provide five constant velocities (0.12–0.96 m·s^–1) of lifting (pulling against the resistance of the motor) and lowering (resisting the downward pull of the motor). For the maximal efforts, position-specific lowering force was greater than lifting force at each respective velocity. In contrast, corresponding IAPs during lowering were less than those during lifting. Highest mean force occurred during slow lowering (1547 N at 0.24 m·s^–1) while highest IAP occurred during the fastest lifts (17.8 kPa at 0.48–0.96 m·s^–1). Among the abdominal muscles, the highest level of activity and the best correlation to variations in IAP (r=0.970 over velocities) was demonstrated by the transversus abdominis muscle. At each velocity the EMG activity of the primary trunk and hip extensors was less during lowering (eccentric muscle action) than lifting (concentric muscle action) despite higher levels of force (r between –0.896 and –0.851). Sub-maximal efforts resulted in IAP increasing linearly with increasing lifting or lowering force (r=0.918 and 0.882, respectively). However, at any given force IAP was less during lowering than lifting. This difference was negated if force and IAP were expressed relative to their respective lifting and lowering maxima. It appears that the IAP increase primarily accomplished by the activation of the transversus abdominis muscle can have the dual function of stabilising the trunk and reducing compression forces in the lumbar spine via its extensor moment. The neural mechanisms involved in sensing and regulating both IAP and trunk extensor activity in relation to the type of muscle action, velocity and effort during the maximal and sub-maximal loading tasks are unknown.     

Evidence for an exposure-response relationship between trunk flexion and impairments in trunk postural control

Prolonged trunk flexion alters passive and active trunk tissue behaviors, and exposure-response relationships between the magnitude of trunk flexion exposure and changes in these behaviors have been reported. This study assessed whether similar exposure-response relationships exist between such exposures and impairments in trunk postural control. Twelve participants (6 M, 6 F) were exposed to three distinct trunk flexion conditions (and a no-flexion control condition), involving different flexion durations with/without an external load, and which induced differing levels of passive tissue creep. Trunk postural control was assessed prior to and immediately following trunk flexion exposures, and during 10 min of standing recovery, by tracking center of pressure (COP) movements during a seated balance task. All COP-based sway measures increased following each flexion exposure. In the anteroposterior direction, these increases were larger with increasing exposure magnitude, whereas such a relationship was not evident for mediolateral sway measures. All measures were fully recovered following 10 min of standing. The present results provide evidence for an exposure-response relationship between trunk flexion exposures and impairments in trunk postural control; specifically, larger impairments following increased exposures (i.e., longer flexion duration and presence of external load). Such impairments in trunk postural control may result from some combination of reduced passive trunk stiffness and altered/delayed trunk reflex responses, and are generally consistent with prior evidence of exposure-dependent alterations in trunk mechanical and neuromuscular behaviors assessed using positional trunk perturbations. Such evidence suggests potential mechanistic pathways through which trunk flexion exposures may contribute to low-back injury risk.     

Controlling for out-of-plane lumbar moments during unidirectional trunk efforts: Learning and reliability issues related to trunk muscle activation estimates

To assess the electromyographic (EMG) activation of trunk muscle during exertions performed in one primary plane (sagittal, frontal, transverse), we previously proposed a protocol allowing minimizing out-of-plane efforts (coupled moments – CMs) with the use of a static dynamometer combined with a visual feedback system. The aims of this study were to go further by testing motor learning and reliability issues related to such a protocol. Three identical sessions were conducted, where maximal voluntary contractions and submaximal ramp contractions were performed in six different directions while standing in the dynamometer. Two feedback conditions were tested, the simple 1D-feedback in the primary plane and the full 3D-feedback in all planes simultaneously. Surface EMG signals were collected from back and abdominal muscles and EMG amplitude and CMs were computed during the ramp contractions. Providing a 3D feedback to minimize CMs did not improve EMG reliability or in other words, did not reduce the within-subject variability. Providing three assessment days had practically no effect (no learning) on CMs and EMG variables. Overall, the reliability of EMG was at best moderate. However, although this limits its use on an individual basis, it still allows within- and between-group comparisons for research applications.     

Torque and force production during shoulder external rotation: differences between transverse and sagittal planes

In joints with 3 degrees of freedom, such as the shoulder joint, the association of different movements results in changes in the behavior of the moment arm of the muscles. The capacity of torque production for the same movement can be changed when movements take place in a different plane. The objective of this study is to quantify differences between torque production and resultant force estimated during the shoulder external rotation in two movement planes: the transverse and sagittal planes (with 90 degrees of shoulder abduction). Eight individuals were evaluated using an isokinetic dynamometer and an eletrogoniometer for movements in the transverse plane and six individuals for movements in the sagittal plane. The results showed that the execution of the external rotation in the sagittal plane allowed greater torque magnitudes and resultant force compared with those in the transverse plane, probably owing to a prestretching of infraspinatus and teres minor.     

The relationship between maximal lifting capacity and maximum acceptable lift in strength-based soldiering tasks

Psychophysical assessments, such as the maximum acceptable lift, have been used to establish worker capability and set safe load limits for manual handling tasks in occupational settings. However, in military settings, in which task demand is set and capable workers must be selected, subjective measurements are inadequate, and maximal capacity testing must be used to assess lifting capability. The aim of this study was to establish and compare the relationship between maximal lifting capacity and a self-determined tolerable lifting limit, maximum acceptable lift, across a range of military-relevant lifting tasks. Seventy male soldiers (age 23.7 ± 6.1 years) from the Australian Army performed 7 strength-based lifting tasks to determine their maximum lifting capacity and maximum acceptable lift. Comparisons were performed to identify maximum acceptable lift relative to maximum lifting capacity for each individual task. Linear regression was used to identify the relationship across all tasks when the data were pooled. Strong correlations existed between all 7 lifting tasks (rrange = 0.87-0.96, p < 0.05). No differences were found in maximum acceptable lift relative to maximum lifting capacity across all tasks (p = 0.46). When data were pooled, maximum acceptable lift was equal to 84 ± 8% of the maximum lifting capacity. This study is the first to illustrate the strong and consistent relationship between maximum lifting capacity and maximum acceptable lift for multiple single lifting tasks. The relationship developed between these indices may be used to help assess self-selected manual handling capability through occupationally relevant maximal performance tests.     

The effects of core muscle activation on dynamic trunk position and knee abduction moments: Implications for ACL injury

Anterior cruciate ligament (ACL) injury is one of the most common serious lower-extremity injuries experienced by athletes participating in field and court sports and often occurs during a sudden change in direction or pivot. Both lateral trunk positioning during cutting and peak external knee abduction moments have been associated with ACL injury risk, though it is not known how core muscle activation influences these variables. In this study, the association between core muscle pre-activation and trunk position as well as the association between core muscle pre-activation and peak knee abduction moment during an unanticipated run-to-cut maneuver were investigated in 46 uninjured individuals. Average co-contraction indices and percent differences between muscle pairs were calculated prior to initial contact for internal obliques, external obliques, and L5 extensors using surface electromyography. Outside tilt of the trunk was defined as positive when the trunk was angled away from the cutting direction. No significant associations were found between pre-activations of core muscles and outside tilt of the trunk. Greater average co-contraction index of the L5 extensors was associated with greater peak knee abduction moment (p=0.0107). Increased co-contraction of the L5 extensors before foot contact could influence peak knee abduction moment by stiffening the spine, limiting sagittal plane trunk flexion (a motion pattern previously linked to ACL injury risk) and upper body kinetic energy absorption by the core during weight acceptance.     

In-vivo motion characteristics of lumbar vertebrae in sagittal and transverse planes

Lumbar vertebrae are complicated in structure and function. The purpose of this study was to investigate the in-vivo motion characteristics of different portions of the lumbar vertebrae during functional activities. Motion of L2, L3 and L4 was reproduced using a combined dual fluoroscopic and MR imaging technique during flexion–extension and left–right twisting of the trunk. The ranges of motion (ROM) of the proximal vertebra with respect to the distal one at 3 representative locations: the center of the vertebral body, the center of the spinal canal and the tip of the spinous process were measured. Centers of rotation (COR) of the vertebrae were then determined by calculation of the points of zero motion in 2D sagittal and transverse planes. During flexion–extension, the center of the vertebral body moved less than 0.6 mm, while the tip of the spinous process moved less than 7.5 mm in the sagittal plane. The CORs of both L23 (L2 with respect to L3) and L34 were located inside the vertebral body, at a distance about one-third the length of the vertebral body from the posterior edge. During left–right twisting, the center of the vertebral body moved less than 1.0 mm, while the tip of the spinous process moved less than 1.6 mm in the transverse plane. The CORs of both L23 and L34 were located approximately 30 mm anterior to the front edge of the vertebral body. The results of this study may be used to define the ideal locations for surgical placement of the disc prosthesis, thus help improve the prosthesis design and surgical treatment of various pathological conditions.     

The relationship between nerves and smooth muscle cells in the rat iris

Summary The dilatator muscle cells form short projections into the stroma of the iris. Close to these projections run several nerve bundles. The unmyelinated axons often show enlargements (varicosities) containing mitochondria and vesicles. Several of the varicosities are partly denuded of the Schwann cell and are covered only by a basement membrane. The varicosities are then separated from the muscle cells only by basement membranes and a 0.1–1 stromal space. The ultrastructure of the iris dilatator muscle thus also fits the view that the autonomic ground plexus with its varicosities forms the real innervation apparatus.The smallest space between axon and muscle has a width of 700–900 Å and is cemented with basement membrane material. It is suggested that the main function of these contact sites is not to transmit a nerve impulse but to anchor the nerves to their effector organ.This study has been supported by grants from the Swedish State Research Council (U 267) and the United States Public Health Service (N B 2854-04).     

The relationship between nerves and smooth muscle cells in the rat iris

Summary The sphincter muscle in the rat iris forms irregular strands in the stroma. Bundles of unmyelinated axons run among the muscle cells. After sympathetic denervation some axons degenerate. This should indicate that sympathetic and parasympathetic nerves are present in the same nerve net. The parasympathetic axons possess varicosities, that is, enlargements containing mitochondria and synaptic vesicles. These varicosities show a similar structural relationship to the muscle cells as do the varicosities of sympathetic nerves. No obvious ultrastructural difference is observed between the sympathetic and parasympathetic varicosities.This study has been supported by research grants (U267 and Y247) from the Swedish Medical Research Council and by a Public Health Service Research Grant (NB05236-01) from the National Institute of Neurological Diseases and Blindness.     

The relationship between migration and chondrogenic potential of trunk neural crest cells in Ambystoma mexicanum

Based on results of transplantation experiments, it has long been believed that trunk neural crest cells are incapable of chondrogenesis. When pigmented trunk neural crest cells of Ambystoma mexicanum are transplanted to cranial levels of albino (a/a) embryos, the graft cells ultimately produce ectopic fins, but are incapable of following the chondrogenic cranial neural crest pathways. Therefore, heterotopic transplantation does not expose these cells to the same environment experienced by cranial neural crest cells, and is neither an adequate nor a sufficient test of chondrogenic potential. However, in vitro culture of trunk neural crest cells with pharyngeal endoderm does provide a direct test of chondrogenic ability. That cartilage does not form under these conditions demonstrates conclusively that trunk neural crest cells possess no chondrogenic potential.     

Comparison between muscle activation measured by electromyography and muscle thickness measured using ultrasonography for effective muscle assessment

In this study, we aimed to compare the intrarater reliability and validity of muscle thickness measured using ultrasonography (US) and muscle activity via electromyography (EMG) during manual muscle testing (MMT) of the external oblique (EO) and lumbar multifidus (MF) muscles. The study subjects were 30 healthy individuals who underwent MMT at different grades. EMG was used to measure the muscle activity in terms of ratio to maximum voluntary contraction (MVC) and root mean square (RMS) metrics. US was used to measure the raw muscle thickness, the ratio of muscle thickness at MVC, and the ratio of muscle thickness at rest. One examiner performed measurements on each subject in 3 trials. The intrarater reliabilities of the % MVC RMS and raw RMS metrics for EMG and the % MVC thickness metrics for US were excellent (ICC = 0.81–0.98). There was a significant difference between all the grades measured using the % MVC thickness metric (p < 0.01). Further, this % MVC thickness metric of US showed a significantly higher correlation with the EMG measurement methods than with the others (r = 0.51–0.61). Our findings suggest that the % MVC thickness determined by US was the most sensitive of all methods for assessing the MMT grade.     

The mechanical role of the trunk and lower extremities in a seated weight-moving task in the sagittal plane

A two-dimensional, sagittally-symmetric biomechanical model was developed to analyze the joint moments required to stabilize the trunk in a seated, dynamic, weight-moving task. Kinematic and reaction force data were measured while subjects moved a hand-held weight (0-4 kgf) at shoulder level to and fro at 1 Hz. These data were then used for model input and validation purposes. A second, simpler model was used to simulate how joint loads varied with weight held, trunk inclination, and movement frequency. The results for this seated task demonstrate a) significant trunk, hip, knee, and ankle joint moments (37, 13, 4, 13 percent of maximum strength values, respectively) were required, b) considerable intersubject differences in mean joint moments (more than 66 percent) were found, which primarily were due to subtle differences in body segment kinematics and lower extremities use, and c) the important role of the lower extremities in stabilizing the trunk in the seated posture.