Model and in vivo studies on human trunk load partitioning and stability in isometric forward flexions

To resolve the trunk redundancy to determine muscle forces, spinal loads, and stability margin in isometric forward flexion tasks, combined in vivo-numerical model studies was undertaken. It was hypothesized that the passive resistance of both the ligamentous spine and the trunk musculature plays a crucial role in equilibrium and stability of the system. Fifteen healthy males performed free isometric trunk flexions of approximately 40 degrees and approximately 65 degrees +/- loads in hands while kinematics by skin markers and EMG activity of trunk muscles by surface electrodes were measured. A novel kinematics-based approach along with a nonlinear finite element model were iteratively used to calculate muscle forces and internal loads under prescribed measured postures and loads considered in vivo. Stability margin was investigated using nonlinear, linear buckling, and perturbation analyses under various postures, loads and alterations in ligamentous stiffness. Flexion postures significantly increased activity in extensor muscles when compared with standing postures while no significant change was detected in between flexed postures. Compression at the L5-S1 substantially increased from 570 and 771 N in upright posture, respectively, for +/-180 N, to 1912 and 3308 N at approximately 40 degrees flexion, and furthermore to 2332 and 3850 N at approximately 65 degrees flexion. Passive ligamentous/muscle components resisted up to 77% of the net moment. In flexion postures, the spinal stability substantially improved due both to greater passive stiffness and extensor muscle activities so that, under 180 N, no muscle stiffness was required to maintain stability. The co-activity of abdominal muscles and the muscle stiffness were of lesser concern to maintain stability in forward flexion tasks as compared with upright tasks. An injury to the passive system, on one hand, required a substantial compensatory increase in active muscle forces which further increased passive loads and, hence, the risk of injury and fatigue. On the other hand, it deteriorated the system stability which in turn could require greater additional muscle activation. This chain of events would place the entire trunk active-passive system at higher risks of injury, fatigue and instability.     

Angle- and gender-specific quadriceps femoris muscle recruitment and knee extensor torque

The objectives were to examine knee angle-, and gender-specific knee extensor torque output and quadriceps femoris (QF) muscle recruitment during maximal effort, voluntary contractions. Fourteen young adult men and 15 young adult women performed three isometric maximal voluntary contractions (MVC), in a random order, with the knee at 0 degrees (terminal extension), 10 degrees, 30 degrees, 50 degrees, 70 degrees, and 90 degrees flexion. Knee extensor peak torque (PT), and average torque (AT) were expressed in absolute (N m), relative (N m kg(-1)) and allometric-modeled (N m kg(-n)) units. Vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscle EMG signals were full-wave rectified and integrated over the middle 3 s of each contraction, averaged over the three trials at each knee angle, and normalized to the activity recorded at 0 degrees. Muscle recruitment efficiency was calculated as the ratio of the normalized EMG of each muscle to the allometric-modeled average torque (normalized to the values at 0 degrees flexion), and expressed as a percent. Men generated significantly greater knee extensor PT and AT than women in absolute, relative and allometric-modeled units. Absolute and relative PT and AT were significantly highest at 70 degrees, while allometric-modeled values were observed to increase significantly across knee joint angles 10-90 degrees. VM EMG was significantly greater than the VL and RF muscles across all angles, and followed a similar pattern to absolute knee extensor torque. Recruitment efficiency improved across knee joint angles 10-90 degrees and was highest for the VL muscle. VM recruitment efficiency improved more than the VL and RF muscles across 70-90 degrees flexion. The findings demonstrate angle-, and gender-specific responses of knee extensor torque to maximal-effort contractions, while superficial QF muscle recruitment was most efficient at 90 degrees, and less dependent on gender.     

The utility of an empirically derived co-activation ratio for muscle force prediction through optimization

Biomechanical optimization models that apply efficiency-based objective functions often underestimate or negate antagonist co-activation. Co-activation assists movement control, joint stabilization and limb stiffness and should be carefully incorporated into models. The purposes of this study were to mathematically describe co-activation relationships between elbow flexors and extensors during isometric exertions at varying intensity levels and postures, and secondly, to apply these co-activation relationships as constraints in an optimization muscle force prediction model of the elbow and assess changes in predictions made while including these constraints. Sixteen individuals performed 72 isometric exertions while holding a load in their right hand. Surface EMG was recorded from elbow flexors and extensors. A co-activation index provided a relative measure of flexor contribution to total activation about the elbow. Parsimonious models of co-activation during flexion and extension exertions were developed and added as constraints to a muscle force prediction model to enforce co-activation. Three different PCSA data sets were used. Elbow co-activation was sensitive to changes in posture and load. During flexion exertions the elbow flexors were activated about 75% MVC (this amount varied according to elbow angle, shoulder flexion and abduction angles, and load). During extension exertions the elbow flexors were activated about 11% MVC (this amount varied according to elbow angle, shoulder flexion angle and load). The larger PCSA values appeared to be more representative of the subject pool. Inclusion of these co-activation constraints improved the model predictions, bringing them closer to the empirically measured activation levels.     

Torso flexion modulates stiffness and reflex response.

Neuromuscular factors that contribute to spinal stability include trunk stiffness from passive and active tissues as well as active feedback from reflex response in the paraspinal muscles. Trunk flexion postures are a recognized risk factor for occupational low-back pain and may influence these stabilizing control factors. Sixteen healthy adult subjects participated in an experiment to record trunk stiffness and paraspinal muscle reflex gain during voluntary isometric trunk extension exertions. The protocol was designed to achieve trunk flexion without concomitant influences of external gravitational moment, i.e., decouple the effects of trunk flexion posture from trunk moment. Systems identification analyses identified reflex gain by quantifying the relation between applied force disturbances and time-dependent EMG response in the lumbar paraspinal muscles. Trunk stiffness was characterized from a second order model describing the dynamic relation between the force disturbances versus the kinematic response of the torso. Trunk stiffness increased significantly with flexion angle and exertion level. This was attributed to passive tissue contributions to stiffness. Reflex gain declined significantly with trunk flexion angle but increased with exertion level. These trends were attributed to correlated changes in baseline EMG recruitment in the lumbar paraspinal muscles. Female subjects demonstrated greater reflex gain than males and the decline in reflex gain with flexion angle was greater in females than in males. Results reveal that torso flexion influences neuromuscular factors that control spinal stability and suggest that posture may contribute to the risk of instability injury.     

Co-activation alters the linear versus non-linear impression of the EMG-torque relationship of trunk muscles

The use of electromyographic signals in the modeling of muscle forces and joint loads requires an assumption of the relationship between EMG and muscle force. This relationship has been studied for the trunk musculature and been shown to be predominantly non-linear, with more EMG producing less torque output at higher levels of activation. However, agonist-antagonist muscle co-activation is often substantial during trunk exertions, yet has not been adequately accounted for in determining such relationships. The purpose of this study was to revisit the EMG-moment relationship of the trunk recognizing the additional moment requirements necessitated due to antagonist muscle activity. Eight participants generated a series of isometric ramped trunk flexor and extensor moment contractions. EMG was recorded from 14 torso muscles, and the externally resisted moment was calculated. Agonist muscle moments (either flexor or extensor) were estimated from an anatomically detailed biomechanical model of the spine and fit to: the externally calculated moment alone; the externally calculated moment combined with the antagonist muscle moment. When antagonist activity was ignored, the EMG-moment relationship was found to be non-linear, similar to previous work. However, when accounting for the additional muscle torque generated by the antagonist muscle groups, the relationships became, in three of the four conditions, more linear. Therefore, it was concluded that antagonist muscle co-activation must be included when determining the EMG-moment relationship of trunk muscles and that previous impressions of non-linear EMG-force relationships should be revisited.     

Lumbar back muscle activity of helicopter pilots and whole-body vibration

Several studies have attributed the prevalence of low back pain (LBP) in helicopter pilots mainly to poor posture in-flight and whole-body vibration, with the latter hypothesis particularly related to a cyclic response of the erector spine (ES) muscle to vibration. This work aims to determine if helicopter vibration and the pilot's normal posture during flight have significant effects on the electromyogram (EMG) of the ES muscle. The bilateral surface EMG of the ES muscle at the L3 level was collected in 10 young pilots before and during a short flight in UH-50 helicopters. The vibration was monitored by a triaxial accelerometer fixed to the pilots' seat. Prior to the flight, the EMG was recorded for relaxed seated and standing postures with 0 degrees (P0) and 35 degrees (P35) of trunk flexion. The effect of the posture during the flight was tested by comparing left and right EMG (normalized with respect to P35). The in-flight muscle stress was evaluated by histograms of EMG activity, and compared to P0 values. Only one pilot in ten showed significant (p<0.05) correlation between the vibration and the EMG over cycles of vibration, and no consistent causal effect was found. The pilots' posture did not show significant asymmetric muscular activity, and low EMG levels were observed during most of the duration of the flight. The results do not provide evidence that LBP in helicopter pilots is caused by ES muscle stress in the conditions studied.     

An empirical approach to characterizing trunk muscle coactivation using simulation input modeling techniques

Accurately describing trunk muscle coactivation is fundamental to quantifying the spine reaction forces that occur during lifting tasks and has been the focus of a great deal of research in the spine biomechanics literature. One limitation of previous approaches has been a lack of consideration given to the variability in these coactivation strategies. The research presented in this paper is an empirical approach to quantifying and modeling trunk muscle coactivation using simulation input modeling techniques. Electromyographic (EMG) data were collected from 28 human subjects as they performed controlled trunk extension exertions. These exertions included isokinetic (10 and 45°/s) and constant acceleration (50°/s/s) trunk extensions in symmetric and asymmetric (30°) postures at two levels of trunk extension moment (30 and 80 Nm). The EMG data were collected from the right and left pairs of the erector spinae, latissimus dorsi, rectus abdominis, external obliques and internal obliques. Each subject performed nine repetitions of each combination of independent variables. The data collected during these trials were used to develop marginal distributions of trunk muscle activity as well as a 10×10 correlation matrix that described how the muscles cooperated to produce these extension torques. These elements were then combined to generate multivariate distributions describing the coactivation of the trunk musculature. An analysis of these distributions revealed that increases in extension moment, extension velocity and sagittal flexion angle created increases in both the mean and the variance of the distributions of the muscular response, while increases in the rate of trunk extension acceleration decreased both the mean and variance of the distributions of activity across all muscles considered. Increases in trunk asymmetry created a decrease in mean of the ipsi–lateral erector spinae and an increase in the mean of all other muscles considered, but there was little change in the variance of these distributions as a function of asymmetry.     

Influence of knee angle and individual flexibility on the flexion-relaxation response of the low back musculature.

In many occupational settings (e.g. agriculture and construction) workers are asked to maintain static flexed postures of the low back for extended periods of time. Recent research indicates that the resulting strain in the viscoelastic, ligamentous tissues may have a deleterious effect on the stability of the spine and the normal reflex response of spinal tissues. The purpose of this study was to evaluate the previously described flexion-relaxation response in terms of the interactive effect of trunk flexion angle (30 degrees, 50 degrees, 70 degrees, 90 degrees ), knee flexion angle (0 degrees (straight knees), 20 degrees, 40 degrees ) and individual flexibiliteky (low, medium, and high). These conditions were tested under two levels of loading: no load (just supporting the weight of the torso) and trunk extension moment equal to 50% of the subject's posture-specific maximum voluntary trunk extension capacity. Surface electromyographic (EMG) data were collected from the multifidus, the longissimus, the iliocostalis, the vastus medialis, the rectus femoris, the vastus lateralis, the biceps femoris, and the gastrocnemius-soleus group from a sample of eight male participants as they performed isometric weight holding tasks in the postures defined by the combinations of trunk angle and knee angle. The results of this study showed that knee angle did have a significant effect on the lumbar extensor muscle activity but only consistently at the 90 degrees trunk angle. Participant flexibility showed a consistent trend of decreasing lumbar extensor muscle activity with decreased flexibility across all trunk angle values. Most interesting was the interactive response of flexibility and knee angle, wherein the flexibility of the participant influenced the trunk angles at which the knee flexion angle affected the flexion-relaxation response. Highly flexible subjects showed an effect of knee angle on the flexion-relaxation response only at the 90 degrees trunk angle; subjects in the medium flexibility category showed a similar response in both the 70 degrees and 90 degrees trunk angles; subject in the low flexibility group showed no knee angle effect on the flexion-relaxation response. Overall the results confirm previous results with regard to the contribution of the passive tissues to the overall trunk extension moment but also show that the tension in the bi-articular biceps femoris, which was influenced by knee flexion angle and flexibility, affects the ratio of active extensor moment contributions of the lumbar extensor musculature to passive extensor moment contributions from the muscular and ligamentous tissues. The results of this study provide empirical data describing this complicated, interactive response.     

The effects of interelectrode distance on electromyographic amplitude and mean power frequency during isokinetic and isometric muscle actions of the biceps brachii.

The purpose of this study was to examine the effects of interelectrode distance (IED) on the absolute and normalized electromyographic (EMG) amplitude and mean power frequency (MPF) versus isokinetic and isometric torque relationships for the biceps brachii muscle. Ten adults [mean+/-SD age=22.0+/-3.4 years] performed submaximal to maximal, isokinetic and isometric muscle actions of the dominant forearm flexors. Following determination of isokinetic peak torque (PT) and the isometric maximum voluntary contraction (MVC), the subjects performed randomly ordered, submaximal step muscle actions in 10% increments from 10% to 90% PT and MVC. Surface EMG signals were recorded simultaneously from bipolar electrode arrangements placed over the biceps brachii muscle with IEDs of 20, 40, and 60mm. Absolute and normalized EMG amplitude (muVrms and %max) increased linearly with torque during the isokinetic and isometric muscle actions (r(2) range=0.988-0.998), but there were no significant changes for absolute or normalized EMG MPF (Hz or %max) from 10% to 100% PT and MVC. In some cases, there were significant (p<0.05) differences among the three IED arrangements for absolute EMG amplitude and MPF values, but not for the normalized values. These findings suggested that for the biceps brachii muscle, IEDs between 20 and 60mm resulted in similar patterns for the EMG amplitude or MPF versus dynamic and isometric torque relationships. Furthermore, unlike the absolute EMG amplitude and MPF values, the normalized EMG data were not influenced by changes in IED between 20 and 60mm. Thus, normalized EMG data can be compared among previous studies that have utilized different IED arrangements.     

Trunk muscle coactivation in preparation for sudden load.

Biomechanical stability of the lumbar spine is an important factor in the etiology and control of low-back disorders. A principle component of biomechanical stability is the musculoskeletal stiffening generated by preparatory muscle coactivation. The goal of this investigation was to quantify preparatory behavior, evaluating trunk muscle activity immediately prior to sudden trunk flexion loading during static extension tasks compared to activity observed when subjects were informed no sudden load would occur. Coactive excitation was also examined as a function of fatigue and gender. Results demonstrated increased extensor muscle and flexor muscle coactivation following static fatiguing exertions, potentially compensating for reduced trunk stiffness. Female subjects produced greater flexor antagonism than in the males. No difference in the preparatory coactive muscle recruitment patterns were observed when subjects were expecting a sudden flexion load compared to recruitment patterns observed in similar static postures when subjects were informed no sudden load would be applied. This indicates the neuromuscular system relies greatly on response characteristics for the maintenance of stability in dynamic loading conditions.     

Trunk strength,muscle activity and spinal loads in maximum isometric flexion and extension exertions: A combined in vivo-computational study

Determination of the trunk maximum voluntary exertion moment capacity and associated internal spinal forces could serve in proper selection of workers for specific occupational task requirements, injury prevention and treatment outcome evaluations. Maximum isometric trunk exertion moments in flexion and extension along with surface EMG of select trunk muscles are measured in 12 asymptomatic subjects. Subsequently and under individualized measured harness-subject forces, kinematics and upper trunk gravity, an iterative kinematics-driven finite element model is used to compute muscle forces and spinal loads in 4 of these subjects. Different co-activity and intra-abdominal pressure levels are simulated. Results indicate significantly larger maximal resistant moments and spinal compression/shear forces in extension exertions than flexion exertions. The agonist trunk muscles reach their maximum force generation (saturation) to greater extent in extension exertions compared to flexion exertions. Local lumbar extensor muscles are highly active in extension exertions and generate most of the internal spinal forces. The maximum exertion attempts produce large spinal compression and shear loads that increase with the antagonist co-activity level but decrease with the intra-abdominal pressure. Intra-abdominal pressure decreases agonist muscle forces in extension exertions but generally increase them in flexion exertions.     

EMG-angle relationship of the hamstring muscles during maximum knee flexion.

The aim of the present study was to investigate the EMG-joint angle relationship during voluntary contraction with maximum effort and the differences in activity among three hamstring muscles during knee flexion. Ten healthy subjects performed maximum voluntary isometric and isokinetic knee flexion. The isometric tests were performed for 5 s at knee angles of 60 and 90 degrees. The isokinetic test, which consisted of knee flexion from 0 to 120 degrees in the prone position, was performed at an angular velocity of 30 degrees /s (0.523 rad/s). The knee flexion torque was measured using a KIN-COM isokinetic dynamometer. The individual EMG activity of the hamstrings, i.e. the semitendinosus, semimembranosus, long head of the biceps femoris and short head of the biceps femoris muscles, was detected using a bipolar fine wire electrode. With isometric testing, the knee flexion torque at 60 degrees knee flexion was greater than that at 90 degrees. The mean peak isokinetic torque occurred from 15 to 30 degrees knee flexion angle and then the torque decreased as the knee angle increased (p<0.01). The EMG activity of the hamstring muscles varied with the change in knee flexion angle except for the short head of the biceps femoris muscle under isometric condition. With isometric contraction, the integrated EMGs of the semitendinosus and semimembranosus muscles at a knee flexion angle of 60 degrees were significantly lower than that at 90 degrees. During maximum isokinetic contraction, the integrated EMGs of the semitendinosus, semimembranosus and short head of the biceps femoris muscles increased significantly as the knee angle increased from 0 to 105 degrees of knee flexion (p<0.05). On the other hand, the integrated EMG of the long head of the biceps femoris muscle at a knee angle of 60 degrees was significantly greater than that at 90 degrees knee flexion with isometric testing (p<0.01). During maximum isokinetic contraction, the integrated EMG was the greatest at a knee angle between 15 and 30 degrees, and then significantly decreased as the knee angle increased from 30 to 120 degrees (p<0.01). These results demonstrate that the EMG activity of hamstring muscles during maximum isometric and isokinetic knee flexion varies with change in muscle length or joint angle, and that the activity of the long head of the biceps femoris muscle differs considerably from the other three heads of hamstrings.     

The relationship between flexibility and EMG activity pattern of the erector spinae muscles during trunk flexion–extension

BackgroundMovements in the lumbar spine, including flexion and extension are governed by a complex neuromuscular system involving both active and passive units. Several biomechanical and clinical studies have shown the myoelectric activity reduction of the lumbar extensor muscles (flexion–relaxation phenomenon) during lumbar flexion from the upright standing posture. The relationship between flexibility and EMG activity pattern of the erector spinae during dynamic trunk flexion–extension task has not yet been completely discovered.ObjectiveThe purpose of this study was to investigate the relationship between general and lumbar spine flexibility and EMG activity pattern of the erector spinae during the trunk flexion–extension task.MethodsThirty healthy female college students were recruited in this study. General and lumbar spine flexibilities were measured by toe-touch and modified schober tests, respectively. During trunk flexion–extension, the surface electromyography (EMG) from the lumbar erector spinae muscles as well as flexion angles of the trunk, hip, lumbar spine and lumbar curvature were simultaneously recorded using a digital camera. The angle at which muscle activity diminished during flexion and initiated during extension was determined and subjected to linear regression analysis to detect the relationship between flexibility and EMG activity pattern of the erector spinae during trunk flexion–extension.ResultsDuring flexion, the erector spinae muscles in individuals with higher toe-touch scores were relaxed in larger trunk and hip angles and reactivated earlier during extension according to these angles (P < 0.001) while in individuals with higher modified schober scores this muscle group was relaxed later and reactivated sooner in accordance with lumbar angle and curvature (P < 0.05). Toe-touch test were significantly correlated with trunk and hip angles while modified schober test showed a significant correlation with lumbar angle and curvature variables.ConclusionThe findings of this study indicate that flexibility plays an important role in trunk muscular recruitment pattern and the strategy of the CNS to provide stability. The results reinforce the possible role of flexibility alterations as a contributing factor to the motor control impairments. This study also shows that flexibility changes behavior is not unique among different regions of the body.     

Kinematic, kinetic and EMG patterns during downward squatting.

The aim of this study was to investigate the kinematic, kinetic, and electromyographic pattern before, during and after downward squatting when the trunk movement is restricted in the sagittal plane. Eight healthy subjects performed downward squatting at two different positions, semisquatting (40 degrees knee flexion) and half squatting (70 degrees knee flexion). Electromyographic responses of the vastus medialis oblique, vastus medialis longus, rectus femoris, vastus lateralis, biceps femoris, semitendineous, gastrocnemius lateralis, and tibialis anterior were recorded. The kinematics of the major joints were reconstructed using an optoelectronic system. The center of pressure (COP) was obtained using data collected from one force plate, and the ankle and knee joint torques were calculated using inverse dynamics. In the upright position there were small changes in the COP and in the knee and ankle joint torques. The tibialis anterior provoked the disruption of this upright position initiating the squat. During the acceleration phase of the squat the COP moved posteriorly, the knee joint torque remained in flexion and there was no measurable muscle activation. As the body went into the deceleration phase, the knee joint torque increased towards extension with major muscle activities being observed in the four heads of the quadriceps. Understanding these kinematic, kinetic and EMG strategies before, during and after the squat is expected to be beneficial to practitioners for utilizing squatting as a task for improving motor function.     

The effect of resistance level and stability demands on recruitment patterns and internal loading of spine in dynamic flexion and extension using a simple trunk model

The effects of external resistance on the recruitment of trunk muscles in sagittal movements and the coactivation mechanism to maintain spinal stability were investigated using a simple computational model of iso-resistive spine sagittal movements. Neural excitation of muscles was attained based on inverse dynamics approach along with a stability-based optimisation. The trunk flexion and extension movements between 60° flexion and the upright posture against various resistance levels were simulated. Incorporation of the stability constraint in the optimisation algorithm required higher antagonistic activities for all resistance levels mostly close to the upright position. Extension movements showed higher coactivation with higher resistance, whereas flexion movements demonstrated lower coactivation indicating a greater stability demand in backward extension movements against higher resistance at the neighbourhood of the upright posture. Optimal extension profiles based on minimum jerk, work and power had distinct kinematics profiles which led to recruitment patterns with different timing and amplitude of activation.     

The effect of resistance level and stability demands on recruitment patterns and internal loading of spine in dynamic flexion and extension using a simple trunk model

The effects of external resistance on the recruitment of trunk muscles in sagittal movements and the coactivation mechanism to maintain spinal stability were investigated using a simple computational model of iso-resistive spine sagittal movements. Neural excitation of muscles was attained based on inverse dynamics approach along with a stability-based optimisation. The trunk flexion and extension movements between 60° flexion and the upright posture against various resistance levels were simulated. Incorporation of the stability constraint in the optimisation algorithm required higher antagonistic activities for all resistance levels mostly close to the upright position. Extension movements showed higher coactivation with higher resistance, whereas flexion movements demonstrated lower coactivation indicating a greater stability demand in backward extension movements against higher resistance at the neighbourhood of the upright posture. Optimal extension profiles based on minimum jerk, work and power had distinct kinematics profiles which led to recruitment patterns with different timing and amplitude of activation.     

Activity of deep abdominal muscles increases during submaximal flexion and extension efforts but antagonist co-contraction remains unchanged

Lumbo-pelvic stability relies, amongst other factors, on co-contraction of the lumbo-pelvic muscles. However, during submaximal trunk flexion and extension efforts, co-contraction of antagonist muscles is limited. It was predicted that activity of the deeper lumbo-pelvic muscles that are often excluded from analysis (transversus abdominis (TrA) and the deep fascicles of multifidus (DM)), would increase with load in each direction. In eleven healthy subjects, electromyographic activity (EMG) was recorded from eight trunk muscles using surface and fine-wire electrodes. Subjects performed isometric flexion and extension efforts to submaximal loads of 50, 100, 150 and 200 N and a maximal voluntary contraction (MVC). Loading tasks were then repeated in trials in which subjects knew that the load would release at an unpredictable time. Compared to the starting position, EMG of all muscles, except DM, increased during MVC efforts in both directions. During the flexion and extension submaximal tasks, there was no increased co-contraction of antagonist muscles. However, TrA EMG increased in both directions. In the unpredictable trials, EMG of all lumbo-pelvic muscles except TrA was decreased. These findings provide further support for a contribution of TrA to lumbo-pelvic stability. In submaximal tasks, TrA activation may enhance stability as a strategy to improve trunk stiffness without requiring a concurrent increase in activity of the larger torque producing trunk muscles.     

Investigation of the Differential Contributions of Superficial and Deep Muscles on Cervical Spinal Loads with Changing Head Postures

Cervical spinal loads are predominately influenced by activities of cervical muscles. However, the coordination between deep and superficial muscles and their influence on the spinal loads is not well understood. This study aims to document the changes of cervical spinal loads and the differential contributions of superficial and deep muscles with varying head postures. Electromyography (EMG) of cervical muscles from seventeen healthy adults were measured during maximal isometric exertions for lateral flexion (at 10°, 20° and terminal position) as well as flexion/extension (at 10°, 20°, 30°, and terminal position) neck postures. An EMG-assisted optimization approach was used to estimate the muscle forces and subsequent spinal loads. The results showed that compressive and anterior-posterior shear loads increased significantly with neck flexion. In particular, deep muscle forces increased significantly with increasing flexion. It was also determined that in all different static head postures, the deep muscle forces were greater than those of the superficial muscle forces, however, such pattern was reversed during peak efforts where greater superficial muscle forces were identified with increasing angle of inclination. In summary, the identification of significantly increased spinal loads associated with increased deep muscle activation during flexion postures, implies higher risks in predisposing the neck to occupationally related disorders. The results also explicitly supported that deep muscles play a greater role in maintaining stable head postures where superficial muscles are responsible for peak exertions and reinforcing the spinal stability at terminal head postures. This study provided quantitative data of normal cervical spinal loads and revealed motor control strategies in coordinating the superficial and deep muscles during physical tasks.     

The influence of task and angle on torque production and muscle activity at the elbow

This study investigated the effect of changing internal mechanical variables and task demands on muscle activity and torque production during high effort isometric contractions of the elbow flexors. The effect of adding a 50% maximal voluntary contraction (MVC) of supination to an MVC of elbow flexion was studied over a range of angles from 30° to 110° of elbow flexion. Surface EMGs were recorded from the biceps brachii (BIC), brachioradialis (BRAD) and triceps brachii (TRI) of 10 healthy subjects. BIC was the only muscle to show a consistent trend of increasing root mean square (rms) EMG with increasing elbow flexion angle. BIC activity also remained constant or increased with the addition of the supination task at all angles. In contrast, BRAD showed decreased activity when supination was added at several angular positions. Maximal flexion torque was reduced when the second task of submaximal supination was added. This torque reduction was statistically significant at all angles except 70° and appeared related to the decreased contribution from BRAD. In a small subset of subjects, however, BRAD activity did not decrease when the second degrees of freedom (df) task was added. These subjects exhibited higher flexion torques averaged over task than the majority, at all angles except 30°. These data support the view that internal mechanical considerations influence the manner in which the central nervous system (CNS) distributes activity to muscular synergists in response to altered task demands. Further, subject-specific patterns exist which must be recognized if these findings are to be incorporated in training or rehabilitation programmes.     

The biomechanical model of the long finger extensor mechanism and its parametric identification

The extensor mechanism of the finger is a structure transmitting the forces from several muscles to the finger joints. Force transmission in the extensor mechanism is usually modeled by equations with constant coefficients which are determined experimentally only for finger extension posture. However, the coefficient values change with finger flexion because of the extensor mechanism deformation. This induces inaccurate results for any other finger postures. We proposed a biomechanical model of the extensor mechanism represented as elastic strings. The model includes the main tendons and ligaments. The parametric identification of the model in extension posture was performed to match the distribution of the forces among the tendons to experimental data. The parametrized model was used to simulate three degrees of flexion. Furthermore, the ability of the model to reproduce how the force distribution in simulated extensor mechanism changes according to the muscle forces was also demonstrated. The proposed model could be used to simulate the extensor mechanism for any physiological finger posture for which the coefficients involved in the equations are unknown.