Achilles tendon stress is more sensitive to subject-specific geometry than subject-specific material properties: A finite element analysis

This study used subject-specific measures of three-dimensional (3D) free Achilles tendon geometry in conjunction with a finite element method to investigate the effect of variation in subject-specific geometry and subject-specific material properties on tendon stress during submaximal isometric loading. Achilles tendons of eight participants (Aged 25–35 years) were scanned with freehand 3D ultrasound at rest and during a 70% maximum voluntary isometric contraction. Ultrasound images were segmented, volume rendered and transformed into subject-specific 3D finite element meshes. The mean (±SD) lengths, volumes and cross-sectional areas of the tendons at rest were 62 ± 13 mm, 3617 ± 984 mm³ and 58 ± 11 mm² respectively. The measured tendon strain at 70% MVIC was 5.9 ± 1.3%. Subject-specific material properties were obtained using an optimisation approach that minimised the difference between measured and modelled longitudinal free tendon strain. Generic geometry was represented by the average mesh and generic material properties were taken from the literature. Local stresses were subsequently computed for combinations of subject-specific and generic geometry and material properties. For a given geometry, changing from generic to subject-specific material properties had little effect on the stress distribution in the tendon. In contrast, changing from generic to subject-specific geometry had a 26-fold greater effect on tendon stress distribution. Overall, these findings indicate that the stress distribution experienced by the living free Achilles tendon of a young and healthy population during voluntary loading are more sensitive to variation in tendon geometry than variation in tendon material properties.     

The effects of muscle architectural change with a pre-motion silent period on the subsequent muscular output during rapid voluntary movement

This study aimed to determine the characteristics of the in vivo behaviour of human muscle architecture during a pre-motion silent period (PMSP) using ultrasonography. Subjects were requested to perform rapid knee extension with vertical jumping. Electromyographic signals were recorded from the vastus lateralis (VL), vastus medialis, and biceps femoris muscles. Ultrasonic images were recorded from the VL. We found that the cross point between the fascicle and deep aponeurosis in the VL moved to the distal side before the rapid vertical jumps with PMSP. This cross point movement with PMSP was of low amplitude (mean: 1.0 ± 0.3 mm) and velocity (22.2 ± 6.1 mm/s). The amplitude and velocity of the cross point movement were significantly positively related to the angular peak velocity of knee extensor during rapid vertical jumping in trials with PMSP. These results suggest that although low levels of pre-movement muscle architectural change with PMSP may be the result of muscle relaxation behaviour rather than the result of muscle stretching behaviour, this pre-movement effect can influence subsequent muscular performance during a rapid voluntary movement. PMSP may allow pre-movement muscle architectural change to generate a better muscular condition to increase neural activation during the subsequent rapid voluntary contraction.     

Spatial distribution of surface action potentials generated by individual motor units in the human biceps brachii muscle

This study analyses the spatial distribution of individual motor unit potentials (MUPs) over the skin surface and the influence of motor unit depth and recording configuration on this distribution. Multichannel surface (13 × 5 electrode grid) and intramuscular (wire electrodes inserted with needles of lengths 15 and 25 mm) electromyographic (EMG) signals were concurrently recorded with monopolar derivations from the biceps brachii muscle of 10 healthy subjects during 60-s isometric contractions at 20% of the maximum torque. Multichannel monopolar MUPs of the target motor unit were obtained by spike-triggered averaging of the surface EMG. Amplitude and frequency characteristics of monopolar and bipolar MUPs were calculated for locations along the fibers’ direction (longitudinal), and along the direction perpendicular (transverse) to the fibers. In the longitudinal direction, monopolar and bipolar MUPs exhibited marked amplitude changes that extended for 16–32 mm and 16–24 mm over the innervation and tendon zones, respectively. The variation of monopolar and bipolar MUP characteristics was not symmetrical about the innervation zone. Motor unit depth had a considerable influence on the relative longitudinal variation of amplitude for monopolar MUPs, but not for bipolar MUPs. The transverse extension of bipolar MUPs ranged between 24 and 32 mm, whereas that of monopolar MUPs ranged between 72 and 96 mm. The mean power spectral frequency of surface MUPs was highly dependent on the transverse electrode location but not on depth. This study provides a basis for the interpretation of the contribution of individual motor units to the interference surface EMG signal.     

Estimation of optimal matching position for orthogonal kV setup images and minimal setup margins in radiotherapy of whole breast and lymph node areas

AimThe aim was to find an optimal setup image matching position and minimal setup margins to maximally spare the organs at risk in breast radiotherapy.BackgroundRadiotherapy of breast cancer is a routine task but has many challenges. We investigated residual position errors in whole breast radiotherapy when orthogonal setup images were matched to different bony landmarks.Materials and methodsA total of 1111 orthogonal setup image pairs and tangential field images were analyzed retrospectively for 50 consecutive patients. Residual errors in the treatment field images were determined by matching the orthogonal setup images to the vertebrae, sternum, ribs and their compromises. The most important region was the chest wall as it is crucial for the dose delivered to the heart and the ipsilateral lung. Inter-observer variation in online image matching was investigated.ResultsThe best general image matching position was the compromise of the vertebrae, ribs and sternum, while the worst position was the vertebrae alone (p ≤ 0.03). The setup margins required for the chest wall varied from 4.3 mm to 5.5 mm in the lung direction while in the superior–inferior (SI) direction the margins varied from 5.1 mm to 7.6 mm. The inter-observer variation increased the minimal margins by approximately 1 mm. The margin of the lymph node areas should be at least 4.8 mm.ConclusionsSetup margins can be reduced by proper selection of a matching position for the orthogonal setup images. To retain the minimal margins sufficient, systematic error of the chest wall should not exceed 4 mm in the tangential field image.     

A simple but reliable method for measuring 3D Achilles tendon moment arm geometry from a single,static magnetic resonance scan

Current methods for measuring in vivo 3D muscle-tendon moment arms generally rely on the acquisition of magnetic resonance imaging (MRI) scans at multiple joint angles. However, for patients with musculoskeletal pathologies such as fixed contractures, moving a joint through its full range of motion is not always feasible. The purpose of this research was to develop a simple, but reliable in vivo 3D Achilles tendon moment arm (ATMA) technique from a single static MRI scan. To accomplish this, for nine healthy adults (5 males, 4 females), the geometry of a cylinder was fit to the 3D form of the talus dome, which was used to estimate the talocrural flexion/extension axis, and a fifth-order polynomial fit to the line of action of the Achilles tendon. The single static scan in vivo 3D ATMA estimates were compared to estimates obtained from the same subjects at the same ankle joint angles using a previously validated 3D dynamic MRI based in vivo ATMA measurement technique. The ATMA estimates from the single scan in vivo 3D method (52.5 mm ± 5.6) were in excellent agreement (ICC = 0.912) to the validated in vivo 3D method (51.5 mm ± 5.1). These data show reliable in vivo 3D ATMA can be obtained from a single MRI scan for healthy adult populations. The single scan, in vivo 3D ATMA technique provides researchers with a simple, but reliable method for obtaining subject-specific ATMAs for musculoskeletal modelling purposes.     

Continuous thickness measurement of rectus femoris muscle in ultrasound image sequences: A completely automated approach

Muscle thickness is one of the most widely used parameters for quantifying muscle function in both diagnosis and rehabilitation assessment. Ultrasound imaging has been frequently used to non-invasively study the thickness of human muscles as a reliable method. However, the measurement is traditionally conducted by manual digitization of reference points at the superior and inferior muscle fascias, thus it is subjective and time-consuming. In this paper, a novel method is proposed to detect the muscle thickness automatically. The superficial and deep fascias of a muscle are detected by line detection algorithm at the first ultrasound frame, and the image regions of interest (ROI) for the fascias are subsequently located and tracked by optical flow technique. The muscle thickness is geometrically obtained based on the location of the fascias for each frame. Six ultrasound sequences (250 frames in each sequence) are used to evaluate this method. The correlation coefficient of the detection results between the proposed method and manual method is 0.95 ± 0.01, and the difference is ?0.05 ± 0.22 mm. The linear regression of the total 1500 detections show that a good linear correlation between the results of the two methods is obtained (R² = 0.981). The automated method proposed here provides an accurate, high repeatable and efficient approach for estimating fascicle thickness during human motion, thus justifying its application in biological sciences.     

Statistical shape modeling predicts patellar bone geometry to enable stereo-radiographic kinematic tracking

Complications in the patellofemoral (PF) joint of patients with total knee replacements include patellar subluxation and dislocation, and remain a cause for revision. Kinematic measurements to assess these complications and evaluate implant designs require the accuracy of dynamic stereo-radiographic systems with 3D-2D registration techniques. While tibiofemoral kinematics are typically derived by tracking metallic implants, PF kinematic measurements are difficult as the patellar implant is radiotransparent and a representation of the resected patella bone requires either pre-surgical imaging and precise implant placement or post-surgical imaging. Statistical shape models (SSMs), used to characterize anatomic variation, provide an alternative means to obtain the representation of the resected patella for use in kinematic tracking. Using a virtual platform of a stereo-radiographic system, the objectives of this study were to evaluate the ability of an SSM to predict subject-specific 3D implanted patellar geometries from simulated 2D image profiles, and to formulate an effective data collection methodology for PF kinematics by considering accuracy for a variety of patient pose scenarios. An SSM of the patella was developed for 50 subjects and a leave-one-out approach compared SSM-predicted and actual geometries; average 3D errors were 0.45 ± 0.07 mm (mean ± standard deviation), which is comparable to the accuracy of traditional segmentation. Further, initial imaging of the patella in five unique stereo radiographic perspectives yielded the most accurate representation. The ability to predict the remaining patellar geometry of the implanted PF joint with radiographic images and SSM, instead of CT, can reduce radiation exposure and streamline in vivo kinematic evaluations.     

Changes in muscle geometry during forearm pronation and supination and their relationships to EMG cross-correlation measures

Geometric artifact may alter the amplitude and frequency of the electromyography (EMG) signal. Artifacts include the changing geometry of muscles with respect to electrodes and potential crosstalk from adjacent muscles. This study addresses: (1) the geometrical relationships between common electrode placement sites for six forearm muscles, (2) the geometrical change of forearm muscles in pronation and supination, and (3) the relationships between EMG cross-correlation and muscle geometry. EMG and ultrasonography images were recorded during pronation, supination, and neutral forearm postures while exerting 20% maximum grip strength. Proportions of anatomical structures were then calculated for 15 mm, 20 mm, and 25 mm radial pick-up zone distances, representing greater than 90% of observed myoelectrical signal energy. We found that guidelines for electrode placements were supported and no single posture maximized the proportion of the target muscle detected. Secondly, other muscles were present in the most conservative 15 mm radius pick up zone; it is unlikely that surface EMG can completely differentiate between forearm muscle activities. Thirdly, forearm orientation did not appear to be an important factor in changing the geometrical relationships between surface electrodes and the muscles studied, and fourthly, certain muscles (e.g., FDS) may be more vulnerable to EMG crosstalk.     

The role of biceps brachii and brachioradialis for the control of elbow flexion and extension movements

How do synergistic muscles interact, when their contraction aims at stabilizing and fine-tuning a movement, which is induced by the antagonistic muscle? The aim of the study was to analyze the interaction of biceps and brachioradialis during fine-tuning control tasks in comparison to load bearing ones. The surface electromyogram of biceps, brachioradialis and triceps were examined in 15 healthy subjects in dynamic flexion and extension movements with different combinations of contraction levels, joint angles and angular velocities. The measurements were conducted in two configurations, where the torque due to an external load opposes the rotational direction of the elbow flexion (load bearing tasks) or the elbow extension (fine-tuning tasks).Whereas during load bearing control tasks, similar muscular activation of biceps and brachioradialis was observed for all joint angles, angular velocities and external loads, during fine-tuning control tasks a significant difference of the muscular activation of both flexors was observed for 1 kg, F(3.639, 47.305) = 2.864, p = 0.037, and 5 kg of external load, F(1.570, 21.976) = 6.834, p = 0.008.The results confirm the synergistic muscular activation of both flexors during load bearing tasks, but suggest different control strategies for both flexors when they comprise a fine-tuning control task.     

Estimating severity of sideways fall using a generic multi linear regression model based on kinematic input variables

Many research groups have studied fall impact mechanics to understand how fall severity can be reduced to prevent hip fractures. Yet, direct impact force measurements with force plates are restricted to a very limited repertoire of experimental falls. The purpose of this study was to develop a generic model for estimating hip impact forces (i.e. fall severity) in in vivo sideways falls without the use of force plates.Twelve experienced judokas performed sideways Martial Arts (MA) and Block (‘natural’) falls on a force plate, both with and without a mat on top. Data were analyzed to determine the hip impact force and to derive 11 selected (subject-specific and kinematic) variables. Falls from kneeling height were used to perform a stepwise regression procedure to assess the effects of these input variables and build the model.The final model includes four input variables, involving one subject-specific measure and three kinematic variables: maximum upper body deceleration, body mass, shoulder angle at the instant of ‘maximum impact’ and maximum hip deceleration. The results showed that estimated and measured hip impact forces were linearly related (explained variances ranging from 46 to 63%). Hip impact forces of MA falls onto the mat from a standing position (3650 ± 916 N) estimated by the final model were comparable with measured values (3698 ± 689 N), even though these data were not used for training the model. In conclusion, a generic linear regression model was developed that enables the assessment of fall severity through kinematic measures of sideways falls, without using force plates.     

Voluntary activation of trapezius measured with twitch interpolation

This study investigated the feasibility of measuring voluntary activation of the trapezius muscle with twitch interpolation. Subjects (n = 8) lifted the right shoulder or both shoulders against fixed force transducers. Stimulation of the accessory nerve in the neck was used to evoke maximal twitches in right trapezius. The twitch-like increments in force (superimposed twitches) evoked during different strength voluntary contractions were linearly related to voluntary force (r = ?0.82 to ?0.99). Hence, voluntary activation could be quantified by twitch interpolation with this stimulus. Comparison of unilateral and bilateral MVCs showed that maximal voluntary force was greater in unilateral than bilateral efforts (92.7 ± 2.9% and 82.3 ± 5.8% MVC, respectively) but voluntary activation was similar (88.6 ± 9.6% and 91.7 ± 5.2%). Trapezius is commonly affected in work-related musculoskeletal disorders. Measurement of voluntary activation will be a useful technique to demonstrate whether the reduced maximal voluntary force reported in such disorders is due to muscular or neural factors.     

Effect of image quality on correlation modeling error using a fiducial marker in a gimbaled linear accelerator

The purpose of this study was to investigate the effect of image quality under various imaging parameters (60, 70, 80, 90, 100, 110, and 120 kV at 200 mA and 10 ms/63, 80, 100, 160, 200, 250, and 320 mA at 120 kV and 10 ms) and the diameter of the fiducial marker (0.25, 0.50, 0.75, and 1.10 mm) on the correlation modeling error for dynamic tumor tracking (DTT) in the Vero4DRT system. Each fiducial marker was inserted into the center of the 30 × 30 × 10 cm³ water-equivalent phantom. A programmable respiratory motion table was used to simulate breathing-induced organ motion, with an amplitude of ±20 mm and a breathing cycle of 4 s. The correlation modeling error was calculated from the absolute difference between the detected and predicted target positions in the cranio-caudal direction. The image contrast of the fiducial marker was enhanced with increasing kV and mA. Increasing the diameter of the fiducial marker also enhanced the image contrast. Correlation-modeling error does not depend on the image quality and fiducial marker diameter. A lower kV setting did not generate a 4D model due to poor image contrast. All fiducial marker diameters were identified as good candidates for DTT in the Vero4DRT system.     

Fingertip forces and completion time for index finger and thumb touchscreen gestures

Users actuate touchscreen computers by applying forces with their fingers to the touchscreen, although the amount and direction of the force is unknown. Our aim was to characterize the magnitude, direction and impulse of the force applied during single finger (tapping and sliding in four directions) and two finger gestures (stretch and pinch). Thirteen subjects performed repeated trials of each gesture. Mean(±SD) resultant force was 0.50(0.09) N for tap, 0.79(0.32) N to 1.18(0.47) N for sliding gestures, 1.47(0.63) N for pinch and 2.05(1.13) N for stretch. Mean resultant force was significantly less (p < 0.04) for tap than for all gestures except slide right. The direction of force application was more vertical for the two-finger gestures as compared to the single- finger gestures. Tap was the fastest gesture to complete at 133(83) ms, followed by slide right at 421(181) ms. On average, participants took the longest to complete the stretch gesture at 920(398) ms. Overall, there are differences in forces, force direction, and completion times among touchscreen gestures that could be used to estimate musculoskeletal exposure and help forge guidelines to reduce risk of musculoskeletal injury.     

Quantitative detection of cartilage surfaces and ligament geometry of the wrist using an imaging cryomicrotome system

Biomechanical models may aid in improving diagnosis and treatment of wrist joint disorders. As input, geometrical information is required for model development. Previous studies acquired some elements of the average wrist joint geometry. However, there is a close geometric functional match between articulating surfaces and ligament geometry. Therefore, biomechanical models need to be fed with the geometric data of individual joints. This study is aimed at acquiring geometric data of cartilage surfaces and ligaments from individual wrist joints by using a cryomicrotome imaging system and the evaluation of inter- and intra-observer variability of the data.The 3D geometry of 30 cartilage surfaces and 15 ligaments in three cadaver wrists was manually detected and quantitatively reconstructed. The inter- and intra-observer variability of the cartilage surface detection was 0.14 and 0.19 mm, respectively. For the position of the radius attachment of the dorsal radiocarpal ligament (DRC), the observer variations were 0.12 and 0.65 mm, for intra-/inter-observer, respectively. For the DRC attachment on the triquetrum, the observer variations were 0.22 and 1.19 mm.Anatomic reconstruction from 3D cryomicrotome images offer a method to obtain unique geometry data of the entire wrist joint for modeling purposes.     

Voluntary activation of the trapezius muscle in cases with neck/shoulder pain compared to healthy controls

Subjects reporting neck/shoulder pain have been shown to generate less force during maximal voluntary isometric contractions (MVC) of the shoulder muscles compared to healthy controls. This has been suggested to be caused by a pain-related decrease in voluntary activation (VA) rather than lack of muscle mass. The aim of the present study was to investigate VA of the trapezius muscle during MVCs in subjects with and without neck/shoulder pain by use of the twitch interpolation technique.Ten cases suffering from pain and ten age and gender matched, healthy controls were included in the study. Upper trapezius muscle thickness was measured using ultrasonography and pain intensity was measured on a 100 mm visual analog scale (VAS). VA was calculated from five maximal muscle activation attempts. Superimposed stimuli were delivered to the accessory nerve at peak force and during a 2% MVC following the maximal contraction.Presented as mean ± SD for cases and controls, respectively: VAS; 16.0 ± 14.4 mm and 2.1 ± 4.1 mm (P = 0.004), MVC; 545 ± 161 N and 664 ± 195 N (P = 0.016), upper trapezius muscle thickness; 10.9 ± 1.9 mm and 10.4 ± 1.5 mm (P = 0.20), VA; 93.6 ± 14.2% and 96.3 ± 6.0% (P = 0.29).In spite of significantly eight-fold higher pain intensity and ∼20% lower MVC for cases compared to controls, no difference was found in VA. Possible explanations for the reduction in MVC could be differences in co-activation of antagonists and synergists as well as muscle quality.     

Experimental validation of finite element predicted bone strain in the human metatarsal

Metatarsal stress fracture is a common injury observed in athletes and military personnel. Mechanical fatigue is believed to play an important role in the etiology of stress fracture, which is highly dependent on the resulting bone strain from the applied load. The purpose of this study was to validate a subject-specific finite element (FE) modeling routine for bone strain prediction in the human metatarsal. Strain gauge measurements were performed on 33 metatarsals from seven human cadaveric feet subject to cantilever bending, and subject-specific FE models were generated from computed tomography images. Material properties for the FE models were assigned using a published density-modulus relationship as well as density-modulus relationships developed from optimization techniques. The optimized relationships were developed with a ‘training set’ of metatarsals (n = 17) and cross-validated with a ‘test set’ (n = 16). The published and optimized density elasticity equations provided FE-predicted strains that were highly correlated with experimental measurements for both the training (r² ≥ 0.95) and test (r² ≥ 0.94) sets; however, the optimized equations reduced the maximum error by 10% to 20% relative to the published equation, and resulted in an X = Y type of relationship between experimental measurements and FE predictions. Using a separate optimized density-modulus equation for trabecular and cortical bone did not improve strain predictions when compared to a single equation that spanned the entire bone density range. We believe that the FE models with optimized material property assignment have a level of accuracy necessary to investigate potential interventions to minimize metatarsal strain in an effort to prevent the occurrence of stress fracture.     

A combined passive and active musculoskeletal model study to estimate L4-L5 load sharing

A number of geometrically-detailed passive finite element (FE) models of the lumbar spine have been developed and validated under in vitro loading conditions. These models are devoid of muscles and thus cannot be directly used to simulate in vivo loading conditions acting on the lumbar joint structures or spinal implants. Gravity loads and muscle forces estimated by a trunk musculoskeletal (MS) model under twelve static activities were applied to a passive FE model of the L4-L5 segment to estimate load sharing among the joint structures (disc, ligaments, and facets) under simulated in vivo loading conditions. An equivalent follower (FL), that generates IDP equal to that generated by muscle forces, was computed in each task. Results indicated that under in vivo loading conditions, the passive FE model predicted intradiscal pressures (IDPs) that closely matched those measured under the simulated tasks (R² = 0.98 and root-mean-squared-error, RMSE = 0.18 MPa). The calculated equivalent FL compared well with the resultant force of all muscle forces and gravity loads acting on the L4-L5 segment (R² = 0.99 and RMSE = 58 N). Therefore, as an alternative approach to represent in vivo loading conditions in passive FE model studies, this FL can be estimated by available in-house or commercial MS models. In clinical applications and design of implants, commonly considered in vitro loading conditions on the passive FE models do not adequately represent the in vivo loading conditions under muscle exertions. Therefore, more realistic in vivo loading conditions should instead be used.     

Sex differences in jaw muscle duty factors during exercise in two environments: A pilot study

It is unknown if females and males use jaw muscles similarly during exercise. This pilot study assessed jaw elevator muscle duty factors (DFs = time of muscle activity/total recording time) at repeated sessions to test if DFs are reliable and different between sexes during exercises in two environments. Ten female and seven male subjects recruited from university soccer teams provided informed consent. Surface electromyography was recorded from masseter and temporalis muscles during biting and leg-extension laboratory exercises. Average activities to produce 20 N bite-forces for each muscle and subject determined thresholds (5–80%·T20 N) for subject-specific DF calculations during exercises performed in laboratory and natural environments. Subjects self-recorded via portable electromyography equipment during in-field leg-extension and weight-lifting exercises. Effects of variables on DFs were assessed via ANOVA (α = 0.05) and simple effects testing (Bonferroni-adjusted p ⩽ 0.012). All subjects used jaw muscles during exercises in both environments. DFs between laboratory sessions were reliable (R = 0.84). During laboratory exercises, male temporalis DFs were significantly higher than female DFs from both muscles (p ⩽ 0.001). During in-field exercises females had higher DFs during weight-lifting while males had higher DFs during leg-extensions. In-field sex differences were significant at most thresholds and showed larger effect sizes for leg-extension compared to weight-lifting exercises.     

Obesity and spinal loads; a combined MR imaging and subject-specific modeling investigation

Epidemiological studies have identified obesity as a possible risk factor for low back disorders. Biomechanical models can help test such hypothesis and shed light on the mechanism involved. A novel subject-specific musculoskeletal-modelling approach is introduced to estimate spinal loads during static activities in five healthy obese (BMI > 30 kg/m²) and five normal-weight (20 < BMI < 25 kg/m²) individuals. Subjects underwent T1 through S1 MR imaging thereby measuring cross-sectional-area (CSA) and moment arms of trunk muscles together with mass and center of mass (CoM) of T1-L5 segments. MR-based subject-specific models estimated spinal loads using a kinematics/optimization-driven approach. Average CSAs of muscles, moment arms of abdominal muscles, mass and sagittal moment arm of CoM of T1-L5 segments were larger in obese individuals (p < 0.05 except for the moment arm of CoMs) but moment arms of their back muscles were similar to those of normal-weight individuals (p > 0.05). Heavier subjects did not necessarily have larger muscle moment arms (e.g., they were larger in 64 kg (BMI = 20.7 kg/m²) subject than 78 kg (BMI = 24.6 kg/m²) subject) or greater T1-L5 trunk weight (e.g., the 97 kg (BMI = 31 kg/m²) subject had similar trunk weight as 109 kg (BMI = 33.3 kg/m²) subject). Obese individuals had in average greater spinal loads than normal-weight ones but heavier subjects did not necessarily have greater spinal loads (117 kg (BMI = 40.0 kg/m²) subject had rather similar L5-S1 compression as 105 kg (BMI = 34.7 kg/m²) subject). Predicted L4-L5 intradiscal pressures for the normal-weight subjects ranged close to the measured values (R² = 0.85–0.92). Obese individuals did not necessarily have greater IDPs than normal-weight ones.     

Active pauses induce more variable electromyographic pattern of the trapezius muscle activity during computer work

The aim of this laboratory study was to evaluate effects of active and passive pauses and investigate the distribution of the trapezius surface electromyographic (SEMG) activity during computer mouse work. Twelve healthy male subjects performed four sessions of computer work for 10 min in one day, with passive (relax) and active (30% maximum voluntary contraction of shoulder elevation) pauses given every 2 min at two different work paces (low/high). Bipolar SEMG from four parts of the trapezius muscle was recorded. The relative rest time was higher for the lower parts compared with the upper of the trapezius (p < 0.01). The centroid of exposure variation analysis (EVA) along the time axis was lower during the computer work with active pause compared with passive one (p < 0.05). The results of this study revealed (i) lower rest time for the upper parts of trapezius compared with the lower parts, in line with previous clinical findings, (ii) active pauses contributed to a more variable muscle activity pattern during computer work that might have functional implications with respect to work-related musculoskeletal disorders.