Crosstalk in surface electromyography of the proximal forearm during gripping tasks.

Electromyographic (EMG) crosstalk was systematically analyzed to evaluate the magnitude of common signal present between electrode pairs around the forearm. Surface EMG was recorded and analyzed from seven electrode pairs placed circumferentially around the proximal forearm in six healthy individuals. The cross-correlation function was used to determine the amount of common signal, which was found to decrease as the distance between electrode pairs increased, but was not significantly altered by forearm posture (pronation, neutral, supination). Overall, approximately 40% common signal was detected between adjacent electrode pairs (3 cm apart), dropping to about 10% at 6 cm spacing and 2.5% at 9 cm. The magnitude of common signal approached 50% between adjacent electrode pairs over the extensor muscles, while over 60% was observed between neighbouring sites on the flexor aspect of the forearm. Although flexor and extensor EMG amplitude was similar, less than 2% common signal was present between flexor and extensor electrode pairs during both pinch and grasp tasks. Maximum grip force production was not affected by forearm rotation for pinch, but reduced 18% from neutral (mid-prone) to pronation during grasp (p=0.01). In spite of differences in grip force, mean muscle activity did not vary between the three forearm postures during maximum pinch or grasp trials. While this study improved our knowledge of crosstalk and electrode spacing issues, further examination of forearm EMG is required to improve understanding of muscle loading, EMG properties and motor control during gripping tasks.     

Quantitative assessment of upper limb muscle fatigue depending on the conditions of repetitive task load

The aim of this study was to discriminate fatigue of upper limb muscles depending on the external load, through the development and analysis of a muscle fatigue index. Muscle fatigue is expressed by a fatigue index based on an amplitude parameter (calculated in the time domain) and a fatigue index based on a frequency parameter (a parameter calculated in the frequency domain). The fatigue index involves a regression function that describes changes in the EMG signal parameter, time elapsing before muscle fatigue and the probability of specific trends in changes in EMG parameters for the population under study.

The experimental study covered a group of 10 young men. During the study, they exerted force at a specific level and for a specific time in 12 load variants. During the study, EMG signals from four muscles of the upper limb were recorded (trapezius pars descendents, biceps brachii caput breve, extensor carpi radialis brevis, flexor carpi ulnaris). For each variant and for each examined muscles, the value of the fatigue index was calculated. Values of that index quantitatively expressed fatigue of a specific muscle in a specific load variant.

A statistical analysis indicated variation in the fatigue of the biceps brachii caput breve, extensor carpi radialis brevis, and flexor carpi ulnaris muscles depending on the external load (load variant) according to the task performed with the upper limb.

The study demonstrated usefulness of the fatigue index in expressing quantitatively muscle fatigue and in discriminating muscle fatigue depending on the external load.     

Evaluating protocols for normalizing forearm electromyograms during power grip

Many studies use a reference task of an isometric maximum voluntary power grip task in a mid-pronated forearm posture to normalize their forearm electromyographic (EMG) signal amplitude. Currently there are no recommended protocols to do this. In order to provide guidance on the topic, we examined the EMG amplitude of six forearm muscles (three flexors and three extensors) during twenty different maximal voluntary efforts that included various gripping postures, force and moment exertions and compared them to a frequently used normalization task of exerting a maximum grip force, termed the reference task. 16 participants (8 male and 8 female, aged 18–26) were recruited for this study. Overall, maximal muscle activity was produced during the resisted moment tasks. When contrasted with the reference task, the resisted moment tasks produced EMG activity that was up to 2.8 times higher (p < 0.05). Although there was no one task that produced greater EMG values than the reference task for all forearm muscles, the resisted flexor and extensor moment tasks produced similar, if not higher EMG activity than the reference task for the three flexors and three extensor muscles, respectively. This suggests that researchers wishing to normalize forearm EMG activity during power gripping prehensile tasks should use resisted flexor and extensor moment tasks to obtain better estimates of the forearm muscles’ maximum electrical activation magnitudes.     

Influence of fatigue on upper limb muscle activity and performance in tennis

The study examined the fatigue effect on tennis performance and upper limb muscle activity. Ten players were tested before and after a strenuous tennis exercise. Velocity and accuracy of serve and forehand drives, as well as corresponding surface electromyographic (EMG) activity of eight upper limb muscles were measured. EMG and force were also evaluated during isometric maximal voluntary contractions (IMVC). Significant decreases were observed after exercise in serve accuracy (−11.7%) and velocity (−4.5%), forehand accuracy (−25.6%) and consistency (−15.6%), as well as pectoralis major (PM) and flexor carpi radialis (FCR) IMVC strength (−13.0% and −8.2%, respectively). EMG amplitude decreased for PM and FCR in serve, forehand and IMVC, and for extensor carpi radialis in forehand. No modification was observed in EMG activation timing during strokes or in EMG frequency content during IMVC. Several hypotheses can be put forward to explain these results. First, muscle fatigue may induce a reduction in activation level of PM and forearm muscles, which could decrease performance. Second, conscious or subconscious strategies could lead to a redistribution of muscle activity to non-fatigued muscles in order to protect the organism and/or limit performance losses. Otherwise, the modifications of EMG activity could also illustrate the strategies adopted to manage the speed-accuracy trade-off in such a complex task.     

Control of the Elbow Extensor Muscles in Slow Targeted Extensions of the Arm in Humans

Relations between the kinematic parameters of slow (non-ballistic) targeted extension movements in the elbow joint of humans and characteristics of the movement-related EMG activity in the two heads of the m. triceps brachii were analyzed. Test movements were performed under conditions of application of non-inertional external loadings directed toward flexion. It was shown that the movement-related EMG activity of the elbow extensors, similarly to what was observed in the flexors at flexion movements with the same parameters, demonstrates a complex structure and includes dynamic and stationary phases. In the former phase, in turn, initial and main components can be differentiated. The rising edge and decay of the main component of the dynamic extensor EMG phase could be approximated by exponential functions; this component was never split into a few subcomponents. Dependences between the amplitudes of m. triceps brachii EMG phases and the amplitude of the movement (or external loading) were, as a rule, nonlinear but monotonic. An increase in the test movement velocity led to an increase in the rate of rise of the rising edge of the dynamic EMG phase, while an increment in the amplitude was less significant. Under the used test conditions, the activity of the elbow extensors was usually accompanied by some coactivation of the antagonists (m. biceps brachii). It is concluded that motor commands coming to the elbow extensors at performance of the extension test movements differ from motor commands to the flexors at analogous flexion test movements by a simpler structure and more tonic pattern. Biomechanical specificities of fixation of the mentioned muscle groups to the arm bones (stability of the moment for application of the extensor force under conditions of changing the joint angle vs variable moment of the flexor force) are considered one of the main reasons for such specificity of the patterns of the extensor and flexor motor commands.     

Effect of biceps-to-triceps transfer on rotator cuff stress during upper limb weight-bearing lift in tetraplegia: A modeling and simulation analysis

Rotator cuff stress during upper limb weight-bearing lifts presumably contribute to rotator cuff disease, which is the most common cause of shoulder pain in individuals with tetraplegia. Elbow extension strength appears to be a key determinant of rotator cuff stress during upper limb weight-bearing lifts since individuals with paraplegia who generate greater elbow extensor moments experience lower rotator cuff stress relative to individuals with tetraplegia. Biceps-to-triceps transfer surgery can increase elbow extension strength in individuals with tetraplegia. The purpose of this study was to determine whether active elbow extension via biceps transfer decreases rotator cuff stress during weight-bearing lifts in individuals with tetraplegia. A forward dynamics computational framework was used to estimate muscle stress during the lift; stress was computed as muscle force divided by the peak isometric muscle force. We hypothesized that rotator cuff stresses would be lower in simulated lifting with biceps transfer relative to simulated lifting without biceps transfer. We found that limited elbow extension strength in individuals with tetraplegia, regardless of whether elbow strength is enabled via biceps transfer or is residual after spinal cord injury, results in muscle stresses exceeding 85% of the peak isometric muscle stress in the supraspinatus, infraspinatus, and teres minor. The rotator cuff stresses we estimated suggest that performance of weight-bearing activities should be minimized or assisted in order to reduce the risk for shoulder pain. Our results also indicate that biceps transfer is unlikely to decrease rotator cuff stress during weight-bearing lifts in individuals with tetraplegia.     

The need for muscle co-contraction prior to a landing

In landings from a flight phase the mass centre of an athlete experiences rapid decelerations. This study investigated the extent to which co-contraction is beneficial or necessary in drop landings, using both experimental data and computer simulations. High speed video and force recordings were made of an elite martial artist performing drop landings onto a force plate from heights of 1.2, 1.5 and 1.8 m. Matching simulations of these landings were produced using a planar 8-segment torque-driven subject-specific computer simulation model. It was found that there was substantial co-activation of joint flexor and extensor torques at touchdown in all three landings. Optimisations were carried out to determine whether landings could be effected without any co-contraction at touchdown. The model was not capable of landing from higher than 1.05 m with no initial flexor or extensor activations. Due to the force–velocity properties of muscle, co-contraction with net zero joint torque at touchdown leads to increased extensor torque and decreased flexor torque as joint flexion velocity increases. The same considerations apply in any activity where rapid changes in net joint torque are required, as for example in jumps from a running approach.     

女子拳击运动员上肢和腰部肌肉力量训练的肌电分析

目的：利用肌电指标分析拳击运动员上肢和腰部肌肉力量训练效果。方法：用Mega公司的ME6000肌电图仪记录分析10名女子拳击运动员上臂肱二头肌（主动肌）与肱三头肌（拮抗肌）、前臂屈肌（主动肌）与伸肌（拮抗肌）和腰部肌群的运动诱发肌电,规定运动为手持2.5 kg的哑铃负荷进行直拳空击运动直至局部肌肉力竭。结果：直拳空击运动至局部肌肉力竭过程中,上肢拮抗肌的中位频率（MF）下降幅度和速度大于相对应的主动肌,且从肌群作功来看,主动肌作功百分比较拮抗肌大。其中9名普通运动员腰肌的肌电频率（MF）均值较1名指定样本世界冠军的下降缓慢,而且其作功百分比都较小。结论：通过对普通女子拳击运动员上肢和腰部肌群肌电指标测试与世界冠军的比较分析,提示本研究中所测普通拳击运动员拮抗肌和腰部肌肉力量训练不足,有待加强该部肌肉的力量训练。     

Correlation of muscle function and bone strain in the hindlimb of the river cooter turtle (Pseudemys concinna)

During terrestrial locomotion, limb muscles must generate mechanical work and stabilize joints against the ground reaction force. These demands can require high force production that imposes substantial loads on limb bones. To better understand how muscle contractile function influences patterns of bone loading in terrestrial locomotion, and refine force platform equilibrium models used to estimate limb bone safety factors, we correlated in vivo recordings of femoral strain with muscle activation and strain in a major propulsive hindlimb muscle, flexor tibialis internus (FTI), of a species with a published model of hindlimb force production (river cooter turtles, Pseudemys concinna). Electromyography (EMG) recordings indicate FTI activity prior to footfall that continues through approximately 50% of the stance phase. Large EMG bursts occur just after footfall when the muscle has reached its maximum length and is beginning to actively shorten, concurrent with increasing compressive strain on the anterior femur. The FTI muscle shortens through 35% of stance, with mean fascicle shortening strains reaching 14.0 ± 5.4% resting length (L₀). At the time of peak compressive strains on the femur, the muscle fascicles remain active, but fascicles typically lengthen until mid‐stance as the knee extends. Influenced by the activity of the dorsal knee extensor femorotibialis, the FTI muscle continues to passively lengthen simultaneously with knee extension and a shift to tensile axial strain on the anterior femur at approximately 40% of stance. The near coincidence in timing of peak compressive bone strain and peak muscle shortening (5.4 ± 4.1% stance) indicates a close correlation between the action of the hip extensor/knee flexor, FTI, and femoral loading in the cooter hindlimb. In the context of equilibrium models of limb bone loading, these results may help explain differences in safety factor estimates observed between previous force platform and in vivo strain analyses in cooters. J. Morphol. 274:1060–1069, 2013. © 2013 Wiley Periodicals, Inc.     

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.     

The sensitivity of endpoint forces produced by the extrinsic muscles of the thumb to posture

This study utilizes a biomechanical model of the thumb to estimate the force produced at the thumb-tip by each of the four extrinsic muscles. We used the principle of virtual work to relate joint torques produced by a given muscle force to the resulting endpoint force and compared the results to two separate cadaveric studies. When we calculated thumb-tip forces using the muscle forces and thumb postures described in the experimental studies, we observed large errors. When relatively small deviations from experimentally reported thumb joint angles were allowed, errors in force direction decreased substantially. For example, when thumb posture was constrained to fall within ±15° of reported joint angles, simulated force directions fell within experimental variability in the proximal–palmar plane for all four muscles. Increasing the solution space from ±1° to an unbounded space produced a sigmoidal decrease in error in force direction. Changes in thumb posture remained consistent with a lateral pinch posture, and were generally consistent with each muscle’s function. Altering thumb posture alters both the components of the Jacobian and muscle moment arms in a nonlinear fashion, yielding a nonlinear change in thumb-tip force relative to muscle force. These results explain experimental data that suggest endpoint force is a nonlinear function of muscle force for the thumb, support the continued use of methods that implement linear transformations between muscle force and thumb-tip force for a specific posture, and suggest the feasibility of accurate prediction of lateral pinch force in situations where joint angles can be measured accurately.     

Muscle architecture of the upper limb in the orangutan

We dissected the left upper limb of a female orangutan and systematically recorded muscle mass, fascicle length, and physiological cross-sectional area (PCSA), in order to quantitatively clarify the unique muscle architecture of the upper limb of the orangutan. Comparisons of the musculature of the dissected orangutan with corresponding published chimpanzee data demonstrated that in the orangutan, the elbow flexors, notably M. brachioradialis, tend to exhibit greater PCSAs. Moreover, the digital II-V flexors in the forearm, such as M. flexor digitorum superficialis and M. flexor digitorum profundus, tend to have smaller PCSA as a result of their relatively longer fascicles. Thus, in the orangutan, the elbow flexors demonstrate a higher potential for force production, whereas the forearm muscles allow a greater range of wrist joint mobility. The differences in the force-generating capacity in the upper limb muscles of the two species might reflect functional specialization of muscle architecture in the upper limb of the orangutan for living in arboreal environments.     

Mechanical effect of rat flexor carpi ulnaris muscle after tendon transfer: does it generate a wrist extension moment?

The mechanical effect of a muscle following agonist-to-antagonist tendon transfers does not always meet the surgeon's expectations. We tested the hypothesis that after flexor carpi ulnaris (FCU) to extensor carpi radialis (ECR) tendon transfer in the rat, the direction (flexion or extension) of the muscle's joint moment is dependent on joint angle. Five weeks after recovery from surgery (tendon transfer group) and in a control group, wrist angle-moment characteristics of selectively activated FCU muscle were assessed for progressive stages of dissection: 1) with minimally disrupted connective tissues, 2) after distal tenotomy, and 3) after maximal tendon and muscle belly dissection, but leaving blood supply and innervations intact. In addition, force transmission from active FCU onto the distal tendon of passive palmaris longus (PL) muscle (a wrist flexor) was assessed. Excitation of control FCU yielded flexion moments at all wrist angles tested. Tenotomy decreased peak FCU moment substantially (by 93%) but not fully. Only after maximal dissection, FCU wrist moment became negligible. The mechanical effect of transferred FCU was bidirectional: extension moments in flexed wrist positions and flexion moments in extended wrist positions. Tenotomy decreased peak extension moment (by 33%) and increased peak flexion moment of transferred FCU (by 41%). Following subsequent maximal FCU dissection, FCU moments decreased to near zero at all wrist angles tested. We confirmed that, after transfer of FCU towards a wrist extensor insertion, force can be transmitted from active FCU to the distal tendon of passive PL. We conclude that mechanical effects of a muscle after tendon transfer to an antagonistic site can be quite different from those predicted based solely on the sign of the new moment arm at the joint.     

Spatio-temporal evaluation of neck muscle activation during postural perturbations in healthy subjects.

The purpose of this study was to examine the spatio-temporal activation of the sternocleidomastoid (SCM) and cervical extensor (CE) muscles with respect to the deltoid muscle onset during rapid voluntary upper limb movement in healthy volunteers. The repeatability and reliability of the spatio-temporal aspects of the myoelectric signals were also examined. Ten subjects performed bilateral and unilateral rapid upper limb flexion, abduction and extension in response to a visual stimulus. EMG onsets and normalised root mean square (nRMS) values were calculated for the SCM and CE muscles. Subjects attended three testing sessions over non-consecutive days allowing the repeatability and reliability of these measures to be assessed. The SCM and CE muscles demonstrated feed-forward activation (activation within 50 ms of deltoid onset) during rapid arm movements in all directions. The sequence and magnitude of neck muscle activation displayed directional specificity, however, the neck flexor and extensor muscles displayed co-activation during all perturbations. EMG onsets demonstrated high repeatability in terms of repeated measure precision (nSEM in the range 1.9-5.7%). This was less evident for the repeatability of nRMS values. The results of this study provide a greater understanding of cervical neuromotor control strategies. During bilateral and unilateral upper limb perturbations, the SCM and CE muscles demonstrate feed-forward co-activation. It seems apparent that feed-forward activation of neck muscles is a mechanism necessary to achieve stability for the visual and vestibular systems, whilst ensuring stabilisation and protection of the cervical spine.     

Modulation of shoulder muscle and joint function using a powered upper-limb exoskeleton

Robotic-assistive exoskeletons can enable frequent repetitive movements without the presence of a full-time therapist; however, human-machine interaction and the capacity of powered exoskeletons to attenuate shoulder muscle and joint loading is poorly understood. This study aimed to quantify shoulder muscle and joint force during assisted activities of daily living using a powered robotic upper limb exoskeleton (ArmeoPower, Hocoma). Six healthy male subjects performed abduction, flexion, horizontal flexion, reaching and nose touching activities. These tasks were repeated under two conditions: (i) the exoskeleton compensating only for its own weight, and (ii) the exoskeleton providing full upper limb gravity compensation (i.e., weightlessness). Muscle EMG, joint kinematics and joint torques were simultaneously recorded, and shoulder muscle and joint forces calculated using personalized musculoskeletal models of each subject’s upper limb. The exoskeleton reduced peak joint torques, muscle forces and joint loading by up to 74.8% (0.113 Nm/kg), 88.8% (5.8%BW) and 68.4% (75.6%BW), respectively, with the degree of load attenuation strongly task dependent. The peak compressive, anterior and superior glenohumeral joint force during assisted nose touching was 36.4% (24.6%BW), 72.4% (13.1%BW) and 85.0% (17.2%BW) lower than that during unassisted nose touching, respectively. The present study showed that upper limb weight compensation using an assistive exoskeleton may increase glenohumeral joint stability, since deltoid muscle force, which is the primary contributor to superior glenohumeral joint shear, is attenuated; however, prominent exoskeleton interaction moments are required to position and control the upper limb in space, even under full gravity compensation conditions. The modeling framework and results may be useful in planning targeted upper limb robotic rehabilitation tasks.     

Muscle mechanical advantage of human walking and running: implications for energy cost.

Muscular forces generated during locomotion depend on an animal's speed, gait, and size and underlie the energy demand to power locomotion. Changes in limb posture affect muscle forces by altering the mechanical advantage of the ground reaction force (R) and therefore the effective mechanical advantage (EMA = r/R, where r is the muscle mechanical advantage) for muscle force production. We used inverse dynamics based on force plate and kinematic recordings of humans as they walked and ran at steady speeds to examine how changes in muscle EMA affect muscle force-generating requirements at these gaits. We found a 68% decrease in knee extensor EMA when humans changed gait from a walk to a run compared with an 18% increase in hip extensor EMA and a 23% increase in ankle extensor EMA. Whereas the knee joint was extended (154-176 degrees) during much of the support phase of walking, its flexed position (134-164 degrees) during running resulted in a 5.2-fold increase in quadriceps impulse (time-integrated force during stance) needed to support body weight on the ground. This increase was associated with a 4.9-fold increase in the ground reaction force moment about the knee. In contrast, extensor impulse decreased 37% (P < 0.05) at the hip and did not change at the ankle when subjects switched from a walk to a run. We conclude that the decrease in limb mechanical advantage (mean limb extensor EMA) and increase in knee extensor impulse during running likely contribute to the higher metabolic cost of transport in running than in walking. The low mechanical advantage in running humans may also explain previous observations of a greater metabolic cost of transport for running humans compared with trotting and galloping quadrupeds of similar size.     

An ankle-foot orthosis powered by artificial pneumatic muscles

We developed a pneumatically powered orthosis for the human ankle joint. The orthosis consisted of a carbon fiber shell, hinge joint, and two artificial pneumatic muscles. One artificial pneumatic muscle provided plantar flexion torque and the second one provided dorsiflexion torque. Computer software adjusted air pressure in each artificial muscle independently so that artificial muscle force was proportional to rectified low-pass-filtered electromyography (EMG) amplitude (i.e., proportional myoelectric control). Tibialis anterior EMG activated the artificial dorsiflexor and soleus EMG activated the artificial plantar flexor. We collected joint kinematic and artificial muscle force data as one healthy participant walked on a treadmill with the orthosis. Peak plantar flexor torque provided by the orthosis was 70 Nm, and peak dorsiflexor torque provided by the orthosis was 38 Nm. The orthosis could be useful for basic science studies on human locomotion or possibly for gait rehabilitation after neurological injury.     

Analysis of a muscular control system in human movements

In this work, we have studied a muscular control system under experimental conditions for analyzing the dynamic behavior of individual muscles and theoretical considerations for elucidating its control strategy. Movement of human limbs is achieved by joint torques and each torque is specified as the sum of torques generated by muscle forces. The behavior of individual muscles is controlled by the neural input which is estimated by means of an electromyogram (EMG). In this study, the EMGs for a flexor and an extensor are measured in elbow joint movements and the dynamic behavior of individual muscles is analyzed. As a result, it is verified that both a flexor and an extensor are activated throughout the entire movement and that the activation of muscles is controlled above a specific limit independent of the hand-held load. Subsequently, a system model for simulating elbow joint movements is developed which includes the muscle dynamic relationship between the neural input and the isometric force. The minimum limit of muscle activation that has been confirmed in experiments is provided as a constraint of the neural input and the criterion is defined by a derivative of the isometric force of individual muscles. The optimal trajectories formulated under these conditions are quantitatively compared with the experimentally observed trajectories, and the control strategy of a muscular control system is studied. Finally, a muscular control system in multi-joint arm movements is discussed with regard to the comparative analysis between observed and optimal trajectories. Received: 7 April 1999 / Accepted in revised form: 27 July 1999     

Effects of vibration on arm and shoulder muscles in three body postures

The electromyographic responses of arm and shoulder muscles to vibrations were studied in three postures similar to the postures of drilling in a ceiling, drilling in a wall and drilling in a floor. This experiment was performed within the defined parameters of: vibrational frequency at 30 Hz, acceleration level 40 m.s-2 (rms), pushing force expressed as percentage maximal voluntary contraction, and gripping force which was set at 100 N. The exposure time for each test was 5 min. The general findings from these three body postures show that all the examined muscles were affected by exposure to vibration. The EMG index increased as follows: trapezius muscle 39% (p less than 0.05), lower-arm flexor muscles 23% (p less than 0.05), infraspinatus muscle 14% (p less than 0.05), lower-arm extensor muscles 14% (p less than 0.1) and biceps muscle 6% (p less than 0.1). The muscle most affected by vibration was found to be the trapezius muscle. It should be taken into consideration that vibration can be a contributing factor in neck/shoulder disorders among power handtool operators. The general conclusion from this study is that changes in working posture give different transmissions of vibration in the upper extremities. It seems as if the prime movers and muscles with an increased muscle length or increased degree of contraction are most affected by vibration.