Neuromuscular assessment in elderly workers with and without work related shoulder/neck trouble: the NEW-study design and physiological findings

Musculoskeletal disorders in the neck and shoulder area are a major occupational concern in the European countries especially among elderly females. The aim was to assess these disorders based on quantitative EMG indicators and functional tests. 252 female computer users (45-68 years) were recruited from four European countries in two contrast groups: (1) 88 neck/shoulder (NS) cases reporting trouble in the neck and/or shoulder region for more than 30 days during the last year, and (2) 164 NS-controls reporting such trouble for no more than 7 days. Questionnaires, functional/clinical tests, and physiological recordings were performed in workplace related field studies. The results showed no differences in anthropometrics but NS-cases reported more strained head positions and more eye problems than controls. The psychosocial working factors were similar, although, NS-controls had slightly better scores on working conditions, general health, and vitality compared to cases. The NS-cases had lower maximal voluntary contraction (MVC) during shoulder elevation (mean (SD) 310 (122) N) compared to the controls (364 (122) N). During 30% MVC electromyography (EMGrms) in the trapezius muscle was lower in NS-cases (194 (105) muV) than in controls (256 (169) muV), while no differences were found regarding endurance time. Estimated conduction velocity was not different between NS-cases and -controls. Four functional computer tests were performed equally well by NS-cases and -controls, and the corresponding EMG variables also did not differ. A major finding in this large-scale epidemiological study is the significantly lower MVC in NS-cases compared with NS-controls together with lower EMGrms value at 30% MVC, while computer tasks were performed at similar relative muscle activation. The study was unable to reveal quantitative EMG indicators and functional tests that could objectively assess disorders in NS-cases.

Voluntary low-force contraction elicits prolonged low-frequency fatigue and changes in surface electromyography and mechanomyography

Controversies exist regarding objective documentation of fatigue development with low-force contractions. We hypothesized that non-exhaustive, low-force muscle contraction may induce prolonged low-frequency fatigue (LFF) that in the subsequent recovery period is detectable by electromyography (EMG) and in particular mechanomyography (MMG) during low-force rather than high-force test contractions. Seven subjects performed static wrist extension at 10% maximal voluntary contraction (MVC) for 10 min (10%MVC10 min). Wrist force response to electrical stimulation of extensor carpi radialis muscle (ECR) quantified LFF. EMG and MMG were recorded from ECR during static test contractions at 5% and 80% MVC. Electrical stimulation, MVC, and test contractions were performed before 10%MVC10 min and at 10, 30, 90 and 150 min recovery. In spite of no changes in MVC, LFF persisted up to 150 min recovery but did not develop in a control experiment omitting 10%MVC10 min. In 5% MVC tests significant increase was found in time domain of EMG from 0.067+/-0.028 mV before 10%MVC10 min to 0.107+/-0.049 and 0.087+/-0.05 mV at 10 and 30 min recovery, respectively, and of the MMG from 0.054+/-0.039 ms(-2) to 0.133+/-0.104 and 0.127+/-0.099 ms(-2), respectively. No consistent changes were found in 80% MVC tests. In conclusion, non-exhaustive low-force muscle contraction resulted in prolonged LFF that in part was identified by the EMG and MMG signals.

Intramuscular pressure and tissue oxygenation during low-force static contraction do not underlie muscle fatigue

AIM

To test the hypothesis that time-wise increase in intramuscular pressure (IMP) and subsequent decrease in muscle tissue oxygenation (TO(2)) results in muscle fatigue development during a non-exhaustive, low-force contraction evidenced by changes in electromyogram (EMG) and particular mechanomyogram (MMG).

METHODS

Seven subjects performed static elbow flexion at 10% maximal voluntary contraction (MVC) for 10 min (10% MVC(10 min)). Surface EMG, MMG, IMP and TO(2) measured by near-infrared spectroscopy was recorded from m. biceps brachii during 10% MVC(10 min) and during 5% MVC test contractions of 1 min duration performed before 10% MVC(10 min), 10 and 30 min post-exercise. EMG and MMG were analysed for root mean square (rms) and mean power frequency (mpf).

RESULTS

During 10% MVC(10 min) MMGrms increased from initial level of 0.04 +/- 0.01 to 0.11 +/- 0.07 m s(-2) in the last minute and MMGmpf and EMGmpf decreased from 34.9 +/- 8.2 to 21.3 +/- 3.8 Hz and from 71.7 +/- 10.9 to 61.7 +/- 10.0 Hz respectively. Similar changes were present in 5% MVC test contractions 30 min post-exercise. Initially, TO(2) decreased by 6.9 +/- 6.5% of resting level but returned to rest within 1 min. IMP remained constant during the contraction after an initial fourfold increase from resting level of 12.2 +/- 10.4 mmHg.

CONCLUSIONS

IMP was anticipated to increase with time of contraction due to e.g. increased muscle water content; but this was not confirmed. Consequently, muscle blood flow was unlikely to be impeded with contraction time, which may account for the maintenance of TO(2). Thus, decreased TO(2) did not underlie either acute or long-term muscle fatigue development evidenced by changes in EMG and particular MMG variables.

Increase in interstitial interleukin-6 of human skeletal muscle with repetitive low-force exercise

Interleukin (IL)-6, which is released from muscle tissue during intense exercise, possesses important metabolic and probably anti-inflammatory properties. To evaluate the IL-6 response to low-intensity exercise, we conducted two studies: 1) a control study with insertion of microdialysis catheters in muscle and determination of interstitial muscle IL-6 response over 2 h of rest and 2) an exercise study to investigate the IL-6 response to 20 min of repetitive low-force exercise. In both studies, a microdialysis catheter (cutoff: 3,000 kDa) was inserted into the upper trapezius muscle of six male subjects, and the catheters were perfused with Ringer-acetate at 5 μl/min. Venous plasma samples were taken in the exercise study. The insertion of microdialysis catheters into muscle resulted in an increase in IL-6 from 8 ± 0 to 359 ± 171 and 484 ± 202 pg/ml after 65 and 110 min, respectively ( P < 0.001). Similarly, in the exercise study, IL-6 increased to 289 ± 128 pg/ml after a 55-min rest ( P < 0.001). During the subsequent repetitive low-force exercise, muscle IL-6 further increased to 1,246 ± 461 pg/ml and reached 2,132 ± 477 pg/ml after a 30-min recovery (all P < 0.001). In contrast to this, plasma IL-6 did not significantly change in response to exercise. We conclude that upper extremity, low-intensity exercise results in a substantial increase in IL-6 in the interstitium of the stabilizing trapezius muscle, whereas no change is seen for plasma IL-6.

Evaluation of models used to study neuromuscular fatigue

The mechanism(s) of neuromuscular fatigue depends on the approach or model used to study fatigue. We evaluate the physiological and functional relevance of the fatigue quantification system, the fatigue protocols, the preparations, and muscle environmental conditions used in various models involving either artificial stimulation or voluntary exercise to induce fatigue.

Interstitial muscle lactate, pyruvate and potassium dynamics in the trapezius muscle during repetitive low-force arm movements, measured with microdialysis

AIM

Local muscle metabolic responses to repetitive low-force contractions and to intense static contractions were studied by microdialysis in humans.

METHODS

Microdialysate and electromyography (EMG) were sampled from the trapezius muscle, mixed venous blood samples were taken and perceived exertion was rated (0-9) before and during 20 min of standardized repetitive arm movement (REP), 60 min recovery (R1), and 10 min 90 degrees sustained arm position (SUS) at 20% maximum voluntary contraction, followed by 60 min recovery (R2) in six healthy male participants (28-33 years).

RESULTS

Average muscle activity was 8 +/- 2% of EMGmax-RMS (mean +/-SEM) during REP and 22 +/- 5% of EMGmax-RMS during SUS. Perceived exertion increased from 0 to 3.2 +/- 0.5 during REP and from 0 to 8.5 +/- 0.3 during SUS. During REP interstitial muscle lactate increased from 2.1 +/- 0.2 to 2.9 +/- 0.2 mmol L(-1) (P < 0.001) and returned to the baseline level during R1, while dialysate [K+] increased from 3.8 +/- 0.2 to 4.7 +/- 0.2 mmol L(-1) (P < 0.002) and returned to 3.8 +/- 0.2 mmol L(-1) during R1. In contrast, plasma lactate and [K+] remained unchanged. During SUS interstitial muscle lactate increased from 2.3 +/- 0.2 to 3.3 +/- 0.3 mmol L(-1) (P < 0.003), increased further to 6.5 +/- 1.3 mmol L(-1) post-exercise (P < 0.001) and returned to baseline levels during R2. Dialysate [K+] increased from 3.9 +/- 0.2 to 4.6 +/- 0.2 mmol L(-1) (P < 0.05) and returned to baseline level during R2. Plasma lactate increased significantly during SUS whereas plasma [K+] was unchanged. During REP and SUS interstitial pyruvate was unchanged but increased in the post-exercise period proportional to the exercise intensity.

CONCLUSIONS

The microdialysis technique was effective in revealing muscle metabolic events that were not found systemically. Furthermore, the trapezius muscle showed an anaerobic metabolism during low-force contraction, which could indicate inhomogeneous muscle activation.

Muscle tissue oxygenation, pressure, electrical, and mechanical responses during dynamic and static voluntary contractions

Dynamic muscle contractions have been shown to cause greater energy turnover and fatigue than static contractions performed at a corresponding force level. Therefore, we hypothesized that: (1) electro- (EMG) and mechanomyography (MMG), intramuscular pressure (IMP), and reduction in muscle oxygen tension (rTO(2)) would be larger during dynamic (DYN) than intermittent static (IST) low force contractions; and that (2) oxygen tension would remain lower in the resting periods subsequent to DYN as compared to those following IST. Eight subjects performed elbow flexions with identical time-tension products: (1) DYN as a 20 degrees elbow movement of 2 s concentric and 2 s eccentric followed by a 4 s rest; and (2) IST with a 4 s contraction followed by a 4 s rest. Each session was performed for 1 min at 10 and 20% of the maximal voluntary contraction (MVC). The force, bipolar surface EMG, MMG, IMP, rTO(2) were measured simultaneously from the biceps brachii, and the data presented as the mean values together with the standard error of the means. Comparison of the corresponding time periods showed the EMG(rms) and MMG(rms) values to be larger during DYN than IST (concentric phase: DYN vs IST were 14.2 vs 9.4, and 22.0 vs 15.9%(max)-EMG(rms); eccentric phase: in DYN, the MMG was approximately 1.5 and approximately 2.0-fold IST at 10 and 20%MVC, respectively). In contrast, the IMP of the concentric phase in DYN was lower than in IST (2.3 vs 29.5 and 10.9 vs 42.0 mmHg at 10 and 20%MVC, respectively), and a similar picture was seen for the eccentric phase. However, no differences were seen in rTO(2) in either the contraction or the rest periods. In a prolonged rest period (8 s) after the sessions, DYN but not IST showed rTO(2) above baseline level. In conclusion, rTO(2) in DYN and IST were similar in spite of major differences in the MMG and EMG responses of the muscle during contraction periods. This may relate to the surprisingly lower IMP in DYN than IST.

Clinical signs and physical function in neck and upper extremities among elderly female computer users: the NEW study

The aim of the study was to present the prevalence of clinical signs and symptoms among female computer users above 45 years, both in a group with self-reported neck/shoulder trouble (NS cases) and in a group without such trouble (NS controls). The hypothesis was that computer users with self-reported neck/shoulder trouble have more clinical findings than those not reporting trouble, and that a corresponding pattern holds true for physical function. In total 42 and 61 questionnaire-defined NS cases and NS controls participated and went through a clinical examination of the neck and upper extremities and five physical function tests: maximal voluntary contraction (MVC) of shoulder elevation, abduction, and handgrip, as well as endurance at 30% MVC shoulder elevation and a physical performance test. Based on clinical signs and symptoms, trapezius myalgia (38%), tension neck syndrome (17%) and cervicalgia (17%) were the most frequent diagnoses among NS cases, and were significantly more frequent among NS cases than NS controls. A total of 60% of the subjects with reported trouble had one or several of the diagnoses located in the neck/shoulder. Physical function of the shoulder was lower in subjects with self-reported trouble as well as in the subgroup of NS cases with clinical diagnoses. In conclusion, the present clinical diagnoses and physical function tests differed between NS cases and NS controls, and are therefore recommended to be included as quantitative objective measures in assessing musculoskeletal health. Physical function tests should be further developed in order to be able to detect pre-stages of work-related disorders for preventive strategies.

Surface mechanomyogram amplitude is not attenuated by intramuscular pressure

The electromyogram (EMG) and intramuscular pressure (IMP) increase linearly with force during voluntary static contractions, while the surface mechanomyogram (MMG) increases linearly only up to approximately 70% of the maximal voluntary contraction (MVC) and then levels off. The aim of this study was to investigate the possible influence of IMP on the non-linear MMG increase with force and hence on the signal generation process. Seven subjects performed static contractions of the elbow flexors during: (1) ramp contractions from 0 to 60% of the MVC, and (2) steps at 10, 20 and 40% of the MVC. An external pressure of 0 and 50 mmHg for the ramps or 0, 20, 40, 60, 80 and 100 mmHg for the steps was applied by means of a sphygmomanometer cuff in separate trials. The EMG and the MMG were detected in the biceps brachii by means of a pair of surface electrodes and an accelerometer. The IMP was measured using a Millar tipped pressure transducer, and the data was presented as the mean and standard deviation in each case. The IMP was strongly and linearly related to the external pressure and contraction force both during ramps and steps. The EMG(rms) and MMG(rms) were never reduced as a consequence of the IMP increments. In contrast, a steeper MMG(rms) versus %MVC relationship during ramps at 50 mmHg cuff pressure, and an influence of the cuff pressure at 40% of MVC on MMG(rms) were evident. We conclude that IMP per se does not attenuate the MMG generation process during voluntary contraction, suggesting that the previously described MMG(rms) decrease at near maximal static efforts must be attributed to other determinants, such as a fusion-like situation due to the high motor unit firing rate.

Pedalling rate affects endurance performance during high-intensity cycling

The purpose of this study into high-intensity cycling was to: (1) test the hypothesis that endurance time is longest at a freely chosen pedalling rate (FCPR), compared to pedalling rates 25% lower (FCPR-25) and higher (FCPR+25) than FCPR, and (2) investigate how physiological variables, such as muscle fibre type composition and power reserve, relate to endurance time. Twenty males underwent testing to determine their maximal oxygen uptake (VO(2max)), power output corresponding to 90% of VO(2max) at 80 rpm (W90), FCPR at W90, percentage of slow twitch muscle fibres (% MHC I), maximal leg power, and endurance time at W90 with FCPR-25, FCPR, and FCPR+25. Power reserve was calculated as the difference between applied power output at a given pedalling rate and peak crank power at this same pedalling rate. W90 was 325 (47) W. FCPR at W90 was 78 (11) rpm, resulting in FCPR-25 being 59 (8) rpm and FCPR+25 being 98 (13) rpm. Endurance time at W90(FCPR+25) [441 (188) s] was significantly shorter than at W90(FCPR) [589 (232) s] and W90(FCPR-25) [547 (170) s]. Metabolic responses such as VO(2) and blood lactate concentration were generally higher at W90(FCPR+25) than at W90(FCPR-25) and W90(FCPR). Endurance time was negatively related to VO(2max), W90 and % MHC I, while positively related to power reserve. In conclusion, at group level, endurance time was longer at FCPR and at a pedalling rate 25% lower compared to a pedalling rate 25% higher than FCPR. Further, inter-individual physiological variables were of significance for endurance time, % MHC I showing a negative and power reserve a positive relationship.

Intramuscular pressure and EMG relate during static contractions but dissociate with movement and fatigue

Intramuscular pressure (IMP) and electromyography (EMG) mirror muscle force in the nonfatigued muscle during static contractions. The present study explores whether the constant IMP-EMG relationship with increased force may be extended to dynamic contractions and to fatigued muscle. IMP and EMG were recorded from shoulder muscles in three sessions: 1) brief static arm abductions at angles from 0 to 90°, with and without 1 kg in the hands; 2) dynamic arm abductions at angular velocities from 9 to 90°/s, with and without 1 kg in the hands; and 3) prolonged static arm abduction at 30° for 30 min followed by recovery. IMP and EMG increased in parallel with increasing shoulder torque during brief static tasks. During dynamic contractions, peak IMP and EMG increased to values higher than those during static contractions, and EMG, but not IMP, increased significantly with speed of abduction. In the nonfatigued supraspinatus muscle, a linear relationship was found between IMP and EMG; in contrast, during fatigue and recovery, significant timewise changes of the IMP-to-EMG ratio occurred. The results indicate that IMP should be included along with EMG when mechanical load sharing between muscles is evaluated during dynamic and fatiguing contractions.

A physiological counterpoint to mechanistic estimates of "internal power" during cycling at different pedal rates

Reported values of "internal power" (IP) during cycling, generated by the muscles to overcome energy changes of moving body segments, are considerably different for various biomechanical models, reflecting the different criteria for estimation of IP. The present aim was to calculate IP from metabolic variables and to perform a physiological evaluation of five different kinematic models for calculating IP in cycling. Results showed that IP was statistically different between the kinematic models applied. IP based on metabolic variables (IP(met)) was 15, 41, and 91 W at 61, 88, and 115 rpm, respectively, being remarkably close to the kinematic estimate of one model (IP(Willems-COM): 14, 43, and 95 W) and reasonably close to another kinematic estimate (IP(Winter): 8, 29, and 81 W). For all kinematic models there was no significant effect of performing 3-D versus 2-D analyses. IP increased significantly with pedal rate - leg movements accounting for the largest fraction. Further, external power (EP) affected IP significantly such that IP was larger at moderate than at low EP at the majority of the pedal rates applied but on average this difference was only 8%.

The effect of physical and psychosocial loads on the trapezius muscle activity during computer keying tasks and rest periods

The overall aim was to investigate the effect of psychosocial loads on trapezius muscle activity during computer keying work and during short and long breaks. In 12 female subjects, surface electromyography (EMG) was recorded bilaterally from the upper trapezius muscle during a standardized one hand keying task-interspaced with short (30 s) and long (4 min) breaks-in sessions with and without a combination of cognitive and emotional stressors. Adding psychosocial loads to the same physical work did not increase the activity of the trapezius muscle on either the keying or the control side, both of which remained at median and static EMG activity levels of around 5% and 2.5% of the maximal voluntary electrical activity (EMG(max)), respectively. The difference between the keying and the control side was significant; and further the control side activity was significantly increased above resting level. During both short and long breaks, exposure to psychosocial loads also did not increase the activity of the trapezius muscle either on the side of the keying or the control hand. Of note is that during long breaks the muscle activity of the keying side as well as that of the control side remained at the same level as during the short breaks, which was increased above resting level. This was to be seen from the static and the median EMG activity levels as well as gap times, the overall mean values being: 0.4%EMG(max), 1.1%EMG(max), and 50% in gap time, respectively.

IN CONCLUSION

psychosocial loads are not solely responsible for increased non-postural muscle activity; and increasing the duration of breaks does not per se cause muscle relaxation.

Evidence of long term muscle fatigue following prolonged intermittent contractions based on mechano- and electromyograms

The focus of the present study is the long term element of muscle fatigue provoked by prolonged intermittent contractions at submaximal force levels and analysed by force, surface electromyography (EMG) and mechanomyogram (MMG). It was hypothesized that fatigue related changes in mechanical performance of the biceps muscle are more strongly reflected in low than in high force test contractions, more prominent in the MMG than in the EMG signal and less pronounced following contractions controlled by visual compared to proprioceptive feedback. Further, it was investigated if fatigue induced by 30 min intermittent contractions at 30% as well as 10% of maximal voluntary contraction (MVC) lasted more than 30 min recovery. In six male subjects the EMG and MMG were recorded from the biceps brachii muscle during three sessions with fatiguing exercise at 10% with visual feedback and at 30% MVC with visual and proprioceptive feedback. EMG, MMG, and force were evaluated during isometric test contractions at 5% and 80% MVC before prolonged contraction and after 10 and 30 min of recovery. MVC decreased significantly after the fatiguing exercise in all three sessions and was still decreased even after 30 min of recovery. In the time domain significant increases after the fatiguing exercise were found only in the 5% MVC tests and most pronounced for the MMG. No consistent changes were found for neither EMG nor MMG in the frequency domain and feedback mode did not modify the results. It is concluded that long term fatigue after intermittent contractions at low force levels can be detected even after 30 min of recovery in a low force test contraction. Since the response was most pronounced in the MMG this may be a valuable variable for detection of impairments in the excitation-contraction coupling.

Muscle performance following fatigue induced by isotonic and quasi-isometric contractions of rat extensor digitorum longus and soleus muscles in vitro

AIM

To study the effect of contraction mode on fatigue development.

METHODS

Muscle fatigue was induced by isotonic and quasi-isometric contractions in rat soleus (SOL) and extensor digitorum longus (EDL) muscles, using identical stimulation protocol (60 Hz, 400 ms s-1) for 100 s in SOL and 60 s in EDL. Fatigue was quantified as the decline in peak values of shortening, shortening velocity, relaxation and work during the isotonic contractions, and, correspondingly, of force, rate of force development, relaxation and work during the quasi-isometric contractions. Maximal test contractions (60 Hz, 1.5 s) performed before and after fatigue were analysed for decline in force development (Fmax), rate of force development (dF/dtmax) and relaxation (-dF/dtmax).

RESULTS

Fmax declined to significantly lower values after isotonic than after quasi-isometric fatiguing contractions (fatigued in percentage of unfatigued): 58.5 +/- 6.4% vs. 64.4 +/- 7.0% in SOL, and 30.4 +/- 4.1% vs. 33.3 +/- 3.6% in EDL, respectively. The same pattern was seen for dF/dtmax which decreased to: 46.3 +/- 9.9% vs. 52.3 +/- 8.5% in SOL, and 19.1 +/- 4.3% vs. 22.3 +/- 3.2% in EDL after isotonic and quasi-isometric contractions, respectively. Similarly, when comparing fatigue development during the two contraction modes, the respective fatigue variables decreased more rapidly and to lower levels during isotonic vs. quasi-isometric contractions. During maximal test contractions, the dynamic fatigue variables (+/-dF/dtmax) declined to significantly lower levels than Fmax.

CONCLUSIONS

Fatigue development was significantly larger during isotonic vs. quasi-isometric contractions. The use of force as the only experimental fatigue variable may underestimate the functional impairment of fatigued muscle, neglecting the fatigue effect on time and length dimensions.

Biomechanical model predicting electromyographic activity in three shoulder muscles from 3D kinematics and external forces during cleaning work

BACKGROUND

The shoulder region is a common site of work-related musculoskeletal disorders. Biomechanical models may reveal the relative importance of force, joint-moments, and angular velocity for predicting muscle activity, thereby contributing to identify risk factors.

OBJECTIVE

The aim of the present study was to predict muscle activity patterns from joint kinetics during cleaning work and to identify the most important variables requesting muscle activity.Design. A comparative study of six cleaners performing five different floor cleaning tasks (combinations of tool and working method) in a laboratory setting.

METHODS

Net forces and moments at the glenohumeral joint were estimated using a video-based 3D link segment model together with 3D force-transducers at each hand, separately. Angular velocities of the upper arm were calculated, and electromyographic activity was recorded bilaterally from the muscles trapezius, deltoideus, and infraspinatus.

RESULTS

The biomechanical model revealed abduction moment in the glenohumeral joint to be the most important factor for development of muscle activity in m. deltoideus and m. infraspinatus, while for m. trapezius vertical force was most important.

CONCLUSION

Muscle specific determinants for shoulder muscle activity could be identified from glenohumeral joint kinetics.

RELEVANCE

This study documents that mechanical work requirements in terms of joint forces, moments of force and angular velocities can predict major fractions of muscle activity patterns in the upper extremities. The biomechanical model used for this prediction revealed different factors of importance for individual muscles. This knowledge is fundamental for work place interventions aiming at minimizing overloading of specific muscles to prevent or rehabilitate muscle disorders.

Muscle fibre type, efficiency, and mechanical optima affect freely chosen pedal rate during cycling

This study investigated the variation in freely chosen pedal rate between subjects and its possible dependence on percentage myosin heavy chain I (%MHC I) in m. vastus lateralis, maximum leg strength and power, as well as efficiency. Additionally, the hypothesis was tested that a positive correlation exists between percentage MHC I and efficiency at pre-set pedal rates but not at freely chosen pedal rate. Twenty males performed cycling at low and high submaximal power output ( approximately 40 and 70% of the power output at which maximum oxygen uptake (VO(2max)) was attained at 80 r.p.m.) with freely chosen and pre-set pedal rates (61, 88, and 115 r.p.m.). Percentage MHC I as well as leg strength and power were determined. Freely chosen pedal rate varied considerably between subjects: 56-88 r.p.m. at low and 61-102 r.p.m. at high submaximal power output. This variation was only partly explained by percentage MHC I (21-97%) as well as by leg strength and power. Interestingly, %MHC I correlated significantly with the pedal rate at which maximum peak crank power occurred (r = -0.81). As hypothesized, %MHC I and efficiency were unrelated at freely chosen pedal rate, which was in contrast to a significant correlation found at pre-set pedal rates (r = 0.61 and r = 0.57 at low and high power output, respectively).

CONCLUSIONS

Subjects with high percentage MHC I chose high pedal rates close to the pedal rates at which maximum peak crank power occurred, while subjects with low percentage MHC I tended to choose lower pedal rates, favouring high efficiency. Nevertheless, the considerable variation in freely chosen pedal rate between subjects was neither fully accounted for by percentage MHC I nor by leg strength and power. Previously recognized relationships between percentage Type I ( approximately %MHC I) and efficiency as well as between pedal rate and efficiency were confirmed for pre-set pedal rates, but for freely chosen pedal rate, these variables were unrelated.

Blood flow and oxygen uptake increase with total power during five different knee-extension contraction rates

Controversies exist regarding quantification of internal power (IP) generated by the muscles to overcome energy changes of moving body segments when external power (EP) is performed. The aim was to 1) use a kinematic model for estimation of IP during knee extension, 2) validate the model by independent calculation of IP from metabolic variables (IP(met)), and 3) analyze the relationship between total power (TP = EP + IP) and physiological responses. IP increased in a curvilinear manner (5, 7, 13, 21, and 34 W) with contraction rate (45, 60, 75, 90, and 105 contractions/min), but it was independent of EP. Correspondingly, IP(met) was 5, 7, 10, 19, and 28 W, supporting the kinematic model. Heart rate, pulmonary oxygen uptake, and leg blood flow plotted vs. TP fell on the same line independent of contraction rate, and muscular mechanical efficiency as well as delta efficiency remained remarkably constant across contraction rates. It is concluded that the novel metabolic validation of the kinematic model supports the model assumptions, and physiological responses proved to be closely related to TP, supporting the legitimacy of IP estimates.

Development of muscle fatigue as assessed by electromyography and mechanomyography during continuous and intermittent low-force contractions: effects of the feedback mode

The goal of the present study was to investigate the significance of low-force continuous or intermittent static contraction and feedback mode (visual or proprioceptive) on the development of muscle fatigue as assessed by electromyography (EMG) and mechanomyography (MMG). Visual (force control) and proprioceptive (displacement control) feedback was investigated during intermittent (6 s contraction, 4 s rest) and continuous static contractions at 10% and 30% of the maximum voluntary contraction (MVC). Mean force, force fluctuation, rating of perceived exertion and root mean square (RMS) and mean power frequency (MPF) of the EMG and MMG signals were analysed. The general pattern for MMG RMS and EMG RMS values and the rating of perceived exertion was an increase with contraction time, while the EMG MPF values decreased ( P<0.05). The increase in RMS values was generally more pronounced for the MMG compared with the EMG, while the decrease in MPF values was more consistent for the EMG compared with the MMG signal. During the intermittent contractions, the main effect was on MPF for both EMG and MMG. Lower force fluctuation and larger rating of perceived exertion ( P<0.05), greater slopes of EMG and MMG RMS and MPF values versus time were observed with proprioceptive feedback compared with visual feedback. The findings suggest that (1) the EMG and MMG signals give complementary information about localised muscle fatigue at low-level contraction: they responded differently in terms of changes in the time and frequency domain during continuous contraction, while they responded in concert in the frequency domain during intermittent contractions, and (2) the different centrally mediated motor control strategies used during fatiguing contraction may be dependent upon the feedback modality.

Assessment of postexercise muscle soreness by electromyography and mechanomyography

Mechanomyography (MMG) and electromyography (EMG) recordings from the first dorsal interosseous muscle of the hand were compared for pre-exercise and immediately after, 24-hour, and 48-hour postexercise muscle soreness. Thirteen healthy male subjects performed progressively increasing number of eccentric contractions from bout 1 (10.34 +/- 1.96) to bout 6 (27.46 +/- 5.01) (P < .03) with 116% maximum voluntary contraction (MVC) for provocation of postexercise muscle soreness. Increased areas of pain, reduced pressure pain threshold, reduced MVC, and reduced range of motion were present immediately after as compared with pre-exercise (P < .05). During intense eccentric exercise, root mean squared amplitude values of MMG increased progressively from bout 1 to bout 6, but EMG root mean squared amplitude decreased as the muscle fatigued (P < .05). Time course changes of MMG and EMG root mean squared amplitude values during single concentric, isometric, and eccentric contractions at 0%, 25%, 50%, 75%, and 100% MVC weights were measured in relation to postexercise muscle soreness. The EMG root mean squared amplitude values showed insignificant changes for concentric, isometric, and eccentric contractions between pre-exercise, immediately after, 24 hours, and 48 hours. MMG root mean squared amplitude values increased during concentric, isometric, and eccentric contractions at immediately after as compared to pre-exercise, 24 hours, and 48 hours. At immediately after, 24 hours, and 48 hours the maximum EMG root mean squared amplitude values were achieved at lower MVC levels as compared with pre-exercise (P < .05). MMG root mean squared amplitude findings suggest changes in viscoelastic properties resulting in significant mechanical muscle vibrations after intense eccentric exercise. This may suggest a role of stimulation of mechanosensitive nociceptors in relation to postexercise muscle soreness. It is concluded that simultaneous recordings of MMG and EMG may serve as an appropriate means of studying the relationship between electrical and mechanical muscle activity occurring with postexercise muscle soreness.

Doublets in motor unit activity of human forearm muscle during simulated computer work

Motor unit firing pattern and characteristics were analyzed during voluntary contractions as they occur during computer work. For comparison controlled standard ramp contractions were performed. Highly diverse firing patterns were identified while the MU characteristics were similar. Of special notice is the frequent occurrence of double discharges during computer like work but not during ramp contractions.

Crank inertial load affects freely chosen pedal rate during cycling

Cyclists seek to maximize performance during competition, and gross efficiency is an important factor affecting performance. Gross efficiency is itself affected by pedal rate. Thus, it is important to understand factors that affect freely chosen pedal rate. Crank inertial load varies greatly during road cycling based on the selected gear ratio. Nevertheless, the possible influence of crank inertial load on freely chosen pedal rate and gross efficiency has never been investigated. This study tested the hypotheses that during cycling with sub-maximal work rates, a considerable increase in crank inertial load would cause (1) freely chosen pedal rate to increase, and as a consequence, (2) gross efficiency to decrease. Furthermore, that it would cause (3) peak crank torque to increase if a constant pedal rate was maintained. Subjects cycled on a treadmill at 150 and 250W, with low and high crank inertial load, and with preset and freely chosen pedal rate. Freely chosen pedal rate was higher at high compared with low crank inertial load. Notably, the change in crank inertial load affected the freely chosen pedal rate as much as did the 100W increase in work rate. Along with freely chosen pedal rate being higher, gross efficiency at 250W was lower during cycling with high compared with low crank inertial load. Peak crank torque was higher during cycling at 90rpm with high compared with low crank inertial load. Possibly, the subjects increased the pedal rate to compensate for the higher peak crank torque accompanying cycling with high compared with low crank inertial load.

The effect of short-term strength training on human skeletal muscle: the importance of physiologically elevated hormone levels

The effect of strength training and endogenously elevated hormone levels (plasma testosterone, growth hormone (GH) and cortisol) was studied in 16 young untrained males, divided into an arm only training group, A, and a leg plus arm training group, LA, in order to increase circulating levels of anabolic hormones. Both groups performed the same one-sided arm training for 9 weeks, twice a week. Group A trained only one arm (AT), the contralateral arm serving as control (AC), whereas group LA additionally trained their legs following the training of the one arm (LAT), with the contralateral arm serving as control (LAC). In spite of the attempt to match the two groups, the initial isometric arm strength was 20-25% lower for group LA compared to group A (significant for the arm to be trained). Isometric strength increased significantly in LAT and LAC by 37% and 10%, respectively, while the 9% and 2% increases in AT and AC, respectively, remained insignificant. Isokinetic strength increased at one out of three velocities tested for the trained arm relative to the untrained arm in both group A and group LA (P<0.05). Functional strength increased significantly by 20% in LAT, 18% in LAC, 19% in AT, and 17% in AC. Hormonal responses were monitored during the first and last training sessions. Resting hormone levels remained unchanged for both groups. However, during the first training session plasma testosterone as well as plasma cortisol increased significantly in group LA but not in group A. Plasma GH rose in all exercise tests, except during the last test in group LA, but was significantly higher in group LA than in group A in the first training session. In conclusion, a larger relative increase in isometric strength was found in the group having the highest hormonal response. However, due to the initial difference in isometric strength caution must be taken with the interpretation of this finding, which may only indicate a possible link between anabolic hormones and muscle strength with training.

Dynamic loads on the upper extremities during two different floor cleaning methods

BACKGROUND

Biomechanical models have been used frequently to estimate the load on the low back and the lower extremities during occupational work tasks. The shoulder region has received much less attention although this area is a common site of work-related musculoskeletal disorders.

OBJECTIVE

The aim of the present study was to evaluate the mechanical loading of the upper extremities during floor cleaning performed with different techniques.

DESIGN

A comparative study of six subjects performing floor cleaning with two different techniques (mopping and scrubbing) in a laboratory setting.

METHODS

A "force handle" equipped with two force dynamometers was used for continuous measurement of three-dimensional forces on each hand, separately, during this two handed asymmetrical task. A link segment model of the upper extremities was used to calculate the mechanical loading of the elbows and shoulders. Electromyograms were recorded from six shoulder muscles.

RESULTS

Despite differences in movement patterns between mopping and scrubbing, only small differences were found in the moments of force. In accordance similar levels of electromyograms were found.

CONCLUSION

In preventive strategies, changing the cleaning tool may not be sufficient to change the load on the shoulder muscles.

RELEVANCE

Combined three-dimensional video recording and force measurements on the hands during a two-handed asymmetric work tasks allow calculation of the mechanical load on the elbow and shoulder. Thus it is possible to evaluate physiologically, new equipment designed to prevent work-related musculoskeletal disorders. Before introducing such equipment in large scale interventions it may be evaluated if the changes in force requirements are sufficient to predict prevention of musculoskeletal disorders.

Motor unit activity during stereotyped finger tasks and computer mouse work

Motor unit (MU) activity pattern was examined in the right-hand extensor digitorum communis muscle (EDC) during standardised finger movements simulating actual computer mouse tasks. Intramuscular recordings were performed with a quadripolar needle electrode. Nine women performed four lifts of their right-hand index finger, middle finger or both as well as a number of double clicks. Additionally, the subjects performed contra lateral activity with their left-hand fingers and for three subjects recordings were also obtained during an interview with no physical activity. Besides the expected close coupling of MU activity with finger movement, activity was observed in three different situations with no physical requirements. Attention related activity was found before or after performance of the finger movement task, contra lateral activity in right EDC during left-hand finger tasks, and activity during mental activity without any finger movements involved. A relatively large number of doublet occurrences suggest they are a natural part of the activation pattern during performance of the rapid finger movement required to perform an efficient double click on the computer mouse.

Aspects of shoulder function in relation to exposure demands and fatigue - a mini review

OBJECTIVE

To discuss the literature on control and function of multiple muscle systems with special focus on shoulder and upper extremity under submaximal conditions.

DESIGN

The paper is a mini review based on 31 studies.

BACKGROUND

Control mechanisms underlying the recruitment and gradation of muscle activity in complex multiple muscle systems during various voluntary exertions is still not fully understood. Load sharing principles may be influenced by several factors like work demand, fatigability, metabolic factors etc.

METHODS

Several methods, e.g., electromyography, intramuscular pressure, and tissue oxygenation are used. The definition of fatigue is discussed.

RESULTS AND CONCLUSION

A relatively fixed load sharing of the shoulder muscles has been found at low load levels, submaximal speeds and with a limited range of movements of the arms for healthy subjects. However, the load sharing of the shoulder muscles can be changed to some extent in response to fatigue, mental demands, visual feedback and in patients suffering from disorders in the upper extremities. It is hypothesized that lack of ability to redistribute muscle activation pattern in the shoulder region or the upper extremity increases the risk of development of work related symptoms.

Muscle involvement during intermittent contraction patterns with different target force feedback modes

OBJECTIVE

Assess the effect of different feedback modes during intermittent contractions on primary and assessory muscle activity.

BACKGROUND

Intermittent contractions and physiological responses have been studied in laboratory settings. However, the feedback given to the subjects regarding timing and force level is generally not specified.

DESIGN

Repeated measure design in which six subjects in randomized order performed two experimental conditions only differing in feedback mode.

METHODS

Intermittent static elbow flexion was performed against either a fixed-force transducer (visual feedback) or a weight to be held in position (proprioceptive feedback) both corresponding to 30% maximal voluntary contraction. Contraction and relaxation timing of 6 and 4 s, respectively, was shown on a VDU screen as colour code identical in both conditions.

RESULTS

Test contractions performed before and following 30 min of intermittent contractions showed larger fatigue development with proprioceptive feedback than visual feedback. Also rating of perceived exertion increased more during proprioceptive feedback than visual feedback. This may in part be explained by small differences in the mechanics during the two different feedback modes. In line with this, EMG recorded from four shoulder/arm muscles analyzed for amplitude and frequency showed similar activity initially; but later, during the 30 min contraction larger amplitudes were attained during proprioceptive feedback than visual feedback.

CONCLUSIONS

Feedback mode significantly effects the muscle involvement and fatigue during intermittent contractions. RelevanceIntermittent contractions are common in many work places and various feedback modes are being given regarding work requirements. The choice of feedback may significantly affect the muscle load and consequently the development muscle fatigue and disorders.

Consistency of motor-unit identification during force-varying static contractions

Due to inter-operator variability, two operators were used to assess the consistency of motor unit (MU) identification during ramp contractions, by the comparison of semi-automatic decompositions of the same recordings. Static shoulder abduction was performed against a force transducer in a position with the upper arms vertical and elbows flexed to 90 degrees. The subjects followed an 8-s force trajectory: 30% maximum voluntary contraction (MVC, 2 s), a reduction in force from 30% to 0% MVC (2 s), 0% MVC (1 s), an increase in force from 0 to 30% MVC (2 s), and 30% MVC (1 s). Muscle activity was recorded from the supraspinatus muscle with a quadripolar needle. From six recordings of 8 s duration, a total of 2527 MU firings were identified by both operators, and 93% of these were identified identically into 31 MUs. Both operators identified 8 of these MUs as continuously firing, 5 as only being active either before or after the 1 s at 0% MVC, and 18 as being de-recruited during force decreases and recruited during force increases. Both operators agreed that 16 of these 18 MUs were de-recruited at a higher force level than that at which they were recruited, which may be due to the electromechanical delay. The coefficient of variation for double determination of the results obtained by operators A and B was 8.5% for the number of MU firings, 4.5% for the MU mean firing rate, and 8.4% for the MU action potential (MUAP) amplitude. Therefore, the operator interactive decomposition method was considered to be valid for studying recruitment and de-recruitment as well as firing rate and MUAP amplitude during static, force-varying ramp contractions.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Motor unit recruitment and rate coding in response to fatiguing shoulder abductions and subsequent recovery

The purpose of the present study was to investigate motor unit (MU) recruitment and firing rate, and the MU action potential (MUAP) characteristics of the human supraspinatus muscle during prolonged static contraction and subsequent recovery. Eight female subjects sustained a 30 degrees shoulder abduction, requiring 11-12% of maximal voluntary contraction (MVC), for 30 min. At 10 and 30 min into the recovery period, the shoulder abduction was repeated for 1 min. The rating of perceived exertion for the shoulder region increased to "close to exhaustion" during the prolonged contraction, and the surface electromyography (EMG) recorded from the deltoid and trapezius muscles showed signs of local muscle fatigue. From the supraspinatus muscle, a total of 23,830 MU firings from 265 MUs were identified using needle electrodes. Of the identified MUs, 95% were continuously active during the 8-s recordings, indicating a low degree of MU rotation. The mean (range) MU firing rate was 11.2 (5.7-14.5) Hz, indicating the relative force contribution of individual MUs to be larger than the overall mean shoulder muscle load. The average MU firing rate remained stable throughout the prolonged abduction, although firing rate variability increased in response to fatigue. The average concentric MUAP amplitude increased by 38% from the beginning (0-6 min) to the end (24-29 min) of the contraction period, indicating recruitment of larger MUs in response to fatigue. In contrast, after 10 min of recovery the average MU amplitude was smaller than seen initially in the prolonged contraction, but not different after 30 min, while the MU firing rate was higher during both tests. In conclusion, MU recruitment plays a significant role during fatigue, whereas rate coding has a major priority during recovery. Furthermore, a low degree of MU rotation in combination with a high relative load at the MU level may imply a risk of overloading certain MUs during prolonged contractions.

Impaired sarcoplasmic reticulum Ca(2+) release rate after fatiguing stimulation in rat skeletal muscle

The purpose of the study was to characterize the sarcoplasmic reticulum (SR) function and contractile properties before and during recovery from fatigue in the rat extensor digitorum longus muscle. Fatiguing contractions (60 Hz, 150 ms/s for 4 min) induced a reduction of the SR Ca(2+) release rate to 66% that persisted for 1 h, followed by a gradual recovery to 87% of prefatigue release rate at 3 h recovery. Tetanic force and rate of force development (+dF/dt) and relaxation (-dF/dt) were depressed by approximately 80% after stimulation. Recovery occurred in two phases: an initial phase, in which during the first 0.5-1 h the metabolic state recovered to resting levels, and a slow phase from 1-3 h characterized by a rather slow recovery of the mechanical properties. The recovery of SR Ca(2+) release rate was closely correlated to +dF/dt during the slow phase of recovery (r(2) = 0.51; P < 0.05). Despite a slowing of the relaxation rate, we did not find any significant alterations in the SR Ca(2+) uptake function. These data demonstrate that the Ca(2+) release mechanism of SR is sensitive to repetitive in vitro muscle contraction. Moreover, the results indicate that +dF/dt to some extent depends on the rate of Ca(2+) release during the slow phase of recovery.

Kinetics and energetics during uphill and downhill carrying of different weights

During physically heavy work tasks the musculoskeletal tissues are exposed to both mechanical and metabolic loading. The aim of the present study was to test a biomechanical model for prediction of whole-body energy turnover from kinematic and anthropometric data during load carrying. Total loads of 0, 10 and 20 kg were carried symmetrically or asymmetrically in the hands, while walking on a treadmill (4.5 km h(-1)) horizontally, uphill, or downhill the slopes being 8%. Mean values for the directly measured oxygen uptake ranged for all trials from 0.5 to 2.1 l O2 min(-1), and analysis of variance showed significant differences regarding slope, load carried, and symmetry. The calculated values of oxygen uptake based on the biomechanical model correlated significantly with the directly measured values, fitting to the line Y = 0.990 X + 0.144, where Y is the estimated and X is the measured oxygen uptake in l min(-1). The close relationship between energy turnover rate measured directly and estimated based on a biomechanical model justifies the assessment of the metabolic load from kinematic data.

Muscle responses to simulated torque reactions of hand-held power tools

The aim of this work was to investigate physiological responses to torque reaction forces produced by hand-held power tools used to tighten threaded fasteners. Such tools are used repetitively by workers in many industries and are often associated with upper limb musculoskeletal complaints. The tools considered for stimulation in this study had straight handles and required from 100 to 400 ms to tighten fasteners to a peak torque of 1.0 to 2.5 Nm and from 50 to 150 ms for the torque to decay to zero. A tool stimulator was constructed to apply a programmed torque profile to a handle similar to that of a straight in-line power screwdriver. Wrist flexor and extensor surface EMGs and handle position were recorded as subjects held handles subjected to controlled torque loads that tended to flex the wrist. It was found that: (1) very high EMG values occurred even though torques were of short duration (50 to 600 ms) and the peak torques were low (7-28% of maximum strength); (2) high EMGs in anticipation of torque are directly related to torque build-up rate and peak torque; (3) high peak flexor and extensor EMGs during and following torque onset are related to torque build-up rate and peak torque; (4) minimum time of peak EMGs of 72-87 ms following the onset of torques with 50 ms build-up suggests the contribution of an extensor muscle stretch reflex component; delayed peak for longer build-ups suggests a central control of muscle force in response to torque; (5) angular excursions of handles increase with decreasing torque build-up time and increasing torque magnitude causes increasing eccentric work; (6) the results show that the slow torque build-up times (450 ms) correspond to minimum peak EMGs; and (7) accumulated EMGs increase with increasing torque and torque build-up times. Further studies are needed to evaluate fatigue and musculoskeletal injuries associated with prolonged periods of tool use.

Effect of speed and precision demands on human shoulder muscle electromyography during a repetitive task

Effects of speed and precision on electromyography (EMG) in human shoulder muscles were studied during a hand movement task where five points were marked repeatedly with a pencil. Six female subjects performed with three precision demands and at four speeds. Three of the speeds were predefined, while the last speed was performed as fast as possible. The EMG were recorded from 13 shoulder muscles or parts of muscles. Elbow velocity, acceleration and rectified EMG were calculated for each task. The mean elbow velocity and acceleration increased with speed and precision demands. There was an increase in EMG as the speed demand increased for all three precision demands (P < 0.001), and as the precision demand increased for the two highest predefined speed demands (P < 0.05). The combination of a high speed and a high precision demand resulted in the highest EMG. Different EMG levels were attained for the 13 muscles and the supraspinatus muscle always showed the highest normalized EMG. However, analysis of variance showed the same relative increase for all muscles with speed and precision demands. The EMG changes in response to precision demand can only be explained in part by the differences in movement velocity and acceleration, and other factors such as increased co-contraction must also be taken into account.

Motor unit activation patterns during concentric wrist flexion in humans with different muscle fibre composition

Muscle activity was recorded from the flexor carpi radialis muscle during static and dynamic-concentric wrist flexion in six subjects, who had exhibited large differences in histochemically identified muscle fibre composition. Motor unit recruitment patterns were identified by sampling 310 motor units and counting firing rates in pulses per second (pps). During concentric wrist flexion at 30% of maximal exercise intensity the mean firing rate was 27 (SD 13) pps. This was around twice the value of 12 (SD 5) pps recorded during sustained static contraction at 30% of maximal voluntary contraction, despite a larger absolute force level during the static contraction. A similar pattern of higher firing rates during dynamic exercise was seen when concentric wrist flexion at 60% of maximal exercise intensity [30 (SD 14) pps] was compared with sustained static contraction at 60% of maximal voluntary contraction [19 (SD 8) pps]. The increase in dynamic exercise intensity was accomplished by recruitment of additional motor units rather than by increasing the firing rate as during static contractions. No difference in mean firing rates was found among subjects with different muscle fibre composition, who had previously exhibited marked differences in metabolic response during corresponding dynamic contractions. It was concluded that during submaximal dynamic contractions motor unit firing rate cannot be deduced from observations during static contractions and that muscle fibre composition may play a minor role.

A model predicting individual shoulder muscle forces based on relationship between electromyographic and 3D external forces in static position

To study the potentiality for developing an EMG-based model for the human shoulder, mapping of relations between static hand forces and electromyographic (EMG) activity of 13 shoulder muscles, were performed. The procedure was to perform by the hands slowly varying isometric forces up to 20% maximum voluntary force in the three-dimensional space. By combining these data with literature values on muscle physiological cross-sectional area and moment arm data, an EMG-based model was developed for estimating muscle forces in the glenohumeral joint. The model was validated for one standardized position by comparing joint moment, calculated from EMG by using the model, with moments from the external force. The highest correlation between these moments was found assuming a linear EMG/force calibration at low force level (< 20% MVC), giving correlations from 0.65 to 0.95 for the abduction/adduction moment and from 0.70 to 0.93 for the flexion/extension moment, for the six subjects. Moments calculated from EMG were for most subjects somewhat lower than the moments from the external force; the mean residual error ranged from 1.6 to 9.9 Nm. Taking this into account, the results can be used for assessment of muscle forces based on recordings of external forces at the hands during submaximal static work tasks without substantially elevated arms.

Muscle injury in repetitive motion disorders

Documentation of causality between repetitive motions and musculoskeletal disorders calls for detailed understanding of the exposure variables and the corresponding physiologic responses in the biologic tissues. Quantification of the kinetics in some jobs characterized by repetitive motions is summarized with the physiologic responses in the muscles. Muscle activity pattern was studied in different shoulder muscles or muscle parts, and in elbow and wrist flexor muscles. Activity pattern was dependent on the kinetics of the work requirements. This holds true for the compound electromyographic signal and for single motor units. Low threshold motor units have been identified that are recruited continuously, the so called Cinderella fibers. The physiologic consequences of prolonged muscle fiber activity are reviewed, revealing mechanisms for the development of necrotic changes in the muscle, which support the likelihood of work relatedness for musculoskeletal disorders.

Motor control and kinetics during low level concentric and eccentric contractions in man

Motor unit (MU) recruitment patterns were studied in 6 female subjects during dynamic contractions at relative workloads corresponding to 10% maximum voluntary contraction. The contractions consisted of a 20 degree elbow flexion (concentric contraction) and extension (eccentric contraction) and MU action potential trains were recorded from the brachial biceps muscle. The mean angular velocity of the dynamic contractions was 10 degrees/s, during which a total of 119 MUs were identified. Additionally, a few contractions were studied at 20 degrees/s during which 30 MUs were identified, and 9 MUs during the 40 degrees/s contraction. About 60% of the identified MUs were active during the concentric as well as the eccentric phase for each of the velocities. Mean firing rate decreased significantly when the contraction changed from concentric to eccentric, whereas the number and properties of identified active MUs were similar. This emphasizes firing rate modulation as important during low level dynamic contractions rather than selective recruitment of different types of MUs in the concentric versus the eccentric phase. Similar kinetic demands occur frequently in occupational tasks, especially during monotonous work. The present data indicate that only a limited pool of MUs are being recruited during such tasks. Extensive recruitment of these MUs may cause fatigue and start a potentially vicious circle leading to work-related muscle disorders.

Musculoskeletal Disorders: Work-related Risk Factors and Prevention

Work-related musculoskeletal disorders cause chronic pain and functional impairment, impose heavy costs on society, and reduce productivity. In this position paper from the Scientific Committee for Musculoskeletal Disorders of the ICOH, the most important risk factors at work are described. Work-related musculoskeletal disorders have high incidences and prevalences among workers who are exposed to manual handling, repetitive and static work, vibrations, and poor psychological and social conditions. The application of ergonomic principles in the workplace is the best method for prevention. International organizations should develop standards, common classifications, and terminologies. Surveillance systems should be further developed nationally and in workplaces, and more effort should be directed to information dissemination, education, and training.

Potassium and fatigue: the pros and cons

A general finding is that muscle activity leads to potassium fluxes across the muscle membrane as well as to muscle fatigue, defined as a reduction in the force-generating capacity of the muscle. However, much controversy exists regarding the causal role of potassium in fatigue development. The experimental model used is decisive in this context, e.g. whether we study intact in vivo organs in situ with voluntary contractions and reflex feedback for cardiorespiratory regulation, or whether we study in vitro isolated muscles or cells-or even skinned fibres. In the latter models, clear evidence has been presented that Ca2+ is the variable significant for force development and that K+ may be ignored. However, in the in situ situation the limiting link in the chain leading to muscle contraction may be one preventing the Ca2+ release from taking place. The sites are the triads, T-tubules, and the surface membrane. The function of the latter two regarding action potential amplitude and propagation depends on [K+] gradients. Again, conflicting results exist regarding the electrophysiological changes and [K+] in fatigue. The activity pattern must then be taken into consideration. During high-intensity (high-frequency) activity the increased interstitial [K+] has been demonstrated to relate to fatigue, while in low-intensity fatiguing protocols, the T-tubule may be the limiting site. This fits with the concept of interstitial [K+] playing an essential role as a regulatory feedback mechanism, e.g. adjusting muscle blood flow to the metabolic load during muscle activity.

Musculoskeletal symptoms among sewing machine operators

OBJECTIVES

A longitudinal study was conducted to describe the prevalences and development of musculoskeletal symptoms among sewing machine operators in relation to age and exposure and among former sewing machine operators who changed exposure by changing occupation.

METHODS

Musculoskeletal symptoms were assessed among 327 sewing machine operators in 1985 with the use of the standardized Nordic questionnaire. A follow-up study in 1991 showed that approximately one-third was still working as a sewing machine operator, one-third had changed occupation, and the rest were out of employment. The exposure was assessed by a questionnaire regarding the type of machine being operated, work organization, workplace design, units produced per day, and payment system.

RESULTS

High prevalences of musculoskeletal symptoms of the neck and shoulders were found, with some associations to exposure variables such as efficiency. Initially symptom-free sewing machine operators were not at a higher risk of developing symptoms when they continued sewing during the six-year follow-up when compared with those who changed to other employment. However, symptomatic sewing machine operators who quit sewing were much more likely to be relieved of their symptoms than were symptomatic operators who continued sewing, odds ratio 3.26 [95% confidence interval (95% CI) 1.38-7.72] for 12-month symptoms and odds ratio 3.90 (95% CI 1.28-11.90) for 7-day symptoms. This trend also applied to long-lasting symptoms.

CONCLUSIONS

The results demonstrate that, for many sewing machine operators, neck and shoulder symptoms are reversible and may be influenced by reallocation to other worktasks.

Intramuscular laser-Doppler flowmetry in the supraspinatus muscle during isometric contractions

To study the regulation of microvascular blood flow in a compartment muscle, laser-Doppler measurements of muscle microcirculation were recorded in the supraspinatus muscle in eight volunteers during and following submaximal isometric muscle contractions. The subjects performed isometric shoulder abductions at five contraction levels from 5% to 50% maximal voluntary contraction for 1 min each and a sustained 30 degrees shoulder abduction for 20 min. The subjects' perceived exertion increased from "no perceived exertion" to "near maximal exertion" during the 20-min period with 30 degrees shoulder abduction. Microcirculation increased during all 1-min contractions. Following the contractions at 20%, 30% and 50% MVC post-exercise reactive hyperaemia was seen for a period of at least 1 min. The reactive hyperaemia increased in magnitude in response to increasing contraction level. The results showed the same time-history of the blood flow at microvascular level as previously seen in larger peripheral vessels in response to muscle contractions. During the 20-min contraction microcirculation increased in line with the findings during the brief contractions. However, in contrast to the brief contractions no postexercise reactive hyperaemia occurred following the prolonged contraction. Lack of postexercise reactive hyperaemia following the prolonged shoulder abduction would suggest insufficient regulation of the vascular resistance. Alternatively, lack of hyperaemia could be taken as an indication of sufficient microcirculation during the preceding contraction. From previous studies on intramuscular pressure and metabolism the latter alternative would seem unlikely.