Lotte Finsen

Musculoskeletal disorders among dentists and variation in dental work

The purpose was to assess risk factors in dentistry which may contribute to musculoskeletal disorders. A questionnaire was used to identify common work tasks, and to estimate one year prevalence for troubles (65% for the neck/shoulder, 59% for the low back). In a field study working postures and electromyography (shoulder/neck) were registered during the three most common work tasks. Prolonged neck flexion and upper arm abduction were found, as well as high static muscle activity levels (splenius and trapezius muscles). No differences between work tasks were found regarding postures, frequencies of movements or muscle activity. Alterations between the three work tasks do not produce sufficient variation to reduce musculoskeletal load on the neck and shoulders.

Musculoskeletal symptoms and duration of computer and mouse use

Abstract The primary aim was to study associations between duration of computer and mouse use and musculoskeletal symptoms among computer users. A questionnaire was delivered to 5033 employees in 11 Danish companies and institutions and 3475 subjects responded (69%). Logistic regression analyses on 2579 full-time working employees showed that working almost the whole working day with a computer was associated with neck symptoms (OR=1.92, CI: 1.21–3.02) and shoulder symptoms (OR=1.83, CI: 1.13–2.95) among women and hand symptoms (OR=2.76, CI: 1.51–5.06) among men. These odds ratios were adjusted for psychosocial factors. Among respondents working almost all of their work time with a computer the gender and age-adjusted odds ratio for mouse use more than half of the work time was 1.68 (CI: 1.22–2.31) for hand/wrist symptoms. Call center and data entry workers experienced the lowest possibilities for development at work. All work tasks involving computer use, except computer maintenance, were characterised by a higher frequency of movements than desk work without using a computer. Call center work was characterised by the highest level of repetitiveness as both work tasks and movements were perceived as repetitive. Relevance to industry Computer users with a long daily duration of computer use and mouse use experienced more musculoskeletal symptoms than those with a short duration of computer use. Computer work in general seemed characterised by repetitive movements, which may be a risk factor for musculoskeletal symptoms.

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.

Upper trapezius muscle activity patterns during repetitive manual material handling and work with a computer mouse.

Firstly, upper trapezius EMG activity patterns were recorded on the dominant side of 6 industrial production workers and on the side operating a computer mouse of 14 computer-aided design (CAD) operators to study differences in acute muscular response related to the repetitiveness of the exposure. The work tasks were performed with median arm movement frequencies ranging from 5 min(-1) to 13 min(-1) and were characterized by work cycle times ranging from less than 30 sec to several days. However, the static and median EMG levels and EMG gap frequencies were similar for all work tasks indicating that shoulder muscle loads may be unaffected by large variations in arm movement frequencies and work cycle times. An exposure variation analyses (EVA) showed that the EMG activity patterns recorded during production work were more repetitive than during CAD work, whereas CAD work was associated with more static muscle activity patterns, both may be associated with a risk of developing musculoskeletal symptoms. Secondly, upper trapezius EMG activity patterns recorded on the mouse side of the CAD operators were compared with those recorded on the non-mouse side to study differences in muscular responses potentially related to the risk of developing shoulder symptoms which were more prevalent on the mouse side. The number of EMG gaps on the mouse side were significantly lower than the values for the upper trapezius on the non-mouse side indicating that more continuous activity was present in the upper trapezius muscle on the mouse side and EVA analyses showed a more repetitive muscle activity pattern on the mouse side. These findings may be of importance to explain differences in the prevalence of shoulder symptoms.

Intramuscular and surface EMG power spectrum from dynamic and static contractions

During sustained static contractions an increase in the root mean square (rms) amplitude and a decrease in mean power frequency (MPF), or median power frequency (MF) of the electromyographic (EMG) signal are indicators for the development of muscle fatigue. However, when studying dynamic contractions the interpretation of these variables has been questioned. Therefore, the purpose was to compare the EMG variables recorded from a non-fatigued muscle during a slow low level dynamic contraction to those during a static contraction of similar force level. Surface and intramuscular EMG registrations were obtained from the brachial biceps muscle during: (a) a static isotonic contraction, (b) a dynamic contraction and (c) a static anisotonic contraction. During contractions (a) and (b) the recruitment pattern was analysed using the precision decomposition method. No differences in rms, MPF or MF between the dynamic and static contractions or between the concentric and eccentric phase of the dynamic contraction were found. Furthermore 60% of the identified motor units were active both in the concentric and the eccentric phase. This indicates that motor control during a slow dynamic contraction at low force level does not influence the power spectrum. We suggest that in occupational studies a possible muscle fatigue development with time can be estimated using EMG recordings from the work tasks.

Muscle Sizes and Moment Arms of Rotator Cuff Muscles Determined by Magnetic Resonance Imaging

Biomechanical models which require information on, e.g., joint torque and muscle force are useful in the estimation of when and how mechanical overload of the musculoskeletal system may lead to disorders. The aim was to study the reliability and validity of magnetic resonance imaging (MRI) to quantify muscle sizes and moment arms by MRI and to test selected anthropometric measures as predictors of muscle sizes and moment arms. A total of 20 healthy Scandinavian women (age 22–58 years) participated in an MRI scanning of their dominant shoulder. With a PC-based program the reliability and the validity of the MRI measurements was estimated to be high, and mean anatomical cross-sectional areas (ACSA) and muscle lengths were measured to be 4.0, 9.8 and 12.1 cm2 and 12.0, 12.6 and 12.8 cm for m. supraspinatus, m. infraspinatus and m. subscapularis, respectively. Volumes were calculated to be 48.8, 125.1 and 153.6 cm3. Moment arms were measured with the upper arm in a neutral position and in a functional position of 34° abduction for m. supraspinatus only, and were 2.4 and 2.6 cm. Physiological cross-sectional area (PCSA) and its fiber force component were estimated from dissected fiber length and pennation angle. MRI volume and PCSA were 1.4–1.7 times higher than dissection data, primarily because of age differences. No external anthropometric measures were found to be predictors of volumes or moment arms.

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.

Muscle activity and cardiovascular response during computer-mouse work with and without memory demands

Computer-mouse work is characterized by repetitive movements combined with mental demands. The present purpose was to study how the body responded to simulated Computer Aided Design (CAD) work without memory demand and when a high short-term memory demand was introduced. Nine female subjects repetitively performed a task which involved 15 s of elevation of the right index and middle fingers followed by 6 s of rest. Every second time the fingers rested, the left index finger was required to type a six-figure number, either ‘123456’ (without memory demand) or a random number shown half a minute before (with memory demand). After 7 min of performing the task without memory demand, the memory demand was introduced and continued for 1 h. Introduction of memory demand resulted in increased heart rate (77 ← 84 beats/min), blood pressure (systolic 129 ← 140 mmHg; diastolic 72 ← 79 mmHg) and forearm extensor muscle activity (wrist, 2.7 ← 4.5% EMGmax; finger, 5.6 ← 7.5% EMGmax) and finger flexor muscle activity (0.7 1.2% EMGmax) indicating increased co-contraction. Hereafter, muscle activity and cardiovascular response tended to decrease. Self-reported stress and rating of perceived exertion (RPE) for the right shoulder increased throughout the period. Two additional sub-studies were inlcuded, which focused on adaptation to the physical load, showing a decrease in muscle activity and arousal, and reintroduction of the memory load, showing a lower response as compared to the initial response. The practical consequences of the findings suggest that job content should have variable mental demands.

Biomechanical aspects of occupational neck postures during dental work

Abstract A typical occupational risk factor for developing neck symptoms is prolonged flexion of the cervical spine. The present aim was to determine joint moments and muscle activity of the neck during forward flexion of the cervical spine to evaluate the load in the neck region. Three dimensional video (3-D) and surface electromyography (EMG) from the splenius muscles were recorded in two common work postures. Using a 3-D static link segment model, moments at the atlanto-occipital (A-O) joint and the seventh cervical-first thoracal (C7-T1) joint were estimated. Maximal extension moments were estimated from maximal neck extension strength. Extension moments at the C7-T1 joint were significantly higher for a highly flexed position (45% of max) compared to a moderately flexed position (32% of max), but remained unchanged at the A-O joint (40% of max). The mean RMS amplitude was 9% of maximal EMG in both positions (no bilateral differences). This difference between mechanical load and muscle load indicates that EMG may seriously underestimate the total loads of the tissue. Lateral flexion influenced the lateral flexion moment while rotation did not influence the rotation moment. The study demonstrates the importance of quantification of joint loads in occupational risk assessment of the neck. Relevance to industry 3-D biomechanical calculations provide information on the mechanical load during work. Because EMG may underestimate total tissue load, calculations of joint moments in combination with information on muscle activity and strength are necessary to estimate different tissue loading of significance for overall risk identification.

Activity patterns of wrist extensor muscles during wrist extensions and deviations

Wrist extensor muscles are prone to certain focal musculoskeletal disorders for which the activation pattern of the extensor carpi radialis (ECR) and ulnaris (ECU) muscles may be important risk factors. Surface and intramuscular EMG of these muscles were recorded during isometric low‐force wrist extension in semipronation and pronation as well as for ulnar/radial deviation, and were analyzed using root mean square (RMS) and decomposition methods. Despite shorter ECR length at semipronation, higher amplitudes of intramuscular EMG and of motor unit action potentials (MUAPs) were found in pronation than in semipronation. However, these changes were not detectable in the surface EMG. Higher ECR activity levels were also found during wrist extension compared to ulnar/radial deviation, and differences in the motor unit (MU) properties were found during ulnar deviation compared to radial deviation and extension. Remarkably, the MUAPs of ECR were almost twice as large as those of the ECU. Overall, the ECR muscle did not respond as predicted from biomechanical considerations, and in general activity level was higher than expected. This may partly explain why the tendon of the ECR often is associated with lateral epicondylitis. Muscle Nerve, 2005