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Biomedical subjects

K Søgaard

Publications and source records attributed to K Søgaard.

31 records · Page 2Linked to original sources
PubMed2001

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.

Computers↗
PubMed2000

Control of the wrist joint in humans.

As one considers changes in motor activity from lower mammals to higher primates, one of the major changes one observes lies in the cortical control of forelimb muscles. There has been a shift from disynaptic control of spinal motoneurons in, for example, the cat, to a greater and greater percentage of monosynaptic control of hand and forelimb motoneurons in the primate. In spite of the species and evolutionary changes in the synaptic connections of the corticospinal tract, it appears that the interneurons identified in the cat are retained in the monkey and human. These interneurons, under the influence of descending pathways, modulate the output of motoneuron pools. Perhaps the control of these interneurons has also changed towards finer control of movement, as has been suggested by recent studies in the monkey. Whether in cat or human, the recruitment pattern for motor units is the same; the change from disynaptic to monosynaptic connections has not changed the recruitment pattern of muscles. Differences in the recruitment patterns of muscles may lie in the finer control of inputs to motoneurons in the primate. This review seeks to integrate the current knowledge of the mechanisms involved in the motor control of the wrist joint and especially in the recruitment patterns of the muscles. These motor control mechanisms include the biomechanics of the wrist joint, recruitment patterns of wrist muscles, interneurons and spinal cord circuits in the cervical regions mediating the output of spinal motoneurons, and the supraspinal control of these muscles.

Biomechanical Phenomena↗
PubMed2000

The influence of experimental muscle pain on motor unit activity during low-level contraction.

In the present study we compared motor unit (MU) activity in a painful extensor carpi ulnaris (ECU) muscle to that of a pain-free control. According to the pain adaptation model the activity of the painful ECU muscle may be inhibited and its antagonist activity increased during wrist extension performed as a pre-defined low-force ramp. The pre-defined low force may then be maintained by increased activity in the pain-free synergist muscles such as the extensor carpi radialis (ECR) muscle. Nine females (31-47 years old) participated in the study. Maximal voluntary contraction (MVC) of the wrist extensors was performed. A catheter was inserted into the ECU muscle to allow the injection of hypertonic saline to evoke muscle pain, and a concentric needle was inserted for the recording of MU activity. Surface electromyograms were recorded from a synergist and an antagonist (ECR and flexor carpi radialis) to the painful ECU muscle. A force ramp of isometric wrist extensions up to 10% MVC, with a force increase of 1% MVC x s(-1), were performed followed by 60 s of sustained contraction at 10% MVC. The number of MUs recruited was almost identical for baseline and with pain, and no effect of experimental muscle pain was found on the properties of the MUs (amplitude, area) or their firing characteristics (mean firing rate, firing variability) during low-force ramp contraction. During the sustained 10% MVC, no effect of pain was found for concentric or surface EMG of the forearm muscles. At low force levels no pain-induced modulations were found in MU activity, when the mechanical condition was similar to that of a control situation.

Adult↗
PubMed2000

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.

Adult↗
PubMed2000

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.

Biomechanical Phenomena↗
PubMed1998

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.

Adult↗
PubMed1998

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.

Cumulative Trauma Disorders↗
PubMed1996

Work load during floor cleaning. The effect of cleaning methods and work technique.

Cardiovascular and muscle load levels were evaluated during floor cleaning. A group of 12 experienced female cleaners participated in the study. Of the subjects 6 used a mopping method and 6 a traditional scrub and cloth method. Heart rate, oxygen consumption, rating of perceived exertion, electromyography from the trapezius muscle and video recordings were obtained during floor cleaning. With respect to relative heart rate and oxygen consumption the two groups did not differ. The results revealed a high cardiovascular load corresponding on average to 53 percent of the individual maximal oxygen uptake. In addition 65 percent of the observed period was spent with the back in a position forward flexed more than 20 degrees C. The time spent in extreme forward back flexion was shorter for the mopping group. Both groups exhibited high static, median and peak shoulder muscle load levels of 10 percent, 25 percent and 54 percent maximal voluntary contraction, respectively. At the same time, however, the mopping group tended to have a higher shoulder load than the group using the scrub and cloth method. Furthermore, electromyographic signs of fatigue in the trapezius muscle indicated a more stereotype activation of the shoulder muscles during mopping than during scrubbing. Based on these results, it was concluded that mopping cannot be recommended as less strenuous than scrubbing.

Adult↗
PubMed1996

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.

Action Potentials↗
PubMed1995

Motor unit recruitment pattern during low-level static and dynamic contractions.

Motor unit (MU) recruitment patterns were studied during dynamic and static contractions at workloads corresponding to 10% of maximal voluntary contraction force. The dynamic contraction consisted of a 20 degrees flexion and extension of the elbow performed with a velocity of 10 degrees/s. Motor unit potential trains were recorded from the brachial biceps muscle of 6 healthy females using a quadripolar needle electrode and a computerized decomposition program. Properties of the identified MUs were derived from concentric needle EMG. A total of 119 MUs were identified during dynamic contractions, 107 MUs during static anisotonic contractions, and 96 MUs during static isotonic contractions. The main result was that MUs recruited during different contractions showed similar properties and may belong to the same part of the motoneuron pool. This indicates that MU recruitment patterns during dynamic contractions may be almost as stereotypical as during static contractions and may even activate the same MUs.

Action Potentials↗
PubMed1993

Shoulder muscle load and muscle fatigue among industrial sewing-machine operators.

Physiological responses to physical work were assessed for 29 female industrial sewing-machine operators during an 8-h working day under ordinary working conditions. During sewing-machine work, the average (left and right) static load in the trapezius muscle was 9% of the maximal electromyogram (EMG) amplitude (% EMGmax), while the average mean load was 15% EMGmax, and the average peak load was 23% EMGmax. The static load level was unrelated to the muscle strength of the sewing-machine operators, which for the group as a whole was within the normal range. The load levels remained unchanged during the working day, while changes in the EMG mean power frequency and zero crossing frequency rate occurred, both indicating the development of muscle fatigue in left and right trapezius muscle during the working day. In line with this, the rating of perceived exertion in the shoulder and neck region increased during the working day. Dividing the group of sewing-machine operators into two groups, those with the highest frequency and those with the lowest frequency of shoulder/neck troubles showed that the former group had significantly lower muscle strength, despite the fact that no differences in the surface EMG during sewing were found between the two groups. It was concluded that industrial sewing-machine work involves a pattern of shoulder muscle activity which induces fatiguing processes in the shoulder and neck regions. Furthermore, since the static shoulder muscle load was independent of muscle strength, factors other than working posture may be of significance for the static shoulder muscle load.

Adult↗
PubMedDate not supplied

Acardiac twins: pathophysiology, diagnosis, outcome and treatment. Six cases and review of the literature.

Twin gestation is often a hazardous pregnancy and especially the monochorionic twin pregnancy significantly contributes to fetal morbidity and mortality. Among the serious complications with twins, the twin-twin transfusion syndrome complicates 5-35% of monozygotic twin pregnancies with monochorionic placentation. Acardiac twinning, earlier known as chorioangiopagus parasiticus, is the most extreme manifestation of this condition. An acardiac twin is a rare complication of multifetal pregnancy, in the literature reported at an incidence of 1% of monochorionic twin pregnancies, i.e. 1 of 35,000 pregnancies. In the following paper we review the literature on the subject and report 6 cases, 5 twins and 1 triplet, that were diagnosed at our department during the period of 1993-1997 and treated conservatively. Only 1 child survived.

Delivery, Obstetric↗