Reference Contraction Research Articles

Evidence of residual force enhancement for multi-joint leg extension

Force enhancement is a well accepted property of skeletal muscle and has been observed at all structural levels ranging from single myofibrils to voluntarily activated m. quadriceps femoris in vivo. However, force enhancement has not been studied for multi-joint movements like human leg extension; therefore knowledge about its relevance in daily living remains limited. The purpose of this study was to determine whether there is force enhancement during maximal voluntary multi-joint leg extension. Human leg extension was studied (n=22) on a motor driven leg press dynamometer where external reaction forces under the feet as well as activity of 8 lower extremity muscles were measured. In addition, torque in the ankle and knee joints was calculated using inverse dynamics. The steady-state isometric force, joint torques, and muscle activation after active stretch (20° stretch amplitude at 60°/s) were compared with the corresponding values obtained during isometric reference contractions. There was consistent force enhancement during and following stretch for both forces and joint torques. Potentiation during stretch reached values between 26% and 30%, while a significant force enhancement of 10.5–12.3% and 4.3–7.4% remained 0.5–1 and 2.5–3s after stretch, respectively. During stretch, EMG signals of m. gastrocnemius medialis and lateralis were significantly increased, while following stretch all analyzed muscles showed the same activity as during the reference contractions. We conclude from these results that force enhancement exists in everyday movements and should be accounted for when analyzing or modelling human movement.

Pharmacological characterization of KR-30988, a novel non-peptide AT1 receptor antagonist, in rat, rabbit and dog.

The pharmacological profile of KR-30988, a non-peptide AT1-selective angiotensin receptor antagonist, has been investigated by use of a variety of experimental models in-vitro and in-vivo. KR-30988 inhibited the specific binding of [125I][Sar1, Ile8]-angiotensin II to the recombinant AT1 receptor from man with a potency similar to that of losartan (IC50 values, the concentrations of drugs displacing 50% of specific binding, 13.6 and 12.3 nM, respectively), but did not inhibit the binding of [125I]CGP 42112A to recombinant AT2 receptor from man (IC50 >10 microM for both drugs). Scatchard analysis showed that KR-30988 interacted competitively with recombinant AT1 receptor from man in the same manner as losartan. In functional studies with rat and rabbit aorta, KR-30988 noncompetitively inhibited the contractile response to angiotensin II (pD2, = -log EC50 (where EC50 is the dose resulting in 50% of a reference contraction), 8.64 and 7.73, respectively) with a 20-85% decrease in the maximum contractile responses, unlike losartan. In pithed rats intravenous KR-30988 resulted in a non-parallel shift to the right of the dose-pressor response curve to angiotensin II (ID50 value, the dose inhibiting the pressor response to angiotensin II by 50%, 0.09 mg kg(-1)) with a dose-dependent reduction in the maximum responses; in this antagonistic effect KR-30988 was 20 times (approx.) more potent than losartan (ID50 1-74 mg kg(-1)). In conscious renal hypertensive rats oral administration of KR-30988 produced a dose-dependent and long-lasting (>24 h) anti-hypertensive effect; the potency was six times that of losartan (ED30 values, the dose reducing mean arterial blood pressure by 30 mmHg, 0.48 and 2.97 mg kg(-1), respectively). In conscious furosemide-treated dogs oral administration of KR-30988 produced a dose-dependent and long-lasting (>8 h) hypotensive effect with a rapid onset of action (time to Emax, the maximum effect, 1-2 h); KR-30988 was eight times more potent than losartan (ED20, the dose reducing mean arterial blood pressure by 20 mm Hg, 1.04 and 7.96 mg kg(-1), respectively). These results suggest that KR-30988 is a potent, orally active selective AT1 receptor antagonist with a mode of insurmountable antagonism.

Using normalized EMG to define the nociceptive flexion reflex (NFR) threshold: Further evaluation of standardized NFR scoring criteria

The nociceptive flexion reflex (NFR) has been used as a psychophysiological tool to study spinal nociceptive processes in numerous clinical and experimental studies. Despite widespread use of the NFR, few attempts have been made to empirically test and compare different scoring criteria to detect the presence/absence of the reflex. The present studies were conducted to address this issue. Study 1 (N=56 healthy participants) examined the reliability of 15 different scoring criteria that were examined in a previous report. Study 2 (N=73 healthy participants) extended this work by examining normalized scoring criteria based on biceps femoris activity unrelated to noxious stimulation (reference contraction, maximal contraction). In both studies, receiver operating characteristics (ROCs) analyses were used to evaluate and compare different scoring methods. The results indicate that a number of different criteria were acceptable for defining an NFR threshold based on the area under the ROC curve and its statistical significance; however, NFR Interval z score [(NFR Interval Mean−baseline mean)/baseline SD] emerged as the scoring criterion with the greatest accuracy and with cut-points that are reliable across samples. These findings support the application of a common NFR scoring criterion to enhance direct comparison of results across different research laboratories and study samples.

An Electromyographic Investigation Of The Biceps Brachii Using Adjustable Variable Resistance

Although variable resistance has been used extensively in the production of strength training equipment for many years, the physiological and biomechanical data analyzing this type of training remains scant. Furthermore, very few investigations have quantified how the use of different cams can alter muscle fiber recruitment. PURPOSE: To investigate differences in the electromyographic (EMG) muscle fiber recruitment of the Biceps Brachii (BB) at various joint angles during the Concentric (Con) and Eccentric (Ecc) phases of exercise on an Adjustable Variable Resistance (AVR) machine. METHODS: Eighteen healthy college aged (20.61±1.42) males performed 2 sets of BB flexion and extension exercise using divergent cam settings (A & B) of a STRIVE AVR machine. EMG and Electrogoniometer (EG) measurements were taken simultaneously to determine the electrical activity of the BB at joint angles corresponding to 40°, 85° and 120° during both the Con and Ecc phases of the movement. The pace of the exercise was controlled using a metronome and the resistance was a weight corresponding to 50% of a predetermined 6 repetition maximum. The Root Mean Square (RMS) EMG scores were rectified and normalized by dividing each score into a maximal isometric reference contraction performed prior to testing. The order in which the cam positions were introduced was randomized and sets were separated by 3:00 minutes of rest. EMG scores were analyzed using a standard T-test with an a-priori value of significance set at p<0.05. RESULTS: RMS EMG scores are reported below for cam positions A & B. CONCLUSIONS: Based upon the limitations of this study, we conclude that muscle activation can indeed be altered within a given range of motion by altering the orientation of the Cam.TABLE: 1No caption provided.

The influence of providing feedback on force production and within-participant reproducibility during maximal voluntary exertions for the anterior deltoid, middle deltoid, and infraspinatus

The normalization of electromyographic signals to a maximum voluntary reference contraction is common practice within the ergonomics research paradigm. However, there is a lack of support for a common protocol for obtaining maximum repeatable exertions. Specifically, there is minimal evidence to support the use of providing force magnitude feedback during the production of voluntary maximum exertions (MVE) in terms of both signal amplitude and repeatability. Therefore, the purpose of this study was to determine (1) if an MVE force magnitude feedback protocol increased both the force exerted and corresponding muscle activity, (2) if force magnitude feedback improved the within-participant reproducibility of the force or activity observed, and (3) if the surface electromyography (sEMG) signal processing method affected the repeatability of determining peak muscle activity. Seventeen participants completed a series of MVEs; first without feedback of the forces they produced, then with feedback of the forces they were producing, and again without feedback to determine if providing force feedback influenced their ability to produce a maximum force. Hand force and sEMG from the anterior deltoid, middle deltoid, and infraspinatus were measured during each exertion. The results showed that the highest forces and muscle activities were achieved when force feedback was provided. Force magnitude feedback resulted in a 7–22% increase in magnitude (for force and activity) and a decrease of 11–46% in the coefficient of variation specifying an improvement in the within-participant reproducibility. Signal processing techniques also affected within-participant reproducibility, however to a much lesser extent. The peak value from a 500-ms moving window average of the linear enveloped or root mean squared sEMG was the most reproducible technique tested.

Force–time history effects in voluntary contractions of human tibialis anterior

When an isometrically activated muscle is stretched or shortened the isometric steady-state force after the length change is increased (residual force enhancement) or decreased (force depression), respectively compared to a purely isometric contraction. This behavior has been observed consistently from the single sarcomere to the whole muscle level. However, the results for voluntary contractions in vivo are controversial and there are no studies for maximal voluntary contractions of medium sized muscles like the human ankle dorsiflexors. We investigated the effect of active muscle stretching and shortening for in vivo human tibialis anterior (n = 12) for maximal voluntary contractions for two magnitudes of stretching (15 degrees and 30 degrees) and two speeds of contraction (10 degrees/s and 45 degrees/s). Torques during stretches were higher compared to the purely isometric reference contractions and peak torques occurred prior to the end of the stretch. During the stretch, muscular activity decreased after peak torque had been reached for the high speed stretch experiments. In the steady-state phase following stretch, torque was increased for all experimental conditions but not for all time periods following stretch. The amount of residual force enhancement was independent of amplitude and speed of stretch. In the steady-state phase following shortening, torques were decreased compared to the isometric reference contractions and force depression was increased with increasing speeds of shortening.

Neuromuscular Recovery of the Biceps Brachii Muscle After Resistance Exercise

The aim of this study was to determine the time to restore the biceps brachii (BB) electromyographic (EMG) activity after the biceps curl (BC) exercise, at different intensities. Ten males performed initially maximal voluntary isometric contractions (MVC) of the elbow flexors, followed by one isometric submaximal contraction at 50% MVC (reference contraction). After this, four bouts of the BC at 25%, 30%, 35%, and 40% 1 RM during 1 minute (randomly assigned, with 10 minutes rest between them) were performed. During the rest intervals at preestablished moments (15 seconds, 1, 3, 5, and 10 min), isometric 50% MVC were performed. The EMG variables (root mean square [RMS], zero crossings [ZC], median frequency, [MF] and peak power [PP]) at rest were compared with reference values. Immediately after the exercise, RMS and PP increased, while ZC and MF decreased, indicating fatigue. After 1 minute most of the variables were similar to the reference. Different load levels did not affect the EMG recovery. In conclusion, the EMG variables recovered after 1 minute rest, indicating the optimal muscular condition for subsequent bouts.

Vastus lateralis surface and single motor unit EMG following submaximal shortening and lengthening contractions

A single shortening contraction reduces the force capacity of muscle fibers, whereas force capacity is enhanced following lengthening. However, how motor unit recruitment and discharge rate (muscle activation) are adapted to such changes in force capacity during submaximal contractions remains unknown. Additionally, there is limited evidence for force enhancement in larger muscles. We therefore investigated lengthening- and shortening-induced changes in activation of the knee extensors. We hypothesized that when the same submaximal torque had to be generated following shortening, muscle activation had to be increased, whereas a lower activation would suffice to produce the same torque following lengthening. Muscle activation following shortening and lengthening (20 degrees at 10 degrees /s) was determined using rectified surface electromyography (rsEMG) in a 1st session (at 10% and 50% maximal voluntary contraction (MVC)) and additionally with EMG of 42 vastus lateralis motor units recorded in a 2nd session (at 4%-47%MVC). rsEMG and motor unit discharge rates following shortening and lengthening were normalized to isometric reference contractions. As expected, normalized rsEMG (1.15 +/- 0.19) and discharge rate (1.11 +/- 0.09) were higher following shortening (p < 0.05). Following lengthening, normalized rsEMG (0.91 +/- 0.10) was, as expected, lower than 1.0 (p < 0.05), but normalized discharge rate (0.99 +/- 0.08) was not (p > 0.05). Thus, muscle activation was increased to compensate for a reduced force capacity following shortening by increasing the discharge rate of the active motor units (rate coding). In contrast, following lengthening, rsEMG decreased while the discharge rates of active motor units remained similar, suggesting that derecruitment of units might have occurred.

Effect of physical training on function of chronically painful muscles: a randomized controlled trial

Pain and tenderness of the upper trapezius muscle is frequent in several occupational groups. The objective of this study is to investigate the effect of three contrasting interventions on muscle function and pain in women with trapezius myalgia. A group of employed women (n = 42) with a clinical diagnosis of trapezius myalgia participated in a 10-wk randomized controlled intervention: specific strength training of the neck/shoulder muscles, general fitness training performed as leg bicycling, or a reference intervention without physical activity. Torque and electromyography (EMG) were recorded during maximal shoulder abductions in an isokinetic dynamometer at -60, 60, 0, and 180 degrees /s. Furthermore, a submaximal reference contraction with only the load of the arms was performed. Significant changes were observed only in the specific strength training group. Pain decreased by 42-49% (P < 0.01-0.05). Whereas the EMG activity of the unaffected deltoid remained unchanged during the maximal contractions, an increase in EMG amplitude (42-86%; P < 0.001-0.05) and median power frequency (19%; P < 0.001) were observed for the painful trapezius muscle. Correspondingly, torque increased by 18-53% (P < 0.001-0.05). EMG during the reference contraction decreased significantly for both the trapezius and deltoid muscles (P < 0.01). In conclusion, specific strength training relieves pain and increases maximal activity specifically of the painful trapezius muscle, leading to increased shoulder abduction strength in women with trapezius myalgia. Furthermore, decreased relative workload may indirectly augment pain reduction.

Activation-induced force enhancement in human adductor pollicis

It has been known for a long time that the steady-state isometric force after muscle stretch is bigger than the corresponding force obtained in a purely isometric contraction for electrically stimulated and maximal voluntary contractions (MVC). Recent studies using sub-maximal voluntary contractions showed that force enhancement only occurred in a sub-group of subjects suggesting that force enhancement for sub-maximal voluntary contractions has properties different from those of electrically-induced and maximal voluntary contractions. Specifically, force enhancement for sub-maximal voluntary contractions may contain an activation-dependent component that is independent of muscle stretching. To address this hypothesis, we tested for force enhancement using (i) sub-maximal electrically-induced contractions and stretch and (ii) using various activation levels preceding an isometric reference contraction at 30% of MVC (no stretch). All tests were performed on human adductor pollicis muscles. Force enhancement following stretching was found for all subjects ( n = 10) and all activation levels (10%, 30%, and 60% of MVC) for electrically-induced contractions. In contrast, force enhancement at 30% of MVC, preceded by 6 s of 10%, 60%, and 100% of MVC was only found in a sub-set of the subjects and only for the 60% and 100% conditions. This result suggests that there is an activation-dependent force enhancement for some subjects for sub-maximal voluntary contractions. This activation-dependent force enhancement was always smaller than the stretch-induced force enhancement obtained at the corresponding activation levels. Active muscle stretching increased the force enhancement in all subjects, independent whether they showed activation dependence or not. It appears that post-activation potentiation, and the associated phosphorylation of the myosin light chains, might account for the stretch-independent force enhancement observed here.

Methodological variance associated with normalization of occupational upper trapezius EMG using sub-maximal reference contractions

Objectives To quantify the variance introduced to trapezius electromyography (EMG) through normalization by sub-maximal reference voluntary exertions (RVE), and to investigate the effect of increased normalization efforts as compared to other changes in data collection strategy on the precision of occupational EMG estimates. Methods Women performed four RVE contractions followed by 30 min of light, cyclic assembly work on each of two days. Work cycle EMG was normalized to each of the RVE trials and seven exposure parameters calculated. The proportions of exposure variance attributable to subject, day within subject, and cycle and normalization trial within day were determined. Using this data, the effect on the precision of the exposure mean of altering the number of subjects, days, cycles and RVEs during data collection was simulated. Results For all exposure parameters a unique component of variance due to normalization was present, yet small: less than 4.4% of the total variance. The resource allocation simulations indicated that marginal improvements in the precision of a group exposure mean would occur above three RVE repeats for EMG collected on one day, or beyond two RVEs for EMG collected on two or more days.

The effect of dietary fat intake on long term vascular function in an animal model of gestational diabetes

ANIMAL MODEL OF GESTATIONAL DIABETES EGLE BYTAUTIENE, ESTHER TAMAYO, ANCIZAR BETANCOURT, MICHEL MAKHLOUF, GARLAND D. ANDERSON, MONICA LONGO, GEORGE SAADE, University of Texas Medical Branch, Obstetrics & Gynecology/Maternal Fetal Medicine, Galveston, Texas OBJECTIVE: To determine the effect of fat intake on maternal long term cardiovascular function in the heterozygous db mouse, an animal model of gestational diabetes. STUDY DESIGN: Leptin mutant, BKS.Cg-mC/ Leprdb/J strain mice maintained on standard (5% fat) or breeder’s diet (9% fat) were bred, allowed to deliver, and used at 4-6 months postpartum for evaluation of blood pressure (BP) and vascular reactivity. BP was measured in the conscious unrestrained animals by telemetric BP catheters inserted into the aortic arch. Mean BP was calculated for each 24-hour period. Vascular reactivity was determined in vitro using carotid artery rings mounted in physiological salt solution in a wire myograph for isometric force recording. To assess the vasodilatory function, the vessels were pre-contracted with phenylephrine (PE) and cumulative concentration-response curves were obtained to the endothelium-dependent vasodilator acetylcholine (ACh), endothelium-independent vasodilator sodium nitroprusside and b-adrenergic agonist isoproterenol (ISO). Vascular contractile function was determined by obtaining cumulative responses to the a1adrenergic agonist PE, thromboxane A2 mimetic (U46619), and serotonin. Vasodilatatory responses were calculated as percent relaxation from PE precontraction, and contractile responses were calculated as percent of the reference contraction to potassium chloride. RESULTS: There were no differences in pre-pregnancy/postpartum weight changes between the groups. Mean BP was significantly higher in postpartum mice on the 9% fat diet compared with the 5% fat diet. Vascular contractile responses to PE were significantly less, and relaxant responses to Ach and ISO were significantly more in the 9% fat compared with the 5% fat. CONCLUSION: In an animal model of gestational diabetes, increased fat intake has significant long term implications on cardiovascular function. This effect is independent of weight change. Women with gestational diabetes may benefit from dietary evaluation and counseling beyond the gestational and immediate postpartum periods.

인체 전경골근의 수의적 수축시 선행 동심성 근수축이 항정상태 등척성 근력에 미치는 영향

The purpose of this study was to investigate steady-state force depression following active muscle shortening in human tibialis anterior muscle during voluntary contractions. Subjects (n = 7; age <TEX>$24{\sim}39$</TEX> years; 7 males) performed isometric reference contractions and isometric-shortening-isometric contractions, using maximal voluntary effort. Force depression was assessed by comparing the steady-state isometric torque produced following active muscle shortening with the purely isometric reference torque obtained at the corresponding joint angle. In order to test for effects of the shortening conditions on the steady-state force depression, the speed of shortening were changed systematically in a random order but balanced design. Ankle dorsiflexion torque and joint angle were continuously measured using a dynamometer. During voluntary contractions, muscle activation of the tibialis anterior and the medical gastrocnemius was recorded using surface electromyography. Force depression during voluntary contractions, with a constant level of muscle activation, was 12 %, on average over all subjects. Force depression was independent of the speeds of shortening (<TEX>$13.8{\pm}2.9%$</TEX>, <TEX>$10.3{\pm}2.6%$</TEX> for 15 and 45 deg/sec over 15 deg of shortening, respectively). The results of this study suggest that steady-state force depression is a basic property of voluntarily-contracting human skeletal muscle and has functional implication to human movements.

The dependence of force enhancement on activation in human adductor pollicis

It has been well recognized that the steady-state isometric force after active muscle/fiber stretch is greater than the corresponding isometric force for electrically stimulated muscles and maximal voluntary contractions (MVC). However, recent evidence obtained for sub-MVC suggests that force enhancement properties are different from those observed for electrically induced and MVC. Specifically, it appears that force enhancement is activation-dependent and that there is a subject-specific threshold for force enhancement in sub-MVC. To address these suggestions, the relationship between force enhancement and voluntary activation during stretch was investigated in 11 healthy subjects. Human adductor pollicis muscles were studied and force enhancement was measured while muscle activation during the steady-state isometric phase was controlled at a level of 30% of MVC. In order to study the effects of activation on force enhancement, subjects performed stretch contractions at 0, 10, 30, 60, and 100% of maximal voluntary effort while the steady-state isometric force after stretch, obtained at 30% of activation in all cases, was compared to the corresponding values measured in the isometric reference contractions. There was no force enhancement if muscle stretching occurred passively but all subjects showed force enhancement when muscle stretching occurred at maximal voluntary effort. When increasing the level of activation during the stretch phase, force enhancement increased, and the number of subjects who showed force enhancement increased as well. We conclude from these results that force enhancement during voluntary contractions is activation-dependent with a threshold that is subject-specific.

Upper trapezius muscle activation patterns in neck–shoulder pain patients and healthy controls

This study aimed at investigating whether patients with neck-shoulder complaints from different aetiologies (work-related musculo-skeletal disorders, WMSD; whiplash associated disorders, WAD) show comparable muscle activation patterns, characterised by higher activation and lower relaxation levels of the trapezius muscles compared to healthy controls. Twenty healthy controls, 21 WMSD and 20 WAD patients with non-acute neck-shoulder pain were recruited for this cross-sectional study. Surface electromyography (sEMG) recordings were performed at the upper trapezius muscles during reference contractions, standardised computer tasks (typing and unilateral stress task), and rest measurements. sEMG was continuously recorded during these measurements. Outcome measures were root mean square (RMS) to study muscle activity, and relative rest time (RRT) to study muscle relaxation. Statistical analysis comprised the bootstrap technique and Kruskall-Wallis tests. Results showed no clear evidence for abnormal muscle activation patterns in WMSD and WAD patients compared to healthy controls. However, a tendency was observed for higher RMS levels during the reference contractions and computer tasks in both patient groups compared to healthy controls, and lower RRT levels at the non-dominant side during stress. Both patient groups also showed larger variability in RMS and RRT values. This variability has more often been reported in literature and may suggest the existence of subgroups of pain patients with corresponding different muscle activation patterns not related to aetiology. Future research may focus on identifying these subgroups of patients with neck-shoulder pain.

Force depression following muscle shortening in sub-maximal voluntary contractions of human adductor pollicis

Mechanical properties of skeletal muscles are often studied for controlled, electrically induced, maximal, or supra-maximal contractions. However, many mechanical properties, such as the force–length relationship and force enhancement following active muscle stretching, are quite different for maximal and sub-maximal, or electrically induced and voluntary contractions. Force depression, the loss of force observed following active muscle shortening, has been observed and is well documented for electrically induced and maximal voluntary contractions. Since sub-maximal voluntary contractions are arguably the most important for everyday movement analysis and for biomechanical models of skeletal muscle function, it is important to study force depression properties under these conditions. Therefore, the purpose of this study was to examine force depression following sub-maximal, voluntary contractions. Sets of isometric reference and isometric-shortening-isometric test contractions at 30% of maximal voluntary effort were performed with the adductor pollicis muscle. All reference and test contractions were executed by controlling force or activation using a feedback system. Test contractions included adductor pollicis shortening over 10°, 20°, and 30° of thumb adduction. Force depression was assessed by comparing the steady-state isometric forces (activation control) or average electromyograms (EMGs) (force control) following active muscle shortening with those obtained in the corresponding isometric reference contractions. Force was decreased by 20% and average EMG was increased by 18% in the shortening test contractions compared to the isometric reference contractions. Furthermore, force depression was increased with increasing shortening amplitudes, and the relative magnitudes of force depression were similar to those found in electrically stimulated and maximal contractions. We conclude from these results that force depression occurs in sub-maximal voluntary contractions, and that force depression may play a role in the mechanics of everyday movements, and therefore may have to be considered in biomechanical models of human movement.

Force enhancement and relaxation rates after stretch of activated muscle fibres

The residual force enhancement following muscle stretch might be associated with an increase in the proportion of attached cross-bridges, as supported by stiffness measurements. In this case, it could be caused by an increase in the attachment or a decrease in the detachment rate of cross-bridges, or a combination of the two. The purpose of this study was to investigate if the stretch-induced force enhancement is related to cross-bridge attachment/detachment kinetics. Single muscle fibres dissected from the lumbrical muscle of frog were place at a length approximately 20% longer than the plateau of the force-length relationship; they were maximally activated, and after full isometric force was reached, ramp stretches were imposed with amplitudes of 5 and 10% fibre length, at a speed of 40% fibre length s(-1). Experiments were performed in Ringer's solution, and with the addition of 2, 5 and 10 nM of 2,3-butanedione monoxime (BDM), a drug that places cross-bridges in a pre-power-stroke, state, inhibiting force production. The total force following stretch was higher than the corresponding force measured after isometric contraction at the corresponding length. This residual force enhancement was accompanied by an increase relaxation time. BDM, which decreases force production during isometric contractions, considerably increased the relative levels of force enhancement. BDM also increased relaxation times after stretch, beyond the levels observed during reference contractions in Ringer's solution, and beyond isometric control tests at the corresponding BDM concentrations. Together, these results support the idea that force enhancement is caused, at least in part, by a decrease in cross-bridge detachment rates, as manifested by the increased relaxation times following fibre stretch.

Observations on force enhancement in submaximal voluntary contractions of human adductor pollicis muscle

It has been observed consistently and is well accepted that the steady-state isometric force after active muscle stretch is greater than the corresponding isometric force for electrically stimulated muscles and maximal voluntary contractions. However, this so-called force enhancement has not been studied for submaximal voluntary efforts; therefore, it is not known whether this property affects everyday movements. The purpose of this study was to determine whether there was force enhancement during submaximal voluntary contractions. Human adductor pollicis muscles (n = 17) were studied using a custom-built dynamometer, and both force and activation were measured while muscle activation and force were controlled at a level of 30% of maximal voluntary contraction. The steady-state isometric force and activation after active stretch were compared with the corresponding values obtained during isometric reference contractions. There was consistent and reliable force enhancement in 8 of the 17 subjects, whereas there was no force enhancement in the remaining subjects. Subjects with force enhancement had greater postactivation potentiation and a smaller resistance to fatigue in the adductor pollicis. We conclude from these results that force enhancement exists during submaximal voluntary contractions in a subset of the populations and suggest that it may affect everyday voluntary movements in this subset. On the basis of follow-up testing, it appears that force enhancement during voluntary contractions is linked to potentiation and fatigue resistance and therefore possibly to the fiber-type distribution in the adductor pollicis muscle.

Precision of measurements of physical workload during standardised manual handling. Part I: Surface electromyography of m. trapezius, m. infraspinatus and the forearm extensors

Though surface electromyography (EMG) has been widely used in studies of occupational exposure, its precision in terms of the variance between-days and between-subjects has seldom been evaluated. This study aimed at such an evaluation. Six women performed three different work tasks: ‘materials picking’, ‘light assembly’, and ‘heavy assembly’, repeated on 3 different days. EMG was recorded from m. trapezius, m. infraspinatus and the forearm extensors. Normalisation was made to a maximal (MVE), and a submaximal (RVE), reference contraction. Variance components between days (within subjects) and between subjects were derived for the 10th, 50th and 90th percentiles, as well as for muscular rest parameters. For the task ‘heavy assembly’, the coefficient of variation between days (CVBD) was 8% for m. trapezius (right side, 50th percentile, MVE normalised values). Larger variabilities were found for m. infraspinatus (CVBD 15%), and the forearm extensors (CVBD 33%). Between-subjects variability (CVBS) was greater, 16% for m. trapezius and 57% for m. infraspinatus, 29% for the forearm extensors. RVE normalisation resulted in larger CVBD, while reducing CVBS. The between-days and between-subjects variability may be used to optimise sampling strategy, and to assess the bias in epidemiological studies. The bias caused by measurement procedures per se is acceptable.

Effects of electrode dislocation on electromyographic activity and relative rest time: effectiveness of compensation by a normalisation procedure.

Myofeedback seems a promising technique to treat myalgia related to computer work. A garment with embedded electrodes allows independent use by the subject. However, this inevitably results in electrode dislocations that can cause such variability in the electromyogram (EMG) parameters that the treatment fails. The objective was to investigate the variability introduced by electrode displacement on EMG root mean square (RMS) and relative rest time (RRT) and the effect of a normalisation procedure using a reference contraction to compensate for this. Dislocation was simulated using simultaneous recordings of pairs of electrodes situated 15mm apart on the trapezius muscle, surrounding a central electrode pair at the standard position. To assess sensitivity for electrode displacement, intra-subject variability was used. To assess the effects of normalisation, both intra and inter-subject variability were used. Results indicated a large sensitivity of RMS to electrode displacements, with no differences between typing (42%) and the reference task (43%). No systematic changes were found relative to a specific electrode dislocation. Normalisation of RMS resulted in a decrease in the sensitivity for electrode displacement (33%), but did not decrease inter-subject variability (27%). The median value of the intra-subject variability of RRT, using a fixed threshold, was 4.9%, whereas the normalised threshold did not decrease the intra-subject variability (7%). The results suggest that RRT is suitable for individual follow-up measurements and applications such as myofeedback, without the need for a normalisation procedure. The use of RMS requires considerably more care, owing to its large sensitivity to electrode displacement and lack of substantial effects of normalisation.