Reference Contraction Research Articles

Torque depression following active shortening is associated with a modulation of cortical and spinal excitation: a history-dependent study.

The reduction in steady‐state isometric torque following a shortening muscle action when compared to a purely isometric contraction at the same muscle length and level of activation is termed torque depression (TD). The purpose of this study was to investigate spinal and supraspinal neural responses during the TD state of a maximal voluntary activation of the ankle dorsiflexors. Thirteen subjects (10 male) were recruited for the study. To explore alterations in corticospinal excitability during voluntary muscle activation in the TD state, motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs), and maximal compound muscle action potentials (Mmax) were elicited during the isometric steady‐state following active shortening (i.e., TD) and the purely isometric condition. A 15% reduction in steady‐state isometric torque (P < 0.05) was observed following isokinetic shortening at 40°/sec. Although mean evoked responses (MEP and CMEP) were not different in the TD state as compared with purely isometric state, the changes in evoked responses were inversely related to one another depending on the level of TD. These findings indicate that supraspinal and spinal responses are interrelated in the TD state. Furthermore, antagonist muscle coactivation during the isometric reference contraction was positively related to TD. These findings suggest the possibility of a relationship between the central nervous system and TD in humans. Further work should be performed to definitively link TD to specific spinal interneurons.

Capacitively coupled EMG detection via ultra-low-power microcontroller STFT.

As motion artefacts are a major problem with electromyography sensors, a new algorithm is developed to differentiate artefacts to contraction EMG. The performance of myoelectric prosthesis is increased with this algorithm. The implementation is done for an ultra-low-power microcontroller with limited calculation resources and memory. Short Time Fourier Transformation is used to enable real-time application. The sum of the differences (SOD) of the currently measured EMG to a reference contraction EMG is calculated. The SOD is a new parameter introduced for EMG classification. The satisfactory error rates are determined by measurements done with the capacitively coupling EMG prototype, recently developed by the research group.

Energy cost of isometric force production after active shortening in skinned muscle fibres.

The steady-state isometric force after active shortening of a skeletal muscle is lower than the purely isometric force at the corresponding length. This property of skeletal muscle is known as force depression. The purpose of this study was to investigate whether the energy cost of force production at the steady state after active shortening was reduced compared with the energy cost of force production for a purely isometric contraction performed at the corresponding length (same length, same activation). Experiments were performed in skinned fibres isolated from rabbit psoas muscle. Skinned fibres were actively shortened from an average sarcomere length of 3.0 µm to an average sarcomere length of 2.4 µm. Purely isometric reference contractions were performed at an average sarcomere length of 2.4 µm. Simultaneously with the force measurements, the ATP cost was measured during the last 30 s of isometric contractions using an enzyme-coupled assay. Stiffness was calculated during a quick stretch-release cycle of 0.2% fibre length performed once the steady state had been reached after active shortening and during the purely isometric reference contractions. Force and stiffness following active shortening were decreased by 10.0±1.8% and 11.0±2.2%, respectively, compared with the isometric reference contractions. Similarly, ATPase activity per second (not normalized to the force) showed a decrease of 15.6±3.0% in the force-depressed state compared with the purely isometric reference state. However, ATPase activity per second per unit of force was similar for the isometric contractions following active shortening (28.7±2.4 mmol l-1mN-1smm3) and the corresponding purely isometric reference contraction (30.9±2.8 mmol l-1mN-1smm3). Furthermore, the reduction in absolute ATPase activity per second was significantly correlated with force depression and stiffness depression. These results are in accordance with the idea that force depression following active shortening is primarily caused by a decrease in the proportion of attached cross-bridges. Furthermore, these findings, along with previously reported results showing a decrease in ATP consumption per unit of force after active muscle stretching, suggest that the mechanisms involved in the steady-state force after active muscle shortening and active muscle lengthening are of distinctly different origin.

The impact of low-frequency, low-force cyclic stretching of human bronchi on airway responsiveness.

BackgroundIn vivo, the airways are constantly subjected to oscillatory strain (due to tidal breathing during spontaneous respiration) and (in the event of mechanical ventilation) positive pressure. This exposure is especially problematic for the cartilage-free bronchial tree. The effects of cyclic stretching (other than high-force stretching) have not been extensively characterized. Hence, the objective of the present study was to investigate the functional and transcriptional response of human bronchi to repetitive mechanical stress caused by low-frequency, low-force cyclic stretching.MethodsAfter preparation and equilibration in an organ bath, human bronchial rings from 66 thoracic surgery patients were stretched in 1-min cycles of elongation and relaxation over a 60-min period. For each segment, the maximal tension corresponded to 80% of the reference contraction (the response to 3 mM acetylcholine). The impact of cyclic stretching (relative to non-stretched controls) was examined by performing functional assessments (epithelium removal and incubation with sodium channel agonists/antagonists or inhibitors of intracellular pathways), biochemical assays of the organ bath fluid (for detecting the release of pro-inflammatory cytokines), and RT-PCR assays of RNA isolated from tissue samples.ResultsThe application of low-force cyclic stretching to human bronchial rings for 60 min resulted in an immediate, significant increase in bronchial basal tone, relative to non-cyclic stretching (4.24 ± 0.16 g vs. 3.28 ± 0.12 g, respectively; p < 0.001). This cyclic stimulus also increased the affinity for acetylcholine (−log EC50: 5.67 ± 0.07 vs. 5.32 ± 0.07, respectively; p p < 0.001). Removal of airway epithelium and pretreatment with the Rho-kinase inhibitor Y27632 and inward-rectifier K+ or L-type Ca2+ channel inhibitors significantly modified the basal tone response. Exposure to L-NAME had opposing effects in all cases. Pro-inflammatory pathways were not involved in the response; cyclic stretching up-regulated the early mRNA expression of MMP9 only, and was not associated with changes in organ bath levels of pro-inflammatory mediators.ConclusionLow-frequency, low-force cyclic stretching of whole human bronchi induced a myogenic response rather than activation of the pro-inflammatory signaling pathways mediated by mechanotransduction.

Relative scapular muscle activity ratios are altered in subacromial pain syndrome

Coordinated muscle activity is needed for synchronized joint motion and stability. Characterizing relative scapular muscle activity deficits in participants with shoulder pain will provide foundational knowledge to develop rehabilitation programs. Participants were recruited with subacromial pain syndrome and an asymptomatic control group matched for age, gender, and dominant arm (N = 56). Surface electromyographic muscle activity was recorded from the upper, middle, and lower trapezius (UT, MT, LT) and serratus anterior (SA) during 5 repetitions of a weighted arm elevation task. Muscle activity was normalized to a reference contraction and then expressed as UT/MT, UT/LT, UT/SA, and LT/SA ratios. Ratios were compared between groups and across 3 arm angle intervals during ascending and descending elevation. A 2 × 3 mixed-model analysis of variance yielded a group main effect for the UT/LT ratio, with a higher ratio in the subacromial pain group during ascending (mean difference, 0.92; P = .008) and descending (mean difference, 0.70; P = .030). For the LT/SA ratio, there was a group effect: a lower ratio in the subacromial group during ascending (mean difference, -0.25; P = .026) and descending (mean difference, -0.51; P = .032). There were no differences for the UT/MT or UT/SA. There is a disruption in coordination between the LT and SA and the UT and LT during an arm elevation task in patients with subacromial pain syndrome. The LT was part of both altered ratios, indicating the relative importance of the LT. Future research should determine if exercises aimed at restoring the dysfunctional LT/SA and UT/LT force couples are beneficial to reduce shoulder pain and disability in patients with unilateral shoulder pain.

Does weightlifting increase residual force enhancement?

The force maintained following stretching of an active muscle exceeds the isometric force at the same muscle length. This residual force enhancement (RFE) is different for various muscles. It is currently unknown whether training induces changes in RFE. Weightlifters perform a large number of eccentric contractions during training, and RFE might be functionally relevant. The aim of this study was to examine whether there is increased RFE in weightlifters versus a reference group.Therefore, we measured external reaction forces during a multi-joint leg extension in weightlifters (n=10) and a reference group (n=11) using a motor driven leg press dynamometer (ISOMED 2000). Steady state isometric forces after stretching were compared to the corresponding forces obtained during isometric reference contractions. Statistical analyses yielded a significant RFE for both groups (p<0.001), but there were no RFE differences between the groups (p=0.320). However, RFE tends to decrease slower in the weightlifting group versus the reference group.We conclude that long-term weightlifting has only a minor influence on RFE. We speculate that the specific training including a combination of eccentric and concentric exercises induced almost no changes in titin-isoform expression which may be responsible for generation of RFE after active muscle stretching.

The role of sarcomere length non-uniformities in residual force enhancement of skeletal muscle myofibrils.

The sarcomere length non-uniformity theory (SLNT) is a widely accepted explanation for residual force enhancement (RFE). RFE is the increase in steady-state isometric force following active muscle stretching. The SLNT predicts that active stretching of a muscle causes sarcomere lengths (SL) to become non-uniform, with some sarcomeres stretched beyond actin–myosin filament overlap (popping), causing RFE. Despite being widely known, this theory has never been directly tested. We performed experiments on isolated rabbit muscle myofibrils (n = 12) comparing SL non-uniformities for purely isometric reference contractions (I-state) and contractions following active stretch producing RFE (FE-state). Myofibrils were activated isometrically along the descending limb of the force–length relationship (mean ± 1 standard deviation (SD) = 2.8 ± 0.3 µm sarcomere−1). Once the I-state was reached, myofibrils were shortened to an SL on the plateau of the force–length relationship (2.4 µm sarcomere−1), and then were actively stretched to the reference length (2.9 ± 0.3 µm sarcomere−1). We observed RFE in all myofibrils (39 ± 15%), and saw varying amounts of non-uniformity (1 SD = 0.9 ± 0.5 µm) that was not significantly correlated with the amount of RFE, but through pairwise comparisons was found to be significantly greater than the non-uniformity measured for the I-state (0.7 ± 0.4 µm). Three myofibrils exhibited no increase in non-uniformity. Active stretching was accompanied by sarcomere popping in four myofibrils, and seven had popped sarcomeres in the I-state. These results suggest that, while non-uniformities are present with RFE, they are also present in the I-state. Furthermore, non-uniformity is not associated with the magnitude of RFE, and myofibrils that had no increase in non-uniformity with stretch still showed normal RFE. Therefore, it appears that SL non-uniformity is a normal associate of muscle contraction, but does not contribute to RFE following active stretching of isolated skeletal muscle myofibrils.

History dependence of the electromyogram: Implications for isometric steady-state EMG parameters following a lengthening or shortening contraction

Residual force enhancement (RFE) and force depression (FD) refer to an increased or decreased force following an active lengthening or shortening contraction, respectively, relative to the isometric force produced at the same activation level and muscle length. Our intent was to determine if EMG characteristics differed in the RFE or FD states compared with a purely isometric reference contraction for maximal and submaximal voluntary activation of the adductor pollicis muscle. Quantifying these alterations to EMG in history-dependent states allows for more accurate modeling approaches for movement control in the future. For maximal voluntary contractions (MVC), RFE was 6–15% (P<0.001) and FD was 12–19% (P<0.001). The median frequency of the EMG was not different between RFE, FD and isometric reference contractions for the 100% and 40% MVC intensities (P>0.05). However, root mean square EMG (EMGRMS) amplitude for the submaximal contractions was higher in the FD and lower in the RFE state, respectively (P<0.05). For maximal contractions, EMGRMS was lower for the FD state but was the same for the RFE state compared to the isometric reference contractions (P>0.05). Neuromuscular efficiency (NME; force/EMG) was lower in the force depressed state and higher in the force enhanced state (P<0.05) compared to the isometric reference contractions. EMG spectral properties were not altered between the force-enhanced and depressed states relative to the isometric reference contractions, while EMG amplitude measures were.

Residual force enhancement during multi-joint leg extensions at joint- angle configurations close to natural human motion

The isometric steady-state forces following lengthening are greater than those produced at the same muscle length and activation level but without prior lengthening. Although residual force enhancement (RFE) has been investigated across a range of conditions, its relevance for daily human movement is still poorly understood. We aimed to study RFE in a setup imitating daily activity, i.e., submaximal activation of the lower extremity’s muscles with slightly flexed knee joints comparable to human walking.A motor-driven leg press dynamometer was used for randomly arranged purely isometric and isometric–eccentric–isometric contractions. Thirteen subjects performed multi-joint leg extensions, which were feedback-controlled at 30% of maximum voluntary vastus lateralis activation. Isometric–eccentric–isometric contractions incorporated a stretch from 30° to 50° knee flexion, while isometric contractions were performed at 50° knee flexion. Isometric contractions following stretch and purely isometric reference contractions were performed at 50° knee flexion. Kinematics, forces, and muscular activity were measured using 3D optical motion tracking, force plates, and surface EMG of 9 lower limb muscles of the right leg and joint torques were calculated by inverse dynamics. Variables of standardization (EMG, joint angles) showed no differences between contraction conditions. Eight of 13 subjects showed RFE of up to 24.8±32.5% for external forces and joint torques. Because the remaining 5 non-responders failed to produce enhanced forces during the stretch, we believe that RFE is functionally relevant for muscle function comparable to everyday human motion but only if there is enhanced force during stretch that sufficiently triggers mechanisms underlying RFE.

Automated detection of uterine contractions in tocography signals – Comparison of algorithms

Monitoring of uterine contractile activity enables to control the progress of labor. Automated detection of contractions is an integral part of the signal analysis implemented in computer-aided fetal surveillance system. Comparison of four algorithms for automated detection of uterine contractions in the signal of uterine mechanical activity is presented. Three algorithms are based generally on analysis of the frequency distribution of signal values. The fourth method relies on analyzing the rate of changes of the uterine activity signal. The reference data in form of beginning and end of contraction episodes were provided by human experts. Obtained results show that all algorithms were capable to detect above 91% reference contractions, and less than 7% of recognized patterns were false. Two algorithms can be distinguished as providing a higher performance expressed by the sensitivity of 95% and the positive predictive value of 97%. Such results could be obtained by optimization of contraction validation criteria.

Biomechanical measures in participants with shoulder pain: Intra-rater reliability

Biomechanical measures are used to characterize the mechanisms of treatment for shoulder pain. The objective was to characterize test-retest reliability and measurement error of shoulder surface electromyographic(sEMG) and kinematic measures. Individuals(n = 12) with subacromial pain syndrome were tested at 2 visits. Five repetitions of shoulder scapular plane elevation were performed while collecting sEMG of the upper trapezius(UT), middle trapezius(MT), lower trapezius(LT), serratus anterior(SA) middle-deltoid, and infraspinatus muscles during ascending and descending phases. Simultaneously, electromagnetic sensors measured 3-dimensional kinematics of scapular internal/external rotation, upward/downward rotation, posterior/anterior tilt, and clavicular elevation/depression and clavicular protraction/retraction. Kinematic and sEMG variables were reduced for the total phase of ascending and descending elevation (30°–120°, 120°–30°), at 30° intervals for sEMG, and at every 30° discrete kinematic angle. The intraclass correlation coefficients(ICC) ranged from 0.08 to 0.99 for sEMG and 0.23–0.95 for kinematics. Correspondingly, the standard error of the measurement(SEM) and minimal detectable change(MDC) for sEMG measures varied from 2.3% to 103.8% of a reference contraction(REF-contraction). For kinematics, the SEM and MDC varied from 1.4° to 5.9°. Between-day reliability was good to very good, except for scapular internal/external rotation kinematics, and sEMG for the LT, UT, and SA. sEMG error values were highest (>25%REF-contraction) for most of the LT, UT, and SA variables. Kinematic error values indicate changes or differences of 2°–3° are meaningful, except for upward/downward rotation and internal/external rotation with MDCs of 4°–6°. Generally, data from the total phase of movement had better reliability and lower error than the data from sEMG interval or kinematic discrete angles.

Calcium Sensitivity after Active Shortening in Rabbit Psoas Fibres

Introduction It has been shown that isometric force after active shortening of muscles is lower than the purely isometric force performed at the corresponding lengths. This property is termed force depression. Force depression has been observed in isolated muscle preparations and muscles in situ and in vivo (Rassier et al., J Applied Physiol 2004). The origin of force depression is still unknown. It has been suggested that active shortening may affect calcium sensitivity by changing calcium-troponin binding equilibrium leading to decreased force after active shortening (Ekelund and Edman, Acta Physiol Scand 1982). Our aim in the present study was to test whether calcium sensitivity was decreased after active shortening. Methods Experiments were performed using skinned fibres (n=7) isolated from rabbit psoas muscle. Calcium sensitivity was characterized by establishing the force-pCa curves for isometric reference contractions performed at an average sarcomere length of 2.4 µm and active shortening contractions from an average sarcomere length of 3.0 µm to an average sarcomere length of 2.4 µm. pCa50 and the coefficient of cooperativity (nH) were compared between reference and active shortening contractions. Results and Discussion pCa50 for the reference contractions was 5.99±0.02. No change in pCa50 was observed in the force depressed state (6.00±0.01). Furthermore, the coefficient of cooperativity did not change after active shortening (3.9±0.3 versus 4.3±0.3 for the reference and force depressed states respectively). These results suggest that calcium sensitivity does not decrease after active shortening and that the binding interactions between troponin and calcium are not altered in the force depressed state.

Residual Force Enhancement in Cardiac Myofibrils

Residual force enhancement (RFE) is a property of muscle where an activated muscle is stretched from a short to a long length resulting in greater force than is produced isometrically at the long length. This history-dependent property has been identified across skeletal muscle hierarchy including whole muscle, fascicles, fibres, and myofibrils. However, RFE has not been investigated in cardiac muscle. Therefore, the purpose of this study was to determine if RFE was present in cardiac myofibrils.Rabbit hearts were dissected and strips of left ventricle were skinned overnight with 1% Triton skinning solution and stored at −20°C. On the day of experiments, the cardiac tissue was blended and a myofibril with a good striation pattern was identified and suspended between a glass needle and a nanolever allowing for length changes and force measurement. The myofibril was set at a sarcomere length (SL) of 2.4 µm and activated to establish a reference contraction before being passively stretched to a SL of 3.2 µm. After a rest period, the myofibril was activated at a SL of 2.4 µm, actively stretched to a SL of 3.2µm, held for one minute and then relaxed. RFE was calculated as the difference between the steady-state force obtained after active stretch and the corresponding predicted isometric force at 3.2 µm based on the reference force, calculated according to the force-length relationship scaled to the filament lengths in rabbit muscles, and accounting for passive force.All myofibrils (n=5) produced more force when stretched actively compared to the calculated reference isometric force, indicating the presence of RFE in cardiac myofibrils. The average RFE was 54.8±10.8%.The presence of RFE in cardiac myofibrils supports that RFE is a universal muscle property. Investigation in cardiac muscle may shed new light into the mechanisms underlying RFE.

Trunk muscle activity during walking in persons with multiple sclerosis: The influence of body weight support1

Although the trunk is important for maintaining balance during walking only very limited information about the trunk muscle activity during walking with body weight support (BWS) is reported in literature. The aim of this study was to measure the effect of BWS on the trunk muscle activity during treadmill walking. 14 persons with multiple sclerosis and 14 healthy persons walked on a treadmill with 0%, 10%, 20%, 30%, 50% and 70% BWS. Bilateral EMG measurements (surface electrodes) on the m. rectus abdominis, m. obliquus externus, m. erector spinae and m. multifidus were performed. The maximal muscle activation was presented as a percentage of a performance related reference contraction. A repeated measures ANOVA with simple contrasts was applied (SPSS20). In general when comparing walking with BWS with walking with 0% BWS there is an increase in m. obliquus externus activity and a decrease in back muscle activity. With increasing percentages of BWS an increase in activity of the abdominal muscles and a decrease in back muscle activity was found, with most changes in high percentages BWS. Based on the results, it is recommended to decrease the percentage BWS as fast as possible beneath 30% BWS.

Trapezius muscle EMG as predictor of mental stress

Stress is a growing problem in society and can cause musculoskeletal complaints. It would be useful to measure stress for prevention of stress-related health problems. An experiment is described in which EMG signals of the upper trapezius muscle were measured with a wireless system during three different stressful conditions: a calculation task (the Norinder test), a logical puzzle task and a memory task. The latter two tests were newly designed and aimed at creating circumstances that are similar to work stress. Amplitudes of the EMG signals were significantly higher during stress compared to rest (+2.6% of reference contraction level) and relative time with EMG gaps was lower during stress (−14.3% of time). Also, mean and median frequencies were significantly lower during stress than during rest (−8.6 and −8.8 Hz, respectively). EMG amplitude increased not only from rest to stress conditions, but also during stressful conditions and decreased during relaxation periods. EMG features correlated with subjectively indicated stress levels (correlations of 0.32 with RMS and −0.32 with relative gaptime). The results indicate that EMG is a useful parameter to detect stress. Together with other physiological sensors, EMG sensors can be included in a wireless system for ambulatory monitoring of stress levels.

Energy cost of force production is reduced after active stretch in skinned muscle fibres

Residual force enhancement has been observed consistently in skeletal muscles. Despite an abundance of experimental observations, there has been no information about the metabolic cost of the force observed after stretch. Our aim was to investigate the energy cost of force production after active stretch in skinned fibres isolated from rabbit psoas muscle, by quantifying the ATPase activity using an enzyme-coupled assay. Fibres were actively stretched from an average sarcomere length of 2.4μm to average sarcomere lengths of 2.8 and 3.2μm. Purely isometric reference contractions were performed at average sarcomere lengths of 2.8 and 3.2μm. Simultaneously with the force measurements, the ATP cost per unit of force produced was measured during the last 40s of isometric contraction. Results showed that ATPase activity per unit of force was reduced by 17.2±4.1% in the isometric contractions after active stretch, compared to the purely isometric contraction at the corresponding lengths for both stretch magnitudes. Fibres stretched to an average sarcomere length of 3.2μm showed a higher reduction in ATPase activity per unit of force compared to fibres stretched to an average sarcomere length of 2.8μm (20.7±4.4 versus 12.4±3.2% respectively). Passive force enhancement was observed in all fibres and was correlated with the decrease in ATPase activity. No difference in stiffness was observed between reference and active stretch contractions. These results suggest that skeletal muscles become more efficient after stretch, either by increasing the amount of force produced per cross bridge or by engaging a passive element.

Perivascular Expression and Potent Vasoconstrictor Effect of Dynorphin A in Cerebral Arteries

BackgroundNumerous literary data indicate that dynorphin A (DYN-A) has a significant impact on cerebral circulation, especially under pathophysiological conditions, but its potential direct influence on the tone of cerebral vessels is obscure. The aim of the present study was threefold: 1) to clarify if DYN-A is present in cerebral vessels, 2) to determine if it exerts any direct effect on cerebrovascular tone, and if so, 3) to analyze the role of κ-opiate receptors in mediating the effect.Methodology/Principal FindingsImmunohistochemical analysis revealed the expression of DYN-A in perivascular nerves of rat pial arteries as well as in both rat and human intraparenchymal vessels of the cerebral cortex. In isolated rat basilar and middle cerebral arteries (BAs and MCAs) DYN-A (1–13) and DYN-A (1–17) but not DYN-A (1–8) or dynorphin B (DYN-B) induced strong vasoconstriction in micromolar concentrations. The maximal effects, compared to a reference contraction induced by 124 mM K+, were 115±6% and 104±10% in BAs and 113±3% and 125±9% in MCAs for 10 µM of DYN-A (1–13) and DYN-A (1–17), respectively. The vasoconstrictor effects of DYN-A (1–13) could be inhibited but not abolished by both the κ-opiate receptor antagonist nor-Binaltorphimine dihydrochloride (NORBI) and blockade of Gi/o-protein mediated signaling by pertussis toxin. Finally, des-Tyr1 DYN-A (2–13), which reportedly fails to activate κ-opiate receptors, induced vasoconstriction of 45±11% in BAs and 50±5% in MCAs at 10 µM, which effects were resistant to NORBI.Conclusion/SignificanceDYN-A is present in rat and human cerebral perivascular nerves and induces sustained contraction of rat cerebral arteries. This vasoconstrictor effect is only partly mediated by κ-opiate receptors and heterotrimeric Gi/o-proteins. To our knowledge our present findings are the first to indicate that DYN-A has a direct cerebral vasoconstrictor effect and that a dynorphin-induced vascular action may be, at least in part, independent of κ-opiate receptors.

The fusimotor and reafferent origin of the sense of force and weight

Signals associated with the command the brain sends to muscles are thought to create the sensation of heaviness when we lift an object. Thus, as a muscle is weakened by fatigue or partial paralysis (neuromuscular blockade), the increase in the motor command needed to lift a weight is thought to explain the increasing subjective heaviness of the lifted object.With different fatiguing contractions we approximately halved the force output of the thumb flexor muscles, which were then used to lift an object. For two deafferented subjects the perceived heaviness of the lifted object approximately doubled, in keeping with the central-signal theory. However, for normal subjects this resulted in objects feeling the same or lighter, inconsistent with the central-signal theory but consistent with the expected effects of the conditioning contractions on the sensitivity of peripheral receptors. In separate experiments we subjected the forearm muscles to complete paralysis with a non-depolarising neuromuscular blocking agent and then allowed them to recover to approximately half-force output. This also resulted in objects feeling lighter when lifted by the semi-paralysed thumb, even though the motor command to the motoneurons must have been greater. This is readily explained by reduced lift-related reafference caused by the prolonged paralysis of muscle spindle intrafusal fibres.We conclude that peripheral signals, including a major contribution from muscle spindles, normally give rise to the sense of exerted force. In concept, however, reafference from peripheral receptors may also be considered a centrally generated signal that traverses efferent and then afferent pathways to feed perceptual centres rather than one confined entirely to the central nervous system. These results therefore challenge the distinction between central- and peripheral-based perception, and the concept that muscle spindles provide only information about limb position and movement.

Changes in Fascicle Lengths and Pennation Angles Do Not Contribute to Residual Force Enhancement/Depression in Voluntary Contractions

Force enhancement following muscle stretching and force depression following muscle shortening are well-accepted properties of skeletal muscle contraction. However, the factors contributing to force enhancement/depression remain a matter of debate. In addition to factors on the fiber or sarcomere level, fiber length and angle of pennation affect the force during voluntary isometric contractions in whole muscles. Therefore, we hypothesized that differences in fiber lengths and angles of pennation between force-enhanced/depressed and reference states may contribute to force enhancement/depression during voluntary contractions. The purpose of this study was to test this hypothesis. Twelve subjects participated in this study, and force enhancement/depression was measured in human tibialis anterior. Fiber lengths and angles of pennation were quantified using ultrasound imaging. Neither fiber lengths nor angles of pennation were found to differ between the isometric reference contractions and any of the force-enhanced or force-depressed conditions. Therefore, we rejected our hypothesis and concluded that differences in fiber lengths or angles of pennation do not contribute to the observed force enhancement/depression in human tibialis anterior, and speculate that this result is likely true for other muscles too.

Effect of bilateral versus unilateral exertion tests on maximum voluntary activity and within-participant reproducibility in the shoulder

Normalized surface EMG requires known muscle activity from a well defined reference contraction, often a maximum voluntary isometric exertion (MVIE). One factor that may impact the activity obtained during a MVIE is the use of a bilateral or unilateral version of the test. The objective of this work was to investigate how performing MVIE tests unilaterally or bilaterally affects maximum voluntary electrical activity (MVE) for selected shoulder muscles. Further, we examined if within-participant reproducibility could be improved with the use of bilateral or unilateral MVIE tests. Sixteen participants completed five repetitions of a palm press, empty can, and external rotation MVIE tests both bilaterally and unilaterally. Muscle activity was recorded from seven muscles acting at the glenohumeral joint during the performance of each test. Repeated measures ANOVAs were used to determine within-participant differences on muscle specific MVEs and hand force measures between the two versions of the test. Gender was compared as a between participants factor. Within-participant reproducibility in muscle activity between bilateral and unilateral MVIEs was compared using coefficients of variation. Performing a bilateral empty can MVIE test elicited significantly higher MVEs for the upper trapezius (14%) and supraspinatus muscles (15%) than a unilateral test in this study population. MVEs during the palm press and external rotation were not affected by performing the tests bilaterally or unilaterally. Reliability was similar between unilateral and bilateral exertions across MVIE tests. Hence, we recommend that the empty can MVIE test be performed bilaterally, while either bilateral or unilateral configurations are equivalent during the palm press and external rotation MVIE tests.