Removal of visual feedback alters muscle activity and reduces force variability during constant isometric contractions

The purpose of this study was to compare force accuracy, force variability and muscle activity during constant isometric contractions at different force levels with and without visual feedback and at different feedback gains. In experiment 1, subjects were instructed to accurately match the target force at 2, 15, 30, 50, and 70% of their maximal isometric force with abduction of the index finger and maintain their force even in the absence of visual feedback. Each trial lasted 22 s and visual feedback was removed from 8–12 to 16–20 s. Each subject performed 6 trials at each target force, half with visual gain of 51.2 pixels/N and the rest with a visual gain of 12.8 pixels/N. Force error was calculated as the root mean square error of the force trace from the target line. Force variability was quantified as the standard deviation and coefficient of variation (CVF) of the force trace. The EMG activity of the agonist (first dorsal interosseus; FDI) was measured with bipolar surface electrodes placed distal to the innervation zone. Independent of visual gain and force level, subjects exhibited lower force error with the visual feedback condition (2.53 ± 2.95 vs. 2.71 ± 2.97 N; P < 0.01); whereas, force variability was lower when visual feedback was removed (CVF: 4.06 ± 3.11 vs. 4.47 ± 3.14, P < 0.01). The EMG activity of the FDI muscle was higher during the visual feedback condition and this difference increased especially at higher force levels (70%: 370 ± 149 vs. 350 ± 143 μV, P < 0.01). Experiment 2 examined whether the findings of experiment 1 were driven by the higher force levels and proximity in the gain of visual feedback. Subjects performed constant isometric contractions with the abduction of the index finger at an absolute force of 2 N, with two distinct feedback gains of 15 and 3,000 pixels/N. In agreement with the findings of experiment 1, subjects exhibited lower force error in the presence of visual feedback especially when the feedback gain was high (0.057 ± 0.03 vs. 0.095 ± 0.05 N). However, force variability was not affected by the vastly distinct feedback gains at this force, which supported and extended the findings from experiment 1. Our findings demonstrate that although removal of visual feedback amplifies force error, it can reduce force variability during constant isometric contractions due to an altered activation of the primary agonist muscle most likely at moderate force levels in young adults.     

Greater amount of visual information exacerbates force control in older adults during constant isometric contractions

The purpose of this study was to compare control of force and modulation of agonist muscle activity of young and older adults when the amount of visual feedback was varied at two different force levels. Ten young adults (25 years ± 4 years, 5 men and 5 women) and ten older adults (71 years ± 5 years, 4 men and 6 women) were instructed to accurately match a constant target force at 2 and 30% of their maximal isometric force with abduction of the index finger. Each trial lasted 35 s, and the amount of visual feedback was varied by changing the visual angle at 0.05, 0.5, and 1.5°. Each subject performed three trials for each visual angle condition. Force variability was quantified as the standard deviation and coefficient of variation (CV) of force. Modulation of the agonist muscle activity was quantified as the normalized power spectrum density of the EMG signal recorded from two pairs of bipolar electrodes placed on the first dorsal interosseus muscle. The frequency bands of interest were between 5 and 100 Hz. There were significant age-associated differences in force control with changes in the amount of visual feedback. The CV of force did not change with visual angle for young adults, whereas it increased for older adults. Although older adults exhibited similar CV of force to young adults at 0.05° (5.95 ± 0.67 vs. 5.47 ± 0.5), older adults exhibited greater CV of force than young adults at 0.5° (8.49 ± 1.34 vs. 5.05 ± 0.5) and 1.5° (8.23 ± 1.12 vs. 5.49 ± 0.6). In addition, there were age-associated differences in the modulation of the agonist muscle activity. Young adults increased normalized power in the EMG signal from 13 to 60 Hz with an increase in visual angle, whereas older adults did not. These findings suggest that greater amount of visual information may be detrimental to the control of a constant isometric contraction in older adults, and this impairment may be due to their inability to effectively modulate the motor neuron pool of the agonist muscle.     

Relation of activity in precentral cortical neurons to force and rate of force change during isometric contractions of finger muscles

Summary The activity of single neurons within the hand area of the precentral motor cortex of primates was recorded during the performance of a maintained precision grip between the thumb and forefinger. The finger opposition forces were exerted against a strain gauge which allowed force changes to be studied under near isometric conditions. Task performance required the generation of a force ramp (the dynamic phase) and thereafter the maintenance of a stable force for one second (the static phase). Intracortical stimulation through the recording electrode was used to verify that the recordings were made from the appropriate somatotopographic area of the motor cortex.From a total of 221 recorded neurons, 76 were found to be either activated or deactivated during performance of the task. Among the 51 activated neurons, most discharged at higher frequencies during the dynamic phase, than during the static phase. The discharge of some of these neurons could be related to both force (F) and rate of force change (df/dt) whereas certain others could only be correlated with df/dt. The change in discharge frequency for these neurons generally occurred prior to the onset of EMG activity. Eight neurons were more active during maintained force than during the force ramp. The discharge frequency could not be correlated with df/dt and only one showed a significant positive relation to force. The change in discharge frequency for these neurons either coincided or occurred after the onset of EMG activity.     

Multiple features of motor-unit activity influence force fluctuations during isometric contractions

To identify the mechanisms responsible for the fluctuations in force that occur during voluntary contractions, experimental measurements were compared with simulated forces in the time and frequency domains at contraction intensities that ranged from 2 to 98% of the maximum voluntary contraction (MVC). The abduction force exerted by the index finger due to an isometric contraction of the first dorsal interosseus muscle was measured in 10 young adults. Force was simulated with computer models of motor-unit recruitment and rate coding for a population of 120 motor units. The models varied recruitment and rate-coding properties of the motor units and the activation pattern of the motor-unit population. The main finding was that the experimental observations of a minimum in the coefficient of variation (CV) for force (1.7%) at approximately 30% MVC and a plateau at higher forces could not be replicated by any of the models. The model that increased the level of short-term synchrony with excitatory drive provided the closest fit to the experimentally observed relation between the CV for force and the mean force. In addition, the results for the synchronization model extended previous modeling efforts to show that the effect of synchronization is independent from that of discharge-rate variability. Most of the power in the force power spectra for the models was contained in the frequency bins below 5 Hz. Only a model that included a low-frequency oscillation in excitation, however, could approximate the experimental finding of peak power at a frequency below 2 Hz: 38% of total power at 0.99 Hz and 43% at 1.37 Hz, respectively. In contrast to the experimental power spectra, all model spectra included a second peak at a higher frequency. The secondary peak was less prominent in the synchronization model because of greater variability in discharge rate. These results indicate that the variation in force fluctuations across the entire operating range of the muscle cannot be explained by a single mechanism that influences the output of the motor-unit population.     

Gradual potentiation of isometric muscle force during constant electrical stimulation

An investigation was carried out into how stimulation frequency and stimulation history affect the potentiation of muscle force during 20s of constant stimulation of the two knee extensors in isometric conditions. Stimulation frequency significantly affected the potentiation pattern: low-frequency (2.5–10 Hz) stimulation showed a reduction and subsequent enhancement of force, and high-frequency (14.3–25 Hz) stimulation showed only enhancement of force. The degree of enhancement in force and time-to-peak decreased with the stimulation frequency. Whereas conditioning stimulation (both 40 Hz and 14.3 Hz) significantly enhanced the muscle force above 85%, following main stimulation (14.3 Hz) after short rest (10 s and 50 s, respectively) induced little force enhancement (below 8%). In particular, when the frequency of the conditioning stimulation was 14.3 Hz, the initial force at the main stimulation showed a very similar value to the final force value of the conditioning stimulation (above 90% similarity). The potentiated twitch force slowly decayed during rest, with an average time constant of 2.4 min. These observations indicate that muscle potentiation depends on the stimulation frequency and stimulation history, and therefore a computer model of potentiation can play an important role in predicting muscle force and body movement induced by electrical stimulation.     

Blood flow does not limit skeletal muscle force production during incremental isometric contractions

It has been suggested that a transient limitation in blood flow during intermittent muscular contractions can contribute to muscle fatigue, and that this limitation is greater as contraction intensity increases. We investigated skeletal muscle blood flow and fatigue in 13 healthy, untrained men (21–27 years) during 16 min of intermittent (4 s contract, 6 s relax) isometric dorsiflexor contractions. Contractions began at 10% of pre-exercise maximal voluntary contraction (MVC) force and increased by 10% every 2 min. Hyperemia (i.e., post-contraction blood flow, measured by venous occlusion plethysmography) and MVC were measured at the end of each stage. Muscle volume measures were obtained using magnetic resonance imaging. After 10 min of exercise, submaximal force and post-contraction hyperemia plateaued. MVC fell from 8 min of exercise onwards (p=0.004), and this onset of fatigue preceded the plateau in submaximal force and hyperemia. Despite a large range in dorsiflexor muscle size (66.3–176.4 cm³) and strength (112.5–421.8 N), neither muscle size nor strength were related to fatigue. The temporal dissociation between changes in blood flow and the onset of fatigue (fall of MVC) suggest that limited blood flow was not a factor in the impaired force production observed during intermittent isometric dorsiflexor contractions in healthy young men. Additionally, post-contraction hyperemia increased linearly with increasing contraction intensity, reflecting a match between blood flow and force production throughout the protocol that was independent of fatigue.     

Energy metabolism in different human skeletal muscles during voluntary isometric contractions

Summary The energy turnover in contracting skeletal muscle was studied by measuring the rate of temperature rise during voluntary, isometric contractions and circulatory arrest in M. soleus, M. sacrospinalis and M. biceps brachii in 14 males, by thermoelements inserted in the muscles. A linear relationship between rate of temperature rise and force intensity given as per cent of maximal voluntary contraction (MVC) was demonstrated in biceps (r=0.95), but not so clearly confirmed in soleus (r=0.73). Muscle biopsies were taken from the same muscles and fibre type distribution was determined histochemically by staining for ATPase. The rate of heat production at MVC showed positive correlation to the percentage of fast twitch (FT) fibres in the muscles (r=0.90). Linear extrapolation indicates that the maximal energy turnover in human FT fibres is approximately six times that of slow twitch (ST) fibres during voluntary isometric contractions.This work was submitted by G. Bolstad as a thesis to the University of Bergen in June 1975, in partial fulfilment of the requirements for the degree of Candidatus realium     

Mechanomyogram and force relationship during voluntary isometric ramp contractions of the biceps brachii muscle

The aim of the present study was to examine the non-stationary mechanomyogram (MMG) during voluntary isometric ramp contractions of the biceps brachii muscles using the short-time Fourier transform, and to obtain more detailed information on the motor unit (MU) activation strategy underlying in the continuous MMG/force relationship. The subjects were asked to exert ramp contractions from 5% to 80% of the maximal voluntary contraction (MVC) at a constant rate of 10% MVC/s. The root mean squared (RMS) amplitude of the MMG began to increase slowly at low levels of force, then there was a slight reduction between 12% and 20% MVC. After that, a progressive increase was followed by a decrease beyond 60% MVC. As to the mean power frequency (MPF), a relatively rapid increase up to 30% MVC was followed by a period of slow increment between 30% and 50% MVC. Then temporary reduction at around 50% MVC and a further rapid increase above 60% MVC was observed. The interaction between amplitude and MPF of the MMG in relation to the MU activation strategy is discussed for five force regions defined on the basis of their inflection points in the RMS-amplitude/force and MPF/force relationships. It was found that the MMG during ramp contractions enables deeper insights into the MU activation strategy than those determined during traditional separate contractions. In addition, this contraction protocol is useful not only to ensure higher force resolution in the MMG/force relationship, but also to markedly shorten the time taken for data acquisition and to reduce the risk of fatigue. Accepted: 31 August 2000     

Actomyosin energy turnover declines while force remains constant during isometric muscle contraction

Energy turnover was measured during isometric contractions of intact and Triton-permeabilized white fibres from dogfish ( Scyliorhinus canicula ) at 12°C. Heat + work from actomyosin in intact fibres was determined from the dependence of heat + work output on filament overlap. Inorganic phosphate (P_i) release by permeabilized fibres was recorded using the fluorescent protein MDCC-PBP, N-(2-[1-maleimidyl]ethyl)-7-diethylamino-coumarin-3 carboxamide phosphate binding protein. The steady-state ADP release rate was measured using a linked enzyme assay. The rates decreased five-fold during contraction in both intact and permeabilized fibres. In intact fibres the rate of heat + work output by actomyosin decreased from 134 ± s.e.m. 28 μW mg⁻¹ ( n = 17) at 0.055 s to 42% of this value at 0.25 s, and to 20% at 3.5 s. The force remained constant between 0.25 and 3.5 s. Similarly in permeabilized fibres the P_i release rate decreased from 5.00 ± 0.39 mmol l⁻¹ s⁻¹ at 0.055 s to 39% of this value at 0.25 s and to 19% at 0.5 s. The steady-state ADP release rate at 15 s was 21% of the P_i rate at 0.055 s. Using a single set of rate constants, the time courses of force, heat + work and P_i release were described by an actomyosin model that took account of the transition from the initial state (rest or rigor) to the contracting state, shortening and the consequent work against series elasticity, and reaction heats. The model suggests that increasing P_i concentration slows the cycle in intact fibres, and that changes in ATP and ADP slow the cycle in permeabilized fibres.     

Maximal force during eccentric and isometric actions at different elbow angles

The purpose of this study was to examine a course of force potentiation and/or inhibition during maximal voluntary eccentric action. Maximal voluntary force (MVC) of elbow flexion of ten healthy male volunteers was measured during isometric and isokinetic eccentric action starting from 80° or 110° and ending at 140° elbow angle. Surface EMG was recorded from biceps brachii (BB) and brachioradialis (BR) muscles. Maximal voluntary eccentric force during the first 10° of the movement was higher (P<0.001) than the maximal voluntary isometric preactivation force both in 80° and in 110° starting position at all three velocities (1, 2, and 4 rad s⁻¹). The relative force potentiation was velocity dependent being smallest at the lowest stretching speed (P<0.01). Average EMG (aEMG) of BB and BR decreased as the joint angle increased both in eccentric and in isometric actions but the decrease in aEMG towards extension was somewhat higher in eccentric actions as compared to isometric. It was concluded that the force measured during the first 10° of eccentric contraction always exceeded the maximal voluntary isometric preactivation force regardless of the joint angle or of the movement velocity. When maximal voluntary preactivation preceded the stretch, the relative force potentiation seemed to be greater at higher stretching velocities (velocity dependent) while at lower preactivation levels, the velocity dependence was not observed. Decreased muscle activation and lower maximal voluntary force towards the end of the movement suggested inhibition during maximal voluntary eccentric actions.     

The effect of number of lengthening contractions on rat isometric force production at different frequencies of nerve stimulation

Aim: To test the effect of 3, 10, 60 and 240 lengthening contractions (LC) on maximal isometric force of rat plantar flexor muscles at different stimulation frequencies. Methods: Using a dynamometer and electrical nerve stimulation, maximally active skeletal muscles were stretched by ankle rotation to produce LC of the plantar flexor muscles in intact female rats. After the lengthening contraction protocols, maximal isometric force was measured at different frequencies of nerve activation to obtain frequency‐dependent force deficits (weakness). Results: The magnitude of the force deficit, measured 1 h after the protocols at 80 Hz, increased as a function of repetition number (three LC, 33.3 ± 1.7%; 10 LC, 37.2 ± 2.3%; 60 LC, 67.6 ± 1.5%; 240 LC, 77.7 ± 1.2%). Force deficits were also measured at each stimulation frequency tested (5:120 Hz). Using a ratio of isometric force at 20:100 Hz stimulation, the relative depression of force at low frequency was determined. The relative depression of isometric force at low frequency was most prominent during the early repetitions. Conclusion: As low‐frequency force depression appears to result primarily from excitation–contraction (E–C) coupling failure, the early LC in a series of repeated contractions probably contribute most to damage of the cellular components involved in E–C coupling.     

Potentiation of isometric and isotonic contractions during high-frequency stimulation

Activity dependent potentiation is an enhanced contractile response resulting from previous contractile activity. It has been proposed that even a maximal effort contraction may be enhanced by prior activity if there is an increase in the peak rate of force development. This should increase the peak active force during a very brief maximal effort contraction. The purpose of these experiments was to evaluate potentiation during brief sequential contractions with high-frequency stimulation. For this experiment, the rat medial gastrocnemius muscle was isolated in situ. Sequential stimulation (two contractions per second for 4 s) with 200, 300, or 400 Hz doublets, triplets, and quadruplets was applied. A small degree of force potentiation was observed in isometric contractions at the reference length (RL), but the activity dependent potentiation of isometric contractions was greater at short muscle length. For example, peak rate of force development for 200 Hz doublets increased significantly from the first to the eighth contraction (from 0.30 ± 0.02 to 0.34 ± 0.02 N·s⁻¹ at RL and from 0.18 ± 0.02 to 0.28 ± 0.01 N·s⁻¹ at RL-3 mm). During isotonic contractions, there were significant increases in peak shortening from the first to the eighth contraction. With 200 Hz doublet stimulation, shortening increased from 0.85 ± 0.14 to 1.14 ± 0.17 mm, and this corresponded with an increase in peak velocity (from 116 ± 18 to 136 ± 19 mm·s⁻¹). Remarkably, even 400 Hz quadruplets showed a significant increase in shortening during repeated contractions (2 s⁻¹). These observations indicate the possibility that activity dependent potentiation can result in significant improvement in both isometric and dynamic contractions, even when activated at very high frequency.     

Rotation of motoneurons during prolonged isometric contractions in humans

Prolonged and weak isometric contractions can result in neuromuscular fatigue. Alternation of discharge of motor units with similar thresholds (termed rotation) could be useful to minimize neuromuscular fatigue by providing periods for metabolic recovery of the contractile elements. In the present study, we investigated the prevalence of motoneuron rotation during prolonged contractions of distal limb muscles. Electromyographic (EMG; needle and surface) was recorded from muscles of the forearm and distal leg. The subject made a slowly increasing isometric contraction to recruit and discharge a motor unit (1) for a prolonged period of time (> 30 min). Sometimes an additional motor unit (2) was recruited in which case the subject relaxed the contraction slightly so that only one motor unit remained tonic. Often it was this newly recruited motor unit (i.e., unit 2) that continued discharging, while motor unit 1 fell silent. Continued contraction would then lead to the resumption of tonic discharge of unit 1 and silence of unit 2. This would complete a rotation between motor units 1 and 2. During prolonged contractions, rotation was observed in approximately 80% of the motor-unit pairs examined. There was no difference in rotation incidence by muscle type. For the unit pairs showing rotation, surface EMG values were significantly higher immediately prior to rotation than after rotation had occurred. Our findings show that rotation of motor units with similar recruitment thresholds during such contractions is common in distal muscles of the arm and leg and may help offset neuromuscular fatigue.     

Movement-related cortical potentials during handgrip contractions with special reference to force and electromyogram bilateral deficit

We investigated movement-related cortical potentials from motor cortex areas (C3 and C4) and isometric force and electromyogram (EMG) activity in association with maximal bilateral (BL) and unilateral (UL) handgrip contraction in eight right-handed subjects. The BL grip exhibited deficits in force [right, –5.2 (SEM 1.1)%; left, –4.5 (SEM 1.9)%] and EMG [right, –9.5 (SEM 2.2)%; left, –7.6 (SEM 2.5)%] compared with the UL grip. In the UL contractions, the amplitudes of the negative slope [NS 2.77 (SEM 0.70) vs 2.40 (SEM 0.76) V·s for left hand,P < 0.05; 2.54 (SEM 0.55) vs 2.23(SEM 0.54) V·s for right hand,P < 0.05 and motor potentials [MP: 1.56 (SEM 0.32) V.s vs 1.23 (SEM 0.35) V·s for left hand,P < 0.01; 1.44 (SEM 0.32) V·s vs 1.10 (SEM 0.25) V·s for right hand,P < 0.01] were greater in the contralateral hemisphere. For the BL contractions, the asymmetry of the larger potentials for the contralateral side disappeared and lower symmetrical potentials [NS, 2.43 (SEM 0.61) V·s for C3 vs 2.43 (SEM 0.63) V·s for C4: MP: 1.31 (SEM 0.35) V·s for C3 vs 1.34 (SEM 0.32) V·s for C4] were observed. It was concluded that the BL deficit in force and EMG is associated with reduced movement-related cortical potentials suggesting that the bilateral force and (EMG) deficit compared with unilateral hand-grip contractions is caused by a mechanism of interhemispheric inhibition.     

Frequency analysis of the surface electromyogram during sustained isometric contractions

Summary Four male and four female volunteers served as subjects in these experiments to assess the frequency components of the surface EMG during and following brief (3 s) and sustained isometric contractions of the handgrip muscles. Two types of fatiguing contractions were performed. Contractions were either maintained to fatigue at a constant tension of up to 100% of their strength or were maintained as a sustained maximal effort in the unfatigued or previously fatigued muscle. The frequency components of the surface EMG were assessed by calculating the power spectra of 1.5 s samples of the EMG from a fundamental frequency of 4 Hz through the first 128 harmonics by Fourier analysis; the centre frequencies of the resultant power spectra were then used as an index of the mean frequency of the EMG. The results of these experiments showed that the centre frequency was independent of the tension exerted by the muscle during brief isometric contractions but decreased linearly with time throughout the duration of fatiguing isometric contractions at tensions between 25 and 100% MVC. During sustained maximal effort, the frequency initially decreased linearly with time. However, once the target tension could no longer be maintained, the centre frequency remained constant throughout the remainder of the contraction. The frequency was found to recover within 1 min following exercise at all tensions examined.     

Physiological modifications of local haemodynamic conditions during bilateral isometric contractions

Summary Nine subjects (five women and four men) simultaneously performed two isometric contractions sustained until exhaustion at different relative forces: 40% of maximum voluntary contraction (MVC) for the right elbow flexors; 50% MVC for the left elbow flexors. Contraction of the left elbow flexors commenced at 50% of the limit time (maximum maintenance time) of isometric contraction of the right elbow flexors. Increase in heart rate during concomitant contraction of the left elbow flexors led to an increase in blood flow to the right elbow flexors. Under these conditions, the limit time of isometric contraction of the right elbow flexors was prolonged with respect to the limit time obtained for an isolated isometric contraction at the same relative tension. The difference was more significant in the female (+40%,P<0.05) than in the male subjects (+20%,P>0.05).     

The effects of massed versus distributed contractions on the variability of maximal isometric force

This study evaluated the effect of a massed versus distributed repetition schedule on the variability of force and surface electromyographic (sEMG) activity during maximal voluntary isometric elbow flexion contractions. The massed group (N = 13) performed 15 contractions on 1 day, while the distributed group (N = 13) performed 15 contractions across three consecutive days (five per day). Two retention tests (five contractions each) occurred 2 weeks and 3 months after the final trial of the initial test sessions. Force and sEMG of the biceps and triceps brachii muscles were recorded concurrently. Both groups had comparable increases in force and biceps brachii sEMG that continued over short- and longer-term retention tests (p < 0.05). Triceps brachii sEMG exhibited a more complicated pattern of successive decreases and increases (p < 0.05). The massed repetition schedule resulted in significantly (p < 0.05) less variability in maintaining a constant force [root mean square (RMS) error]. There was a significant decrease in the variability of the force–time and sEMG–time curves as assessed by the variance ratio (VR) (p < 0.05). Only biceps sEMG and VR correlated highly with force VR for the distributed group. Total (biceps + triceps) sEMG magnitude and variability correlated highly with both RMS error and force VR for the massed group. It was concluded that the massed contraction pattern allowed participants to learn how to regulate joint stiffness in addition to the variability of muscle activity. This allowed for greater decreases in RMS error than could be obtained by regulating the variability of muscle activity alone.