Effect of prior isometric muscle action on concentric torque output during plantar flexion

The influence of different levels of prior isometric muscle action on the concentric torque output during plantar flexion was examined at two angular velocities (60°·s^–1 and 120°·s^–1) in ten healthy female subjects. The levels of the prior muscle actions were 25%, 50%, 75% and 100% of the maximal voluntary isometric contraction (MVIC). A KINetic-COMmunicator II dynamometer was used to measure torque output during plantar flexion within a range of motion of 78°-120° of the ankle joint. Simultaneous recordings of electromyograms (low-pass filtered and rectified) were obtained from the gastrocnemius medialis muscle and the soleus muscle. Torque-angle curves were made for the plantar flexions using different prior muscle actions. Up to 75% of MVIC, the torque output in the first part of the range of motion increased with the level of the prior isometric muscle action; at higher levels of MVIC the torque did not appear to increase any further. Later in the range of motion, after 24° in the plantar flexion at a velocity of 60°·s^–1 and 31° at 120°·s^–1, the prior muscle actions had no further influence. No increase was found in the electromyograms, with one exception, during the concentric movements when preceded by higher levels of MVIC. It would seem therefore that the increase in torque output early in the range of motion cannot be explained on the basis of differences in electrical muscle activation in this study.     

Isotonic lengthening and shortening movements of cat soleus muscle

1. By supplying pulses to different subdivisions of the ventral nerve roots in rotation it was possible to obtain smooth contractions of cat soleus with low rates of stimulation.2. After contracting isometrically the muscle was subjected to ;step' changes in tension after which it lengthened or shortened isotonically.3. Isotonic lengthening movements usually began relatively slowly but proceeded with increasing velocity; this acceleration was most conspicuous when low rates of stimulation were used.4. At low rates of stimulation the isotonic lengthening movement often continued beyond the length at which the muscle could have generated that tension in an isometric contraction. The muscle then shortened slowly back toward that length.5. Isotonic shortening movements began relatively rapidly, but as shortening continued the movement became slower, and often had an irregular oscillatory course.6. The isotonic movements are discussed in relation to the sliding filament theory of muscle contraction.7. The compliance of the series elastic elements was calculated from the relative amplitudes of the ;step' changes in tension and length. The stiffness of this component increased with increasing muscle tension.     

The mechanical properties of cat soleus muscle during controlled lengthening and shortening movements

1. By supplying pulses to different subdivisions of the ventral nerve roots in rotation, it was possible to obtain smooth contractions of cat soleus with low rates of stimulation.2. After contracting isometrically the muscle was subjected to constant velocity lengthening or shortening movements.3. During shortening the tension always fell below the isometric value. The fall in tension was usually greatest when low rates of stimulation were used.4. The effect of lengthening on tension depended on the rate of stimulation. At high rates of stimulation the tension during lengthening always rose above the isometric tension. At lower rates of stimulation (5-15 pulses/sec) the tension rose at the beginning of an extension, but decreased later in the movement to a level that was often less than the isometric tension corresponding to that muscle length. At these stimulus rates the tension during isometric contraction was usually higher than during a sustained movement in either direction.5. At low rates of stimulation longitudinal vibratory movements of more than 0.1 mm also reduced the tension far below the isometric value, whereas the reduction was quite slight when the rate of stimulation was high.6. The isometric tension during smooth contractions at low stimulus rates was remarkable in the following respects: it developed rather slowly, it was higher than the tension during or immediately after movements, and it was only slowly regained after movement had ceased.7. The results are discussed in relation to the sliding filament theory of muscle contraction, which, with certain assumptions, provides an explanation for many of the findings.     

Changes in oxygen consumption of human muscle and tendon following repeat muscle contractions

The purpose of this study was to investigate changes in the oxygen consumption (VO₂) of muscle and tendon following repeat muscle contractions. During endurance tests (50 repetitions at 70% of the maximum voluntary contraction with 5-s contractions and 5-s rest) and the recovery period (0–10 min), we measured the blood volume and oxygen saturation (StO₂) of the medial gastrocnemius muscle and Achilles tendon using near infrared spectroscopy and red laser light. Nine male subjects performed the endurance tests three times on separate days (tests-1, 2, and 3). Before and after (test-1: immediately after, test-2: at the 5-min recovery point, test-3: at the 10-min recovery point) the endurance tests, the rate of StO₂ during 8-min period of arterial occlusion was measured to estimate the VO₂ of muscle and tendon. In test-3, after the end of exercise, the THb and StO₂ of the Achilles tendon increased gradually, and these values were higher than the pre-exercise levels until the end of the recovery period. The VO₂ of tendon as well as muscle increased significantly after the repeat muscle contractions. Furthermore, the VO₂ of tendon returned to the pre-exercise level at the 10-min point of recovery, although that of muscle was significantly higher compared to the pre-exercise level until the end of the recovery period. These results indicate that the difference between oxygen supply and consumption within tendon was greater after compared to before exercise.     

Acoustic myography reflects force changes during dynamic concentric and eccentric contractions of the human biceps brachii muscle

Summary The relationship between acoustic myography (AMG), electromyography (EMG) and force during submaximal dynamic contractions was examined in the biceps brachii muscles of eight healthy males (aged 17–26 years). Different weights were lifted and lowered at a constant speed, using a wall pulley system, to perform concentric and eccentric contractions, respectively. Integrated AMG (iAMG) and integrated EMG (iEMG) activity both increased linearly with force during concentric (iAMGr=0.94; iEMGr=0.99) and eccentric (iAMGr=0.90; iEMGr=0.94) contractions. The slopes of the concentric regression lines were significantly different from the eccentric slopes (P<0.01) for both iAMG and iEMG with concentric contractions showing greater levels of activity. The results indicated that AMG can be used to detect changes in force during dynamic contractions which has important implications for the use of AMG in rehabilitation. The differences in iAMG activity between concentric and eccentric contractions are discussed in relationship to the origin of the AMG signal.     

Contraction characteristics of the human quadriceps muscle during percutaneous electrical stimulation

Percutaneous electrical stimulation was used to study the force response of the quadriceps muscle. The normal frequency dependence of force was investigated in muscles at rest and after fatiguing contractions. A comparison between force response during fatigue induced by electrical stimulation at different frequencies and by voluntary work suggested equal changes in contractility, irrespective of the fatigue-inducing procedure. In fresh muscle we found a linear relation between stimulation period (10–100 ms) and force. At fatigue the relation changes with maximal deviation from linearity at a 50-ms period (20 Hz). There is a rapid recovery of high frequency force whereas the low frequency response remains low even after 30 min rest. At very low frequencies there is initially unexpectedly high force in fatigued muscle. This could be a result of increased fusion of twitches with initially prolonged relaxation time. To study the twitch summation we compared experimental results in a wide frequency range with computer-simulated twitch summations and present the frequency dependence of summation processes in human quadriceps muscle.     

Study of human muscle contraction using electrically evoked twitch responses during passive shortening and lengthening movements

Summary Electrically evoked twitch properties of the human plantarflexor muscles were measured with the muscles at a constant length (static) and during passive shortening and lengthening. A Kin-Com dynamometer system was used to passively move the ankle joint at 0.52 rad s⁻¹ (30‡ s⁻¹), as well as to record the twitch responses which were elicited by supramaximal electric shocks applied over the tibial nerve in the popliteal fossa. In the lengthening and shortening conditions, twitches were evoked by triggering the shocks so that the twitch response occurred at a similar angular position for all three conditions. The lengthening twitch peak torque was about twice as large as that recorded for the shortening condition. There was, however, no statistical difference in the twitch time course in these three testing conditions. This twofold increase in the peak twitch torque during lengthening, compared to shortening, is much greater than the torque increase reported during eccentric, as compared to concentric maximal voluntary contractions. These findings suggest that a deactivation process of the contractile system occurs during muscle shortening, while in contrast, during passive lengthening a potentiation mechanism is acting, and that both these mechanisms are independent of volitional muscle activation. The present study is the first to demonstrate the possibility of electrically evoked contractions of human muscles during passive lengthening and shortening. We believe that the use of such evoked contractions may be promising for the study of contractile behaviour of human skeletal muscles during eccentric and concentric conditions. This work was supported by a grant from l'Institut de Recherche en Santé et en Sécurite du Travail du Québec (IRSST)     

Reduced voluntary activation of human skeletal muscle during shortening and lengthening contractions in whole body hyperthermia

This study examined the effect of whole body hyperthermia on the voluntary activation of exercised and non-exercised skeletal muscle performing a series of lengthening and shortening contractions. Thirteen subjects exercised on a cycle ergometer at 60% of maximal oxygen consumption until voluntary exhaustion in ambient conditions of approximately 40 degrees C and 60% relative humidity. Before and immediately following the cycle protocol, subjects performed a series of 25 continuous isokinetic shortening and lengthening maximal voluntary contractions (MVCs) of the leg extensors and forearm flexors. Voluntary activation for shortening and lengthening contractions for the forearm and leg was assessed prior to and following the 25 MVCs by superimposing a paired electrical stimulus to the femoral nerve and the biceps brachii during additional MVCs. Exercise to exhaustion increased rectal temperature to 39.35+/-0.50 degrees C. Voluntary activation remained unchanged following the prehyperthermia endurance set of shortening and lengthening maximal contractions in both the forearm flexors and leg extensors. Similarly, voluntary activation remained at prehyperthermic levels for the single MVCs immediately following the cycle trial. However, by the time of completion of the posthyperthermia endurance contractions, voluntary activation had declined significantly by 5.87+/-7.56 and 8.46+/-9.26% in the shortening and lengthening phases, respectively, for the leg extensors but not for the forearm flexors. These results indicate that the central nervous system (CNS) reduces voluntary drive to skeletal muscle performing both shortening and lengthening contractions following exercise-induced hyperthermia. The reductions in voluntary activation were only observed following a series of dynamic movements, indicating that the CNS allows for initial and brief 're-activation' of skeletal muscle following exercise-induced hyperthermia.     

Glucose infusion attenuates fatigue without sparing glycogen in rat soleus muscle during prolonged electrical stimulation in situ

Carbohydrate administration increases endurance in man, and this could be associated with a reduction in muscle glycogen utilization in type I but not in type II fibres. Glucose infusion also attenuates fatigue in the rat plantaris muscle (94% type II fibres) stimulated indirectly in situ, but this is not associated with a glycogen sparing effect. The aims of this study were to verify if glucose infusion would attenuate fatigue and would reduce glycogen utilization in a muscle predominantly composed of type I fibres. For this purpose, the soleus muscle (84% type I fibres) was indirectly stimulated in situ in anaesthetized rats for 60 min while infusing either saline or glucose (1 g.kg^–1.h^–1; plasma glucose 7.7 mmol.l^–1 vs. ~5 mmol.l^–1 with saline only). The experimental data were expressed as the means (SD). With and without glucose, the dynamic force decreased by ~20% in the first minute of stimulation. With the infusion of saline, the dynamic force further decreased to 55% of the initial value at the end of the 60-min period of stimulation, but when glucose was infused for 60 min, the dynamic force remained constant at 78% of the initial value. When glucose was infused starting at min 30, dynamic force was partially restored. However, muscle glycogen utilization was not significantly different with the infusion of glucose compared to with the infusion of saline. These results suggest that glucose infusion attenuates fatigue in type I muscle fibres, but that this is not associated with any muscle glycogen sparing.     

Cortical control of human soleus muscle during volitional and postural activities studied using focal magnetic stimulation

The surface-recorded electromyographic (EMG) responses evoked in the ankle musculature by focal, transcranial, magnetic stimulation of the motor cortex were studied in healthy human subjects. Such soleus evoked motor responses (EMRs) were characterised over a wide range of background levels of motor activity and using different stimulus intensities. EMRs were recorded during predominantly (1) volitional and (2) postural tasks. In the former task subjects were seated and voluntarily produced prescribed levels of soleus activation by reference to a visual monitor of EMG. In the latter task subjects assumed standing postures without EMG feedback. Comparison of the EMRs of soleus, traditionally considered a slow anti-gravity extensor muscle, during these tasks was used to evaluate its cortical control in primarily volitional versus primarily postural activities. The form of soleus EMRs produced by single magnetic cortical stimuli comprised an initial (approx. 30 ms) increase and subsequent (approx. 50 ms) depression of EMG. Cortical stimulation could elicit substantial excitatory soleus EMG responses; for example, responses evoked by mild, magnetic stimuli (125% threshold for inducing a response in the relaxed muscle) as subjects exerted full voluntary plantarflexor effort averaged almost 20% of the maximum M-wave which could be elicited by an electrical stimulus to the posterior tibial nerve. Excitatory EMRs could be elicited in the voluntarily relaxed soleus muscle of the majority of subjects during sitting. The amplitude of soleus responses, induced by threshold stimuli for the relaxed state or approximately 125% threshold intensity, increased approximately linearly with background EMG over a wide range of volitional contraction levels. By contrast, there was no systematic change in the latency of excitatory soleus EMRs with increasing voluntary effort. The excitatory responses evoked in the voluntarily relaxed soleus of seated subjects by magnetic stimulation were regularly facilitated by incremental, voluntary contraction of the contralateral ankle extensors in a graded manner. However, such facilitation of responses was not observed when subjects voluntarily activated the muscle in which EMRs were elicited. The pattern of the responses elicited in soleus by magnetic stimulation during the postural task generally resembled that found during the volitional task. The amplitudes of excitatory soleus EMRs at a given stimulus intensity, obtained when subjects stood quietly, leaned forwards or stood on their toes to produce differing levels of ankle extensor contraction, increased with background EMG. Overall, the relationship between the size of cortically evoked soleus responses and the tonic level of motor activity, observed in individual subjects at matched stimulus intensities, did not consistently differ between postural and volitional tasks. The present results suggest that the motor cortex is potentially capable of exerting rapid regulation of the soleus muscle, and presumably other ankle extensors, not only when the muscle participates in volitional tasks but also when it is engaged in postural maintenance.     

Effect of joint angle on mechanomyographic amplitude during unfused and fused tetani in the human biceps brachii muscle

The influence of muscle length-dependent changes in contractile properties on the mechanomyogram (MMG) was investigated during evoked contractions of the biceps brachii muscle. The biceps of nine healthy subjects was stimulated by single twitches, unfused (10 Hz), and fused (30 Hz) frequencies at elbow joint angles of 75, 90, 105, 120, 135, and 150°. During evoked contractions, the longitudinal movement was estimated by the fluctuation of torque signal, and the lateral movement was detected by MMG. Contraction time and half relaxation time were calculated from the single twitches. For repetitive stimulations, followed by eliminating DC component from torque signals, the root mean square values calculated from the torque and MMG signals, which were torque fluctuation and RMS–MMG, respectively. For the 10 Hz stimulation condition, reductions in the torque fluctuation and RMS–MMG were concomitantly observed with increasing elbow angle, and there was a significant correlation between the torque fluctuation and RMS–MMG. On the other hand, for 30 Hz stimulation, there were no significant differences in the torque fluctuation and RMS–MMG over all elbow angles, and no significant correlation between the two parameters. Moreover, the torque fluctuation and RMS–MMG for 10 Hz stimulation were correlated with the contraction time and half relaxation time obtained at each elbow angle, while there were no correlations for the 30 Hz condition. These data suggest that MMG could be a reliable tool to study the development of fusion and the changes in muscle contractile properties during repetitive unfused contractions.     

Toward the optimal waveform for electrical stimulation of human muscle

Electrical stimulation of the quadriceps muscle was used to elicit 4-min isometric contractions at 10% of the maximal voluntary contraction (MVC) in four male and three female subjects. The effect of four waveforms, including Russian, interferential, sine, and square, on the mean stimulation current required to achieve the desired contraction force, subjective comfort, and physiological responses was studied. Interferential stimulation, even at full power, could not elicit a sustained contraction at 10% MVC. The contractions elicited by electrical stimulation utilizing the sine waveform required significantly less mean stimulation current to maintain the desired force of contraction with consistently lower verbal rating scale scores and greater increases in oxygen consumption than either the Russian or square waveform stimulations. Russian waveform stimulation produced a significantly greater rise in galvanic skin resistance than the sine or square waveform while the changes in respiratory quotient were similar between waveforms. The data support sine wave stimulation as working the best by producing the desired muscle tension with the least mean stimulation current and therefore, the least tissue trauma while providing the most subjective comfort. Electronic Publication     

Response of the human triceps surae muscle to electrical stimulation during varying levels of blood flow restriction

The changes in muscle force associated with varying degrees of lower-limb ischaemia were investigated. Isometric torque production by the triceps surae muscle was measured during a 5-min continuous train of 2-Hz electrical stimulation in six healthy young adults under different thigh cuff occlusion pressures. The reproducibility of this protocol when performed under complete ischaemia (tested five times over a 2-week period) was assessed as having a coefficient of variation (CV) for fatigue (end/initial force) of [mean (SEM) 12 (1)%; n=5]. This compares favourably with that obtained for maximum voluntary contraction torque [CV 9 (1)%]. In six subjects, triceps surae muscle fatigue was assessed under thigh cuff pressures of 0, 6.7 kPa (50 mmHg, venous occlusion) and 28 kPa (210 mmHg, complete ischaemia), as well as two intermediate levels of occlusion that were established by cuff pressures of 13.4 (0.5) and 20.3 (1.1) kPa [103 (4) and 152 (8) mmHg, respectively]. These corresponded to ankle-brachial pressure indices of 1.3 and 0.8, respectively when subjects were seated, or 0.8 and 0.36 when supine. With undisturbed lower-leg circulation, force potentiated steadily over the 5 min of stimulation such that the final force was 135 (8)% of the initial value. With complete ischaemia, force fell to 47 (2)% of the initial value. Stimulation under thigh occlusion pressures of 6.7, 13.4 and 20.3 kPa elicited intermediate levels of reduction in force, graded according to the increasing restriction of perfusion. The results show that low-force twitch contractions, which themselves do not occlude blood flow, are extremely sensitive to impaired perfusion and may represent a viable alternative to established methods of muscle performance assessment in patients with blood flow insufficiency. Accepted: 5 November 1999     

Relaxation and force during fatigue and recovery of the human quadriceps muscle: relations to metabolite changes

Force and relaxation were measured during electrical stimulation of the quadriceps muscle of 14 volunteers. Stimulation produced 51.2 s of intermittent ischaemic contractions either as 16 3.2-s tetani or as 64 0.8-s tetani. Changes during recovery were followed for 180 s. On 8 subjects muscle biopsies were taken during work and after the rest period for determination of ATP, phosphocreatine and intermediates in glucolysis. The stimulation using 0.8-s contractions gave more pronounced fatigue and slowing of relaxation. There was a good correlation between force and relaxation during work but this relation changed during recovery, indicating that no general relation exists between these two contraction characteristics. In the 0.8-s stimulation more ATP was utilized and there were more profound changes in metabolite levels. We found a correlation between estimated [H₂PO ₄ ^– ] and relaxation covering both work and recovery and hypothesize that inorganic phosphate and its removal by phosphocreatine resynthesis during recovery might be important. Since stimulation patterns differ in force and relaxation even after the recovery period we suggest that additional factors, such as pH, are of importance in this work model.     

Action of single dynamic fusimotor neurones on cat soleus Ia afferents during muscle shortening

Summary 1. The ability of single dynamic fusimotor (d) fibres to sustain the firing of muscle spindle primary (Ia) afferents during shortening was investigated in soleus muscles of anaesthetised cats. 2. Of 11 d fibres, 10 could maintain Ia firing during 10 mm/s shortening. Of the 7 tested at greater velocities, 5 could maintain Ia firing during shortening at velocities greater than 50 mm/s. 3. This ability was, however, critically dependent upon the timing of the stimulation. In particular, it rapidly reduced with increasing duration of stimulation before the onset of shortening. Furthermore, if appreciable stretch occurred between the onset of _d stimulation and the onset of shortening, this could greatly reduce the ability of _d fibres to sustain Ia discharge. 4. If _d neurones are on occasion phasically activated during voluntary shortening movements, their action could be an important determinant of Ia firing, even in the presence of weak _s action. Therefore in chronic recordings, observation of Ia firing during muscle shortening is not an adequate criterion for inferring _d activity.     

Modulation of corticospinal excitability during lengthening and shortening contractions in the first dorsal interosseus muscle of humans

Lengthening and shortening contractions are the fundamental patterns of muscle activation underlying various movements. It is still unknown whether or not there is a muscle-specific difference in such a fundamental pattern of muscle activation. The purpose of this study was, therefore, to investigate whether or not the relationship between lengthening and shortening contractions in the modulation of corticospinal excitability in the first dorsal interosseus (FDI) muscle is the same as that of previously tested muscles because the hand muscles are anatomically and functionally different from the other muscles. To this end, we investigated the relationship between the input-output curves of the corticospinal pathway (i.e., the relationship between the stimulus intensities vs. the area of motor-evoked potentials) during lengthening and shortening contractions in 17 healthy subjects. The shape of this relationship was sigmoidal and characterized by a plateau value, maximum slope, and threshold. The plateau value was at the same level between lengthening and shortening contractions. However, the maximum slope (P < 0.01) and threshold (P < 0.01) were significantly higher during lengthening contractions than during shortening contractions. These findings were different from the results of other muscles tested in previous studies (i.e., the soleus muscle and the elbow flexors). That is to say, the plateau value and the maximum slope during lengthening contractions were significantly lower than those during shortening contractions in previous studies. This study provides tentative evidence that the relationship between lengthening and shortening contractions in the modulation of corticospinal excitability differs between muscles, indicating that the underlying neural control is not necessarily the same even though the fundamental patterns of muscle activation are carried out.     

Estimation of the distribution of intramuscular current during electrical stimulation of the quadriceps muscle

Electrical stimulation is commonly used for strengthening muscle but little evidence exists as to the optimal electrode size, waveform, or frequency to apply. Three male and three female subjects (22–40 years old) were examined during electrical stimulation of the quadriceps muscle. Two self adhesive electrode sizes were examined, 2 cm × 2 cm and 2 cm × 4 cm. Electrical stimulation was applied with square and sine waveforms, currents of 5, 10 and 15 mA, and pulse widths of 100–500 μs above the quadriceps muscle. Frequencies of stimulation were 20, 30, and 50 Hz. Current on the skin above the quadriceps muscle was measured with surface electrodes at five positions and at three positions with needle electrodes in the same muscle. Altering pulse width in the range of 100–500 μs, the frequency over a range of 20–50 Hz, or current from 5 to 15 mA had no effect on current dispersion either in the skin or within muscle. In contrast, the distance separating the electrodes caused large changes in current dispersion on the skin or into muscle. The most significant finding in the present investigation was that, while on the surface of the skin current dispersion was not different between sine and square wave stimulation, significantly more current was transferred deep in the muscle with sine versus square wave stimulation. The use of sine wave stimulation with electrode separation distances of less then 15 cm is recommended for electrical stimulation with a sine wave to achieve deep muscle stimulation.