Mechanical behavior of skeletal muscle during intermittent voluntary isometric contractions in humans

Vøllestad, N. K., I. Sejersted, and E. Saugen. Mechanical behavior of skeletal muscle duringintermittent voluntary isometric contractions in humans.J. Appl. Physiol. 83(5):1557-1565, 1997.Changes in contractile speed and force-fusionproperties were examined during repetitive isometric contractions withthe knee extensors at three different target force levels. Sevenhealthy subjects were studied at target force levels of 30, 45, and60% of their maximal voluntary contraction (MVC) force. Repeated 6-s contractions followed by 4-s rest were continued until exhaustion. Contractile speed was determined for contractions elicited by electrical stimulation at 1-50 Hz given during exercise and a subsequent 27-min recovery period. Contraction time remained unchanged during exercise and recovery, except for an initial rapid shift in thetwitch properties. Half relaxation time(RT_1/2) decreased gradually by 20-40% during exercise at 30 and 45% of MVC. In the recovery period, RT_1/2 values werenot fully restored to preexercise levels. During exercise at 60% MVC,the RT_1/2 decreased for twitches and increased for the 50-Hz stimulation. In the recovery period after60% MVC, RT_1/2 values declinedtoward those seen after the 30 and 45% MVC exercise. The forceoscillation amplitude in unfused tetani relative to the mean forceincreased during exercise at 30 and 45% MVC but remained unalteredduring the 60% MVC exercise. This altered force-fusion was closelyassociated with the changes inRT_1/2. The faster relaxation mayat least partly explain the increased energy cost of contractionreported previously for the same type of exercise.

     

Proprioception during voluntary movement

In the last decade, a number of laboratories have accumulated data on the firing of single afferent fibres from muscle and skin during movement in awake cats, monkeys and human subjects. While there is general agreement on the firing behaviour of skin afferents and tendon organ (Ib) afferents during movement, there remains a significant divergence of opinion regarding the way in which the response of muscle spindle afferents (Ia and II) to length changes is modified by fusimotor action (e.g., alpha-gamma linkage versus "fusimotor set"). The controversies surrounding the fusimotor system have tended to overshadow the emergence of several important characteristics of proprioceptive behaviour, corroborated in separate laboratories. (i) Mean Ia firing rates during active movements are nearly always higher than at rest. Thus, activation of the fusimotor system is reserved for the control of, or preparation for, movement. In animals, there is now strong evidence that there is usually a tonic component of fusimotor action during rhythmical movements. (ii) During fast, unloaded movements (peak muscle speeds, 0.2 resting lengths/s or more), the firing of both Ia and II afferents usually increases during lengthening and decreases during shortening. Ib afferents fire during even the most rapid active shortening of their parent muscles. (iii) During powerful shortening contractions performed against significant loads, Ia firing is often appreciable, suggesting that there is at least some underlying alpha-gamma coactivation. (iv) During fast imposed muscle stretches, Ia afferents respond with segmented bursts of firing (threshold speed for segmentation, 0.5-1.0 resting length/s). Ib afferents show far less segmentation of discharge under similar circumstances.(ABSTRACT TRUNCATED AT 250 WORDS)     

History dependence of human muscle-fiber conduction velocity during voluntary isometric contractions

The conduction velocity (CV) of a muscle fiber is affected by the fiber's discharge history going back ～1 s. We investigated this dependence by measuring CV fluctuations during voluntary isometric contractions of the human brachioradialis muscle. We recorded electromyogram (EMG) signals simultaneously from multiple intramuscular electrodes, identified potentials belonging to the same motor unit using EMG decomposition, and estimated the CV of each discharge from the interpotential interval. In 12 of 14 subjects, CV increased by ～10% during the first second after recruitment and then fluctuated by about ±2% in a way that mirrored the fluctuations in the instantaneous firing rate. The CV profile could be precisely described in terms of the discharge history by a simple mathematical model. In the other two subjects, and one subject retested after cooling the arm, the CV fluctuations were inversely correlated with instantaneous firing rate. In all subjects, CV was additionally affected by very short interdischarge intervals (<25 ms): it was increased in doublets at recruitment, but decreased in doublets during continuous firing and after short interdischarge intervals in doubly innervated fibers. CV also exhibited a slow trend of about -0.05%/s that did not depend on the immediate discharge history. We suggest that measurements of CV fluctuations during voluntary contractions, or during stimulation protocols that involve longer and more complex stimulation patterns than are currently being used, may provide a sensitive approach for estimating the dynamic characteristics of ion channels in the human muscle-fiber membrane.     

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

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

Nonisometric behavior of fascicles during isometric contractions of a human muscle

Fascicle length, pennation angle, and tendonelongation of the human tibialis anterior were measured in vivo byultrasonography. Subjects (n = 9) wererequested to develop isometric dorsiflexion torque gradually up tomaximal at the ankle joint angle of 20° plantarflexion from theanatomic position. Fascicle length shortened from 90 ± 7 to 76 ± 7 (SE) mm, pennation angle increased from 10 ± 1 to 12 ± 1°, and tendon elongation increased up to 15 ± 2 mm with gradedforce development up to maximum. The tendon stiffness increased withincreasing tendon force from 10 N/mm at 0-20 N to 32 N/mm at240-260 N. Young's modulus increased from 157 MPa at 0-20 Nto 530 MPa at 240-260 N. It can be concluded that, in isometriccontractions of a human muscle, mechanical work, some of which isabsorbed by the tendinous tissue, is generated by the shortening ofmuscle fibers and that ultrasonography can be used to determine thestiffness and Young's modulus for human tendons.

     

The muscle sound properties of different muscle fiber types during voluntary and electrically induced contractions.

Soundmyogram (SMG) and electromyogram signals were recorded simultaneously from the relatively fast medial gastrocnemius (MG) and slow soleus (SOL) during voluntary and electrically induced contractions. Using a spike-triggered averaging technique, the averaged elementary sound and corresponding MU spikes were also obtained from about 35 different MUs identified. The rms-SMG of MG increased as a function of force (P < 0.01). On the contrary, these values for SOL increased up to 60% MVC (P < 0.01), but decreased at 80% MVC. The relationship between the peak to peak amplitude of SMG and MU spike indicated significant positive correlations (r = 0.631 to approximately 0.657, P < 0.01). During electrical stimulation at 5 Hz, the SMG power spectral peak frequency (PF) was matched with stimulation frequency in both muscles. At higher stimulation frequencies, e.g., > 15 Hz, only in the MG was SMG-PF synchronized with stimulation frequency; the slow SOL did not show such synchronization. Our data suggest that the SMG frequency components might reflect active motor unit firing rates, and that the SMG amplitude depends upon mechanical properties of contraction, muscle fiber composition, and firing rate during voluntary and electrically induced contractions.     

Motor unit firing rates during isometric voluntary contractions performed at different muscle lengths

Firing rates of motor units and surface EMG were measured from the triceps brachii muscles of able-bodied subjects during brief submaximal and maximal isometric voluntary contractions made at 5 elbow joint angles that covered the entire physiological range of muscle lengths. Muscle activation at the longest, midlength, and shortest muscle lengths, measured by twitch occlusion, averaged 98%, 97%, and 93% respectively, with each subject able to achieve complete activation during some contractions. As expected, the strongest contractions were recorded at 90 degrees of elbow flexion. Mean motor unit firing rates and surface EMG increased with contraction intensity at each muscle length. For any given absolute contraction intensity, motor unit firing rates varied when muscle length was changed. However, mean motor unit firing rates were independent of muscle length when contractions were compared with the intensity of the maximal voluntary contraction (MVC) achieved at each joint angle.     

Effects of speed and distance of muscle shortening on force depression during voluntary contractions

The purpose of this study was to determine the influence of speed and distance of muscle shortening on the amount of force depression for voluntary contractions. Two experimental tests were performed. In the first test, subjects performed isometric knee extensor contractions following muscle shortening produced by isokinetic knee extensions over the range 25-50 degrees. In the second test, subjects performed isometric knee extensor contractions following muscle shortening produced by isokinetic knee extensions at two speeds: 20 and 240 degrees /s. Knee extensor moments, surface electromyographical (EMG) signals of quadriceps femoris, and interpolated twitch moments were measured during all contractions and were compared with the corresponding values obtained during purely isometric contractions. Force depression following muscle shortening for the voluntary contractions tested in this study did not depend on the distance or the speed of muscle shortening. These results are in contrast to the corresponding results in the literature obtained using artificial electrical stimulation in which force depression was always found to be directly related to the distance of shortening and inversely related to the speed of shortening. The difference in force depression as a function of the distance and speed of muscle shortening between voluntary and artificial electrical stimulation may be associated with changes in activation following the voluntary shortening contractions, whereas activation is controlled and constant in all artificial stimulation protocols.     

Contralateral activity in a homologous hand muscle during voluntary contractions is greater in old adults.

This study compared the amount of contralateral activity produced in a homologous muscle by young (18-32 yr) and old (66-80 yr) adults when they performed unilateral isometric and anisometric contractions with a hand muscle. The subjects were not aware that the focus of the study was the contralateral activity. The tasks involved the performance of brief isometric contractions to six target forces, slowly lifting and lowering six inertial loads, and completing a set of 10 repetitions with a heavy load. The unintended force exerted by the contralateral muscle during the isometric contractions increased with target force, but the average force was greater for the old adults (means +/- SD; 12.6 +/- 15.3%) compared with the young adults (6.91 +/- 11.1%). The contralateral activity also increased with load during the anisometric contractions, and the average contralateral force was greater for the old subjects (5.28 +/- 6.29%) compared with the young subjects (2.10 +/- 3.19%). Furthermore, the average contralateral force for both groups of subjects was greater during the eccentric contractions (4.17 +/- 5.24%) compared with the concentric contractions (3.20 +/- 5.20%). The rate of change in contralateral activity during the fatigue task also differed between the two groups of subjects. The results indicate that old subjects have a reduced ability to suppress unintended contralateral activity during the performance of goal-directed, unilateral tasks.     

Effect of vibration during fatiguing resistance exercise on subsequent muscle activity during maximal voluntary isometric contractions

This investigation was designed to determine if vibration during fatiguing resistance exercise would alter associated patterns of muscle activity. A cross-over design was employed with 8 subjects completing a resistance exercise bout once with a vibrating dumbbell (V) (44 Hz, 3 mm displacement) and once without vibration (NV). For both exercise bouts, 10 sets were performed with a load that induced concentric muscle failure during the 10th repetition. The appropriate load for each set was determined during a pretest. Each testing session was separated by 1 week. Electromyography (EMG) was obtained from the biceps brachii muscle at 12 different time points during a maximum voluntary contraction (MVC) at a 170 degrees elbow angle after each set of the dumbbell exercise. The time points were as follows: pre (5 minutes before the resistance exercise bout), T1-T10 (immediately following each set of resistance exercise), and post (15 minutes after the resistance exercise bout). EMG was analyzed for median power frequency (MPF) and maximum (mEMG). NV resulted in a significant decrease in MPF at T1-T4 (p < or 0.05) and a significant increase in mEMG at T2 during the MVC. V had an overall trend of lower mEMG in comparison to NV. The mEMG and MPF values associated with NV were similar to previously reported investigations. The lower mEMG values and the higher MPF of V in comparison to NV are undocumented. The EMG patterns observed with vibration may indicate a more efficient and effective recruitment of high threshold motor units during fatiguing contractions. This may indicate the usage of vibration with resistance exercise as an effective tool for strength training athletes.     

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

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

Supraspinal fatigue during intermittent maximal voluntary contractions of the human elbow flexors.

Responses to transcranial magnetic stimulation in human subjects (n = 9) were studied during series of intermittent isometric maximal voluntary contractions (MVCs) of the elbow. Stimuli were given during MVCs in four fatigue protocols with different duty cycles. As maximal voluntary torque fell during each protocol, the torque increment evoked by cortical stimulation increased from approximately 1.5 to 7% of ongoing torque. Thus "supraspinal" fatigue developed in each protocol. The motor evoked potential (MEP) and silent period in the elbow flexor muscles also changed. The silent period lengthened by 20-75 ms (lowest to highest duty cycle protocol) and recovered significantly with a 5-s rest. The MEP increased in area by >50% in all protocols and recovered significantly with 10 s, but not 5 s, of rest. These changes are similar to those during sustained MVC. The central fatigue demonstrated by the torque increments evoked by the stimuli did not parallel the changes in the electromyogram responses. This suggests that part of the fatigue developed during intermittent exercise is "upstream" of the motor cortex.     

Repeatability of surface EMG variables during voluntary isometric contractions of the biceps brachii muscle.

The repeatability of initial value and rate of change of mean spectral frequency (MNF), average rectified values (ARV) and muscle fiber conduction velocity (CV) was investigated in the dominant biceps brachii of ten normal subjects during sustained isometric voluntary contractions. Four levels of contraction were studied: 10%, 30%, 50% and 70% of the maximal voluntary contraction level (MVC). Each contraction was repeated three times in each of three different days for a total of nine contractions/level/subject and 90 contractions per level across the ten subjects. Repeatability was investigated using the Intraclass Correlation Coefficient (ICC) and the standard error of the mean (SEM) of the estimates for each subject. Contrary to observations in other muscles, CV estimates appeared to be very repeatable both within and between subjects. CV showed a small but significant increase when contraction force increased from 10% to 50% MVC but no change for further increase of force. As force increased, MNF showed a slight decrease possibly related to a wider spreading of the CV values. The rate of time decrement of MNF and CV increased with the level of contraction. The normalized decrement (% of initial value per second) was in general higher for MNF than for CV and was more repeatable between subjects at 10% MVC than at 70% MVC. A final observation is that a resting time of 5 minutes may not be sufficient after a contraction at 50% or 70% MVC.     

Fluctuations in acceleration during voluntary contractions lead to greater impairment of movement accuracy in old adults.

The purpose of the study was to assess the effect of movement velocity on the relation between fluctuations in acceleration and the ability to achieve a target velocity during voluntary contractions performed by young (29.5 +/- 4.3 yr) and old (74.9 +/- 6.2 yr) adults. Subjects performed concentric and eccentric contractions with the first dorsal interosseus muscle while lifting a submaximal load (15% of maximum) at six movement velocities (0.03-1.16 rad/s). Fluctuations in acceleration, the accuracy of matching the target velocity, and electromyographic (EMG) activity were determined from three trials for each contraction type and movement velocity. The fluctuations in acceleration increased with movement velocity for both concentric and eccentric contractions, but they were greatest during fast eccentric contractions ( approximately 135%) when there was stronger modulation of acceleration in the 5- to 10-Hz bandwidth. Nonetheless, EMG amplitude for first dorsal interosseus increased with movement velocity only for concentric and not eccentric contractions. Consistent with the minimum variance theory, movement accuracy was related to the fluctuations in acceleration for both types of contractions in all subjects. For a given level of fluctuations in acceleration, however, old subjects were three times less accurate than young subjects. Although the EMG amplitude at each speed was similar for young and old adults, only the young adults modulated the power in the EMG spectrum with speed. Thus the fluctuations in acceleration during voluntary contractions had a more pronounced effect on movement accuracy for old adults compared with young adults, probably due to factors that influenced the frequency-domain characteristics of the EMG.     

Impulse propagation and muscle activation in long maximal voluntary contractions

With fatigue, force generation may be limited by several factors, including impaired impulse transmission and/or reduced motor drive. In 5-min isometric maximal voluntary contraction, no decline was seen in the peak amplitude of the tibialis anterior compound muscle mass action potential (M wave) either during or immediately after the voluntary effort, provided maximal nerve stimulation was retained. For first dorsal interosseous (FDI) muscle, M wave amplitudes declined by 19.4 +/- 1.6% during the first 2 min but did not change significantly thereafter, despite the continued force reduction (up to 94% in 5 min for both muscles). The duration of the FDI M waves increased (greater than 30%), suggesting that the small decline in amplitude was the result of increased dispersion between the responses of different motor units. Some subjects kept FDI maximally activated throughout, but when they used tibialis anterior, twitch occlusion and tetanic muscle stimulation showed that most subjects were usually only able to do so for the first 60 s and thereafter only during brief "extra efforts." Thus force loss during isometric voluntary contractions sustained at the highest intensities results mainly from failure of processes within the muscle fibers.     

Nonuniform strain of human soleus aponeurosis-tendon complex during submaximal voluntary contractions in vivo.

The distribution of strain along the soleus aponeurosis tendon was examined during voluntary contractions in vivo. Eight subjects performed cyclic isometric contractions (20 and 40% of maximal voluntary contraction). Displacement and strain in the apparent Achilles tendon and in the aponeurosis were calculated from cine phase-contrast magnetic resonance images acquired with a field of view of 32 cm. The apparent Achilles tendon lengthened 2.8 and 4.7% in 20 and 40% maximal voluntary contraction, respectively. The midregion of the aponeurosis, below the gastrocnemius insertion, lengthened 1.2 and 2.2%, but the distal aponeurosis shortened 2.1 and 2.5%, respectively. There was considerable variation in the three-dimensional anatomy of the aponeurosis and muscle-tendon junction. We suggest that the nonuniformity in aponeurosis strain within an individual was due to the presence of active and passive motor units along the length of the muscle, causing variable force along the measurement site. Force transmission along intrasoleus connective tissue may also be a significant source of nonuniform strain in the aponeurosis.     

Postactivation potentiation in a human muscle: effect on the load-velocity relation of tetanic and voluntary shortening contractions.

Recently it was demonstrated that postactivation potentiation (PAP), which refers to the enhancement of the muscle twitch torque as a result of a prior conditioning contraction, increased the maximal rate of torque development of tetanic and voluntary isometric contractions (3). In this study, we investigated the effects of PAP and its decay over time on the load-velocity relation. To that purpose, angular velocity of thumb adduction in response to a single electrical stimulus (twitch), a high-frequency train of 15 pulses at 250 Hz (HFT(250)), and during ballistic voluntary shortening contractions, performed against loads ranging from 10 to 50% of the maximum torque, were recorded before and after a conditioning 6-s maximal voluntary contraction (MVC). The results showed an increase of the peak angular velocity for the different loads tested after the conditioning MVC (P < 0.001), but the effect was greatest for the twitch ( approximately 182%) compared with the HFT(250) or voluntary contractions ( approximately 14% for both contraction types). The maximal potentiation occurred immediately following the conditioning MVC for the twitch, whereas it was reached 1 min later for the tetanic and ballistic voluntary contractions. At that time, the load-velocity relation was significantly shifted upward, and the maximal power of the muscle was increased ( approximately 13%; P < 0.001). Furthermore, the results also indicated that the effect of PAP on shortening contractions was not related to the modality of muscle activation. In conclusion, the findings suggest a functional significance of PAP in human movements by improving muscle performance of voluntary dynamic contractions.