High-threshold Motor Units Research Articles

Muscle strength of trunk flexion-extension in post-stroke hemiplegic patients.

This study was undertaken to determine muscle strength of trunk flexion-extension in hemiplegic patients after stroke compared with that of normal controls. The design consisted of a nonrandomized control trial in a secondary care setting (a rehabilitation unit at a hospital facility). The subjects included 25 post-stroke male hemiplegic patients and 25 male healthy controls. The maximal peak torques of trunk flexion-extension at angular velocities of 0 degrees (isometric contraction), 60 degrees, 120 degrees, and 150 degrees/s were measured by using an isokinetic dynamometer (Cybex Trunk Extension-Flexion Unit, Cybex, Ronkonkoma, NY). Peak torque of trunk flexion and extension in hemiplegic patients was significantly smaller than that of healthy controls (P < 0.05), except isometric trunk flexion (P > 0.05). The weakness of trunk flexion-extension muscles in hemiplegic patients might be accounted for by the bilateral innervation from the motor cortex, the insufficient use of high threshold motor units, and disuse atrophy.

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. II. Motoneuron firing-rate modulation.

1. The aim of this study was to examine the nature of motoneuron firing-rate modulation in type-identified motor units during smoothly graded contractions of the cat medial gastrocnemius (MG) muscle evoked by stimulation of the mesencephalic locomotor region (MLR). Motoneuron discharge patterns, firing rates, and the extent of firing-rate modulation in individual units were studied, as was the extent of concomitant changes in firing rates within pairs of simultaneously active units. 2. In 21 pairs of simultaneously active motor units, studied during 41 evoked contractions, the motoneurons' discharge rates and patterns were measured by processing the cells' recorded action potentials through windowing devices and storing their timing in computer memory. Once recruited, most motoneurons increased their firing rates over a limited range of increasing muscle tension and then maintained a fairly constant firing rate as muscle force continued to rise. Most motoneurons also decreased their firing rates over a slightly larger, but still limited, range of declining muscle force before they were derecruited. Although this was the most common discharge pattern recorded, several other interesting patterns were also seen. 3. The mean firing rate for slow twitch (type S) motor units (27.8 imp/s, 5,092 activations) was found to be significantly different from the mean firing rate for fast twitch (type F) motor units (48.4 imp/s, 11,272 activations; Student's t-test, P < 0.001). There was no significant difference between the mean firing rates of fast twitch, fatigue-resistant (type FR) and fast twitch, fatigable (type FF) motor units. When the relationship between motoneuron firing rate and whole-muscle force was analyzed, it was noted that, in general, smaller, lower threshold motor units began firing at lower rates and reached lower peak firing rates than did larger, higher threshold motor units. These results confirm both earlier experimental observations and predictions made by other investigators on the basis of computer simulations of the cat MG motor pool, but are in contrast to motor-unit discharge behavior recorded in some human motor-unit studies. 4. The extent of concomitant changes in firing rate within pairs of simultaneously active motor units was examined to estimate the extent of simultaneous motoneuron firing-rate modulation across the motoneuron pool. A smoothed (5 point sliding average) version of the two motoneurons' instantaneous firing rates was plotted against each other, and the slope and statistical significance of the relationship was determined. In 16 motor-unit pairs, the slope of the motoneurons' firing-rate relationship was significantly distinct from 0. Parallel firing-rate modulation (< 10-fold difference in firing rate change reflected by a slope of > 0.1) was noted only in pairs containing motor units of like physiological type and then only if they were of similar recruitment threshold. 5. Other investigators have demonstrated that changes in a motoneuron's "steady-state" firing rate predictably reflect changes in the amount of effective synaptic current that cell is receiving. The finding in the present study of limited parallel firing-rate modulation between simultaneously active motoneurons would suggest that changes in the synaptic drive to the various motoneurons of the pool is unevenly distributed. This finding, in addition to the findings of orderly motor-unit recruitment and the relationship between motor-unit recruitment threshold and motoneuron firing rate, cannot be adequately accommodated for by the existing models of the synaptic organization in motoneuron pools. Therefore a new model of the synaptic organization within the motoneuron pool has been proposed.

Motor unit firing behavior in slow and fast contractions of the first dorsal interosseous muscle of healthy men

The motor unit recruitment threshold and firing rate were evaluated during slow and fast contraction of the first dorsal interosseous (FDI) muscle by healthy young men. Using a special quadrifilar electrode myoelectric activity was recorded during voluntary isometric contraction. Motor unit action potentials (MUAPs) were decomposed into individual MUAP trains by the electromyography (EMG) signal decomposition technique. Recruitment thresholds of the motor units decreased with the increase in the speed of contraction, and there was no recruitment reversal despite the increase. In terms of rate coding, the firing rates of the motor units increased as the speed of contraction increased; however, a high threshold motor unit always had a lower firing rate than a low threshold motor unit regardless of the contraction speed.At all contraction speeds, recruitment and rate coding may act through the same mechanism. If excitation of the motoneuron pool occurs rather than excitation of an individual motoneuron, a low threshold motor unit is easier to recruit and fire repetitively than a high threshold one. The motor unit firing behavior during fast contraction basically may be the same as during slow contraction.

Mitochondrial function during heavy exercise

Maximal rates, coupling, and control of oxidative phosphorylation were studied in isolated skeletal muscle mitochondria from rat and rabbit. Mitochondria were incubated under various conditions of temperature, pH, and substrate availability. A 20% decrease in coupling (ADP/O) was observed at 43 degrees C as compared to 37 degrees C in rat mixed skeletal muscle mitochondria. Changes in pH from 7.00 to 6.20 affected neither coupling nor maximal (state 3) respiration rates. Changing the substrate supply from pyruvate to palmitoyl-carnitine (+ malate) did not alter ADP/O, but markedly degraded the energy state sustained at submaximal ATP turnover. Thus, carbohydrate depletion may be associated with inhibition of contractile function and the recruitment of less economical higher threshold motor units. State 3 respiration of mitochondria from rabbit Type IIb fibers oxidizing pyruvate+malate+alpha-glycerophosphate was 27% higher than that of mitochondria from Type I rabbit skeletal muscle. However, the ADP/O ratio in the Type IIb preparation was 18% lower. The experimental findings suggest that temperature, substrate supply, and energetic differences between slow twitch and fast twitch motor units may impact the economy of mitochondrial oxygen utilization during heavy aerobic exercise, and thus contribute to the slow component of oxygen uptake.

Motor unit firing behavior in man.

Studies on motor unit firing behavior in man by the decomposition technique are described. The decomposition technique identifies motor unit firing with 100% accuracy at force levels of greater than 80% maximal voluntary contraction (MVC). In all muscles examined, the higher the recruitment threshold of the motor unit, the lower the rate at which it fired at the target level. Smaller muscles, such as those in the hand, recruit their motor units at 0-50% MVC and rely exclusively on firing rate increases to augment force output at 50-100% MVC. Larger muscles, such as those in the leg or arm, recruit motor units at least to 90% MCV, and possibly higher. Their firing rates have a relatively smaller dynamic swing. Thus, smaller muscles rely primarily on firing rate and larger muscles rely primarily on recruitment to modulate their force. High cross-correlation functions in firing rate behavior within a muscle were observed between individual motor unit firing rates at constant force isometric contraction. Thus the nervous system does not control the firing rate of motor units individually. Instead, it acts on the pool of homonymous motoneurons in a uniform fashion. Electrical stimulation of cutaneous receptors tends to increase the recruitment thresholds of low-threshold motor units and to decrease their firing rates, while high-threshold motor units generally exhibit a decrease in recruitment threshold and an increase in firing rate.

Computer simulations of motoneuron firing rate modulation

1. As a human subject slowly increases the amount of force exerted by a muscle, the discharge rates of low-threshold motor units saturate at a rather low level, whereas higher-threshold units continue to be recruited and undergo increases in their discharge rates. The presently known intrinsic properties of motor units do not produce this "rate limiting." 2. Using computer simulations of a model motoneuron pool, we tested the hypothesis that rate limiting can be accounted for on the basis of the known distributions of synaptic input from different sources. The properties of the simulated motor units and their synaptic inputs were based as closely as possible on the available experimental data. A variety of simulated synaptic input organizations were applied to the pool, and the resulting outputs were compared with the data on rate limiting in human subjects. 3. We found that the data on rate limiting in human subjects greatly constrained the possible organizations of characterized synaptic input systems. Only when the synaptic organization included a gradual "crossover" between two specific types of input systems could the human data be accurately reproduced. Low input/output levels relied on a system organized like the monosynaptic Ia input, which produces greater effective synaptic currents in low- than in high-threshold motor units. Above a sharply defined crossover level, all further increases in output were produced by a system organized like the oligosynaptic rubrospinal input, which generates the opposite pattern.

Activation of high-threshold motor units in man by transcranial magnetic stimulation

Motor units (MUs) with low voluntary recruitment thresholds are the first to be activated by transcranial magnetic stimulation. It is not clear, however, if high-threshold MUs can also be activated and if they contribute to motor evoked potentials (MEPs). We therefore studied 11 high-threshold motor units in the first dorsal interosseous muscle of 11 healthy subjects. Voluntary recruitment thresholds ranged from 22 to 41% (29.5 ± 5.6%; mean ± S.D.) of maximal muscle force. When MUs were driven at their recruitment thresholds, transcranial magnetic stimuli were applied to the vertex. Peri-stimulus time histograms of MU discharges were constructed. All MUs studied revealed a period of increased firing probability at 19–27 ms after the stimulus (primary peak). Stimulus intensities were lower by 10–57% of the maximal stimulator output than required to produce near maximal MEPs in conventional surface EMG recordings in the same subjects. We conclude that high-threshold MUs can be activated by transcranial magnetic stimulation and that they contribute to conventionally recorded MEPs.

Effects of percutaneous stimulation on motor unit firing behavior in man

Motor unit firing behavior in human first dorsal interosseous (FDI) muscle was studied during controlled constant force isometric contractions. The threshold at which motor units were recruited and the mean firing rate at 50% of maximal voluntary contraction (MVC) were evaluated following stimulation of the skin area over the second digit. Stimulation of cutaneous receptors tended to increase the recruitment threshold of most of the motor units recruited under 20% MVC, while high-threshold motor units (those recruited over 30% MVC) generally exhibited a decrease in recruitment threshold. Less dramatic changes in motor unit firing rates were observed, but those motor units recruited over 30% MVC exhibited some increase in firing rate. The relationship between the change in recruitment threshold and change in motor unit firing rate is not rigid and seems to be susceptible to considerable synaptic noise.

Disuse of anterior tibial muscle during locomotion and increased proportion of type II fibres in hemiplegia

The tibialis anterior (TA) is a muscle activated mainly during walking. Its use during the step cycle was studied in 10 patients (55.8 ± 8.8 years) with chronic hemiplegia (duration 3–18 years) and related to the muscle fibre composition, size and expression of isoforms of myosin heavy chains (MHCs). In the average step cycle the integrated surface EMG of the paralysed TA did in the majority of the hemiplegic patients not exceed 10% of that recorded during maximal contraction of the normal leg. The type I fibre percentage in the paralysed TA subject was 57.4% as compared with 79.4% in normal muscles ( P < 0.05). The range of axonal conduction velocities in the peroneal nerve did not differ in paralysed and non-paralysed leg, suggesting that there was no selective loss of one class of motoneurons. The type II fibres consisted of IIA (66%) and IIB (31%), in contrast to the normal TA muscle where less than 1% of the muscle fibres are of type IIB. The incidence of fibres in the biopsies with both slow and fast MHCs had a mean value of 3.5% (range 0.7–9%). The type I and type II muscle fibres had normal sizes with cross-sectional area 4511 ± 962 μm 2 and 6181 ± 1062 μm 2. No selective type II atrophy was seen. Occasional hypertrophic type I and II fibres were seen in 4 patients. The findings indicate that mainly the low threshold motor units are active in the hemiplegic patients and suggest that the selective disuse of high threshold motor units predisposes for a transformation from type I to type II fibres with the occurrence of type IIB fibres.

Technique for detecting MUAP propagation from high-threshold motor units

A technique was developed to identify the propagation pattern of high-threshold motor unit action potentials (MUAPs) in human skeletal muscles. A linear surface array and a selective needle electrode were used simultaneously for detection of myoelectric signals during a constant-force isometric voluntary contraction. The needle signals were decomposed into trains of single MUAPs and were used to trigger-average the surface signals. After averaging 16 channels of surface signals derived with the linear array electrode, we obtained the propagation of single MUAPs along the muscle fibers. The special quadrifilar needle electrode and the algorithm for decomposing the needle signals made it possible to detect high-threshold MUs recruited at ≤57.5% of the maximal voluntary contraction (MVC) during 75% MVC sustained contractions and consequently the propagation pattern of APs from those MUs. The position of the innervation zones within single MUs was estimated from the location of the conduction reversals in the surface myoelectric signals. With this technique, we were able to study the configuration of innervation zones of high-threshold MUs in conjunction with their recruitment threshold.

急激な足関節底屈動作前にみられる筋放電休止期のメカニズムについて

The present study was designed to investigate the neurophysiological mechanism of electromyographic silent periods(SP) preceding a ballistic voluntary movement. Ten male subjects were asked to respond to the flashing light by performing a plantar flexion as strongly and quickly as possible. The myoelectric signals from agonist (m. gastrocnemius, caput laterale: LG, m. soleus: SOL) and antagonist (m. tibialis anterior: TA) were simultaneously recorded together with the isometric contraction force signal. The excitability of the alpha motoneuron pools by means of the H reflex analysis was also determined at various phases of the movement. The results obtained are as follows: 1) In five out of 10 subjects, the electromyographic silent period was consistently observed (SP Group), while the rest of the subjects did not show such changes (NSP Group). 2) The maximum rate of tension raise (dF/dt) in SP Group (32.3 +/- 24.9 N/ms) was significantly (p less than .05) greater than NSP Group (15.1 +/- 12.0 N/ms). 3) In SP Group, the averaged H-wave mean amplitude decreased about 40ms prior to the appearance of SP. From these results, it can be concluded that SP in this study was attributable to the supraspinal influences so as to allow a specific motor program to be executed for selective recruitment of phasic (high threshold) motor units.

Prolonged muscle vibration reducing motor output in maximal voluntary contractions in man.

1. We observed in a previous study on the human foot dorsiflexor muscles that the fatigue-induced decline in motor output during sustained maximal voluntary contractions (MVCs) was temporarily counteracted during the initial phase of superimposed high-frequency (150 Hz) muscle vibration, whereas prolonged muscle vibration seemed to accentuate the fatigue-induced decline in gross EMG activity and motor unit firing rates. A more extensive investigation of this late effect of muscle vibration on MVCs was performed in the present study. 2. Prolonged periods of superimposed muscle vibration caused a reduction of EMG activity, motor unit firing rates and contraction force in both intermittent and sustained MVCs. This vibration-induced effect had the following main characteristics: (i) it developed slowly during the course of about 1 min of sustained vibration and subsided within 10-20 s after the end of vibration; (ii) it was much more pronounced in some subjects than in others (not age-dependent) and it was accentuated by preceding muscle exercise; (iii) it affected primarily the subject's ability to generate and/or maintain high firing rates in high-threshold motor units. 3. Since the effect developed while vibration at the same time exerted a tonic excitatory influence on the alpha-motoneurone pool (as evidenced by the presence of a tonic vibration reflex) it is argued that the vibration-induced suppression of motor output in MVCs probably does not depend on alpha-motoneurone inhibition, but on a reduced accessibility of these neurones to the voluntary commands. It is suggested that contributing mechanisms might be vibration-induced presynaptic inhibition and/or 'transmitter depletion' in the group Ia excitatory pathways which constitute the afferent link of the gamma-loop.

Antigravity posture for analysis of motor unit recruitment: The “45 degree test”

The maximum number of different motor unit action potentials (MUAPs), their firing rates, and total MUAP spikes/second recorded by monopolar needle electrode were determined for the biceps brachii muscle during 45-degree elbow flexion. There were 4.2 +/- 1.6 different MUAPs exceeding 100 microV. Mean firing rate was 10.0 +/- 1.7 Hz, and total MUAP spikes/second were 40.3 +/- 18. Recordings from 16 patients with neurogenic atrophy (NA) and just detectable weakness revealed corresponding values of 3.1 +/- 1.7 different MUAPs, a mean rate of 10.2 +/- 1.5 Hz and 30.6 +/- 19 total MUAP spikes/second, not different from normal. In these patients, increased force of muscle contraction was required to activate high threshold motor units firing at high rates. In each of 4 patients just able to hold the arm against gravity, 1 or 2 "overdriven" motor units firing at a mean rate greater than 20 Hz were recorded. In 8 patients with myopathy and just detectable weakness, greater than 100 total MUAP spikes/second were recorded. Antigravity posture as a reference level of innervation has the advantage that motor unit firing rate is set about that of physiologic tremor (10-13 Hz). Its application was helpful in quantifying recruitment.

Effects of immobilization on contractile properties, recruitment and firing rates of human motor units.

1. The contractile properties, recruitment and firing rates of motor units from the human adductor pollicis and the first dorsal interosseous were studied during voluntary isometric contractions after 6-8 weeks' immobilization of the corresponding limbs. 2. In both muscles, motor units of different force thresholds showed a proportionally identical twitch tension decrease and slowing of their time course after immobilization. 3. When expressed as a percentage of the maximal voluntary contraction, more high-threshold motor units were recorded in disused muscles than in control muscles, but the order of recruitment was maintained. 4. The motor unit firing rate at recruitment was identical in control and disused muscles, but the maximal firing rate decreased in all motor units after immobilization. This decrease of the maximal firing rate was greater in motor units of lower threshold than in those of higher threshold. 5. The results further document motoneuronal plasticity in human muscles of different fibre type composition.

Effects of periodic weight support on medial gastrocnemius fibers of suspended rat

Based on the current view that muscle fiber types reflect, at least to some degree, the probability of excitation of motor units in most normal movements, it was hypothesized that brief moderate periods of weight support would have little effect on a muscle that consists predominantly of high-threshold motor units. To test this hypothesis, the effects of 7 days of hindlimb suspension (HS) and HS plus intermittent weight support activity on the size and metabolic properties of individual fibers in the medial gastrocnemius (MG) were studied. HS resulted in a 40% decrease in the mean cross-sectional area of fibers that stain either dark or light for myosin adenosinetriphosphatase (ATPase) at an alkaline preincubation and are located in the deep region (i.e., close to the bone) of the MG. Dark ATPase fibers located in the superficial region were 17% smaller than controls (P greater than 0.05). Although the mean succinate and alpha-glycerophosphate dehydrogenase activities (optical density/min) per muscle fiber were not significantly (P greater than 0.05) affected by HS, it appeared that selected fibers of the deep MG region of HS rats had elevated enzyme activities. HS plus walking on a treadmill for 10 min every 6 h at 5 m/min and at a 19 degrees incline (total of 40 min/day) resulted in mean fiber cross-sectional area and enzyme activities nearer to control than the HS values. All adaptations were much less obvious in the fibers in a superficial (i.e., away from the bone) MG region.(ABSTRACT TRUNCATED AT 250 WORDS)

Unusual motor unit firing behavior in older adults

Motor unit firing behavior was studied in the first dorsal interosseus (FDI) and tibialis anterior (TA) muscles of 10 aged subjects during slow, isometric contractions. Previous study in younger individuals had shown that motor units are recruited and derecruited in an orderly manner whereby the early-recruited units are the last to be derecruited. However, there were several examples in the old subjects in which some high-threshold motor units were derecruited at much lower levels of force. Concurrent antagonist firing in an effort to maintain the required precision is considered a likely candidate for such prolonged motor unit activation.

Muscle fibre type composition, motoneuron firing properties, axonal conduction velocity and refractory period for foot extensor motor units in dystrophia myotonica.

Seven patients with dystrophia myotonica were investigated using neurophysiological combined with histochemical techniques to elucidate motor unit properties in foot extensor muscles, which are often involved in the early stages of this disorder. For the 25 extensor digitorum brevis motor units studied the axonal conduction velocity, the axonal refractory period and the voluntary firing properties were within normal limits. However, high threshold motor units were not observed and the mean value of the axonal conduction velocities was lower (p less than 0.02) for the dystrophia myotonica motor units when compared with corresponding data from healthy subjects. There were also signs of impaired impulse propagation in the terminal part of the motor unit. In muscle biopsy specimens from the anterior tibial muscle, fibre type composition and structure were demonstrated using enzyme histochemical techniques for adenosine-triphosphate and immunohistochemical techniques for identification of the types of myosin isoform present. The histochemical findings indicated a type I fibre dominance, which was most obvious in the more seriously affected muscles. Neonatal myosin was observed preferentially in small but also in some normal sized fibres. Furthermore, some ring fibres were present and these showed staining with antineonatal myosin in their superficial portion. This indicates that an abnormal regeneration is one cause of the myopathic appearance of the muscle fibres in dystrophia myotonica. These investigations show that there is a reduced proportion of type II motor units in foot extensor muscles involved in the myopathy in dystrophia myotonica although it cannot definitely be established whether this is due to a loss of high threshold type II motor units or type II to type I transformation.

Myoelectric signal conduction velocity and spectral parameters: influence of force and time.

Reports on measurement of muscle fiber conduction velocity in humans are scarce. Inferences on the behavior of conduction velocity have been drawn from the behavior of myoelectric spectral parameters. The present report contains information on conduction velocity and spectral parameters studied at various muscle contraction levels and during and after sustained contractions. The following results have been obtained from measurements on the tibialis anterior muscle. Conduction velocity demonstrated a positive correlation with limb circumference and with muscle force output. Thus we conclude that the diameters of the muscle fibers of high-threshold motor units are, on an average, larger than those of low-threshold motor units. The study of a sustained contraction and on the recovery after such a contraction revealed that conduction velocity consistently decreased during a strong contraction as did various myoelectric spectral parameters. However, the spectral parameters decreased approximately twice as much as did the conduction velocity, and we conclude that factors other than the conduction velocity along the muscle fibers affect the myoelectric signal during a high-level contraction. These other factors appertain to changes in the firing statistics of individual motor units as well as the correlation between the firings of different motor units.

Firing properties of single human motor units during locomotion.

The discharge properties of single motor units in the short extensors of the toes were studied during normal locomotion using electromyographic recordings which permitted the identification of single motor unit potentials. The single motor units were recruited in the same order during slowly increasing speed of locomotion as they were during slowly increasing voluntary tension, i.e. motor units with low axonal conduction velocity were recruited before motor units with higher conduction velocity. Low threshold units fired five to ten times at 80-40 ms intervals in each step cycle during walking at normal speed. Motor units with moderately high thresholds fired once or only a small number of times in each cycle at shorter interspike intervals. High threshold motor units did not participate in the step cycle during walking at normal speeds but fired in short high-frequency bursts in corrective movements.

High threshold motor unit discharges during prolonged voluntary contraction in man

High threshold motor unit discharges during prolonged voluntary contraction in man