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In patients with prior polio there was an excessive use of remaining motor units and an absence of type II muscle fibres in the tibialis anterior (TA). In the present study, eight subjects with prior polio with more than 90% type I fibres in the TA were examined. The aim was to elucidate whether the lack of type II muscle fibres was due to a selective loss of motoneurons with high threshold and high axonal conduction velocity or due to a muscle fibre transition from type II to type I. There was no decrease of the proportion of motoneurons with high threshold and high axonal conduction velocity. Monoclonal antibodies against fast and slow myosin heavy chains (MHC) were used as histochemical markers and many muscle fibres of type I according to ATPase stainability showed a binding of both anti-fast and anti-slow MHC. It is suggested that the type I muscle fibre dominance in prior polio subjects with excessive use of TA during walking is due to a muscle fibre transition from type II to type I and not to a loss of one class of motor units. [ABSTRACT FROM PUBLISHER]

The recruitment order of motor units was studied with an electromyographic technique for secure identification of single motor unit potentials. It has been shown in previous studies of normal subjects that the recruitment order in sustained voluntary contraction is predominantly stable, and that motor units which increase slowly in discharge rate with increasing contraction strength and which already attain regular discharge intervals at low frequencies are always recruited before motor units which increase more rapidly in discharge rate and which do not attain regular discharge intervals until at higher frequencies. In this study 15 patients with severe cerebellar ataxia were examined. It was shown that the recruitment order in sustained voluntary contraction in attaxia is unstable and that low- and high-frequency motor units may alternate as the unit of lowest threshold. [ABSTRACT FROM AUTHOR]

The recruitment of motor units is studied with an electromyographic technique for secure identification of single motor unit potentials. It has been shown in previous studies that the recruitment order is different in tonic and in phasic activities; in tonic activity the recruitment order is stable and low frequency units are always recruited before high frequency units; in phasic activity, however, the recruitment order is unstable and units with a higher frequency range may be recruited before units with lower frequency range. In this investigation the shifts between tonic and phasic recruitment order in voluntary contraction were compared in normal subjects and in patients with severe bradykinesia of Parkinsonism. Upon initiation of a voluntary contraction in a normal subject, phasic recruitment order may be used for a few 100 msec but tonic recruitment order then takes over. In bradykinetic patients, however, this shift from phasic to tonic recruitment order is delayed. After termination of tonic voluntary contraction in a normal subject, phasic recruitment order can again be used after a few seconds. In bradykinetic patients, however, the shift back from tonic to phasic recruitment order is also delayed. In favourable experimental situations the shift from phasic to tonic recruitment order can be normalized by passive stretch of the muscle and the shift from tonic to phasic recruitment pattern by unloading the muscle. It is discussed whether the pathological recruitment in bradykinesia might be due to disturbed gamma loop function. [ABSTRACT FROM AUTHOR]

The recruitment order of motor units in the tibialis anterior muscle upon fatigue of tonic voluntary contraction was studied in 20 patients with severe spastic paraparesis. An electromyographic technique for secure identification of single motor units was used. Before fatigue the recruitment order is stable and low-frequency units are recruited before high-frequency units; this recruitment pattern agrees with that in normal voluntary activity. When fatigue appears, however, the recruitment order becomes indefinite and high-frequency units can be recruited before low-frequency units; this recruitment pattern agrees with that in normal phasic voluntary activity. Finally, all voluntary activation power disappears, even in the case of units which have never been active. Contraction ability and original recruitment order are restored upon rest but also upon tonic reflex support of the voluntary drive. Whether the fatigue reaction may be due to insufficient gamma motoneurone innervation and whether it is related to spasticity are discussed. The practical physiotherapeutic implications are reviewed. [ABSTRACT FROM AUTHOR]

It has been shown in previous investigations that the recruitment order of motor units is different in tonic and in phasic voluntary activity. The significance of the pre-existing state of facilitation in the motoneurone pool for the recruitment of units is studied, using the phasic flexion reflex in the anterior tibial muscle as test reflex. It is shown that the recruitment order of units in a series of reflexes (1) is unstable if the subject does not expect the stimulus; (2) is stable and identical with that in tonic activity if the subject subliminally facilitates the motoneurone pool before the reflex activation; (3) is stable and almost identical with that in tonic activity if the subject expects the stimulus and therefore involuntarily influences the motoneurone pool; (4) is stable and similar to that in phasic voluntary activity if the subject inhibits the motoneurone pool before the activation and the stimulus strength thus consequentially is increased; and (5) is influenced by blockade of the proprioceptive afferent impulses from the muscle. It is concluded that normal man can select in advance the recruitment order of motor units most appropriate for the work intended. [ABSTRACT FROM AUTHOR]

The recruitment order of motoneurones in muscle contractions has been held to be largely constant and determined by the size of the cell. However, as shown in a previous investigation using electromyographic techniques, the order in which different motor units are activated during voluntary muscle contractions changes in normal human subjects on shifts from phasic to tonic contraction. In order to investigate these two types of activity also in cases in which the cerebral influence on the motoneurone pool is blocked, an analysis was made of the recruitment order in phasic and tonic flexion reflexes in 10 patients with total interruption of the spinal cord. The following four principles were found to apply and presumed to be generally valid for the isolated human spinal cord: (1) in the phasic exteroceptive reflex, the order of recruitment varies despite application of a standardized stimulus; (2) in the tonic reflex, the first unit to be recruited is usually the same even with widely different types of stimuli; (3) a shift from phasic to tonic reflex activation may result in considerable changes in recruitment order; (4) after facilitation by a subliminal long-lasting stimulus, the first unit to be recruited in the phasic reflex is also the first to be recruited in the tonic reflex. It is suggested that a tonic influence on the motoneurone pool is required for the presupposed constancy of the recruitment order. [ABSTRACT FROM AUTHOR]

The firing of single motor units (MUs) in musculus tibialis anterior (TA) was studied during maximal voluntary effort and maximal speed of walking in 10 patients with severe chronic hemiparesis and the findings compared to normal data. As shown in a previous study, the paralysed TA exhibited an increase in proportion of type 2 fibres as compared with normal muscle. Thus, 57% of the muscle fibres were type 1 and 43% type 2, while the normal percentages were 80% and 20% respectively. The present findings indicate that in the paralysed muscles a little less than half of the fibres, i.e. roughly the equivalent of the type 2 population, was not tonically active either during sustained voluntary contraction or during locomotion. Normally high threshold MUs reached high rates during both modes of activation. The findings paralysed muscles also indicate that a little more than half of the fibres, i.e. roughly the equivalent of the type 1 population, could be brought into tonic firing during voluntary contraction as well as during walking. Their maximal firing rate was, however, no more than two thirds of that of normal low threshold MUs.

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 less than 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 microns 2 and 6181 +/- 1062 microns 2. No selective type II atrophy was seen. Occasional hypertrophic type I and II fibres were seen in 4 patients.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of tibial anterior (TA) motor units in lower motoneurone disorders was studied during comfortable walking that was the main function of TA. The findings were compared to those in normal subjects. The loss of TA power was compensated either by increases of recruitment and firing rate of residual TA units or by a change from the normal plantigrade gait pattern to the infantile digitigrade pattern putting less strain on TA. Moderately paralysed subjects usually maintained plantigrade gait by excessive use of residual TA units but tended to change to digitigrade gait on fatigue thus economizing with remaining TA power. Severely paralysed subjects used digitigrade gait but plantigrade gait could be restored by application of a toe using string, i.e. the string might increase the strain on TA instead of decreasing it.

The firing properties of single anterior tibial motor units were studied (during walking) with electromyogram (EMG) techniques in five normal subjects and in 10 patients who had footdrop due to an acute lesion of the common peroneal nerve. In the normal subjects, high threshold units did not fire in the ordinary step cycle and low threshold units fired at 10-25 Hz. In half of the paralyzed subjects, all motor units fired at 40-50 Hz for more than 200 msec in each step cycle, which could be sufficient for maximal force. In the other half of the paralyzed subjects, high threshold units were not recruited in each step cycle and low threshold units fired at 15-35 Hz, indicating that there were force reserves which were not used to prevent footdrop in each stride.

1. Single human motor units from the short extensors of the toes were classified by their voluntary threshold and axonal conduction velocity. The voluntary discharge intervals, and the shortest discharge intervals at which action potentials were conducted to the muscle fibres, were studied during maximal voluntary effort maintained for 1 min, and were compared with the intervals calculated to be necessary for full fusion.2. Electromyographic techniques were used. High selectivity of the recordings was obtained after blocking of the main motor nerve with lidocaine in subjects with an accessory innervation of just a few motor units or 6 months after lesions to intramuscular nerve twigs, when the muscle-fibre density within the motor units was increased.3. For a whole minute of maximal voluntary effort, motor units with low threshold and low conduction velocity fired tonically at a rate that should have been sufficient for maximal tension. These intervals were longer than the intervals at which blocking occurred.4. In ordinarily motivated and untrained subjects, motor units with high threshold and high conduction velocity fired tonically at intervals sufficient for full tension for some seconds. Under these circumstances the voluntary intervals were longer than the critical ones.5. Through extraordinary motivation and training, motor units with high threshold and high conduction velocity could also be driven tonically for 20-60 s at the high rates required for full tension. The critical intervals then approached the voluntary ones. Blockings occurred and increased in frequency as the contraction continued.6. When motor units with high threshold and high conduction velocity were tetanized electrically at 20 Hz, which was the lowest discharge rate of these units in tonic voluntary contraction, blockings occurred within 60 s.7. The role of the peripheral block is discussed in relation to loss of tension in prolonged voluntary contraction.

The discharge properties on maintained maximal voluntary effort, axonal conduction velocity (a.c.v.) and contraction time (c.t.) of single human motor units were studied. Electromyographic (EMG) techniques were used and sufficient selectivity was obtained after repeated lesions to the terminal nerve twigs and consequent collateral sprouting, or by blocking the main muscle nerve in subjects with an accessory nerve supplying just one or a few units, or using high impedance wire electrodes. Most units with a.c.v. above 45 m/s and c.t. below 50 ms had maximal voluntary firing rates of about 50 Hz. On prolonged maximal effort, however, their rates rapidly decreased and after some seconds to a minute they ceased to respond tonically. As long as their motoneurons fired their EMG potentials were mainly intact and their twitch tension was significant. Most units with a.c.v. below 40 m/s and c.t. above 60 ms fired initially at 30 Hz, decreased slowly in firing rate and continued at 20 Hz for some minutes. We conclude that there is a central fatigue of units with short c.t. and high a.c.v. which protects their peripheral regions from severe exhaustion, but that there is no significant central fatigue of units with long c.t. and low a.c.v. We emphasize, however, that most units are intermediate in their properties.

The use of the musculus tibialis anterior during walking and the type and size of its muscle fibers were determined in 15 young normal subjects (29 +/- 4 years) and in 13 old healthy subjects (70 +/- 3 years). In the average step cycle 16 +/- 6% of the maximal surface EMG was recorded during the swing phase and 44 +/- 15% during 100 msec at the heel strike. Single motor unit recordings showed that low-threshold units fired at 10-15 Hz during the swing and at 20-25 Hz during the heel strike peak. Usually, high-threshold units did not participate in the ordinary step cycle. A significantly higher mean use was found in the old subjects as compared with the young ones presumably due to lower muscle power in relation to body weight. In the young subjects 76 +/- 7% of the muscle fibers were type 1 and 22 +/- 7% were type 2A. In the old subjects 84 +/- 9% were type 1 and 15 +/- 8% were type 2A. In the young subjects the mean cross-sectional area of type 1 fibers was 3950 +/- 950 microns2 and of type 2 fibers was 8070 +/- 1850 microns2. In the old subjects the corresponding numerals were 4050 +/- 890 microns2 and 5700 +/- 1970 microns2, respectively. The significance of physiological variations in use for muscle fiber type composition and size is discussed.

The voluntary discharge properties and axonal conduction velocity of single motor units were studied in patients with neuromuscular diseases with retained differentiation of the muscle fibers into type 1 and type 2, and in patients with late-onset hereditary distal myopathy in which muscle fibers have only intermediate histochemical properties. In the patients with muscle fiber differentiation, the findings were similar to those in normal subjects; that is, there was a continuum between motor units which fired tonically at low rates and had a low axonal conduction velocity, and motor units which fired phasically at high rates and had a high axonal conduction velocity. In the patients without muscle fiber differentiation, all motor units had intermediate firing properties and a low axonal conduction velocity. It is suggested that in chronic pathologic states, the differentiation of the muscle fiber histochemistry remains only as long as the differentiation of the motor neurons remains.

The voluntary recruitment order of anterior tibial motor units was studied in electromyographic recordings using both low and high impedance electrodes. It was shown in previous papers, that the recruitment order in sustained voluntary contraction is normally stable and that motor units with low thresholds discharge at low, regular frequencies and that motor units with high thresholds discharge at higher and more irregular frequencies. In this paper the consequences for the discharge properties of reduced afferent inflow in sustained voluntary contraction were studied, using partial ischemic or lidocain blockades of the muscle nerve and local cooling of the muscle belly. 1. The recruitment order of certain low and certain high frequency units was reversed. 2. The muscle tension and sense of effort necessary for tonic firing were increased for certain low frequency units but decreased for certain high frequency units. 3. On severe reduction of the afferent inflow, the differences in discharge pattern between low and high frequency units were decreased. It is concluded that the recruitment of low frequency units before high frequency units in sustained voluntary contraction is partly due to proprioceptive afferent activity favouring low frequency units and disfavouring high frequency units. It is also concluded that the afferent inflow is involved in restricting certain units to low frequency discharge and others to high frequency discharge.

1. The discharge properties of single motor units during prolonged maximal voluntary effort have been studied using electromyographic recordings, mainly in the short big toe extensor muscle but also in the anterior tibial muscle.2. The required selectivity of the e.m.g. recordings was achieved in the short big toe extensor muscle after previous mechanical lesions to the terminal nerve twigs and muscle fibres and consequent collateral sprouting, and in the anterior tibial muscle with the use of a high impedance wire electrode.3. During the first few hundred milliseconds of sustained maximal effort the motor units fired at rates ranging from about 30-60 Hz, and the tension was the same as that obtained on electrical tetanization of the nerve to the toe extensor muscles above 50 Hz.4. During prolonged maximal effort the firing rates and the proportion of motor units firing successively decreased. Motor units initially firing at 30 Hz continued to fire tonically but at 15-20 Hz. Motor units initially firing at 60 Hz ceased to fire tonically but could still be made to discharge phasically. The period of time during which all motor units responded tonically could be increased from some seconds up to 20 sec by long-term training.5. Motor units with a limited endurance fired at a lower tension in the early than in the late stages of maintained contraction.6. It is suggested that motoneurones innervating slow twitch muscle fibres respond continuously to prolonged voluntary drive at rates sufficient for full fusion but that the threshold of motoneurones innervating fast twitch muscle fibres increases so that they finally mainly fire phasically thus protecting the peripheral excitation and contractile mechanisms from excessive exhaustion.

Electromyographic potentials of fasciculations were studied in ten patients with amyotrophic lateral sclerosis (ALS). The EMG recordings were made from the extensor digitorum brevis muscle. The EMG recording was so selective that only one motor unit potential appeared on maximal voluntary effort and on supramaximal electrical stimulation of the peroneal nerve. In a series of fasciculations, the shapes of the EMG potentials varied, while in a series of voluntary twitch activations of electrical nerve stimulations the EMG potentials were mainly constant. Fasciculations were followed by antidromic impulses in the test unit axon as judged from collision tests, and they persisted after lidocaine blockades of the nerve to the muscle. The findings are compatible with a conclusion of distal multifocal triggering of fasciculation. Fasciculating motor units had voluntary firing properties close to those of normal low-threshold motor units. Widespread fasciculations were abolished by a nonparalytic dose of a synthetic curare derivative (Pavulon) and augmented by administration of neostigmine in two cases. The fasciculations in ALS thus have the same characteristics as experimental fasciculations evoked by cholinesterase inhibitors, and there is reason to believe that the underlying pathophysiological mechanism is similar in the two cases.

The motoneuron part of this review deals with the changes in recruitment and firing rates of the motor unit types upon changes from a physically inactive life to endurance or strength training. The muscle fibers react to prolonged exercise by adaptation to a higher level of performance. A matter of discussion is the prerequisites for a transformation between the basic muscle fiber types, slow twitch and fast twitch, during voluntary (transsynaptic) activity, which is demonstrated after artificial nerve stimulation. The review includes current knowledge of muscle fiber transformation as an adaptive response to increased usage either by electrical stimulation or by transsynaptic neuronal activity. The metabolic adaptation related to increased endurance is reviewed with special reference to effects on muscle fibers. The increase in strength as a result of high resistance training is mainly the result of increased muscle cross-section. Whether this is solely the result of an increase in size of individual fibers or an increased fiber number is a controversial matter.

The firing ranges of 50 normal anterior tibial or short toe extensor motor units in sustained isometric voluntary contraction were studied in electromyographic recordings. The afferent inflow was decreased by compression of the sciatic nerve, thereby blocking large afferents before alpha efferents. Motor units with low minimum and low maximum rates before blockade had higher minimum and maximum rates before blockade had a lower minimum but an unchanged maximum rate on blockade. The firing range recorded for both types of motor unit on blockade was also recorded for all tonically firing motor units during early reinnervation after traumatic lesion to the muscle nerve in 14 patients. It is concluded that both facilitating and inhibiting proprioceptive afferent activity is involved in the differentiation of motor unit firing ranges in sustained isometric voluntary contraction.

Twenty-three subjects with weakness of musculus tibialis anterior (TA) due to prior poliomyelitis (n = 19) or prior LV lesion (n = 4) were examined regarding the use of remaining TA motor units during walking and the consequences of long-term overuse for the TA muscle fibers. Subjects with an excessive overuse exhibited almost only type 1 muscle fibers and with marked hypertrophy. Subjects who did not use remaining TA fibers because of too-severe paralysis had a normal fiber type differentiation and fiber atrophy. The use of remaining TA motor units was correlated to the type 1 muscle fiber percentage (r = 0.69) and to the type 1 fiber size (r = 0.78). Identical changes were observed in subjects with prior poliomyelitis and in subjects with prior LV lesion and were, thus, not specific for poliomyelitis. The changes in fiber type composition may be explained by a transition of type 2 to type 1 muscle fibers in accordance with previously reported experimental transneuronal stimulation through implanted electrodes in animals.