Biomedical subjects

B Bigland-Ritchie

Publications and source records attributed to B Bigland-Ritchie.

At least 19 recordsLinked to original sources

PubMed2005

EMG changes in human thenar motor units with force potentiation and fatigue.

Few studies have analyzed activity-induced changes in EMG activity in individual human motor units. We studied the changes in human thenar motor unit EMG that accompany the potentiation of twitch force and fatigue of tetanic force. Single motor unit EMG and force were recorded in healthy subjects in response to selective stimulation of their motor axons within the median nerve just above the elbow. Twitches were recorded before and after a series of pulse trains delivered at frequencies that varied between 5 and 100 Hz. This stimulation induced significant increases in EMG amplitude, duration, and area. However, in relative terms, all of these EMG changes were substantially smaller than the potentiation of twitch force. Another 2 min of stimulation (13 pulses at 40 Hz each second) induced additional potentiation of EMG amplitude, duration, and area, but the tetanic force from every unit declined. Thus activity-induced changes in human thenar motor unit EMG do not indicate the alterations in force or vice versa. These data suggest that different processes underlie the changes in EMG and force that occur during human thenar motor unit activity.

Action Potentials↗

PubMed2000

Discharge behaviour of single motor units during maximal voluntary contractions of a human toe extensor.

1. While it is known that the average firing rate of a population of motoneurones declines with time during a maximal voluntary contraction, at least for many muscles, it is not known how the firing patterns of individual motoneurones adapt with fatigue. To address this issue we used tungsten microelectrodes to record spike trains (mean +/- s.e.m., 183 +/- 27 spikes per train; range, 100-782 spikes) from 26 single motor units in extensor hallucis longus during sustained (60-180 s) maximal dorsiflexions of the big toe in seven human subjects. 2. Long spike trains were recorded from 13 units during the first 30 s of a maximal voluntary contraction (mean train duration, 9.6 +/- 1.2 s; range, 3.6-21.9 s) and from 13 units after 30 s (mean train duration, 16.6 +/- 3.7 s; range, 7.1-58.1 s). Maximal isometric force generated by the big toe declined to 78.3 +/- 6.3 % of its control level by 60-90 s and to 39.5 +/- 1.4 % of control by 120-150 s. Despite this substantial fatigue, mean firing rates did not change significantly over time, declining only slightly from 15.8 +/- 0.7 Hz in the first 30 s to 14.0 +/- 0.5 Hz by 60-90 s and 13.6 +/- 0.3 Hz by 120-150 s. 3. To assess fatigue-related adaptation in discharge frequency and variability of individual motor units, each spike train was divided into 2-15 equal segments containing at least 50 interspike intervals. Discharge variability was measured from the coefficient of variation (s.d. /mean) in the interspike intervals, with the s.d. being calculated using a floating mean of 19 consecutive intervals. Adaptation was computed as the average change in firing rate or variability that would occur for each 1 s of activity. There were no systematic changes in either firing rate or variability with time. 4. We conclude that single motoneurones supplying the extensor hallucis longus, a muscle comprised primarily of slow twitch muscle units, show little adaptation in firing with fatigue, suggesting that a progressive reduction in firing rate is not an invariable consequence of the fatigue associated with sustained maximal voluntary contractions.

Action Potentials↗

PubMed2000

Muscle fatigue induced by stimulation with and without doublets.

Muscles are usually stimulated by shocks delivered at some constant rate. However, human thenar motor units generate optimum force per pulse when excited by impulse trains that begin with one or two short interpulse intervals ("doublets"), followed by longer intervals. Our aim was to determine whether the rate of force and force-time integral reduction during fatigue of thenar muscles is influenced by an initial doublet, and/or the number of pulses per train. We first matched thenar force-time integral using two different pulse patterns, one of which began with a doublet. Fatigue induced by trains that contained a doublet resulted in slower rates of force and force-time integral reduction and smaller increases in half-relaxation time than that evoked by bursts of 40-HZ stimulation. When the force was measured in each protocol after equal numbers of pulses had been delivered, the force loss was still significantly less for pulse trains containing a doublet. These results have useful implications when designing stimulation to strengthen weak muscles or to drive paralyzed muscles.

Adult↗

PubMed1999

Force-frequency and fatigue properties of motor units in muscles that control digits of the human hand.

Modulation of motor unit activation rate is a fundamental process by which the mammalian nervous system encodes muscle force. To identify how rate coding of force may change as a consequence of fatigue, intraneural microstimulation of motor axons was used to elicit twitch and force-frequency responses before and after 2 min of intermittent stimulation (40-Hz train for 330 ms, 1 train/s) in single motor units of human long finger flexor muscles and intrinsic hand muscles. Before fatigue, two groups of units could be distinguished based on the stimulus frequency needed to elicit half-maximal force; group 1 (n = 8) required 9.1 +/- 0.5 Hz (means +/- SD), and group 2 (n = 5) required 15.5 +/- 1.1 Hz. Twitch contraction times were significantly different between these two groups (group 1 = 66. 5 ms; group 2 = 45.9 ms). Overall 18% of the units were fatigue resistant [fatigue index (FI) > 0.75], 64% had intermediate fatigue sensitivity (0.25 </= FI </= 0.75), and 18% were fatigable (FI < 0. 25). However, fatigability and tetanic force were not significantly different among groups. Therefore unlike findings in some other mammals, fast-contracting motor units were neither stronger nor more susceptible to fatigue than slowly contracting units. Fatigue, however, was found to be greatest in those units that initially exerted the largest forces. Despite significant slowing of contractile responses, fatigue caused the force-frequency relation to become displaced toward higher frequencies (44 +/- 41% increase in frequency for half-maximal force). Moreover, the greatest shift in the force-frequency relation occurred among those units exhibiting the largest force loss. A selective deficit in force at low frequencies of stimulation persisted for several minutes after the fatigue task. Overall, these findings suggest that with fatigue higher activation rates must be delivered to motor units to maintain the same relative level of force. Questions regarding classification of motor units and possible mechanisms by which fatigue-related slowing might coexist with a shift in the force-frequency curve toward higher frequencies are discussed.

Action Potentials↗

PubMed1999

Pattern of pulses that maximize force output from single human thenar motor units.

We assessed the sequence of nerve impulses that maximize force output from individual human thenar motor units. When these motor units were stimulated intraneurally by a variable sequence of seven pulses, the pattern of pulses that elicited maximum force always started with a short (5-15 ms) interpulse interval termed a "doublet. " The twitch force summation caused by this "doublet" elicited, on average, 48 +/- 13% (SD) of the maximum tetanic force. The peak amplitude of "doublet" forces was 3.5 times that of the initial twitches, and twitch potentiation appeared to have little influence on twitch force summation elicited by the "doublets." For some units, the second optimal interpulse interval was also short. Peak forces elicited by the third to sixth interpulse intervals did not change substantially when the last interpulse interval was varied between 5 to 55 ms, so maximum force could not be attributed to any unique interpulse interval. Each successive pulse contributed a smaller force increment. When five to seven pulses were delivered in an optimal sequence, the evoked force was close to that recorded during maximal tetanic stimulation. In contrast, maximal force-time integral was evoked with one short interpulse interval (5-15 ms) then substantially longer interpulse intervals (>100 ms). Maximum force and force-time integrals were therefore elicited by different patterns of stimuli. We conclude that a brief initial interpulse interval (5-15 ms) is required to elicit maximum "doublet" force from human thenar motor units and that near-maximal tetanic forces can be elicited by only five or six additional post-"doublet" pulses if appropriately spaced in time. However, the rate at which these post-"doublet" stimuli must be provided is fairly uncritical. In contrast, maximum post-"doublet" force-time integrals were obtained at intervals corresponding to motoneuronal firing rates of approximately 7 Hz, rates close to that typically used to recruit motor units and to maintain weak voluntary contractions.

Adult↗

PubMed1998

Voluntary muscle weakness and co-activation after chronic cervical spinal cord injury.

Muscle strength was assessed from the maximum force that could be exerted voluntarily by triceps brachii muscles of 72 people with chronic cervical spinal cord injury (SCI) at or above C7, and 18 able-bodied (A-B) subjects. The magnitude of co-activation was estimated from the ratio of biceps brachii surface EMG to triceps plus biceps brachii surface EMG (biceps EMG/ triceps + biceps EMG). Maximum voluntary forces exerted by triceps brachii muscles of SCI subjects were significantly lower than those of controls (p < 0.01). Strength differences between muscles of SCI men and women were not evident. Significant positive relationships were found (linear or curvilinear) between triceps surface EMG and force for all control muscles (n = 19) and for 54% of the muscles of SCI subjects (n = 73). The remaining muscle of SCI subjects (n = 63) were either so weak that only one EMG and force value could be measured or EMG occurred without detectable force. For control muscles (n = 19), the mean triceps-biceps EMG ratio was 0.15+/-0.05 for all voluntary contraction force levels. For muscles of SCI subjects, 41 had EMG ratios similar to those of controls, co-activity largely attributed to EMG cross talk; 19 muscles had constant EMG ratios, but these were three standard deviations above the control means; 13 muscles had EMG ratios that decreased or increased as force increased. Muscles of SCI subjects with greater than control levels of co-activity during maximum voluntary contractions (high EMG ratios) were as strong as muscles with EMG ratios similar to controls. These results provide quantitative descriptions of voluntary muscle weakness after SCI and a database from which to evaluate improvements in muscle strength. These data also show that, for many SCI subjects, any triceps-biceps co-activation is similar to that of controls and does not necessarily distort muscle control unduly.

Adult↗

PubMed1996

Contractile properties of single motor units in human toe extensors assessed by intraneural motor axon stimulation.

1. Single motor axons innervating human toe extensor muscles were selectively stimulated through a tungsten microelectrode inserted percutaneously into the peroneal nerve. Twitch and tetanic forces were measured from a strain gauge over the proximal phalanx of the toe generating the greatest force. Twitch data were obtained from 19 single motor units in nine subjects: 8 motor units supplied extensor hallucis longus (EHL), 5 motor units supplied extensor digitorum longus (EDL), and 6 motor units supplied extensor digitorum brevis (EDB). Unpotentiated twitch forces ranged from 6.3 to 78.1 mN (20.0 +/- 4.0 mN, mean +/- SE), with the distribution highly skewed toward small forces. Twitch contraction and half-relaxation times were 74.8 +/- 3.9 and 78.6 +/- 6.0 ms, respectively. Compared with motor units in human thenar muscles, those in human toe extensor muscles were stronger but slower. However, as in thenar motor units, twitch force and contraction time were not related. 2. Force-frequency relationships were determined for 13 units (5 EDL, 5 EHL, 3 EDB) by stimulating each unit with short trains (1.0-5.0 s) of constant frequency (2-100 Hz). Peak force was related to stimulus frequency in a sigmoid fashion. The steep region of the curve extended from 5.5 +/- 0.7 (SE) Hz to 16.3 +/- 1.1 Hz for all units, and the stimulus frequency required to generate half-maximal force (9.6 +/- 0.6 Hz) was close to the center of the steep range. This frequency, which was inversely related to twitch contraction time, was lower than the frequency required to develop half-maximal force of human thenar motor units (12 +/- 4 Hz, mean +/- SD). The slopes of the regression lines relating force to frequency, computed over the steep range for each unit, were also lower for the toe extensors (3.7 +/- 0.7 mN/Hz) than for the thenar muscles (6 +/- 1 mN/Hz). 3. Maximal tetanic forces ranged from 29.9 to 188.1 mN (89.0 +/- 16.5 mN, mean +/- SE), and were generated at stimulus frequencies from 15 to 100 Hz (median 50 Hz). The stimulation frequency required for fused tetani (absence of noticeable force fluctuation) was generally less than that required for maximum tetanic force. The mean twitch-tetanus ratio, calculated for unpotentiated twitches, was 0.22 +/- 0.02 (range 0.15-0.41). This ratio was higher than for human thenar motor units (0.14 +/- 0.06, mean +/- SE). After twitch potentiation of 10 units, the mean twitch-tetanus ratio increased to 0.28 +/- 0.04. 4. The effects of preceding each stimulus train with a short interstimulus interval (10 ms) on force production at each frequency were examined in nine motor units. Peak forces at the onset of each contraction were higher when such an "initial doublet" preceded stimulus trains of < or = 20 Hz, but the mean force at the end of each stimulus train was not significantly affected at any frequency. 5. Eight units were stimulated with a train that increased in frequency continuously from 2 to 80 Hz, and then decreased symmetrically. This pattern resulted in peak forces that were higher on the descending limb of the stimulus train, the force-frequency relationship tracing a hysteresis loop. Hysteresis was exhibited because damping in the neuromuscular system causes the mechanical output of muscle to lag behind neural input. Thus, in non-steady-state conditions (as in most forms of natural activity), somewhat higher firing rates may be required to attain a particular level of force; once attained, force output will be transiently unresponsive to diminution of firing rate. 6. We conclude that there are differences in the contractile properties of single motor units in human toe extensor muscles (involved in posture and locomotion) and thenar muscles (involved in prehension and manipulation). Twitch-tetanus ratios were greater for motor units in the toe extensors, and this property accounted for the lower force sensitivity of these units to increases in frequency. (ABSTRACT TRUNCATED)

Adult↗

PubMed1995

Task-dependent factors in fatigue of human voluntary contractions.

This chapter explores the hypothesis that fatigue is not caused uniquely by any common set of factors, but rather that the amount of stress placed on each site depends on the type of exercise from which fatigue develops. Evidence supporting this idea is presented by comparing results from various studies in which fatigue was caused by different exercise protocols. However, the way in which human endurance capacity changes with the type or intensity of the task performed suggest a unitary process. Thus, perhaps the neuromuscular system as a whole is so well adjusted that any task-related additional impairment at one site is compensated by corresponding functional improvements at others. We suggest that nature has had a long time in which to "get it right".

Exercise↗

PubMed1995

Motor unit activity during isometric and concentric-eccentric contractions of the human first dorsal interosseus muscle.

1. Motor unit activity was recorded with intramuscular fine wire electrodes during isometric, concentric, and eccentric activity in the human first dorsal interosseus muscle. Twenty-one units from 11 subjects were sampled. 2. During isotonic cycles of shortening and lengthening, 18 of 21 units were recruited during the concentric phase, increased their discharge rates as the concentric movement progressed, then decreased their discharge rate during the eccentric phase, and were derecruited. 3. A different pattern of recruitment was observed in recordings from three units. These units were recruited during the eccentric phase, at a time when other units were decreasing their discharge rate or being derecruited. In two of the units selectively recruited during the eccentric phase, it was possible to determine their isometric thresholds, which were higher than those of units exhibiting the more common pattern of recruitment. 4. For two of the three units exhibiting selective recruitment during eccentric contraction, the unit was recorded simultaneously with different pairs of recording wires separated by 5-10 mm. Each discharge of these units was detected by both electrodes, making it unlikely that movement artifact was responsible for the initiation or cessation of discharge. 5. The recruitment patterns observed suggest that changes in the type or distribution of synaptic inputs to motoneurons during movement can, in some instances, override pre- and postsynaptic factors that shape recruitment order in isometric conditions.

Adolescent↗

PubMed1993

Motoneuronal output and gradation of effort in attempts to contract acutely paralysed leg muscles in man.

1. The study was designed to determine the degree to which normal subjects can control motoneurones innervating a leg muscle when acutely deprived of muscle afferent feedback. Microneurographic recordings were made from eighteen motor fascicles in the common peroneal nerve, of which thirteen innervated tibialis anterior and five toe dorsiflexor muscles. The nerve was then blocked completely at a distal site near the fibular head with local anaesthetic. A sequence of tests was performed with each fascicle to determine the degree to which the subject could control the motoneuronal drive to the paralysed muscle. 2. During a complete distal block of the common peroneal nerve, motoneurones innervating tibialis anterior were frequently activated during weak attempted contraction of the synergist toe extensors and vice versa. 3. When subjects attempted contractions of the paralysed muscles at a constant effort, pressure applied to the dorsum of the foot caused relatively small changes in the level of neural output, producing a small increase in motoneuronal drive to tibialis anterior, but no consistent change in the drive to toe extensor fascicles. 4. Subjects were able to increase the motoneuronal drive to the paralysed tibialis anterior in five steps of effort each lasting 10 s. The level of motor output increased linearly with step number, but declined as the step was maintained, more so when auditory feedback was withdrawn. 5. There was hysteresis in the relationship between motoneuronal output and force (measured on the contralateral side) during attempts to make slowly increasing then decreasing ramps of effort on both sides over 20-120 s. Motor drive to the paralysed muscle increased disproportionately rapidly compared with contralateral force when subjects attempted bilaterally symmetrical increasing efforts. 6. Subjects attempted to activate the paralysed muscle group maximally for 20-30 s with auditory feedback of the neurogram and verbal encouragement. There was a small statistically significant reduction in the motoneuronal output 5-10 s into the 30 s effort but, with further encouragement, it recovered towards the end of the effort. 7. When compared directly in the same recording sequences, attempts to make rapid brief maximal efforts (2-3 s duration) produced the same motoneuronal output as attempts to make sustained efforts. 8. Similar results occurred when the motoneuronal output to tibialis anterior was recorded during a selective distal block of tibialis anterior sparing toe dorsiflexors.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1993

The firing rates of human motoneurones voluntarily activated in the absence of muscle afferent feedback.

1. To quantify the net influence of muscle afferent feedback on the firing rates of human motoneurones, the discharge frequencies of single motor axons in the common peroneal nerve were recorded during sustained voluntary efforts performed in the absence of feedback from the target muscle. These data were compared with the firing rates of single motor units in the intact tibialis anterior muscle. In five subjects, recordings were made from fifty-two motor axons innervating tibialis anterior during acute deafferentation and paralysis of the dorsiflexor muscles produced by anaesthetic block of the nerve distal to the recording site. 2. Maximal sustainable firing rates were determined for twenty-four motoneurons, twelve of which were classified as relatively low threshold (estimated recruitment level < or = 10% maximal) and six as high threshold. Mean firing rates of the low-threshold motoneurones (21.7 +/- 2.7 Hz; +/- S.E.M.) were significantly higher than those of the high-threshold motoneurones (14.0 +/- 4.4 Hz). The mean firing rate of the twenty-four deafferented motoneurones during maximal efforts to contract the paralysed muscle was 18.6 +/- 1.9 Hz, significantly lower than the maximal firing rates of single motor units recorded from the normally innervated tibialis anterior muscle (28.2 +/- 0.6 Hz). 3. During half-maximal efforts, the mean firing rate of eight deafferented motoneurones (10.8 +/- 1.1 Hz) was significantly lower than that of intact motor units (16.5 +/- 0.2 Hz). A similar finding was apparent during minimal efforts; the mean discharge frequency of seven deafferented motoneurones during weak voluntary efforts was 6.0 +/- 0.9 Hz, compared with 7.3 +/- 0.13 Hz for intact motor units. Overall, the range of motoneurone firing rates (from minimal to maximal levels of voluntary effort) was significantly affected by the acute deafferentation, but was shifted significantly to lower rates. 4. During sustained maximal voluntary efforts of at least 30 s duration the firing rate of deafferented motoneurones decreased over the first 5 s but was then maintained, i.e. there was no progressive decline as occurs with normally innervated motor units during fatiguing contractions. This observation supports a reflex origin for the normal decline in motoneurone discharge. 5. It is concluded that muscle afferents in the common peroneal nerve provide a net facilitation to the tibialis anterior motoneurone pool, reflexly increasing the motor output at all levels of voluntary drive by approximately one-third.

Action Potentials↗

PubMed1992

Neuromuscular responses of patients with multiple sclerosis.

Muscle weakness, studied in 4 patients with multiple sclerosis (MS), was compared with values from normal subjects. Twitch occlusion showed that normal subjects could activate their muscles maximally, but patients rarely achieved greater than 60% activation. In both groups, motoneuron firing rates increased linearly with force. Consistent with the reduced level of activation, MCV firing rates in MS muscles rarely exceeded 17 Hz (compared with approximately 24 Hz for normals). However, for right and left muscles of one patient, mean maximum firing rates were 14.2 +/- 2 Hz and 8.0 +/- 2 Hz, but her muscles, could be activated to levels greater than 92% and 60%, respectively. This patient's ability to achieve higher than expected forces at low firing rates was probably due to her slow muscle contractile speeds, especially 1/2-relaxation time (75 to 115 ms, cf. approximately 60 ms for normals), and high twitch/tetanus ratio (0.4, cf. 0.2).

Adult↗

PubMed1992

Central and peripheral fatigue of human diaphragm and limb muscles assessed by twitch interpolation.

1. This study used a sensitive modification of the twitch interpolation technique to compare the extent of voluntary neural drive to the diaphragm and the elbow flexors during fatigue. For the diaphragm both inspiratory and expulsive efforts were tested, and fatigue was induced by expulsive efforts which were either maximal voluntary contractions (MVCs, 10 s duration, 50% duty cycle) or submaximal contractions (50% MVC, 3 s duration, 60% duty cycle). 2. Over the series of thirty MVCs peak elbow torque declined to 57.9 +/- 3.0% (mean +/- S.E.M.) of the initial value while maximal inspiratory pressure declined to 78.7 +/- 7.3% (P < 0.05). For the diaphragm the relative decline in voluntary peak inspiratory (and expulsive) force was similar to the decline in twitch responses to single and twin (10 ms interval) stimuli. However, for the elbow flexors the decline in twitch force was disproportionately greater than the decline in maximal voluntary force. The decline in twitch force for the diaphragm could not be attributed to failure at the neuromuscular junction. 3. At the start of the exercise, twitch potentiation (following three brief MVCs) was significantly less for the diaphragm than for the elbow flexors (20% versus 61%, P < 0.01). 4. In the unfatigued state maximal voluntary efforts by subjects activated 98.4 +/- 0.4% of the stimulated elbow flexors compared with 95.0 +/- 1.5% of the diaphragm (P < 0.05). During the exercise period there was a progressive failure in the ability to activate the limb muscle ('central fatigue'; voluntary drive declined from 98.4 +/- 0.4 to 86.8 +/- 2.2%, P < 0.01) whereas the decline in voluntary activation during inspiratory contractions was not significant (from 95.0 +/- 15 to 91.5 +/- 2.5%). 5. Voluntary activation during attempted maximal efforts was less complete for both muscles when stimuli were delivered without warning. The index of voluntary activation for unwarned stimuli was lower for the diaphragm (performing expulsive efforts, 81.0 +/- 2.8%) than for the limb muscle (89.9 +/- 1.5%, P < 0.01). 6. During repeated submaximal expulsive efforts we confirmed that subjects develop a marked inability to contract the diaphragm voluntarily, but when the diaphragm performed inspiratory manoeuvres at the same level of contractile fatigue, the index of voluntary drive was greater than 94%. 7. In conclusion, when tested with inspiratory efforts the diaphragm developed less central fatigue than the limb muscle over the same exercise period.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1992

Muscle temperature, contractile speed, and motoneuron firing rates during human voluntary contractions.

A study was made of motoneuron firing rates and mechanical contractile parameters during maximum voluntary contraction of human hand muscles. A comparison of muscles that had been fatigued after a 60-s maximum voluntary contraction (MVC) with muscles that were cooled by approximately 5 degrees C showed that the contractile properties, in particular the rates of contraction and relaxation, were similarly affected in both conditions. In contrast, the motoneuron firing rate was affected differently by the two treatments. In the case of the fatigued muscles the motoneuron firing rate was reduced by 36%, as was expected from previous studies, but in the case of the cooled muscles, there was no significant change in the motoneuron firing rate. We conclude that the reflex reduction in the motoneuron firing rate seen in the fatigued muscle is not triggered directly by a change in the mechanical properties of the muscle.

Adult↗

PubMed1991

Attempts to physiologically classify human thenar motor units.

1. This study was designed to determine whether human thenar motor units can be classified into types by the same physiological criteria used for other mammalian limb motor units and to consider whether such classification is functionally relevant. 2. Contractile responses of 25 human thenar single motor units were examined when their motor axons were stimulated intraneurally at rates from 1 to 100 Hz and intermittently at 40 Hz in a conventional 2-min fatigue test. Twitch and tetanic forces were measured together with various indexes of contractile rate. 3. Twitch contraction times and subtetanic to maximum tetanic force ratios were both distributed continuously. "Sag" in tension was not evident in unfused force profiles. Thus these units could not be divided into fast and slow types by the use of traditional contractile rate criteria. 4. Most units were fatigue resistant, with force fatigue indexes (FI) ranging from 0.33 to 1.14. None could be classified as fatiguable (FI less than 0.25). Seven units (28%) fell into the fatigue-intermediate (FI = 0.25-0.75) category, whereas 18 units (72%) had FI greater than 0.75, i.e., they were fatigue-resistant units. However, these units could not be classified by conventional FI and contractile rate criteria, because fatigue-resistant and fatigue-intermediate units had similar contractile rates. 5. Additional FI were calculated to describe changes in contractile rate. During the fatigue test, units behaved in one of three ways, showing 1) little change in either force or rate; 2) contractile slowing during the contraction and relaxation phases, with little or no force loss; or 3) both force and rate reduction.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

Matching focal and non-focal magnetic coil stimulation to properties of human nervous system: mapping motor unit fields in motor cortex contrasted with altering sequential digit movements by premotor-SMA stimulation.

Possible classifications of effects of magnetic coil (MC) stimulation are discussed, with the conclusion that the most useful is focal versus non-focal excitation. The mode of excitation of peripheral motor axons by the longitudinal-orthogonally orientated round MC is deduced from the insignificant latency shift in motor unit response when the current direction is reversed either by rotating the coil or by a switching device. A hypothesis is advanced of how peak membrane current entry and exit could occur only 1-3 nodes apart. The mode of excitation of cerebral cortex is more complex. Related to the orientation of the round MC, corticospinal neurons are: (1) directly excited and; or (2) indirectly excited through stimulation of corticocortical and other presynaptic inputs. Although the round MC can directly excite monkey corticospinal neurons at two sites, i.e. the initial segment and the node, the node is believed to be the main target in humans. The meaning of 'focality' of excitation is discussed as applied to MC stimulation of peripheral nerve and cerebral cortex. A potential conflict exists between focality and magnitude of response to MC excitation. However, by appropriately orientating the round MC, activation of all motor axons in one nerve (e.g., the median nerve at the wrist) can be achieved without coactivation of another (e.g., the ulnar nerve). By contrast, no orientation of the round MC, or use of a specially designed MC (e.g., double coil) over motor cortex permits all members of a defined set to be activated in isolation. Nevertheless, some members of the set can be activated in isolation with the MC over motor cortex. Response properties of individual motor units in the extensors of the digits when focally stimulating motor cortex with the figure '8' MC include: (1) Responses are variable to a given stimulus a little above threshold. Comparing responses by individual motor units with that of the population, or with other simultaneously recorded units revealed both coherent and independent sources of variability. (2) The scalp field from which the motor units could be driven by suprathreshold stimulation was of the order of 4-6 cm2. The fields were elongated in the antero-posterior axis, possibly related to the similar orientation of the junction region of the figure '8' MC. (3) Motor units initially excited by threshold MC stimulation were typically recruited early during voluntary contraction (confirming Hess et al. 1987).(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1990

Measurement of contractile and electrical properties of single human thenar motor units in response to intraneural motor-axon stimulation.

1. A method is described for measuring contractile properties of single human motor units. Conventional human microneurographic techniques were adapted to stimulate individual motor axons in the median nerve, with the use of negative current pulses and a tungsten microelectrode, while recording motor-unit electromyographic activity (EMG) and isometric force responses from the thenar muscles. 2. EMG signals were recorded from both proximal and distal thenar muscle surfaces. Force was recorded in two directions (thumb flexion and abduction). This allowed calculation of the direction and magnitude of resultant force exerted by each unit. 3. Data accepted as originating from a single unit satisfied all the traditional "all-or-none" criteria. Additional criteria also required the following: 1) a wide safety margin between the threshold for unit activation and the current intensity needed to elicit responses from other units; 2) that the characteristic direction in which each unit generated force did not change during the recording period; and 3) whenever F-responses were encountered, the second EMG waveform was identical to the first--a highly improbable event if more than one unit had been excited. 4. Respiration and blood pressure waves introduced baseline fluctuations that distorted the force measurements. These fluctuations were minimized by synchronizing stimuli to the pulse pressure cycle and resetting the baseline electronically just before stimulus onset. 5. Combining motor-axon stimulation at a site remote from the muscle with electronic resetting of the force baseline and delivery of stimuli at fixed intervals after the pulse pressure waves allowed the full time course of human motor-unit twitch and tetanic force and EMG signals to be recorded accurately without signal averaging.

Axons↗

PubMed1990

Twitch properties of human thenar motor units measured in response to intraneural motor-axon stimulation.

1. The twitch properties of human thenar motor units were examined in response to intraneural motor-axon stimulation. Force components of thumb abduction and flexion were measured before and after tetanic stimulation. The magnitude, direction, and time derivatives of resultant forces, together with axon conduction velocities, were calculated for each unit. 2. Various indexes of contraction and relaxation rate were measured including contraction time (time from force onset to peak), one-half relaxation time (time from peak force to one-half that value), normalized maximum contraction and normalized maximum relaxation rates (peak positive and negative time derivatives of the force signal normalized to twitch force), and the times at which these maximum rates occurred. 3. For different units, the directions of resultant forces were approximately evenly distributed between thumb abduction and flexion. At the onset of the experiment, initial twitch forces ranged from 3 to 34 mN, contraction times from 35 to 80 ms, and one-half relaxation times from 25 to 108 ms. 4. Resultant twitch forces were positively correlated to normalized maximum relaxation rates, but not to other rate indexes or to conduction velocity. The various contraction rate measures were correlated to each other, but generally not to relaxation rates. 5. After the first test involving tetanic stimulation, the twitches of most units were potentiated and slowed, especially their relaxation phase. However, the extent of these changes varied considerably between units. In general, units with weak initial forces potentiated most, some up to three-fold. These changes in twitch properties were denoted posttetanic twitch potentiation.(ABSTRACT TRUNCATED AT 250 WORDS)

Axons↗