Stimulation pattern that maximizes force in paralyzed and control whole thenar muscles

The pattern of seven pulses that elicited maximal thenar force was determined for control muscles and those that have been paralyzed chronically by spinal cord injury. For each subject group (n = 6), the peak force evoked by two pulses occurred at a short interval (5-15 ms; a "doublet"), but higher mean relative forces were achieved in paralyzed versus control muscles (41.4 +/- 3.9% vs. 22.7 +/- 2.0% maximal). Thereafter, longer intervals evoked peak force in each type of muscle (mean: 35 +/- 1 ms, 36 +/- 2 ms, respectively). With seven pulses, paralyzed and control muscles reached 76.4 +/- 5.6% and 57.0 +/- 2.6% maximal force, respectively. These force differences resulted from significantly greater doublet/twitch and doublet/tetanic force ratios in paralyzed (2.73 +/- 0.08, 0.35 +/- 0.03) compared with control muscles (2.07 +/- 0.07, 0.25 +/- 0.01). The greater force enhancement produced in paralyzed muscles with two closely spaced pulses may relate to changes in muscle stiffness and calcium metabolism. Peak force-time integrals were also achieved with an initial short interpulse interval, followed by longer intervals. The postdoublet intervals that produced peak force-time integrals in paralyzed and control muscles were longer than those for peak force, however (77 +/- 3 ms, 95 +/- 4 ms, respectively). These data show that the pulse patterns that maximize force and force-time integral in paralyzed muscles are similar to those that maximize these parameters in single motor units and various whole muscles across species. Thus the changes in neuromuscular properties that occur with chronic paralysis do not strongly influence the pulse pattern that optimizes muscle force or force-time integral.     

Twitch and tetanic properties of human thenar motor units paralyzed by chronic spinal cord injury

Little is known about how human motor units respond to chronic paralysis. Our aim was to record surface electromyographic (EMG) signals, twitch forces, and tetanic forces from paralyzed motor units in the thenar muscles of individuals (n = 12) with chronic (1.5-19 yr) cervical spinal cord injury (SCI). Each motor unit was activated by intraneural stimulation of its motor axon using single pulses and trains of pulses at frequencies between 5 and 100 Hz. Paralyzed motor units (n = 48) had small EMGs and weak tetanic forces (n = 32 units) but strong twitch forces, resulting in half-maximal force being achieved at a median of only 8 Hz. The distributions for cumulative twitch and tetanic forces also separated less for paralyzed units than for control units, indicating that increases in stimulation frequency made a smaller relative contribution to the total force output in paralyzed muscles. Paralysis also induced slowing of conduction velocities, twitch contraction times and EMG durations. However, the elevated ratios between the twitch and the tetanic forces, but not contractile speed, correlated significantly with the extent to which unit force summated in response to different frequencies of stimulation. Despite changes in the absolute values of many electrical and mechanical properties of paralyzed motor units, most of the distributions shifted uniformly relative to those of thenar units obtained from control subjects. Thus human thenar muscles paralyzed by SCI retain a population of motor units with heterogeneous contractile properties because chronic paralysis influenced all of the motor units similarly.     

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)     

Effect of ageing on the force development in tetanic contractions of motor units in rat medial gastrocnemius muscle

The purpose of this study was to determine the effect of ageing on the rate of force generation of motor units, and the mechanical efficiency of contraction produced by a doublet discharge. The study was carried out on isolated motor units of rat medial gastrocnemius muscle of young (5-10 mo) and two groups of old (24-25 and 28-30 mo) Wistar rats. Motor units were classified into the fast fatigable (FF), fast resistant (FR) and slow (S) ones. The force output and rate of force development were determined for non-doublet unfused tetanic contractions evoked by a series of a constant-rate trains of pulses and corresponding doublet contractions starting with an initial brief interpulse interval of 5 ms, and for maximal tetanic contraction. In FF motor units the rate of force development and the force produced by the doublet discharge increased transiently at the age of 24-25 mo, while in S and FR motor units this increase was observed at the age of 28-30 mo. Age-related decrease in the rate of force development of skeletal muscle cannot be attributed to a decline in efficiency of force production by functioning motor units.     

The influence of changes in the stimulation pattern on force and fusion in motor units of the rat medial gastrocnemius muscle

The effects of irregularity in the pattern of stimulation on the tension produced by motor units in the rat medial gastrocnemius muscle were investigated. The effects of decreasing as well as increasing the interpulse intervals were observed for each motor unit in tetani fused to different degrees. For each motor-unit type, it was found that the effects of these changes depended on the extent of tetanic fusion. Decreasing the interpulse interval produced an increase in tension during the tetanus: the more fused the profile of tetanus, the smaller the tension increase. Increasing the interpulse interval resulted in a decrease in tetanic tension. This effect was most prominent when the tetanic fusion index was approximately 0.75. This phenomenon resulted from the prolongation in relaxation when tetanic fusion increased, thereby preventing a decrease in tension when the interpulse interval increased. We also investigated the effects of introducing a short interpulse interval (”doublet”) at the beginning of the stimulation. The doublets produced increased tetanic tension with a more fused profile. However, the doublet enhanced the sensitivity of the tetanus to increases in interpulse interval and decreased its sensitivity to decreases in interpulse intervals. Slow-twitch motor units appeared to be significantly less sensitive to both increases and decreases in interpulse interval than fast-twitch units. This suggests that slow-twitch units are better suited for producing long-lasting contractions with a constant tension level. Conversely, the high sensibility of fast-twitch units to changes in stimulation frequency enhances their participation in regulation of tension of the muscular contraction. Received: 25 March 1998 / Accepted: 24 March 1999     

Sag during unfused tetanic contractions in rat triceps surae motor units.

Contractile properties and conduction velocity were studied in 202 single motor units of intact rat triceps surae muscles activated by intra-axonal (or intra-myelin) current injection in L5 or L6 ventral root to assess the factors that determine the expression of sag (i.e., decline in force after initial increase during unfused tetanic stimulation). Sag was consistently detected in motor units with unpotentiated twitch contraction times <20 ms. However, the range of frequencies at which sag was expressed varied among motor units such that there was no single interstimulus interval (ISI), with or without adjusting for twitch contraction time, at which sag could be detected reliably. Further analysis indicated that using the absence of sag as a criterion for identifying slow-twitch motor units requires testing with tetani at several different ISIs. In motor units with sag, the shape of the force profile varied with tetanic frequency and contractile properties. Simple sag force profiles (single maximum reached late in the tetanus followed by monotonic decay) tended to occur at shorter ISIs and were observed more frequently in fatigue-resistant motor units with long half-relaxation times and small twitch amplitudes. Complex sag profiles reached an initial maximum early in the tetanus, tended to occur at longer ISIs, and were more common in fatigue-sensitive motor units with long half-relaxation times and large twitch amplitudes. The differences in frequency dependence and force maximum location suggested that these phenomena represented discrete entities. Successive stimuli elicited near-linear increments in force during tetani in motor units that never exhibited sag. In motor units with at least one tetanus displaying sag, tetanic stimulation elicited large initial force increments followed by rapidly decreasing force increments. That the latter force envelope pattern occurred in these units even in tetani without sag suggested that the factors responsible for sag were expressed in the absence of overt sag. The time-to-peak force (TTP) of the individual contractions during a tetanus decreased in tetani with sag. Differences in the pattern of TTP change during a tetanus were consistent with the differences in force maximum location between tetani exhibiting simple and complex sag. Tetani from motor units that never exhibited sag did not display a net decrease in TTP during successive contractions. These data were consistent with the initial force decrement of sag resulting from a transient reduction in the duration of the contractile state.     

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.     

A comparison of human thenar motor-unit properties studied by intraneural motor-axon stimulation and spike-triggered averaging

1. Measurements of twitch contractile properties of human motor units recorded by spike-triggered averaging may be distorted by partial fusion between twitches, because motor units seldom fire at rates below 8-10 Hz. The effects of this fusion were examined by comparing the responses of 27 human thenar motor units when their motor axons were stimulated at 1, 8, and 10 Hz. 2. Resultant forces were calculated from the abduction and flexion force components, together with various contraction and relaxation rate indexes as reported previously. Values for single twitches were compared to measurements made from the unfused force fluctuations ("apparent twitches") of the same units recorded during 8 and 10 Hz stimulation. 3. For all units, stimulation at 8 and 10 Hz caused partial twitch fusion. At 10 Hz, mean values for "apparent twitch" forces, contraction times (CT), and one-half relaxation times (1/2RT) were reduced to 44, 76, and 52% of the corresponding values measured from separate twitches evoked by 1 Hz stimulation. Similar but smaller reductions were seen at 8 Hz. 4. Slow units, with initial twitch CT greater than 60 ms, showed significantly more distortion of all "apparent twitch" parameters when stimulated at both 8 and 10 Hz, compared to fast units (less than 50 ms). 5. The potentiated abduction force component data were compared with abduction forces obtained previously by spike-triggered averaging from the same muscle group. Mean force obtained by spike-triggered averaging ("STA twitch" force: 21 mN) was significantly larger than that measured in abduction in response to either 1 or 10 Hz motor-axon stimulation (14 mN, 6 mN, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Summation of forces from multiple motor units in the cat soleus muscle

Nearly all muscle models and most motor control concepts assume that forces from individual muscle fibers and motor units sum in an additive manner once effects of in-series tendon compliance are taken into account. Due to the numerous mechanical linkages between individual fibers, though, it is unclear whether this assumption is warranted. This work examined motor unit force summation over a wide range of muscle forces in the cat soleus. Nonadditive summation implies a nonlinear summation of motor unit forces. Summation nonlinearities were quantified during interactions of 10 individual motor units and 4 motor unit bundles containing approximately 10 units each. These protocols allowed motor unit force summation to be examined from approximately 0 to 25% of tetanic muscle force. Nonlinear summation was assessed by comparing the actual forces to the algebraic sum of individual units and bundles stimulated in isolation. Superadditive summation meant that the actual force exceeded the algebraic sum, whereas subadditive summation meant that the actual force was smaller than the algebraic sum. Experiments tested the hypothesis that superadditive summation occurs at low force levels when few motor units are recruited, whereas subadditive summation prevails above 10% of tetanic force. Results were consistent with this hypothesis. As in previous studies, nonlinear summation in the soleus was modest, but a clear transition from predominately superadditive to predominantly subadditive summation occurred in the range of 6-8% of tetanic force. The largest nonlinearities were transient and appeared at the onset of recruitment and derecruitment of groups of motor units. The results are discussed in terms of the mechanical properties of the connective tissue forming the tendon and linking muscle fibers.     

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.     

Force-frequency relationships of human thenar motor units

1. Force-frequency relationships were examined in 30 human thenar single motor units. The technique of intraneural stimulation was used to stimulate the motor axon in the median nerve proximal to the elbow with a tungsten microelectrode. 2. The stimulation consisted of either single shocks or trains of pulses (1 or 2 s duration) at constant rates varying between 5 and 100 Hz. To control various mechanical artifacts, the stimuli were delivered after electronically resetting the force baseline, and the stimuli were phased to the pulse pressure wave. Thumb flexion and abduction force components were recorded and the magnitude and direction of the resultant force calculated. Electromyographic responses (EMG) were recorded from both the proximal and distal thenar muscle surfaces. 3. For all units, twitch force began to fuse between 5 and 8 Hz and maximum tetanic force was achieved between 30 and 100 Hz. Half-maximum tetanic force was produced at 12 +/- 4 (SD) Hz, as assessed by interpolation using the nearly linear force-frequency relationship between 8 and 30 Hz (on logarithmic frequency coordinates). 4. For the majority of units (n = 19), the strongest force changes in response to variations in stimulation frequency occurred between 5 and 10 Hz (sensitivity 6 +/- 1 mN/Hz). Fewer units showed highest force-frequency sensitivity between 8 and 15 Hz (n = 7; 4 +/- 3 mN/Hz) or 10 and 20 Hz (n = 4; 5 +/- 2 mN/Hz).(ABSTRACT TRUNCATED AT 250 WORDS)     

The tetanic depression in fast motor units of mammalian skeletal muscle can be evoked by lengthening of one initial interpulse interval

A lower than expected tetanic force (the tetanic depression) is regularly observed in fast motor units (MUs) when a higher stimulation frequency immediately follows a lower one. The aim of the present study was to determine whether prolongation of only the first interpulse interval (IPI) resulted in tetanic depression. The experiments were carried out on fast MUs of the medial gastrocnemius muscle in cats and rats. The tetanic depression was measured in each case as the force decrease of a tetanus with one IPI prolonged in relation to the tetanic force at the respective constant stimulation frequency. Force depression was observed in all cases studied and was considerably greater in cats. For cats, the mean values of force depression amounted to 28.64% for FR and 10.86% for FF MUs whereas for rats 9.30 and 7.21% for FR and FF motor units, respectively. Since the phenomenon of tetanic depression in mammalian muscle is commonly observed even after a change in only the initial interpulse interval within a stimulation pattern, it can effectively influence processes of force regulation during voluntary activity of a muscle, when motoneurones progressively increase the firing rate.     

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)     

Force and fatiguability of sprouting motor units in partially denervated rat plantaris

Summary The contractile properties of single motor units of rat plantaris were measured in situ 7 days following muscle partial denervation, achieved by section of radicular nerve L4. Partially denervated muscles weighed less, generated weaker twitch and tetanic forces, and contained denervated fibers, as evidenced by indirect/direct stimulation force ratios less than 1. Fast motor units (over 90% of unit pool) showed elevated twitch and tetanic responses (222% and 171% of controls, respectively) and elevated twitch-to-tetanic force ratios. Although partial denervation did not alter the mean fatiguability of fast motor units, fewer proportions of units remained in the extreme categories of fatigue resistance, with a clustering of units in the intermediate ranges. Slow units, while showing elevated twitch and tetanic responses, did not change in fatiguability. Glycogen depletion of the fibers of two fast motor units in partially denervated muscles revealed the presence of fibers varying in size, and in staining intensities for succinate dehydrogenase and ATPase, within the same motor unit, as a result of motoneurone sprouting.     

Relation between isometric force and stimulus rate in cat's hindlimb motor units of different twitch contraction time

The relation between isometric force and rate (or pulse interval) of repetitive stimulation was studied for 77 motor units from m. peroneus longus of the cat. The units were activated by constant-frequency bursts of 1 s, and the stimulus interval needed for producing half the maximum tension was strongly correlated to twitch contraction time (twitch CT, non-potentiated values 13-42 ms). This remained true for comparisons within groups of fast and slow units respectively (fast/slow classification according to criteria of Burke et al. 1973). A mean contractile force of half maximum amplitude (0.5 PO) was produced by repetitive stimuli with a pulse interval of about 1.5 CT in fast and 2 CT in slow units. Among both kinds of unit, however, these stimulus rates corresponded to pulse intervals of about 1.4 times the half-relaxation time of the twitch. At half-maximum force, the rise of tension per Hz rise of stimulus frequency was about 2.5% PO for fast and 5.8% PO for slow units. Fast-twitch fatigue-sensitive (FF) and twitch fatigue-resistant (FR) units showed similar tension-frequency relations. Comparisons to results from m. gastrocnemius medialis showed that, for corresponding types of fast units (FF units), the twitch CT tended to be about 25% longer for gastrocnemius than for peroneus. The stimulus rate needed for a half-maximum contraction was, however, not lower for FF units from gastrocnemius than for those from the peroneus muscle.     

Concomitant changes in afterhyperpolarization and twitch following repetitive stimulation of fast motoneurones and motor units

The study aimed at determining changes in a course of motoneuronal afterhyperpolarization (AHP) and in contractile twitches of motor units (MUs) during activity evoked by increasing number of stimuli (from 1 to 5), at short interspike intervals (5 ms). The stimulation was applied antidromically to spinal motoneurones or to isolated axons of MUs of the medial gastrocnemius muscle within two separate series of experiments on anesthetized rats. Alterations in the amplitude and time parameters of the AHP of successive spikes were compared to changes in force and time course of successive twitches obtained by mathematical subtraction of tetanic contractions evoked by one to five stimuli. The extent of changes of the studied parameters depended on a number of applied stimuli. The maximal modulation of the AHP and twitch parameters (a prolongation and an increase in the AHP and twitch amplitudes) was typically observed after the second pulse, while higher number of pulses at the same frequency did not induce so prominent changes. One may conclude that changes observed in parameters of action potentials of motoneurons are concomitant to changes in contractile properties of MU twitches. This suggests that both modulations of the AHP and twitch parameters reflect mechanisms leading to force development at the beginning of MU activity.     

Motor-unit synchronization alters spike-triggered average force in simulated contractions

The purpose of the study was to quantify the effect of motor-unit synchronization on the spike-triggered average forces of a population of motor units. Muscle force was simulated by defining mechanical and activation characteristics of the motor units, specifying motor neuron discharge times, and imposing various levels of motor-unit synchronization. The model comprised 120 motor units. Simulations were performed for motor units 5-120 to compare the spike-triggered average responses in the presence and absence of motor-unit synchronization with the motor-unit twitch characteristics defined in the model. To synchronize motor-unit activity, selected motor-unit discharge times were adjusted; this kept the number of action potentials constant across the three levels of synchrony for each motor unit. Because there was some overlap of motor-unit twitches even at minimal discharge rates, the simulations indicated that spike-triggered averaging underestimates the twitch force of all motor units and the contraction time of motor units with contraction times longer than 49 ms. Although motor-unit synchronization increased the estimated twitch force and decreased the estimated contraction time of all motor units, spike-triggered average force changed systematically with the level of synchrony in motor units 59-120 (upper 90% of the range of twitch forces). However, the reduction in contraction time was similar for moderate and high synchrony. In conclusion, spike-triggered averaging appears to provide a biased estimate of the distribution of twitch properties for a population of motor units because twitch fusion causes an underestimation of twitch force for slow units and motor-unit synchronization causes an overestimation of force for fast motor units.     

Variability of the twitch parameters of the rat medial gastrocnemius motor units--experimental and modeling study

In the present study a previously proposed model of a twitch based on an analytical function with four-parameters (lead, contraction and half-relaxation times and maximum force of the twitch) was validated on 115 motor units (MUs), divided into slow (S), fast-fatigue resistant (FR) and fast fatigable (FF) types. The original records were collected from electrophysiological experiments performed on MUs from the medial gastrocnemius muscle of five rats. Besides the easy calculation of the twitch parameters and their variability, the usefulness of the model was confirmed by eliminating artifacts and noise in the original twitch records, as well as by calculations of the velocity of force increase and decrease, the area under force records, and by normalization of all twitches with respect to the maximal force and contraction time. It was concluded that: (1) the four-parameter twitch model describes precisely the individual contractions of various MUs; (2) all physiological twitch parameters are distributed continuously and located within overlapping intervals for different MU types; this distribution is not linear, but exponential; (3) S MUs can be distinguished from fast ones on the basis of some twitch parameters (contraction and half-relaxation times, velocity of contraction), but the same cannot be applied for FF and FR MUs; (4) the analysis of the normalized twitches reveals the differences in shapes for different types of MUs, which shows that twitches of different MUs cannot be obtained from one standard pattern scaled in time and force. These results may have functional implications for studying effectiveness of twitch summation during tetanic contractions and the work performed by various types of MUs.     

Electromyographic and contractile properties of rabbit masseter motor units during fatiguing stimulation

Trigeminal motoneurons were electrically stimulated in order to investigate the electromyographic (EMG) behavior in relation to the contractile properties of motor units of the masseter muscle. A total of 80 motor units were studied in situ in male New Zealand White rabbits (n=46). The motor units were separated into two groups, each exposed to a specific fatiguing stimulation regimen. Motor unit action potential (MUAP) features, which comprised the amplitude (AMP) and inter-peak time (IPT), and the tetanic force were measured. All motor units were classified as fast (F) units. Forty-one motor units underwent a prolonged standard fatigue regimen of 40-Hz trains at 1 Hz for 20 min. While the MUAP showed an immediate decrease of mean AMP at the beginning of the stimulation, the mean force and IPT increased. After 2 min, the force declined, while the IPT continued to increase until 20 min. Only after 3 min of stimulation, did the degree of force decrease parallel the decline in MUAP AMP. After 20 min of stimulation, the majority of motor units (n=34) still generated a force larger than 50% of the initial value, but only 17 motor units showed MUAP AMP of less than 50% of the initial EMG response. A more intensive fatigue regimen (40-Hz trains at 1.5 Hz) was applied to another group of 39 motor units. A rapid decline of force and MUAP amplitude to almost 50% was observed within the first 5 min of stimulation. After 20 min, only four motor units were still able to produce a tetanic force of more than 50% of the initial. Most strikingly, motor units with twitch contraction times faster than 22 ms exhibited a decrease in force more than in MUAP AMP, whereas the reverse was seen for units slower than 22 ms; motor units with a twitch contraction time of 22 ms showed equal decrease in AMP and force. This finding is suggestive of a division of fast masseter motor units into two classes, those which fatigue more rapidly mechanically and those which fatigue more readily electrically. Electronic Publication     

Gradation of force output in normal fast and slow muscles of the rat

The discharge patterns of 16 motor units in extensor digitorum longus (EDL) and soleus (SOL) muscles of freely moving adult rats, described by Hennig & L?mo (1985), were further analysed with respect to their role in grading muscle force output. The units fell into three distinct classes, termed EDL-1, EDL-2 and SOL-1, probably corresponding to type FF, FR and S units. The EDL-1 units generated only single impulses or impulse trains of short duration (phasic firing) which had high frequency and usually started with a short interspike interval (initial doublet). The EDL-2 and SOL-1 units generated single impulses and impulse trains of both short and long durations (phasic and tonic firing) without initial doublets. The frequency was high in EDL-2 and low in SOL-1 units. In EDL-2 and SOL-1 units, the mean durations of the first interspike intervals in a train decreased as the number of impulses per train increased. In EDL-1 units they did not change. Both SOL and EDL muscles were simulated through the nerve at different regular frequencies and tension-frequency (T-F) curves constructed. The EDL-2 units fired naturally most often at frequencies corresponding to the steepest part of the EDL T-F curve. The EDL-1 and SOL-1 units fired naturally most often at frequencies where the T-F curves of their respective muscles began to flatten before maximum tetanic tension was reached. Stimulus trains starting with an initial doublet produced maximum rate of tension development (optimum impulse pattern). At optimum intervals the force increased from about 20 to 85% of maximum tetanic tension when the number of stimuli was increased from 1 to 7. It is concluded that the natural firing pattern of EDL-1 units and the contractile properties of EDL muscle fibres are normally matched so that the force can develop at maximum rate to maximum levels at the start of contractions. Tension output is apparently regulated primarily through varying number of impulses per train in EDL-1 units; in SOL-1 and EDL-2 units both rate and number of impulses are important.