D. Kernell

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

SummaryThe 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.

Time Course and Properties of Late Adaptation in Spinal Motoneurones of the Cat

SummaryIn the spinal cord of anaesthetized cats, motoneurones of m.gastrocnemius medialis were stimulated to repetitive firing by very long-lasting steady currents injected through an intracellular microelectrode (maximum duration 4 min). In such discharges, a gradual decline of impulse frequency was found to occur during several tens of seconds. Most of this “late adaptation” occurred during the first 30 s of firing. Comparisons between the responses of different cells showed that the frequency-drop during late adaptation was strongly correlated to the impulse rate at the beginning of the discharge. For one and the same cell, late adaptation was more prominent at strong than at weaker intensities of stimulation (i.e., at high than at lower initial firing rates). In cells capable of discharging continuously for several minutes, a semi-stationary discharge rate tended to be reached after about 1 min or less.

Motor fatigue and cognitive task performance in humans

During fatiguing submaximal contractions a constant force production can be obtained at the cost of an increasing central command intensity. Little is known about the interaction between the underlying central mechanisms driving motor behaviour and cognitive functions. To address this issue, subjects performed four tasks: an auditory choice reaction task (CRT), a CRT simultaneously with a fatiguing or a non‐fatiguing submaximal muscle contraction task, and a fatiguing submaximal contraction task alone. Results showed that performance in the single‐CRT condition was relatively stable. However, in the fatiguing dual‐task condition, performance levels in the cognitive CRT deteriorated drastically with time‐on‐task. Moreover, in the fatiguing dual‐task condition the rise in force variability was significantly larger than during the fatiguing submaximal contraction alone. Thus, our results indicate a mutual interaction between cognitive functions and the central mechanisms driving motor behaviour during fatigue. The precise nature of this interference, and at what level this interaction takes place is still unknown.

Motoneurone properties and motor fatigue

SummaryGastrocnemius motoneurones with different types of muscle unit were compared with respect to their repetitive discharges during 4 min periods of steady intracellular stimulation. The cells were activated by a constant injected current of 5 nA above threshold. Among neurones capable of discharging for 10 s or more, the discharge duration showed no significant correlation to the contraction time or amplitude of the muscle unit twitch. Neither was there any obvious correlation between discharge duration and the sensitivity to contractile fatigue. The slow drop in discharge rate, as measured from the 2nd to the 26th s of firing, was more pronounced for fast-twitch units than for the ones with a slower twitch. Among fast-twitch neurones with about the same initial discharge rate, no difference in the extent of slow frequency drop was found between cells with fatigue-resistant and fatigue-sensitive muscle units. For fast-twitch neurones, measurements and calculations showed that, if the effects of peripheral potentiation and fatigue were disregarded, the drop in firing rate was great enough to cause a decrease in force by more than 60% during the first minute of constant stimulation. Among the fast-twitch units studied, the mean recorded fall in contractile force was initially less than expected (potentiation dominating) and it had become about equal to the expected one at 1 min after the onset of the discharge. It is concluded that, particularly with respect to fast-twitch motoneurones, the late adaptation is likely to be a significant factor for the development of central “fatigue” in voluntary or reflex contractions. Thanks to their small amount of late adaptation, slow-twitch motoneurones are par ticularly suitable for producing a steady postural contraction.

Somatotopic relations between spinal motoneurones and muscle fibres of the cat's musculus peroneus longus

The cats m.peroneus longus was analyzed with respect to the somatotopic relation between the rostro-caudal site of emergence of ventral root filaments (i.e. rostro-caudal site of motoneurones) and the intramuscular distribution of innervation. Rostro-caudally distinct fractions of ventral roots were stimulated repetitively in order to deplete their respective muscle fibres of glycogen. The intramuscular position of glycogen-depleted fibres was analyzed in transverse sections from different proximo-distal levels. At each level, depleted muscle fibres were dispersed across the whole muscle. No consistent relation was found between the spinal site of origin of a ventral root filament and the proximo-distal distribution of its fibres within the pennate muscle. A significant and evident tendency was found, however, for rostral root filaments (i.e. rostral motoneurones) to innervate a greater number of muscle fibres in anterior than in posterior muscle portions. For caudal root filaments, the opposite pattern of innervation was observed.

Motor unit categorization on basis of contractile properties: An experimental analysis of the composition of the cat's m. peroneus longus

SummaryRecordings were made of isometric contractions of single motor units of the cats m. peroneus longus (PerL). The units were activated by stimulation of dissected filaments of ventral roots. In accordance with the general principles introduced by Burke et al. (1973), the 80 isolated PerL units were classified into three or four type-categories according to their contractile speed and endurance. Three currently used varieties of a “fatigue index” were calculated and found to give equivalent results. Units with a high, intermediate and low resistance to fatigue were responsible for about 22.5, 25.4 and 52.1% respectively of the total muscle force. Two alternative methods for fast/slow categorization were compared: (i) classifying all units as slow that failed to show a “sag” in partly fused contractions (“sagcriterion”, Burke et al. 1973) and (ii) classifying all units as slow that had a more prolonged twitch contraction time than that of fatigue-sensitive units (‘FF vs. S-criterion’). The relative contribution of slow units to total muscle force was about 2.8 times as great (14 versus 5%) for a classification by the FF vs. S-criterion than for a subdivision according to sagging behaviour. When compared to equivalent data from previously published studies of feline hindlimb muscles, peroneus longus was found to resemble gastrocnemius medialis in relative motor unit composition. The maximum force of individual PerL units was, however, on average ≤ 50% of that reported for corresponding types of gastrocnemius units.

Daily durations of spontaneous activity in cat's ankle muscles

Abstract For an understanding of how various degrees of altered use (training, disuse) affect the properties of skeletal muscles, it is important to know how much they are used normally. The main aim of the present project was to produce such background knowledge for hindlimb muscles of the cat. In four adult female cats, each one being studied in several experimental sessions, ankle muscles were chronically implanted with electrodes for electromyographic (EMG) recording. The muscles recorded from were: extensor digitorum longus (EDL), peroneus longus (PL), tibialis anterior (TA), lateral gastrocnemius (LG) and soleus (SOL). For PL, TA and LG, there were anterior as well as posterior recording sites. During 24-h experimental sessions, the studied animal stayed, together with another cat, in a box large enough for playing and walking around. Using telemetric techniques, samples of EMG signals were recorded on tape for 4 min every 30 min. In an off-line analysis, measurements were made of the total accumulated duration of activity from each one of the studied muscle regions. These ”duty times” were expressed as a percentage of total sampling duration. When averaged over the whole 24-h experimental period, the mean duty times per muscle region varied from 1.9% for EDL up to about 13.9% for SOL. Also, among predominantly fast muscles of mixed-fibre composition (i.e. all studied muscles except SOL), marked and statistically significant differences in duty time were found, mean values varying fivefold from 1.9% (EDL) to 9.5% (PL, posterior site). For all three muscles with simultaneous recordings from different sites, consistent and statistically significant differences in daily duty time were found between anterior and posterior regions (anterior less than posterior for TA and PL; anterior more than posterior for LG). We also measured the extent to which each 4-min sampling period was filled with activity (if any). As compared to muscles with a low mean 24-h duty time, those with high duty times were not active during more sampling periods per day, but, whenever being used, their activity lasted relatively longer. Such results were consistent with the view that differences in mean 24-h duty time might largely reflect differences in the extent to which the various muscles and muscle regions were used for long-lasting stabilizing contractions.

Influence of a voluntary fatigue test on the contralateral homologous muscle in humans

Influences of a submaximal endurance test in the right first dorsal interosseus on force and fatigue-related parameters of activating the contralateral muscle were studied. The test consisted of a 30% maximum voluntary contraction (MVC), regularly interrupted by maximal contractions and brief rest periods. Despite the induced central fatigue, as tested with the MVC-superimposed twitch technique, and substantial peripheral fatigue, only minor effects of the previous fatigue test were seen for the contralateral hand. No significant influence was found on endurance time, the perceived effort for maintaining 30% MVC force or the MVC-superimposed twitch. Thus, our fatigue protocol induced both central and peripheral fatigue but only minor cross-over effects of fatigue were found for the homologous contralateral muscle.

Spatial differences in fatigue‐associated electromyographic behaviour of the human first dorsal interosseus muscle.

1. Fatigue‐associated electromyographic (EMG) reactions of intrinsic hand muscles were studied during maintained isometric voluntary contractions of normal subjects. Most measurements concerned actions of the first dorsal interosseus (FDI). In a smaller number of subjects, complementary measurements were obtained for adductor pollicis (AP). 2. Measurements were made of isometric force (thumb adduction, index finger abduction and flexion) and of surface EMG amplitudes (AP and FDI) after rectification and smoothing (rsEMG). 3. In the analysis of fatigue, the subjects were required to maintain a steady isometric force (index finger abduction or thumb adduction) of half their maximum voluntary contraction (1/2MVC test) for as long as possible. Average endurance times were 88 +/‐ 19 s (mean +/‐ S.D.) for FDI and 119 +/‐ 29 s for AP (Students t test, P < 0.02). 4. Pronounced differences in fatigue‐associated EMG behaviour were observed between AP and FDI. In AP the reaction was as expected: a rise of EMG during maintained force (mean rsEMG at end of fatigue test/mean rsEMG at start of test (rsEMG‐FI): 181 +/‐ 64%). In FDI this reaction was seen in half of the recorded cases, the remainder displaying bidirectional changes or a more or less marked decrease of EMG during the endurance task (mean for all cases together: rsEMG‐FI, 103 +/‐ 15%; difference between AP vs. FDI significant, P < 0.01). 5. The unexpected EMG variability of the FDI reactions was further analysed with multiple bipolar recordings of surface EMG. For all the four thoroughly studied subjects, recordings were obtained which showed simultaneously occurring EMG changes in opposite directions (decrease and increase) at different sites of FDI while force was kept constant at 50% of the maximum voluntary contraction (MVC). 6. Further observations on FDI showed that EMGs simultaneously obtained from different recording sites could show dramatic differences in their responses depending on ‘synergistic context’ (e.g. in relation to changes in index finger extension force during maintained abduction at 50% MVC). Evidence for ‘task switching’ (shift in rsEMG distribution, shift in hand muscle synergy) was frequently observed during the performance of the 1/2MVC test. 7. The results indicate that FDI is not handled in a topographically homogeneous manner during the execution of an isometric constant force endurance test. Furthermore, the results suggest that this seemingly simple motor performance can be executed in several alternative manners associated with the activation of different muscle synergies and with different distributions of activity within the FDI.

Contralateral muscle activity and fatigue in the human first dorsal interosseous muscle

During effortful unilateral contractions, muscle activation is not limited to the target muscles but activity is also observed in contralateral muscles. The amount of this associated activity is depressed in a fatigued muscle, even after correction for fatigue-related changes in maximal force. In the present experiments, we aimed to compare fatigue-related changes in associated activity vs. parameters that are used as markers for changes in central nervous system (CNS) excitability. Subjects performed brief maximal voluntary contractions (MVCs) with the index finger in abduction direction before and after fatiguing protocols. We followed changes in MVCs, associated activity, motor-evoked potentials (MEP; transcranial magnetic stimulation), maximal compound muscle potentials (M waves), and superimposed twitches (double pulse) for 20 min after the fatiguing protocols. During the fatiguing protocols, associated activity increased in contralateral muscles, whereas afterwards the associated force was reduced in the fatigued muscle. This force reduction was significantly larger than the decline in MVC. However, associated activity (force and electromyography) remained depressed for only 5-10 min, whereas the MVCs stayed depressed for over 20 min. These decreases were accompanied by a reduction in MEP, MVC electromyography activity, and voluntary activation in the fatigued muscle. According to these latter markers, the decrease in CNS motor excitability lasted much longer than the depression in associated activity. Differential effects of fatigue on (associated) submaximal vs. maximal contractions might contribute to these differences in postfatigue behavior. However, we cannot exclude differences in processes that are specific to either voluntary or to associated contractions.