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

1983 ◽  
Vol 50-50 (2-3) ◽  
Author(s):  
D. Kernell ◽  
O. Eerbeek ◽  
B.A. Verhey
Keyword(s):  
Isometric Force ◽  
Motor Units ◽  
Contraction Time ◽  
Stimulus Rate ◽  
Twitch Contraction

Anatomic and physiological characteristics of the ferret lateral rectus muscle and abducens nucleus

2007 ◽  
Vol 103 (5) ◽  
pp. 1706-1714 ◽  
Author(s):  
Keith N. Bishop ◽  
J. Ross McClung ◽  
Stephen J. Goldberg ◽  
Mary S. Shall
Keyword(s):  
Motor Units ◽  
Lateral Rectus ◽  
Contraction Time ◽  
Fatigue Index ◽  
Muscle Twitch ◽  
Abducens Nucleus ◽  
Single Motor Unit ◽  
Twitch Contraction ◽  
Whole Muscle ◽  
Individual Motor

The ferret has become a popular model for physiological and neurodevelopmental research in the visual system. We believed it important, therefore, to study extraocular whole muscle as well as single motor unit physiology in the ferret. Using extracellular stimulation, 62 individual motor units in the ferret abducens nucleus were evaluated for their contractile characteristics. Of these motor units, 56 innervated the lateral rectus (LR) muscle alone, while 6 were split between the LR and retractor bulbi (RB) muscle slips. In addition to individual motor units, the whole LR muscle was evaluated for twitch, tetanic peak force, and fatigue. The abducens nucleus motor units showed a twitch contraction time of 15.4 ms, a mean twitch tension of 30.2 mg, and an average fusion frequency of 154 Hz. Single-unit fatigue index averaged 0.634. Whole muscle twitch contraction time was 16.7 ms with a mean twitch tension of 3.32 g. The average fatigue index of whole muscle was 0.408. The abducens nucleus was examined with horseradish peroxidase conjugated with the subunit B of cholera toxin histochemistry and found to contain an average of 183 motoneurons. Samples of LR were found to contain an average of 4,687 fibers, indicating an LR innervation ratio of 25.6:1. Compared with cat and squirrel monkeys, the ferret LR motor units contract more slowly yet more powerfully. The functional visual requirements of the ferret may explain these fundamental differences.

Velocity of shortening of single motor units from rat soleus

1992 ◽  
Vol 67 (5) ◽  
pp. 1133-1145 ◽  
Author(s):  
S. R. Devasahayam ◽  
T. G. Sandercock
Keyword(s):  
Motor Unit ◽  
Motor Units ◽  
Surrounding Tissue ◽  
Passive Tension ◽  
Contraction Time ◽  
Single Motor ◽  
Twitch Contraction ◽  
Single Motor Units ◽  
Velocity Of Shortening ◽  
Force Velocity

1. The force-velocity relationship of a motor unit can provide insight into the contractile proteins of its constituent fibers as well as fundamental information about the function and use of the motor unit. Although the force-velocity profiles of whole muscle and skinned mammalian fibers have been studied, technical difficulties have prevented similar studies on motor units. A technique is presented to directly measure the velocity of shortening of individual motor units from in vivo rat soleus muscle. 2. The soleus muscles of anesthetized rats were dissected free of surrounding tissue while their nerve and blood supplies were preserved. Both tendons were cut, and the distal tendon was attached to a servomechanism to control muscle length, whereas the proximal tendon was attached to a force transducer. Single motor units were stimulated via the ventral roots. 3. The major problem encountered in measuring the force-velocity profile of a motor unit was that the force from the large number of passive fibers and connective tissue in the soleus confounded the force produced by the small number of active fibers in the motor unit. This problem was minimized by measuring active motor unit tension during an isovelocity ramp. This allowed experimental measurement of the passive tension by shortening the muscle with an identical isovelocity ramp without, however, stimulating the motor unit. Active tension was estimated by subtracting the passive tension waveform from the waveform recorded when the motor unit was active. 4. The method substantially reduced the noise from the passive fibers; however, problems remained. The probable sources of error are discussed, with the most significant being the elasticity associated with the blood and nerve connections to surrounding tissue. The elasticity prevents uniform shortening velocities along the length of the active fibers, thereby introducing a systematic bias to measurements made at high velocities. These errors are most pronounced when the data are extrapolated to determine the maximum velocity of shortening (Vmax). Determination of velocity at peak power (Vpp) is a more robust measure; however, of the 34 motor units studied, only 19 exhibited a distinct peak in the power-force curve, indicating residual noise. 5. To assess the validity of using twitch contraction time as an index of the velocity of shortening, when possible, Vmax and Vpp of each motor unit were correlated with the inverse of its twitch contraction time. The correlation was poor (r less than 0.2), indicating that, although widely used, twitch contraction time is a poor index of contractile speed.

Nonuniform fatigue characteristics of slow-twitch motor units activated at a fixed percentage of their maximum tetanic tension

1991 ◽  
Vol 66 (5) ◽  
pp. 1483-1492 ◽  
Author(s):  
T. C. Cope ◽  
C. B. Webb ◽  
A. K. Yee ◽  
B. R. Botterman
Keyword(s):  
Relaxation Time ◽  
Conduction Velocity ◽  
Motor Units ◽  
Contraction Time ◽  
Stimulation Rate ◽  
Axonal Conduction ◽  
Slow Twitch ◽  
Force Clamp ◽  
Highly Correlated ◽  
Tetanic Tension

1. The endurance of slow-twitch motor units from the soleus (SOL) and medial gastrocnemius (MG) muscles of the cat were tested by determining the length of time (endurance time, Et) that a unit could maintain its tension output at 85% of maximum. Motor-unit tension was clamped at the target level by altering the stimulation rate of a unit's motor axon through computer feedback control. Tested in this way, units of both muscles displayed a wide range of Ets, approximately 40- to 50-fold. 2. Electromyographic (EMG) waveforms of motor units subjected to force-clamp contractions were analyzed to access whether any activity-dependent changes in their waveform shape might predict Et. Three measurements of waveform shape were determined: baseline-to-baseline duration, peak-to-peak amplitude, and area. Typically, amplitude decreased and duration increased as a contraction proceeded, whereas area remained fairly constant. Because changes in each measure were very similar for units of widely different Ets, it was concluded that neuromuscular junction failure and changes in the excitability of the sarcolemma (excluding the t-tubule system) play a minor role in determining Et. 3. Et was highly correlated with the mean stimulation rate (Et/number of stimuli) used during the force-clamp contractions. Mean rate was seen to progressively decrease with increasing Et. This correlation could not be explained by measures of isometric contractile speed or relaxation (e.g., twitch contraction time or half-relaxation time) measured before the force-clamp contractions. Both contraction time and half-relaxation time were found to be unrelated to both Et and the rate used to stimulate the unit during the force-clamp contraction. 4. Among type S units of SOL and MG, maximum tetanic tension and Et were not related. A significant relation (r = -0.49) was found between axonal conduction velocity and Et for SOL units (n = 38). In addition, a significant correlation (r = 0.47) was found between conduction velocity and tetanic tension for SOL units. Perhaps because of the small sample of type S units from MG (n = 10), conduction velocity was found not be related to either Et or tetanic tension. 5. Others have shown that a motor unit's maximum tetanic tension and axonal conduction velocity are correlated with its order of recruitment among motoneurons innervating a muscle. Recent work has further shown that among type F units the order in which a motoneuron is recruited is highly correlated with the fatigue resistance of its muscle unit.(ABSTRACT TRUNCATED AT 400 WORDS)

Contributions of motor-unit recruitment and rate modulation of compensation for muscle yielding

1982 ◽  
Vol 47 (5) ◽  
pp. 797-809 ◽  
Author(s):  
P. J. Cordo ◽  
W. Z. Rymer
Keyword(s):  
Motor Unit ◽  
Stretch Reflex ◽  
Muscle Force ◽  
Motor Units ◽  
Muscle Stiffness ◽  
Motor Unit Recruitment ◽  
Reflex Response ◽  
Rate Modulation ◽  
Stimulus Rate ◽  
Wide Range

1. Subdivided portions of the cut ventral root innervation of the soleus muscle were electrically stimulated in 14 anesthetized cats. The stimulus trains imposed on these nerves simulated the recruitment and rate-modulation patterns of single motor units recorded during stretch-reflex responses in decerebrate preparations. Each activation pattern was evaluated for its ability to prevent muscle yield. 2. Three basic stimulus patterns, recruitment, step increases in stimulus rate, and doublets were imposed during the course of ramp stretches applied over a wide range of velocities. The effect of each stimulus pattern on muscle force was compared to the force output recorded without stretch-related recruitment or rate modulation. 3. Motor-unit recruitment was found to be most effective in preventing yield during muscle stretch. Newly recruited motor units showed no evidence of yielding for some 250 ms following activation, at which time muscle stiffness declined slightly. This time-dependent resistance to yield was observed regardless of whether the onset of the neural stimulus closely preceded or followed stretch onset. 4. Step increases in stimulus rate arising shortly after stretch onset did not prevent the occurrence of yield at most stretch velocities, but did augment muscle stiffness later in the stretch. Doublets in the stimulus train were found to augment muscle stiffness only when they occurred in newly recruited motor units. 5. These results suggest that at low or moderate initial forces, the prevention of yield in lengthening, reflexively intact muscle results primarily from rapid motor-unit recruitment. To a lesser extent, the spring-like character of the stretch-reflex response also derives from step increases in firing rate of motor units active before stretch onset and doublets in units recruited during the course of stretch. Smooth rate increases appear to augment muscle force later in the course of the reflex response.

Adaptations in physiological properties of rat motor units following 5 weeks of whole-body vibration

2013 ◽  
Vol 38 (9) ◽  
pp. 913-921 ◽  
Author(s):  
Dawid Łochyński ◽  
Marcin Bączyk ◽  
Dominik Kaczmarek ◽  
Maria Jolanta Rędowicz ◽  
Jan Celichowski ◽  
...  
Keyword(s):  
Myosin Heavy Chain ◽  
Heavy Chain ◽  
Whole Body Vibration ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Whole Body ◽  
Force Frequency ◽  
Body Vibration ◽  
Myosin Heavy Chain Isoform ◽  
Twitch Contraction

The purpose of the study was to determine the effects of 5-week whole-body vibration (WBV) on contractile parameters and force–frequency relationship of functionally isolated motor units of the rat medial gastrocnemius muscle: fast fatigable (FF), fast fatigue-resistant (FR), and slow (S). Moreover, myosin heavy chain isoform content was quantified. Following WBV, the maximum tetanic force of FF units was increased by ∼25%. The twitch half-relaxation time in all types of motor units and the twitch contraction time in FR units were shortened. The twitch-to-tetanus force ratio was decreased and the force–frequency curves were shifted rightwards in S and FR units. Myosin heavy chain distribution was not changed. These findings suggest modifications of the excitation–contraction coupling towards shortening of a twitch contraction. The observed increase in force of FF units may contribute to gains in muscle dynamic strength reported following WBV treatment.

Motor-Unit Synchronization Alters Spike-Triggered Average Force in Simulated Contractions

2002 ◽  
Vol 88 (1) ◽  
pp. 265-276 ◽  
Author(s):  
Anna M. Taylor ◽  
Julie W. Steege ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Motor Units ◽  
Contraction Time ◽  
Average Force ◽  
Twitch Force ◽  
Discharge Rates ◽  
Spike Triggered Averaging ◽  
Motor Unit Synchronization ◽  
Contraction Times ◽  
Twitch Characteristics

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.

scholarly journals Neural and muscular determinants of maximal rate of force development

2020 ◽  
Vol 123 (1) ◽  
pp. 149-157 ◽  
Author(s):  
Jakob L. Dideriksen ◽  
Alessandro Del Vecchio ◽  
Dario Farina
Keyword(s):  
Motor Unit ◽  
Maximum Rate ◽  
Rapid Development ◽  
Motor Units ◽  
Rate Of Force Development ◽  
Time Range ◽  
Limiting Factor ◽  
Force Development ◽  
Twitch Contraction ◽  
Discharge Rates

The ability to produce rapid forces requires quick motor unit recruitment, high motor unit discharge rates, and fast motor unit force twitches. The relative importance of these parameters for maximum rate of force development (RFD), however, is poorly understood. In this study, we systematically investigated these relationships using a computational model of motor unit pool activity and force. Across simulations, neural and muscular properties were systematically varied in experimentally observed ranges. Motor units were recruited over an interval starting from contraction onset (range: 22–233 ms). Upon recruitment, discharge rates declined from an initial rate (range: 89–212 pulses per second), with varying likelihood of doublet (interspike interval of 3 ms; range: 0–50%). Finally, muscular adaptations were modeled by changing average twitch contraction time (range: 42–78 ms). Spectral analysis showed that the effective neural drive to the simulated muscle had smaller bandwidths than the average motor unit twitch, indicating that the bandwidth of the motor output, and thus the capacity for explosive force, was limited mainly by neural properties. The simulated RFD increased by 1,050 ± 281% maximal voluntary contraction force per second from the longest to the shortest recruitment interval. This effect was more than fourfold higher than the effect of increasing the initial discharge rate, more than fivefold higher than the effect of increasing the chance of doublets, and more than sixfold higher than the effect of decreasing twitch contraction times. The simulated results suggest that the physiological variation of the rate by which motor units are recruited during ballistic contractions is the main determinant for the variability in RFD across individuals. NEW & NOTEWORTHY An important limitation of human performance is the ability to generate explosive movements by means of rapid development of muscle force. The physiological determinants of this ability, however, are poorly understood. In this study, we show using extensive simulations that the rate by which motor units are recruited is the main limiting factor for maximum rate of force development.

Progress of Age-Related Changes in Properties of Motor Units in the Gastrocnemius Muscle of Rats

2004 ◽  
Vol 92 (3) ◽  
pp. 1357-1365 ◽  
Author(s):  
Miho Sugiura ◽  
Kenro Kanda
Keyword(s):  
Gastrocnemius Muscle ◽  
Muscle Tension ◽  
Muscle Fibers ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Motor Nucleus ◽  
Twitch Contraction ◽  
Age Related ◽  
Old Rats ◽  
Whole Muscle

The mechanical properties of individual motor units in the medial gastrocnemius muscle, as well as the whole muscle properties and innervating motor nucleus, were investigated in dietary-restricted, male Fischer 344/DuCrj rats at ages of 4, 7, 12, 21/22, 27, 31, and 36 mo. The tetanic tension of the type S units continuously increased until the age of 36 mo. Those of type FF and FR units declined from 21/22 to 27 mo of age but did not change further while the whole muscle tension decreased greatly. The atrophy of muscle fibers, the decline in motoneuron number and axonal conduction velocity, and the decrease in the posttetanic potentiation of twitch contraction of motor units seemed to start after 21/22 mo of age and were accelerated with advancing age. Prolongation of twitch contraction time was evident for only type S and FR units in 36-mo-old rats. The fatigue index was greatly increased for type FF units in 36-mo-old rats. These findings indicated that the progress of changes in various properties occurring in the senescent muscle was different in terms of their time course and degree and also dependent on the types of motor unit. The atrophy and decrease in specific tension of muscle fibers affected the decline in tension output of motor units. This was effectively compensated for by the capture of denervated muscle fibers over time.

scholarly journals Behaviour of Motor Units during Submaximal Isometric Contractions in Chronically Strength-Trained individuals

2021 ◽  
Author(s):  
Andrea Casolo ◽  
Alessandro Del Vecchio ◽  
Thomas Grant Balshaw ◽  
Sumiaki Maeo ◽  
Marcel Bahia Lanza ◽  
...  
Keyword(s):  
Prolonged Exposure ◽  
Biceps Brachii ◽  
Isometric Force ◽  
Force Production ◽  
Motor Units ◽  
Surface Emg ◽  
Muscle Size ◽  
Training Experience ◽  
Cross Sectional ◽  
Neural Input

Neural and morphological adaptations combine to underpin the enhanced muscle strength following prolonged exposure to strength training, although their relative importance remains unclear. We investigated the contribution of motor unit (MU) behaviour and muscle size to submaximal force production in chronically strength-trained athletes (ST) vs. untrained controls (UT). Sixteen ST (age, 22.9±3.5 yr; training experience, 5.9±3.5 yr) and fourteen UT (age, 20.4±2.3 yr) performed maximal voluntary isometric force (MViF) and ramp contractions (at 15, 35, 50, 70%MViF) with elbow flexors, whilst high-density surface EMG (HDsEMG) was recorded from the biceps brachii (BB). Recruitment thresholds (RT) and discharge rates (DR) of MUs identified from the submaximal contractions were assessed. The neural drive-to-muscle gain was estimated from the relation between changes in force (ΔFORCE, i.e. muscle output) relative to changes in MU DR (ΔDR, i.e. neural input). BB maximum anatomical cross-sectional area (ACSAMAX) was also assessed by MRI. MViF (+64.8% vs. UT, P<0.001) and BB ACSAMAX (+71.9%, P<0.001) were higher in ST. Absolute MU RT was higher in ST (+62.6%, P<0.001), but occurred at similar normalized forces. MU DR did not differ between groups at the same normalized forces. The absolute slope of the ΔFORCE-ΔDR relationship was higher in ST (+66.9%, P=0.002), whereas it did not differ for normalized values. We observed similar MU behaviour between ST athletes and UT controls. The greater absolute force-generating capacity of ST for the same neural input, demonstrates that morphological, rather than neural, factors are the predominant mechanism for their enhanced force generation during submaximal efforts.

Effect of velocity and mechanical history on the forces of motor units in the cat medial gastrocnemius muscle

1992 ◽  
Vol 68 (5) ◽  
pp. 1503-1515 ◽  
Author(s):  
C. J. Heckman ◽  
J. L. Weytjens ◽  
G. E. Loeb
Keyword(s):  
Motor Unit ◽  
Gastrocnemius Muscle ◽  
Motor Behavior ◽  
Isometric Force ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Stimulation Rate ◽  
Force Velocity ◽  
Isometric Forces ◽  
Medial Gastrocnemius Muscle

1. Two fundamental aspects of the dynamic behavior of motor units of the cat medial gastrocnemius (MG) muscle were measured. Force-velocity (FV) relationships were measured with the use of constant velocity shortening and lengthening movements. Effects of mechanical history were assessed via comparisons of forces immediately after or during slow movements with standard isometric forces. Isometric force-length (FL) relations were also measured, and the effect of different stimulation rates on both FV and FL data was assessed. 2. Prior or concurrent movement greatly potentiated motor-unit force, but this movement potentiation was highly dependent on the amplitude of the unit's force. The smallest twitch forces of type S units (< 10 mN) were potentiated more than threefold, but no potentiation occurred for unit forces > 200 mN. It was tentatively concluded that movement potentiation may play little role in normal movements because it does not occur at forces > 1% of maximal isometric force of the MG. 3. During shortening, the normalized FV relations of type S units were relatively steeper than those of type FR or FF units. For lengthening, there was no evident relation between FV steepness and motor-unit type. 4. Stimulation rate affected both the FV and FL relationships of the motor units. The peak of the FL relationship (Lo) clearly shifted to shorter muscle lengths as stimulation rate was increased. The steepness of the FV relationship for shortening was decreased by increasing stimulation rate, but this effect was modest. 5. The shift in motor-unit Lo and the differences in motor-unit FV relationships were hypothesized to play significant roles during normal motor behavior. Realistic computer simulations of FL and FV functions for a population of motor units undergoing normal steady-state recruitment and rate modulation supported these hypotheses. As the level of simulated neural drive increased, the population Lo shifted to considerably shorter lengths, and the normalized FV function became much less steep. The significance of these results for models of muscle are discussed.

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