scholarly journals Robust estimation of average twitch contraction forces of populations of motor units in humans

2017 ◽  
Vol 37 ◽  
pp. 132-140 ◽  
Author(s):  
Francesco Negro ◽  
Claudio Orizio
Keyword(s):  
Robust Estimation ◽  
Motor Units ◽  
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.

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.

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.

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.

Sag During Unfused Tetanic Contractions in Rat Triceps Surae Motor Units

1999 ◽  
Vol 81 (6) ◽  
pp. 2647-2661 ◽  
Author(s):  
J. S. Carp ◽  
P. A. Herchenroder ◽  
X. Y. Chen ◽  
J. R. Wolpaw
Keyword(s):  
Relaxation Times ◽  
Motor Units ◽  
Contractile Properties ◽  
Triceps Surae ◽  
Initial Increase ◽  
Tetanic Stimulation ◽  
Twitch Contraction ◽  
Time To Peak ◽  
Single Motor Units ◽  
Initial Force

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.

Cross-reinnervated motor units in cat muscle. II. Soleus muscle reinnervated by flexor digitorum longus motoneurons

1985 ◽  
Vol 54 (4) ◽  
pp. 837-851 ◽  
Author(s):  
R. P. Dum ◽  
M. J. O'Donovan ◽  
J. Toop ◽  
P. Tsairis ◽  
M. J. Pinter ◽  
...  
Keyword(s):  
Muscle Fibers ◽  
Motor Units ◽  
Fat Content ◽  
Type Ii ◽  
Glycogen Depletion ◽  
Flexor Digitorum Longus ◽  
Twitch Contraction ◽  
Contraction Times ◽  
Flexor Digitorum ◽  
Individual Motor

The properties of whole soleus (SOL) muscles and of individual motor units were studied in cats 30-50 wk after self-reinnervation by soleus (SOL) motoneurons (SOL----SOL) or cross-reinnervation by flexor digitorum longus (FDL) motoneurons (FDL----SOL). As in the preceding paper (22), intracellular and glycogen-depletion methods were used to examine the physiological and histochemical properties of individual motor units. The results were compared with data from normal SOL motor units (8, 12). Intentionally self-reinnervated SOL muscles (SOL----SOL; n = 6) were normal in size and wet weight, and all of the five SOL----SOL motor units studied had physiological and histochemical characteristics that matched those of normal SOL units. Cross-reinnervation of SOL by FDL alpha-motoneurons (FDL----SOL; n = 7) produced muscles with wet weights and appearance essentially identical to normal SOL. However, whole-muscle twitch contraction times were much shorter (mean 60.4 ms) than those of normal (mean 136.9 ms, n = 18) or SOL----SOL muscles (mean 115.3 ms; n = 6). Despite this difference, none of the FDL----SOL muscles contained more than 7% histochemical type II muscle fibers, all of which were type IIA. Normal cat SOL muscles can contain up to 5% type IIA fibers, but none of our SOL----SOL muscles showed any type II fibers. Two FDL----SOL muscles had significant amounts of unintended self-reinnervation, permitting side-by-side comparison of FDL----SOL and SOL----SOL muscle fibers. The twitch contraction times of the two populations differed markedly, but they were histochemically indistinguishable except for the fact that SOL----SOL fibers had high neutral fat content (as do normal SOL fibers), whereas FDL----SOL showed much lower fat content. The 23 FDL----SOL muscle units studied were classified as physiological type S by criteria ("sag" test and fatigue resistance) used to identify motor-unit types in normal cat muscles. All five of the FDL----SOL units studied histochemically after glycogen depletion showed the type I histochemical profile, which is characteristic of the normal cat SOL. In marked contrast to the preceding study, cross-reinnervation of cat SOL by FDL motoneurons produced no conversion of muscle-unit properties into those associated with fast-twitch unit types, despite significant decreases in isometric twitch contraction time. The altered twitch speed was not associated with evident changes in conventional myofibrillar adenosine triphosphatase (ATPase) histochemistry.(ABSTRACT TRUNCATED AT 400 WORDS)

Muscle spindle discharge in response to contraction of single motor units

1983 ◽  
Vol 49 (2) ◽  
pp. 291-302 ◽  
Author(s):  
B. McKeon ◽  
D. Burke
Keyword(s):  
Muscle Spindle ◽  
Discharge Rate ◽  
Human Subjects ◽  
Motor Units ◽  
Muscle Spindles ◽  
Single Motor Unit ◽  
Single Motor ◽  
Twitch Contraction ◽  
Single Motor Units ◽  
Tendon Organ

1. In human subjects, microelectrode recordings were made from 25 muscle spindle afferents and two tendon organ afferents coming from muscles innervated by the peroneal nerve. 2. Stimulation at low intensity through the recording microelectrode activated efferent axons innervating motor units in close proximity to the muscle spindle or tendon organ. There was a clear alteration in the discharge of 17 afferents (15 muscle spindle, 2 tendon organ) in response to twitch contractions that involved only one, two, or three motor units. With three other afferents there was a less overt but statistically significant alteration in discharge rate by the twitch contraction of a single motor unit. 3. The sensitivity of 21 receptors (20 spindles, 1 tendon organ) to twitch contractions of anatomically close motor units was contrasted with their sensitivity to twitches of more remote motor units in the muscle. In no instance was the sensitivity to the contraction of remote motor units greater than that to the contraction of local motor units stimulated through the microelectrode; with remote stimulation many units usually had to be activated before the resulting twitch contraction altered the discharge of an afferent. 4. It is concluded that muscle spindles as well as tendon organs can play a role in monitoring the activity of motor units anatomically close to the receptor.

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