scholarly journals Recruitment and derecruitment characteristics of motor units in a hand muscle of young and old adults

2010 ◽  
Vol 108 (6) ◽  
pp. 1659-1667 ◽  
Author(s):  
Mark Jesunathadas ◽  
Adam R. Marmon ◽  
James M. Gibb ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Discharge Rate ◽  
Age Groups ◽  
Motor Units ◽  
Motor Unit Recruitment ◽  
Force Variability ◽  
Old Adults ◽  
Hand Muscle ◽  
The Difference ◽  
Ramp Contractions

The significant decline in motor neuron number after ∼60 yr of age is accompanied by a remodeling of the neuromuscular system so that average motor unit force increases and the ability of old adults to produce an intended force declines. One possible explanation for the loss of movement precision is that the remodeling increases the difference in recruitment forces between successively recruited motor units in old adults and this augments force variability at motor unit recruitment. The purpose of the study was to compare the forces and discharge characteristics of motor units in a hand muscle of young and old adults at motor unit recruitment and derecruitment. The difference in recruitment force between pairs of motor units did not differ between young ( n = 54) and old adults ( n = 56; P = 0.702). However, old adults had a greater proportion of contractions in which motor units discharged action potentials transiently before discharging continuously during the ramp increase in force (young: 0.32; old: 0.41; P = 0.045). Force variability at motor unit recruitment was greater for old adults compared with young adults ( P ≤ 0.010), but discharge rate and discharge variability did not differ between age groups ( P ≥ 0.729). These results suggest that the difference in force between the recruitment of successive motor units does not differ between age groups, but that motor unit recruitment may be more transient and could contribute to the greater variability in force observed in old adults during graded ramp contractions.

scholarly journals Motor unit activity when young and old adults perform steady contractions while supporting an inertial load

2013 ◽  
Vol 109 (4) ◽  
pp. 1055-1064 ◽  
Author(s):  
Michael A. Pascoe ◽  
Jeffrey R. Gould ◽  
Roger M. Enoka
Keyword(s):  
Young Adults ◽  
Motor Unit ◽  
Action Potentials ◽  
Biceps Brachii ◽  
Discharge Rate ◽  
Age Groups ◽  
Motor Units ◽  
Old Adults ◽  
Target Force ◽  
Force Difference

The purpose of the study was to compare the discharge characteristics of biceps brachii motor units of young and old adults when they performed steady, submaximal contractions while the arm supported different inertial loads. Young (28 ± 4 yr; n = 16) and old (75 ± 4 yr; n = 14) adults performed steady contractions with the elbow flexors at target forces set at either small (11.7 ± 4.4% maximum) or large (17.8 ± 6.5% maximum) differences below the recruitment threshold force of the motor unit ( n = 40). The task was to maintain an elbow angle at 1.57 rad until the motor unit was recruited and discharged action potentials for ∼120 s. Time to recruitment was longer for the larger target force difference (187 ± 227 s vs. 23 ± 46 s, P < 0.001). Once recruited, motor units discharged action potentials either repetitively or intermittently, with a greater proportion of motor units exhibiting the repetitive pattern for old adults. Discharge rate at recruitment and during the steady contraction was similar for the two target force differences for old adults but was greater for the small target force difference for young adults. Discharge variability was similar at recruitment for the two age groups but less for the old adults during the steady contraction. The greatest difference between the present results and those reported previously when the arm pulled against a rigid restraint was that old adults modulated discharge rate less than young adults across the two contraction intensities for both load types.

Change in Muscle Fascicle Length Influences the Recruitment and Discharge Rate of Motor Units During Isometric Contractions

2005 ◽  
Vol 94 (5) ◽  
pp. 3126-3133 ◽  
Author(s):  
Benjamin Pasquet ◽  
Alain Carpentier ◽  
Jacques Duchateau
Keyword(s):  
Motor Unit ◽  
Ankle Joint ◽  
Discharge Rate ◽  
Motor Units ◽  
Motor Unit Recruitment ◽  
Isometric Contractions ◽  
Neutral Position ◽  
Proprioceptive Information ◽  
Fascicle Length ◽  
Muscle Fascicle

This study examines the effect of fascicle length change on motor-unit recruitment and discharge rate in the human tibialis anterior (TA) during isometric contractions of various intensities. The torque produced during dorsiflexion and the surface and intramuscular electromyograms (EMGs) from the TA were recorded in eight subjects. The behavior of the same motor unit ( n = 59) was compared at two ankle joint angles (+10 and −10° around the ankle neutral position). Muscle fascicle length of the TA was measured noninvasively using ultrasonography recordings. When the ankle angle was moved from 10° plantarflexion to 10° dorsiflexion, the torque produced during maximal voluntary contraction (MVC) was significantly reduced [35.2 ± 3.3 vs. 44.3 ± 4.2 (SD) Nm; P < 0.001] and the average surface EMG increased (0.47 ± 0.08 vs. 0.43 ± 0.06 mV; P < 0.05). At reduced ankle joint angle, muscle fascicle length declined by 12.7% ( P < 0.01) at rest and by 18.9% ( P < 0.001) during MVC. Motor units were activated at a lower recruitment threshold for short compared with long muscle fascicle length, either when expressed in absolute values (2.1 ± 2.5 vs. 3.6 ± 3.7 Nm; P < 0.001) or relative to their respective MVC (5.2 ± 6.1 vs. 8.8 ± 9.0%). Higher discharge rate and additional motor-unit recruitment were observed at a given absolute or relative torque when muscle fascicles were shortened. However, the data indicate that increased rate coding was mainly present at low torque level (<10% MVC), when the muscle-tendon complex was compliant, whereas recruitment of additional motor units played a dominant role at higher torque level and decreased compliance (10–35% MVC). Taken together, the results suggest that the central command is modulated by the afferent proprioceptive information during submaximal contractions performed at different muscle fascicle lengths.

Eccentric Muscle Damage Has Variable Effects on Motor Unit Recruitment Thresholds and Discharge Patterns in Elbow Flexor Muscles

2009 ◽  
Vol 102 (1) ◽  
pp. 413-423 ◽  
Author(s):  
Tamara J. Dartnall ◽  
Nigel C. Rogasch ◽  
Michael A. Nordstrom ◽  
John G. Semmler
Keyword(s):  
Muscle Damage ◽  
Motor Unit ◽  
Eccentric Exercise ◽  
Biceps Brachii ◽  
Discharge Rate ◽  
Motor Units ◽  
Motor Unit Recruitment ◽  
Recruitment Threshold ◽  
Flexor Muscles ◽  
Elbow Flexor Muscles

The purpose of this study was to determine the effect of eccentric muscle damage on recruitment threshold force and repetitive discharge properties of low-threshold motor units. Ten subjects performed four tasks involving isometric contraction of elbow flexors while electromyographic (EMG) data were recorded from human biceps brachii and brachialis muscles. Tasks were 1) maximum voluntary contraction (MVC); 2) constant-force contraction at various submaximal targets; 3) motor unit recruitment threshold task; and 4) minimum motor unit discharge rate task. These tasks were performed on three separate days before, immediately after, and 24 h after eccentric exercise of elbow flexor muscles. MVC force declined (42%) immediately after exercise and remained depressed (29%) 24 h later, indicative of muscle damage. Mean motor unit recruitment threshold for biceps brachii was 8.4 ± 4.2% MVC, ( n = 34) before eccentric exercise, and was reduced by 41% (5.0 ± 3.0% MVC, n = 34) immediately after and by 39% (5.2 ± 2.5% MVC, n = 34) 24 h after exercise. No significant changes in motor unit recruitment threshold were observed in the brachialis muscle. However, for the minimum tonic discharge rate task, motor units in both muscles discharged 11% faster (10.8 ± 2.0 vs. 9.7 ± 1.7 Hz) immediately after ( n = 29) exercise compared with that before ( n = 32). The minimum discharge rate variability was greater in brachialis muscle immediately after exercise (13.8 ± 3.1%) compared with that before (11.9 ± 3.1%) and 24 h after exercise (11.7 ± 2.4%). No significant changes in minimum discharge rate variability were observed in the biceps brachii motor units after exercise. These results indicate that muscle damage from eccentric exercise alters motor unit recruitment thresholds for ≥24 h, but the effect is not the same in the different elbow flexor muscles.

scholarly journals Load Type Influences Motor Unit Recruitment in Biceps Brachii During a Sustained Contraction

2009 ◽  
Vol 102 (3) ◽  
pp. 1725-1735 ◽  
Author(s):  
Stéphane Baudry ◽  
Thorsten Rudroff ◽  
Lauren A. Pierpoint ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Biceps Brachii ◽  
Discharge Rate ◽  
Motor Units ◽  
Motor Unit Recruitment ◽  
Time To Failure ◽  
Single Motor Unit ◽  
Recruitment Threshold ◽  
Task Failure ◽  
Position Task

Twenty subjects participated in four experiments designed to compare time to task failure and motor-unit recruitment threshold during contractions sustained at 15% of maximum as the elbow flexor muscles either supported an inertial load (position task) or exerted an equivalent constant torque against a rigid restraint (force task). Subcutaneous branched bipolar electrodes were used to record single motor unit activity from the biceps brachii muscle during ramp contractions performed before and at 50 and 90% of the time to failure for the position task during both fatiguing contractions. The time to task failure was briefer for the position task than for the force task ( P = 0.0002). Thirty and 29 motor units were isolated during the force and position tasks, respectively. The recruitment threshold declined by 48 and 30% ( P = 0.0001) during the position task for motor units with an initial recruitment threshold below and above the target force, respectively, whereas no significant change in recruitment threshold was observed during the force task. Changes in recruitment threshold were associated with a decrease in the mean discharge rate (−16%), an increase in discharge rate variability (+40%), and a prolongation of the first two interspike intervals (+29 and +13%). These data indicate that there were faster changes in motor unit recruitment and rate coding during the position task than the force task despite a similar net muscle torque during both tasks. Moreover, the results suggest that the differential synaptic input observed during the position task influences most of the motor unit pool.

Electromyogram power spectra frequencies associated with motor unit recruitment strategies

1990 ◽  
Vol 68 (3) ◽  
pp. 1177-1185 ◽  
Author(s):  
M. Solomonow ◽  
C. Baten ◽  
J. Smit ◽  
R. Baratta ◽  
H. Hermens ◽  
...  
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Discharge Rate ◽  
Power Spectra ◽  
Motor Units ◽  
Motor Unit Recruitment ◽  
Power Density Spectrum ◽  
Median Frequency ◽  
Action Potential Firing ◽  
Density Spectrum

The isolated contributions of motor unit recruitment and firing rate variations to the median frequency of the electromyogram's power density spectrum were determined. Orderly stimulation of the cat gastrocnemius motor units via nerve electrodes gave rise to linearly increasing median frequency regardless of the action potential firing rate of the active motor units. Increase in the discharge rate of all the motor units resulted in nearly constant median frequency. It was concluded that the increasing average conduction velocity during motor unit recruitment is the major contributor to variations in the electromyogram median frequency. The possibility of using the median frequency as the index to identify the recruitment control strategies employed by various muscles during increasing force contraction is suggested.

Discharge Rate Variability Influences the Variation in Force Fluctuations Across the Working Range of a Hand Muscle

2005 ◽  
Vol 93 (5) ◽  
pp. 2449-2459 ◽  
Author(s):  
Chet T. Moritz ◽  
Benjamin K. Barry ◽  
Michael A. Pascoe ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Discharge Rate ◽  
Motor Units ◽  
Peak Discharge ◽  
High Threshold ◽  
Force Variability ◽  
Recruitment Threshold ◽  
Unit Model ◽  
Wide Range ◽  
Significant Difference

The goal of this study was to improve the ability of a motor unit model to predict experimentally measured force variability across a wide range of forces. Motor unit discharge characteristics were obtained from 38 motor units of the first dorsal interosseus muscle. Motor unit discharges were recorded in separate isometric contractions that ranged from 4 to 85% of the maximal voluntary contraction (MVC) force above recruitment threshold. High-threshold motor units exhibited both greater minimal and peak discharge rates compared with low-threshold units ( P < 0.01). Minimal discharge rate increased from 7 to 23 pps, and peak discharge rate increased from 14 to 38 pps with an increase in recruitment threshold. Relative discharge rate variability (CV) decreased exponentially for each motor unit from an average of 30 to 13% as index finger force increased above recruitment threshold. In separate experiments, force variability was assessed at eight force levels from 2 to 95% MVC. The CV for force decreased from 4.9 to 1.4% as force increased from 2 to 15% MVC ( P < 0.01) and remained constant at higher forces (1.2–1.9%; P = 0.14). When the motor unit model was revised using these experimental findings, discharge rate variability was the critical factor that resulted in no significant difference between simulated and experimental force variability ( P = 0.22) at all force levels. These results support the hypothesis that discharge rate variability is a major determinant of the trends in isometric force variability across the working range of a muscle.

Rate Coding Is Compressed But Variability Is Unaltered for Motor Units in a Hand Muscle of Old Adults

2007 ◽  
Vol 97 (5) ◽  
pp. 3206-3218 ◽  
Author(s):  
Benjamin K. Barry ◽  
Michael A. Pascoe ◽  
Mark Jesunathadas ◽  
Roger M. Enoka
Keyword(s):  
Young Adults ◽  
Motor Unit ◽  
Discharge Rate ◽  
Motor Units ◽  
Peak Discharge ◽  
Old Adults ◽  
Single Motor Units ◽  
Discharge Rates ◽  
Rate Coding ◽  
Reported Data

The discharge of single motor units ( n = 34) in the first dorsal interosseus muscle and the fluctuations in force during steady contractions were measured across a range of index finger abduction forces in old adults (77.1 ± 6.9 yr, n = 20). These results were compared with previously reported data on 38 motor units from young adults (25.7 ± 5.7 yr). Both minimal and peak discharge rates increased with recruitment threshold, but the strength of these relations was notably weaker for the old adults. Minimal discharge rates were similar for young and old adults ( P = 0.77), whereas peak discharge rates were lower for old adults ( P < 0.01). Consequently, the range of rate coding for each motor unit was substantially less for the old adults (7.1 pps) compared with the young adults (12.1 pps, P < 0.01). However, the variability in motor-unit discharge was similar for young and old adults; the coefficient of variation of the interspike intervals was similar at recruitment (old: 25.4%, young: 27.1%, P = 0.39) and declined with an increase in discharge rate (old: 13.2%, young: 14.2%, P = 0.21). Furthermore, the fluctuations in force during steady isometric contractions (2–95% of maximal force) were similar for young and old adults, except that the relative variability at the lowest force was greater for the old adults. A computational model of motor-unit recruitment and rate coding incorporated the experimental observations and was able to match the measured and simulated values for force steadiness across the operating range of the muscle.

scholarly journals Prolonged muscle vibration increases stretch reflex amplitude, motor unit discharge rate, and force fluctuations in a hand muscle

2005 ◽  
Vol 99 (5) ◽  
pp. 1835-1842 ◽  
Author(s):  
Minoru Shinohara ◽  
Chet T. Moritz ◽  
Michael A. Pascoe ◽  
Roger M. Enoka
Keyword(s):  
Motor Unit ◽  
Stretch Reflex ◽  
Discharge Rate ◽  
Motor Units ◽  
Muscle Vibration ◽  
Constant Force ◽  
Unit Discharge ◽  
Force Fluctuations ◽  
Motor Unit Discharge ◽  
Hand Muscle

The purpose of this study was to compare the influence of prolonged vibration of a hand muscle on the amplitude of the stretch reflex, motor unit discharge rate, and force fluctuations during steady, submaximal contractions. Thirty-two young adults performed 10 isometric contractions at a constant force (5.0 ± 2.3% of maximal force) with the first dorsal interosseus muscle. Each contraction was held steady for 10 s, and then stretch reflexes were evoked. Subsequently, 20 subjects had vibration applied to the relaxed muscle for 30 min, and 12 subjects received no vibration. The muscle vibration induced a tonic vibration reflex. The intervention (vibration or no vibration) was followed by 2 sets of 10 constant-force contractions with applied stretches (After and Recovery trials). The mean electromyogram amplitude of the short-latency component of the stretch reflex increased by 33% during the After trials ( P < 0.01) and by 38% during the Recovery trials ( P < 0.01). The standard deviation of force during the steady contractions increased by 21% during the After trials ( P < 0.05) and by 28% during the Recovery trials ( P < 0.01). The discharge rate of motor units increased from 10.3 ± 2.7 pulses/s (pps) before vibration to 12.2 ± 3.1 pps ( P < 0.01) during the After trials and to 11.9 ± 2.6 pps during the Recovery trials ( P < 0.01). There was no change in force fluctuations or stretch reflex magnitude for the subjects in the Control group. The results indicate that prolonged vibration increased the short-latency component of the stretch reflex, the discharge rate of motor units, and the fluctuations in force during contractions by a hand muscle. These adjustments were necessary to achieve the target force due to the vibration-induced decrease in the force capacity of the muscle.

Determining patterns of motor recruitment during locomotion

2002 ◽  
Vol 205 (3) ◽  
pp. 359-369 ◽  
Author(s):  
James M. Wakeling ◽  
Motoshi Kaya ◽  
Genevieve K. Temple ◽  
Ian A. Johnston ◽  
Walter Herzog
Keyword(s):  
Motor Unit ◽  
Frequency Band ◽  
Fibre Type ◽  
Low Frequency ◽  
Motor Units ◽  
Motor Unit Recruitment ◽  
Medial Gastrocnemius ◽  
Fibre Types ◽  
Myoelectric Signals ◽  
Frequency Bands

SUMMARY Motor units are the functional units of muscle contraction in vertebrates. Each motor unit comprises muscle fibres of a particular fibre type and can be considered as fast or slow depending on its fibre-type composition. Motor units are typically recruited in a set order, from slow to fast, in response to the force requirements from the muscle. The anatomical separation of fast and slow muscle in fish permits direct recordings from these two fibre types. The frequency spectra from different slow and fast myotomal muscles were measured in the rainbow trout Oncorhynchus mykiss. These two muscle fibre types generated distinct low and high myoelectric frequency bands. The cat paw-shake is an activity that recruits mainly fast muscle. This study showed that the myoelectric signal from the medial gastrocnemius of the cat was concentrated in a high frequency band during paw-shake behaviour. During slow walking, the slow motor units of the medial gastrocnemius are also recruited, and this appeared as increased muscle activity within a low frequency band. Therefore, high and low frequency bands could be distinguished in the myoelectric signals from the cat medial gastrocnemius and probably corresponded, respectively, to fast and slow motor unit recruitment. Myoelectric signals are resolved into time/frequency space using wavelets to demonstrate how patterns of motor unit recruitment can be determined for a range of locomotor activities.

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.

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