Motor-unit activation patterns in lengthening and isometric contractions of hindlimb extensor muscles in the decerebrate cat

1982 ◽  
Vol 47 (5) ◽  
pp. 782-796 ◽  
Author(s):  
P. J. Cordo ◽  
W. Z. Rymer
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Stretch Reflex ◽  
Single Unit ◽  
Motor Units ◽  
Isometric Contractions ◽  
Active Force ◽  
Lengthening Contractions ◽  
Rate Modulation ◽  
Initial Force

1. Multiunit integrated electromyographic (EMG) signals and single-unit EMG potentials were recorded during isometric and lengthening (stretch reflex) contractions of soleus and medial gastrocnemius (MG) muscles in 20 decerebrate cats. Patterns of motor-unit recruitment and rate modulation were examined in isometric muscles and during constant-velocity stretches. 2. Analysis of multiunit EMG activity and its relationship to active force revealed a marked difference between isometric and lengthening contractions. While the force-EMG relationship for isometric contractions was characteristically linear, the relation recorded during stretch-reflex responses showed a disproportionate early EMG increase, which was most obvious at low force levels, suggesting that the efficacy of force production is reduced in lengthening muscle. 3. Single-unit recruitment patterns were found to be qualitatively similar in isometric and lengthening contractions. In each case, motor units were recruited in order of increasing spike voltage. The numbers of newly recruited units declined steeply with each successive increment in active force. For a given unit, the force at which recruitment occurred was found to be greater in lengthening contractions than in isometric contractions, and in lengthening contractions it was also found to depend on the level of initial force. 4. Two patterns of motor-unit rate modulation were observed during muscle stretch, depending on whether a given unit was firing before the beginning of stretch or whether it was recruited during the course of stretch. Motor units that were active prior to stretch were found to increase firing rate at stretch onset and to vary their rate very little thereafter. Motor units recruited in the course of stretch began firing at an initial rate proportional to their force threshold, gradually increased their firing rate with increasing force, and sometimes reached an apparent maximum rate. 5. These results are discussed in terms of the mechanical properties of lengthening muscle and reflex regulation of these properties. Each identified pattern of motor-unit recruitment and rate modulation is evaluated for its potential contribution to the regulation of muscle properties, especially the prevention of muscle yield. We conclude that at low to moderate levels of initial force, recruitment of new motor units is likely to be the most effective compensatory mechanism.

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. III. Muscle-unit force modulation

1996 ◽  
Vol 75 (1) ◽  
pp. 51-59 ◽  
Author(s):  
K. E. Tansey ◽  
A. K. Yee ◽  
B. R. Botterman
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Muscle Force ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Rate Variation ◽  
Unit Force ◽  
Rate Modulation ◽  
Force Modulation ◽  
Whole Muscle

1. The aim of this study was to examine the extent of muscle-unit force modulation due to motoneuron firing-rate variation in type-identified motor units of the cat medial gastrocnemius (MG) muscle, and to investigate the contribution of muscle-unit force modulation to whole-muscle force regulation. The motoneuron discharge patterns recorded from 8 pairs of motor units during 12 smoothly graded muscle contractions evoked by stimulation of the mesencephalic locomotor region (MLR) were used to reactivate those units in isolation to estimate what their force profiles would have been like during the evoked whole-muscle contractions. 2. For most motor units, muscle-unit force modulation was similar to motoneuron firing-rate modulation, in that muscle-unit force increased over a limited range (120-600 g) of increasing whole-muscle tension and was then maintained at a near maximal (> 70%) output level as muscle force continued to rise. Most muscle units also decreased their force outputs over a slightly larger range of declining whole-muscle force before relaxing. This second finding was best explained by the counterclockwise hysteresis recorded in the motor units' frequency-tension (f-t) relationships. 3. In those instances when whole-muscle force fluctuated just above the recruitment threshold of a motor unit, a substantial percentage (10-25%) of the change in whole-muscle force could be accounted for by force modulation in that motor unit alone. This finding suggested that few motor units in the pool were simultaneously simultaneously undergoing force modulation. To evaluate this possibility, the extent of parallel muscle-unit force modulation within the 8 pairs of simultaneously active motor units was evaluated. As with parallel motoneuron firing-rate modulation, the extent of parallel muscle-unit force modulation was limited to unit pairs of the same physiological type and recruitment threshold. In several instances, pairs of motor units displayed parallel motoneuron firing-rate modulation but did not show parallel muscle-unit force modulation because of the nature of the motor units' f-t relationships. 4. The limited extent of parallel muscle-unit force modulation seen in these experiments implies that the major strategy for force modulation in the cat MG muscle, involving contractions estimated to reach 30-40% of maximum, may be motor-unit recruitment rather than motor-unit firing-rate variation with resulting force modulation. Given, however, that the majority of motor units are already recruited at these output levels (< 40%), it is proposed that motor-unit firing-rate variation with resulting force modulation may take over as the major muscle force modulating strategy at higher output levels.

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.

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. II. Motoneuron firing-rate modulation

1996 ◽  
Vol 75 (1) ◽  
pp. 38-50 ◽  
Author(s):  
K. E. Tansey ◽  
B. R. Botterman
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Muscle Force ◽  
Motor Units ◽  
Medial Gastrocnemius ◽  
Firing Rates ◽  
Rate Modulation ◽  
The Mean ◽  
Fast Twitch ◽  
The Relationship

1. The aim of this study was to examine the nature of motoneuron firing-rate modulation in type-identified motor units during smoothly graded contractions of the cat medial gastrocnemius (MG) muscle evoked by stimulation of the mesencephalic locomotor region (MLR). Motoneuron discharge patterns, firing rates, and the extent of firing-rate modulation in individual units were studied, as was the extent of concomitant changes in firing rates within pairs of simultaneously active units. 2. In 21 pairs of simultaneously active motor units, studied during 41 evoked contractions, the motoneurons' discharge rates and patterns were measured by processing the cells' recorded action potentials through windowing devices and storing their timing in computer memory. Once recruited, most motoneurons increased their firing rates over a limited range of increasing muscle tension and then maintained a fairly constant firing rate as muscle force continued to rise. Most motoneurons also decreased their firing rates over a slightly larger, but still limited, range of declining muscle force before they were derecruited. Although this was the most common discharge pattern recorded, several other interesting patterns were also seen. 3. The mean firing rate for slow twitch (type S) motor units (27.8 imp/s, 5,092 activations) was found to be significantly different from the mean firing rate for fast twitch (type F) motor units (48.4 imp/s, 11,272 activations; Student's t-test, P < 0.001). There was no significant difference between the mean firing rates of fast twitch, fatigue-resistant (type FR) and fast twitch, fatigable (type FF) motor units. When the relationship between motoneuron firing rate and whole-muscle force was analyzed, it was noted that, in general, smaller, lower threshold motor units began firing at lower rates and reached lower peak firing rates than did larger, higher threshold motor units. These results confirm both earlier experimental observations and predictions made by other investigators on the basis of computer simulations of the cat MG motor pool, but are in contrast to motor-unit discharge behavior recorded in some human motor-unit studies. 4. The extent of concomitant changes in firing rate within pairs of simultaneously active motor units was examined to estimate the extent of simultaneous motoneuron firing-rate modulation across the motoneuron pool. A smoothed (5 point sliding average) version of the two motoneurons' instantaneous firing rates was plotted against each other, and the slope and statistical significance of the relationship was determined. In 16 motor-unit pairs, the slope of the motoneurons' firing-rate relationship was significantly distinct from 0. Parallel firing-rate modulation (< 10-fold difference in firing rate change reflected by a slope of > 0.1) was noted only in pairs containing motor units of like physiological type and then only if they were of similar recruitment threshold. 5. Other investigators have demonstrated that changes in a motoneuron's "steady-state" firing rate predictably reflect changes in the amount of effective synaptic current that cell is receiving. The finding in the present study of limited parallel firing-rate modulation between simultaneously active motoneurons would suggest that changes in the synaptic drive to the various motoneurons of the pool is unevenly distributed. This finding, in addition to the findings of orderly motor-unit recruitment and the relationship between motor-unit recruitment threshold and motoneuron firing rate, cannot be adequately accommodated for by the existing models of the synaptic organization in motoneuron pools. Therefore a new model of the synaptic organization within the motoneuron pool has been proposed.

scholarly journals Absence of lateral gastrocnemius activity and differential motor unit behavior in soleus and medial gastrocnemius during standing balance

2014 ◽  
Vol 116 (2) ◽  
pp. 140-148 ◽  
Author(s):  
Martin E. Héroux ◽  
Christopher J. Dakin ◽  
Billy L. Luu ◽  
John Timothy Inglis ◽  
Jean-Sébastien Blouin
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Standing Position ◽  
Standing Balance ◽  
Triceps Surae ◽  
Medial Gastrocnemius ◽  
Isometric Contractions ◽  
Functional Requirements ◽  
Vertical Projection ◽  
Lateral Gastrocnemius

In a standing position, the vertical projection of the center of mass passes in front of the ankle, which requires active plantar-flexor torque from the triceps surae to maintain balance. We recorded motor unit (MU) activity in the medial (MG) and lateral (LG) gastrocnemius muscle and the soleus (SOL) in standing balance and voluntary isometric contractions to understand the effect of functional requirements and descending drive from different neural sources on motoneuron behavior. Single MU activity was recorded in seven subjects with wire electrodes in the triceps surae. Two 3-min standing balance trials and several ramp-and-hold contractions were performed. Lateral gastrocnemius MU activity was rarely observed in standing. The lowest thresholds for LG MUs in ramp contractions were 20–35 times higher than SOL and MG MUs ( P < 0.001). Compared with MUs from the SOL, MG MUs were intermittently active ( P < 0.001), had higher recruitment thresholds ( P = 0.022), and greater firing rate variability ( P < 0.001); this difference in firing rate variability was present in standing balance and isometric contractions. In SOL and MG MUs, both recruitment of new MUs ( R2 = 0.59–0.79, P < 0.01) and MU firing rates ( R2 = 0.05–0.40, P < 0.05) were associated with anterior-posterior and medio-lateral torque in standing. Our results suggest that the two heads of the gastrocnemius may operate in different ankle ranges with the larger MG being of primary importance when standing, likely due to its fascicle orientation. These differences in MU discharge behavior were independent of the type of descending neural drive, which points to a muscle-specific optimization of triceps surae motoneurons.

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.

scholarly journals Common Synaptic Input to the Human Hypoglossal Motor Nucleus

2011 ◽  
Vol 105 (1) ◽  
pp. 380-387 ◽  
Author(s):  
Christopher M. Laine ◽  
E. Fiona Bailey
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Synaptic Input ◽  
Motor Units ◽  
Motor Nucleus ◽  
Tongue Protrusion ◽  
Ongoing Research ◽  
Airway Patency ◽  
Significant Difference ◽  
Common Drive

The tongue plays a key role in various volitional and automatic functions such as swallowing, maintenance of airway patency, and speech. Precisely how hypoglossal motor neurons, which control the tongue, receive and process their often concurrent input drives is a subject of ongoing research. We investigated common synaptic input to the hypoglossal motor nucleus by measuring the coordination of spike timing, firing rate, and oscillatory activity across motor units recorded from unilateral (i.e., within a belly) or bilateral (i.e., across both bellies) locations within the genioglossus (GG), the primary protruder muscle of the tongue. Simultaneously recorded pairs of motor units were obtained from 14 healthy adult volunteers using tungsten microelectrodes inserted percutaneously into the GG while the subjects were engaged in volitional tongue protrusion or rest breathing. Bilateral motor unit pairs showed concurrent low frequency alterations in firing rate (common drive) with no significant difference between tasks. Unilateral motor unit pairs showed significantly stronger common drive in the protrusion task compared with rest breathing, as well as higher indices of synchronous spiking (short-term synchrony). Common oscillatory input was assessed using coherence analysis and was observed in all conditions for frequencies up to ∼5 Hz. Coherence at frequencies up to ∼10 Hz was strongest in motor unit pairs recorded from the same GG belly in tongue protrusion. Taken together, our results suggest that cortical drive increases motor unit coordination within but not across GG bellies, while input drive during rest breathing is distributed uniformly to both bellies of the muscle.

scholarly journals Blood Flow Restriction Alters Motor Unit Behavior During Resistance Exercise

2019 ◽  
Vol 40 (09) ◽  
pp. 555-562 ◽  
Author(s):  
Pedro Fatela ◽  
Goncalo V. Mendonca ◽  
António Prieto Veloso ◽  
Janne Avela ◽  
Pedro Mil-Homens
Keyword(s):  
Blood Flow ◽  
Firing Rate ◽  
Motor Unit ◽  
Motor Units ◽  
Blood Flow Restriction ◽  
Motor Unit Recruitment ◽  
Recruitment Threshold ◽  
Firing Rates ◽  
Flow Restriction ◽  
Voluntary Contractions

AbstractWe aimed to determine whether blood flow restriction (BFR) alters the characteristics of individual motor units during low-intensity (LI) exercise. Eight men (26.0±3.8 yrs) performed 5 sets of 15 knee extensions at 20% of one-repetition maximum (with and without BFR). Maximal isometric voluntary contractions (MVC) were performed before and after exercise to quantify force decrement. Submaximal isometric voluntary contractions were additionally performed for 18 s, matching trapezoidal target-force trajectories at 40% pre-MVC. EMG activity was recorded from the vastus lateralis muscle. Then, signals were decomposed to extract motor unit recruitment threshold, firing rates and action potential amplitudes (MUAP). Force decrement was only seen after LI BFR exercise (–20.5%; p<0.05). LI BFR exercise also induced greater decrements in the linear slope coefficient of the regression lines between motor unit recruitment threshold and firing rate (BFR: –165.1±120.4 vs. non-BFR: –44.4±33.1%, p<0.05). Finally, there was a notable shift towards higher values of firing rate and MUAP amplitude post-LI BFR exercise. Taken together, our data indicate that LI BFR exercise increases the activity of motor units with higher MUAP amplitude. They also indicate that motor units with similar MUAP amplitudes become activated at higher firing rates post-LI BFR exercise.

VASTUS LATERALIS MOTOR UNIT RECRUITMENT THRESHOLDS ARE COMPRESSED TOWARDS LOWER FORCES IN OLDER MEN

2020 ◽  
pp. 1-6
Author(s):  
R.M. Girts ◽  
J.A. Mota ◽  
K.K. Harmon ◽  
R.J. MacLennan ◽  
M.S. Stock
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Vastus Lateralis ◽  
Motor Units ◽  
Older Men ◽  
Force Level ◽  
Recruitment Threshold ◽  
Rate Versus ◽  
Younger Men ◽  
The Mean

Background: Aging results in adaptations which may affect the control of motor units. Objective: We sought to determine if younger and older men recruit motor units at similar force levels. Design: Cross-sectional, between-subjects design. Setting: Controlled laboratory setting. Participants: Twelve younger (age = 25 ± 3 years) and twelve older (age = 75 ± 8 years) men. Measurements: Participants performed isometric contractions of the dominant knee extensors at a force level corresponding to 50% maximal voluntary contraction (MVC). Bipolar surface electromyographic (EMG) signals were detected from the vastus lateralis. A surface EMG signal decomposition algorithm was used to quantify the recruitment threshold of each motor unit, which was defined as the force level corresponding to the first firing. Recruitment thresholds were expressed in both relative (% MVC) and absolute (N) terms. To further understand age-related differences in motor unit control, we examined the mean firing rate versus recruitment threshold relationship at steady force. Results: MVC force was greater in younger men (p = 0.010, d = 1.15). Older men had lower median recruitment thresholds in both absolute (p = 0.005, d = 1.29) and relative (p = 0.001, d = 1.53) terms. The absolute recruitment threshold range was larger for younger men (p = 0.020; d = 1.02), though a smaller difference was noted in relative terms (p = 0.235, d = 0.50). These findings were complimented by a generally flatter slope (p = 0.070; d = 0.78) and lower y-intercept (p = 0.009; d = 1.17) of the mean firing rate versus recruitment threshold relationship in older men. Conclusion: Older men tend to recruit more motor units at lower force levels. We speculate that recruitment threshold compression may be a neural adaptation serving to compensate for lower motor unit firing rates and/or denervation and subsequent re-innervation in aged muscle.

Motor-unit responses in human wrist flexor and extensor muscles to transcranial cortical stimuli

1987 ◽  
Vol 58 (5) ◽  
pp. 1168-1185 ◽  
Author(s):  
B. Calancie ◽  
M. Nordin ◽  
U. Wallin ◽  
K. E. Hagbarth
Keyword(s):  
Motor Unit ◽  
Single Unit ◽  
Motor Units ◽  
Onset Latency ◽  
Size Principle ◽  
Single Motor Unit ◽  
Motor Responses ◽  
Wrist Flexor ◽  
Extensor Muscles ◽  
Single Unit Recordings

1. Transcranial cortical stimuli (TCCS) were used to elicit motor responses in contralateral wrist flexor and extensor muscles of healthy adult subjects. The motor responses were assessed by surface EMG recordings, by needle recordings of single motor-unit discharges, and by measurements of wrist twitch force. Our main aim was to analyze the single-unit events underlying those changes in latency, amplitude, and duration of the compound EMG responses, which could be induced by voluntary preactivation of target muscles and by changes in stimulation strength. 2. Different stimulus strengths were tested with and without background contractions in the flexor or extensor muscles. For each test (consisting of a series of 20 stimuli) the compound EMG responses were averaged and displayed together with the averaged wrist force signals. Responses of individual flexor and extensor motor units were displayed in raster diagrams and peristimulus time histograms. For units exhibiting a background firing, the mean background interdischarge interval was calculated and compared with the subsequent poststimulus intervals. 3. In relaxed muscles, a shortening of onset latency of evoked compound EMG responses was observed when raising stimulation strength. A similar latency reduction was not seen in any of the single-unit recordings. This would be consistent with the size principle of motoneuron recruitment. 4. A shortening of onset latency of evoked EMG potentials was observed also as a result of a voluntary preactivation. Such latency shifts, which were seen also in single-unit recordings, might be attributed to variations in the time required for D and I wave temporal summation at the anterior horn cell. 5. When raising stimulation strength or when adding voluntary background contraction, the evoked compound EMG potential grew not only in amplitude but also in duration, as later peaks of activity were added to the initial ones. Under optimal conditions (strong stimulus + background contraction), the period of excitation (termed E1) had an onset latency of approximately 15 ms and a duration of approximately 35 ms and was similar for wrist flexor and extensor muscles. 6. We never saw the same flexor or extensor unit fire more than once during the E1 period. For units preactivated by a background contraction, the stimulus-triggered impulse exhibited latency shifts, which, to a large extent, depended on the timing of the stimulus in relation to a preceding background discharge and which could be influenced by a change in stimulation strength.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of Aging on Motor-Unit Control Properties

1999 ◽  
Vol 82 (5) ◽  
pp. 2081-2091 ◽  
Author(s):  
Zeynep Erim ◽  
M. Faisal Beg ◽  
David T. Burke ◽  
Carlo J. de Luca
Keyword(s):  
Firing Rate ◽  
Motor Unit ◽  
Motor Units ◽  
Elderly Subjects ◽  
Compensatory Mechanism ◽  
Type I ◽  
Firing Behavior ◽  
Firing Rates ◽  
Age Related ◽  
Young Subjects

It was hypothesized that the age-related alterations in the morphological properties of a motor unit would be accompanied by modifications in the control aspects of the motor unit, as either an adaptive or compensatory mechanism to preserve smooth force production. In specific, the objective of the study was to investigate the age-related alterations in the concurrent firing behavior of multiple motor units in the first dorsal interosseous (FDI) muscle in isometric contractions at 20 and 50% of the subject's voluntary contraction level. Analysis of the data collected from 10 young (24–37 yr of age) and 10 elderly (65–88 yr of age) subjects led to three novel observations regarding the firing behavior of aged motor units. 1) Among elderly subjects, there is a decrease in the common fluctuations that are observed among the firing rates of motor units in the young. 2) The relationship observed between the firing rate and recruitment threshold of young subjects is disturbed in the elderly. Although in young subjects, at any point in a given submaximal contraction, earlier recruited motor units have higher firing rates than later-recruited units; in aged subjects this dependency of firing rate on recruitment rank is compromised. 3) The progressive decrease observed in the firing rates of concurrently active motor units in constant-force contractions in the young is not seen in the aged. In addition to these original findings, this study provided support for earlier reports of 1) decreased average firing rates probably reflecting the slowing of the muscle, 2) a shift in recruitment thresholds toward lower force levels in line with the shift toward type I fibers, and 3) multiphasic action potential shapes indicative of the reinnervation process that takes place during aging. Taken as a whole, these findings indicate significant age-related modifications in the control properties of human motor units.

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